Submitted to the Proceedings of the US Community Study

on the Future of Particle Physics (Snowmass 2021)

SLAC-PUB-17660

C3 Demonstration Research and Development Plan

Editors:

Emilio A. Nanni7, Martin Breidenbach7, Caterina Vernieri7, Sergey
Belomestnykh3,8, Pushpalatha Bhat3 and Sergei Nagaitsev3,11

Authors:

Mei Bai7, Tim Barklow7, William J. Berg1, John Byrd1, Ankur Dhar7, Ram C.
Dhuley3, Chris Doss10, Joseph Duris7, Auralee Edelen7, Claudio Emma7, Josef
Frisch7, Annika Gabriel7, Spencer Gessner7, Carsten Hast7, Chunguang Jing1,
Arkadiy Klebaner3, Anatoly K. Krasnykh7, John Lewellen7, Matthias Liepe2,
Michael Litos10, Xueying Lu1, Jared Maxson2, David Montanari3, Pietro
Musumeci9, Alireza Nassiri1, Cho-Kuen Ng7, Mohamed A. K. Othman7, Marco
Oriunno7, Dennis Palmer7, J. Ritchie Patterson2, Michael E. Peskin7, Thomas J.
Peterson7, John Power1, Ji Qiang4, James Rosenzweig9, Vladimir Shiltsev,
Evgenya Simakov5, Emma Snively7, Bruno Spataro6, Sami Tantawi7, Brandon
Weatherford7, Glen White7, and Kent P. Wootton1

1Argonne National Laboratory
2Cornell University
3Fermi National Accelerator Laboratory
4Lawrence Berkeley National Laboratory
5Los Alamos National Laboratory
6National Laboratory of Frascati, INFN-LNF
7SLAC National Accelerator Laboratory, Stanford University
8Stony Brook University
9University of California, Los Angeles
10University of Colorado, Boulder
11University of Chicago

###### Contents

  1. 1 Introduction
  2. 2 Objectives & Timeline
  3. 3 C3 Demonstration Research and Development Plan Topics
    1. 3.1 Direct R&D
      1. 3.1.1 Accelerating Structures for Main Linac and Source Linac
      2. 3.1.2 Cryomodule Cryogenics
      3. 3.1.3 Cryomodule Internals Design
      4. 3.1.4 Cryomodule Assembly
      5. 3.1.5 Beam Dynamics
      6. 3.1.6 DC Polarized Electron Gun and Injector
      7. 3.1.7 RF High-Brightness Photo-Injector for the Demonstrator
      8. 3.1.8 RF High-Brightness Polarized Photo-Injector
    2. 3.2 High Power Klystron Upgrades
      1. 3.2.1 Low Level RF and Klystron Controls
      2. 3.2.2 Precision Timing / Phase Distribution
      3. 3.2.3 Beam Diagnostics
      4. 3.2.4 Accelerator Raft Alignment
      5. 3.2.5 Vibrations
      6. 3.2.6 High Power RF Distribution
        1. 3.2.7 Start-to-end simulation
    3. 3.3 Parallel Research and Development
      1. 3.3.1 Damping Rings
      2. 3.3.2 Beam Delivery System and Final Focus (BDS/FF)
      3. 3.3.3 Levitated Positron Target - Radiatively Cooled
      4. 3.3.4 Advanced RF Source Research and Development
      5. 3.3.5 RF Pulse Compression
    4. 3.4 Industrialization
    5. 3.5 Additional HEP Research and Development
      1. 3.5.1 Artificial Intelligence and Machine Learning (AI/ML)
      2. 3.5.2 Plasma Lens
      3. 3.5.3 XCC
      4. 3.5.4 Polarized Positrons
  4. 4 Infrastructure Requirements for Demonstrator R$\&$D
    1. 4.1 Basic Requirements
    2. 4.2 Siting Options
      1. 4.2.1 ESB at SLAC
      2. 4.2.2 FAST at Fermilab
      3. 4.2.3 UCLA Research and Infrastructure
      4. 4.2.4 CERF-NM and LEDA building at LANL
      5. 4.2.5 LEA at Argonne
    3. 4.3 Hardware
  5. 5 Acknowledgements

## Executive Summary

C3 is an opportunity to realize an $e^{+}e^{-}$ collider for the study of the
Higgs boson at $\sqrt{s}=250$ GeV, with a well defined upgrade path to 550 GeV
while staying on the same short facility footprint [1, 2]. C3 is based on a
fundamentally new approach to normal conducting linear accelerators that
achieves both high gradient and high efficiency at relatively low cost. Given
the advanced state of linear collider designs, the key system that requires
technical maturation for C3 is the main linac. This white paper presents the
staged approach towards a facility to demonstrate C3 technology with both
Direct (source and main linac) and Parallel (beam delivery, damping ring,
ancillary component) R&D. The white paper also includes discussion on the
approach for technology industrialization, related HEP R&D activities that are
enabled by C3 R&D, infrastructure requirements and siting options.

The primary goal of the C3 Demonstration R&D Plan is to reduce technical and
cost risk by building and operating the key components of C3 at an adequate
scale. Each stage, see Table 1 has clear technical objective and deliverables
that are synergistic with other accelerator based research fields. This R&D
plan starts with the engineering design, and demonstration of one cryomodule
and will culminate in the construction of a 3 cryomodule linac with pre-
production prototypes. This R&D program would also demonstrate the linac rf
fundamentals including achievable gradient and gradient stability over a full
electron bunch train and breakdown rates. It will also investigate beam
dynamics including energy spread, wakefields, and emittance growth. This work
will be critical to confirm the suitability of the C3 beam parameters for the
physics reach and detector performance in preparation for a Conceptual Design
Report (CDR), as well as for follow-on technology development and
industrialization.

The C3 Demonstration R&D Plan will open up significant new scientific and
technical opportunities based on development of high-gradient and high-
efficiency accelerator technology. It will push this technology to operate
both at the GeV scale and mature the technology to be reliable and provide
high-brightness electron beams.

The timeline for progressing with C3 technology development will be governed
by practical limitations on both the technical progress and resource
availability. It consists of four stages: Stage 0) Ongoing fundamental R&D on
structure prototypes, damping and vibrations. Stage 1) Advancing the
engineering maturity of the design and developing start-to-end simulations
including space-charge and wakefield effects. Stage 1 will terminate with
submission of a full proposal for the C3 Demonstration R&D Plan by the end of
2024. Stage 2) Production and testing of the first cryomodule at cryogenic
temperatures. This would provide sufficient experimental data to compile a CDR
and it is anticipated for Stage 2 to last 3 years (2025-2027) and to culminate
with the transport of photo-electrons through the first cryomodule. Stage 3)
Updates to the engineering design of the cryomodules, production of the second
and third cryomodule and their installation. Beam tests would be performed
with increasing beam current to test full beam loading. Lower charge and lower
emittance beams will be used to investigate emittance growth. The successful
full demonstration of the 3 cryomodules to deliver up to a 3 GeV beam and
achieve the C3-550 gradient will allow a comprehensive and robust evaluation
of the technical design of C3 as well as mitigate technical, schedule, and
cost risks required to proceed with a Technical Design Report (TDR).

|  Time Frame |  Key R&D |  Synergy and Spin-Offs
---|---|---|---
Stage 0 |  Ongoing |  Fundamental structure R&D with prototype structure demonstration with beam and corresponding industrialization |  Cost effective compact linacs for medical, security and industrial applications (irradiation with electrons, x-rays)
Stage 1 |  2022-2024 |  Beamline and cryogenics design study for demonstrator. Cryomodule engineering design and raft prototyping. |  High brightness electron source and photo injector feasibility. Linacs for injection at scientific facility (injectors, booster, capture, etc.)
Stage 2 |  2025-2027 |  First high-gradient test with cryomodule. Implement one-cryomodule based linac to allow test with beam. |  C3 based next generation X-FEL, beam dynamics study including beam loading, compact light sources
Stage 3 |  2027-2029 |  Develop the second and third cryomodules, demonstration with beam up to full beam loading. |  Future facility studies: Beam dynamics, positron targets, advanced concept based final focusing for linear collider, PWFA experiments, etc.
Table 1: Staging of C3 Demonstration R&D Plan. A more detailed timeline is
presented in Figure 3

##  1 Introduction

Translating the high-gradients and exceptional rf efficiency of cryogenic-
copper rf accelerator technology from laboratory prototypes to practical and
scalable technology with GeV-class beams will have enormous benefits for HEP
and many related accelerator applications. However, the technical and
operational feasibility demonstration will require a dedicated and integrated
R&D plan that develops the modular units of the rf accelerator, control
system, and rf sources, while addressing the key technical concerns for
cryomodule engineering design, gradient, power, alignment, manufacturability,
and industrialization.

C3 is an opportunity to realize an $e^{+}e^{-}$ collider with this technology
for the study of the Higgs boson at $\sqrt{s}=250$ GeV, with a well defined
upgrade path to 550 GeV to study the top-Higgs Yukawa coupling and Higgs boson
pair production, while staying on the same short facility footprint [1]. The
target beam parameters for C3 are listed in Table 2. C3 can also be extended
to TeV scale energies to probe new physics in unexplored/underexplored weakly
coupled states that may well escape detection at the HL-LHC. C3 is based on a
fundamentally new approach to normal conducting linear accelerators that
achieves both high gradient and high efficiency at relatively low cost. This
design is described in [2]. The primary goal of the C3 Demonstration R&D Plan
is to reduce technical and cost risk by building and operating the key
components of C3 at an adequate scale.

Given the advanced state of linear collider designs, the key system that
requires technical maturation for C3 is the main linac. We propose a staged
R&D plan that will culminate in the construction of a 3 cryomodule linac with
the cryomodules being pre-production prototypes. This linac would be fed by an
S-band rf photo-injector, a booster linac and a magnetic bunch compressor. The
injector would utilize accelerator technology that would be suitable for the
electron source and positron capture linac. The linac would feed an energy
spectrometer for direct gradient measurements and then send the beam to a
dump. This linac and electron source technology will serve as direct
prototypes for FELs and other instruments requiring a high brightness, high
gradient, compact linac. The full demonstration linac is shown in Fig. 1. The
target tolerances for the injector and cryomodule are listed in Table 3. A
more detailed view of a single cryomodule is shown in Figure 2.

The completion of the C3 demonstration R&D program would demonstrate:

  * •

the linac rf fundamentals including achievable gradient and gradient stability
over a full electron bunch train and breakdown rates

  * •

beam dynamics including energy spread, wakefields, and emittance growth

  * •

cooling performance under liquid nitrogen (LN2) with direct measurements of
vibrations from several accelerators fitted with sensitive cryogenic
accelerometers

  * •

small quantity component costs providing a reliable basis for extrapolation to
C3 scale production

  * •

prototype assembly tooling and exploration of industrial production and
assembly

  * •

module-to-module assembly, possibly of simple robotic welding for cryostat
connections

Parameter [Unit] | Value | Value
---|---|---
CM Energy [GeV] | 250 | 550
Num. Bunches per Train | 133 | 75
Train Rep. Rate [Hz] | 120 | 120
Bunch Spacing [ns] | 5.26 | 3.5
Bunch Charge [nC] | 1 | 1
Beam Power [MW] | 2 | 2.45
Gradient [MeV/m] | 70 | 120
Effective Gradient [MeV/m] | 63 | 108
Shunt Impedance [M$\Omega$/m] | 300 | 300
Length [km] | 8 | 8
Table 2: Target beam parameters for C3 .

The injector for the demonstrator linac will consist of an rf photo-injector
with a removable CsTe cathode [3] or another cathode material capable of
producing suitable electron bunches. The goal will be to extract up to 133 nC
of charge over 700 ns from the photo-injector to match the beam loading of the
C3 main linac. The individual bunch charge will vary depending on the beam
test that is being conducted. Beam dynamics simulations of the injector and
demonstrator linac will determine the target bunch charge and bunch spacing.
The photo-injector will operate at S-band (a sub-harmonic of the C-band
accelerator frequency). The photo-injector will produce 3-5 MeV electron
bunches. This will be followed by matching optics to inject the beam into two
one-meter long S-band accelerating structures operating at cryogenic
temperatures. These linacs will operate at 50 MeV/m and will have a large
transverse aperture to minimize any short- or long-range wakefields. The
S-band section will bring the beam energy to 100 MeV. A magnetic chicane will
be used to compress the electron bunches prior to injection into the first
C-band cryomodule.

Figure 1: Schematic of the full 3 cryomodule C3 demonstrator capable of
achieving all of the technical milestones for the R&D plan. The full length of
the layout it $\sim$50 m. Figure 2: Rendering of the 9 m C3 cryomodule with 8
one meter accelerating structures, 4 two meter support rafts, 4 permanent
magnetic quadrupoles and integrated beam position monitors.

The C3 Demonstration R&D Plan will open up significant new scientific and
technical opportunities based on development of high-gradient and high-
efficiency accelerator technology. Accelerator technology that forms the basis
of C3 is presently being pursued and developed for multiple small-scale
accelerator projects. The C3 Demonstration R&D Plan will push this technology
to operate both at the GeV level and mature the technology to be reliable and
provide high-brightness electron beams. The rf photo-injector for the
demonstration will be a new design based on the recent advances in cryogenic
copper technology with significantly higher peak field at the emission
surface. These GeV-class high-brightness beams produced as part of the C3
demonstration plan could be utilized for a variety of follow-on experiments
targeting HEP applications, for beam physics studies, advanced accelerator
concepts (plasma-wakefield staging, plasma lenses), positron target
development, exploring novel concepts in generation of polarized beams, and
developing advanced techniques for machine control with AI/ML. Already with
GeV-class high-brightness beams many applications outside of HEP would be
directly impacted by C3 technology. For example, FEL, gamma-ray or x-ray
inverse Compton sources could be developed. The high-brightness high-charge
electron source could be utilized for ultra-fast electron
diffractometry/microscopy or incorporated as a new electron source for
existing x-ray FELs to push pulse energy, x-ray energy or repetition rate.

Goal arrival time stability (fs) | 20
---|---
Goal relative energy stability | $5\cdot 10^{-4}$
Goal peak current stability (%) | 0.5
Injector tolerances |
Photo cathode laser pointing stability ($\mu$m) | $\leq 10$
Photo cathode laser energy stability (%) | 0.84
Photo cathode laser jitter (fs) | 55
S-band RF phase stability (deg) | 0.02
S-band RF amplitude stability (%) | 0.04
Main Linac tolerances |
C-band RF phase stability (deg) | 0.3
C-band RF amplitude stability (%) | 0.1
Quadrupole alignment ($\mu$m) | 10
Structure alignment ($\mu$m) | 10
Table 3: Target tolerances for electron injector and cryomodules for the C3
demonstration.

##  2 Objectives & Timeline

The objective of the C3 Demonstration R&D Plan is aimed at mitigating C3
technical, schedule, and cost risks by achieving the following Technical
Milestones and the Critical Parameters in Table 4:

  * •

High-gradient ($\geq$155 MeV/m) test with gradient margins of a damped and
detuned C3 accelerating structures at S-band and C-band

  * •

Testing of the cryomodule at gradients required for C3-550 (goal is to reach
$\geq$155 MeV/m for margin)

  * •

Demonstration of a fully engineered cryomodule with full beam loading in at
least one cryomodule

  * •

Demonstration of full C3 LN2 and vapor flow in 3 cryomodules as well as
verification of vibrations and alignment stability.

  * •

Experimental verification of the tolerances and specifications for production
of the main linac cryomodules, including assembly procedures.

  * •

Demonstration of a viable rotation and cooling system for a positron
production target

Accelerating Structure |
---|---
Gradient (MeV/m) | 70/120
Flat Top (ns) | 700/250
Shunt Impedance (M$\Omega$/m) | 300
Breakdown Rate (/pulse/m) | 3$\times 10^{-7}$
Beam Loading (%) | 45
Alignment $\&$ Vibrations |
Main Linac Components ($\mu$m) | 10
Beam Delivery System Components ($\mu$m) | 10
Vertical Stabilization Linac Quad (nm $>$ 1 Hz) | 1.5
Table 4: Critical Parameters

The timeline for progressing with C3 technology development will be governed
by practical limitations on both the technical progress and resource
availability. Here we present the technically limited timeline for the
Demonstrator R&D Plan. The detailed breakdown is shown in Figure 3.

Figure 3: C3 Demonstration R&D Plan timeline. The timeline is undertaken in
four stages. Stage 0 consists of ongoing fundamental R&D for structure design,
manufacture and test. Including operation at high gradient, damping and
detuning designs, vibrations and beam test. Stage 1 is the preparatory phase
for preliminary engineering studies and submission of a full proposal. Stage 2
delivers the first cryomodule, the injector and and beam tests at the full C3
accelerating gradient. Stage 3 completes two additional cryomodules and
completes beam tests. Many applications will be enable by this technology
development plan and the rough timeline for those applications is also
highlighted.

Stage 0 At present, fundamental rf accelerator R&D that is relevant to C3 is
ongoing with the investigation of high-gradient accelerating structures, rf
design, rf components for power distribution and compression, frequency and
temperature scaling, materials, manufacturing techniques, wakefield
suppression, novel rf source concepts, and experimental prototypes at the
meter scale. This fundamental R&D is directly impacting the development of
small-scale and compact accelerators for industrial, security and medical
applications ranging from 10-100 MeV. These advances are also impacting larger
scale facilities, for example with the adoption of new concepts for pulse
compression and transverse deflectors. This technology is also being adopted
for both the rf gun and linac for compact x-ray and FEL sources.

Through the Snowmass process we are actively exploring how these innovations
in rf accelerator technology impact the design of C3 facilities that scale
from Higgs factory to a few TeV. This has allowed us to identify the key
parameters and milestones for advancing the C3 concept.

Stage 1 for the C3 Demonstration R&D Plan will serve to advance the
development of C3 technology to a level of engineered maturity to form the
basis for a demonstrator proposal. In particular, we will focus on the
engineering of the cryomodules that would be tested as part of the
demonstrator. This will include a damped-detuned accelerating structure,
prototype support rafts with alignment and vibration suppression, and full
consideration of the cryogenics design of the cryomodule. This will also
include the design of the injector and C3 demonstrator beamline design with
start-to-end simulations including space-charge and wakefield effects. In
addition to providing the technical information needed to produce a full
demonstrator proposal, Stage 1 activities will directly impact applications
requiring compact accelerators such as the activities described in Sections
4.2.3 and 4.2.4 targeting compact x-ray and gamma-ray sources. The injector
design could also be utilized for applications targeting very high charge
electron bunch sources (for examples as an upgrade to the EIC injector[4]) or
the high-brightness rf photo-injector (3.1.7) design could be utilized for a
an upgrade to LCLS-I to produce very high energy soft x-ray pulses [5], as
discussed in 3.5.3. In particular, the distributed-coupling rf linac
technology would also impact the realization of medium energy electron beam
injectors as it provides for a resilient design against failures in the rf
subsystems, i.e., klystrons. A demonstration S-band prototype accelerator
structure for electron injectors is currently under construction at SLAC to
demonstrate the attainable gradient, optimized beam aperture and operational
reliability [4].

Stage 1 will terminate with submission of a full proposal for the C3
Demonstration R&D Plan. We aim for a decision to proceed at the end of 2024.

Stage 2 of the C3 Demonstration R&D Plan will focus on the production and
testing of the first cryomodule. The first engineered design for a full
cryomodule will be tested initially with high power rf. The system will be
operated at cryogenic temperatures. The gradients that are achieved would
accelerate a beam to by 1 GeV in 9 m. Captured dark current would allow for
confirmation of this energy gain and gradient. In parallel we will build and
commission the electron source for the C3 demonstrator. This will consist of a
high-brightness rf photo-injector and two meters of S-band linac that can
support high charge. This linac would be adopted in the electron and positron
sources for C3 up until the damping ring. After commissioning the injector,
the photo-emitted beam would be compressed and transported into the first
cryomodule and the first beam tests would commence. This would be a major
milestone for C3 and provide sufficient experimental data to compile a CDR,
but also deliver a high-gradient accelerator technology that could be mass
produced and used for compact FELs, XFELs, compact gamma ray sources, or a
beam driver for PWFA. Stage 2 of the C3 Demonstration R&D Plan is anticipated
to take 3 years (2025-2027) and concludes with the transport of photo-
electrons through the first cryomodule.

Stage 3 of the C3 Demonstration R&D Plan includes updates to the engineering
design of the cryomodules, production of the second and third cryomodule and
their installation. The LN2 boiler would also be installed to allow for full
thermal and vibration tests. After the installation of each cryomodule, beam
tests would be performed with increasing beam current to test full beam
loading. Lower charge and lower emittance beams would be used to investigate
emittance growth. Stage 3 would be completed with the operation of the final
cryomodule with beam at the C3-550 gradient. Sufficient information would be
produced mitigating C3 technical, schedule, and cost risks required to proceed
with a TDR. After Stage 3, the C3 demonstrator could be utilized for a wide
variety of R&D related to technological improvements for C3 , related HEP R&D
or other applications such as a compact FEL as discussed in Sections 3.3 and
3.5.

Completion of the C3 Demonstration R&D Plan will allow us to achieve:

  * •

Demonstration of feasibility and first optimization of the C3 linac technology

  * •

Design and optimization of the linear collider accelerator complex based on C3
technology

  * •

Confirm the suitability of the C3 beam parameters for the physics reach and
detector performance

  * •

Preparation for a Conceptual Design Report (CDR)

  * •

Preparation for follow-on technology development and industrialization

##  3 C3 Demonstration Research and Development Plan Topics

###  3.1 Direct R&D

####  3.1.1 Accelerating Structures for Main Linac and Source Linac

The implementation of new approaches to designing, building and operating a
normal conducting accelerator structure has been key to the development of the
technology for C3 . The discovery process began with an ab initio study of
cavity shapes to maximize the on-axis accelerating field while minimizing
probability of breakdown. After the first optimization, some suitable cavity
shapes were proposed, however the optimized shapes had very small beam irises
that precluded travelling-wave cavity coupling for the fundamental
accelerating mode. As an alternative to on-axis coupling a distributed rf
coupling scheme was proposed, implemented by parallel manifold rf waveguides
providing side-coupling into each cavity with the proper rf phase and fraction
of the inlet rf power. It was concluded that this relatively complex structure
could be machined in two halves (or 4 quarters) by low-cost numerically-
controlled milling machines. In addition to allowing for fabrication of the
complex cavity shapes, this novel milling process results in ultra-high-vacuum
(UHV) quality surfaces that need no further finishing apart from a standard
copper surface etch. The final element of the proposed approach is operating
the copper accelerator at a reduced temperature to increase the electrical
conductivity of the material and improve the material strength and reduce
probability of breakdown. The benefit of increasing electrical conductivity is
in the reduction of the required rf power, as the rf power sources are a
costly and complex part of the linac infrastructure. Increased conductivity
also reduces thermal stresses in the material that result in cyclic fatigue of
the material exposed to rf pulses, crystal growth, motion of dislocations and
eventually in electrical breakdown. The onset of breakdown limits the
achievable gradient for the accelerating structure and sets the practical
gradient limit that the main linac can be operated at without degrading
luminosity.

The C3 structures will operate in a bath of LN2 cooled to a cryogenic
temperature of approximately $\sim$80 K. The increased conductivity of copper
at this temperature results in a shunt impedance of the optimized accelerator
cavity of 300 M$\Omega$/m, six times the effective shunt impedance of the Next
Linear Collider (NLC) X-band accelerating structure.

The C3 accelerating structure utilizes a copper standing wave distributed
coupling rf structure. The first meter-scale prototype C3 structure is shown
in Fig. 4. The aperture of the cavity is determined by considering short-range
and long-range wakefield effects for the nominal bunch charge of 1 nC. The
baseline phase advance between cells is $\pi$. The frequency of operation for
the main linac will be 5.712 GHz (C-band) in order to provide a high shunt
impedance. For the electron source, positron capture linac and booster linacs
into the damping ring, the operational frequency will be at S-band (2.856 GHz)
to accommodate the longer electron and positron bunch length before
compression. Prototype one meter accelerating structures have been fabricated
and tested at high gradient and at cryogenic temperatures. A similar C-band
accelerating structure was recently tested at Radiabeam and their room-
temperature high gradient test setup is shown in Fig. 5.

Figure 4: Both halves of the C3 prototype structure prior to braze. The one
meter structure consists of 40 cavities. A rf manifold that runs parallel to
the structure feeds 20 cavities on each side. The structure operates at 5.712
GHz. Figure 5: The C3 prototype structure in test at Radiabeam. Both room
temperature and cryogenic tests will be conducted as part of the C3
Demonstration R&D Plan to quantify improvements to the structure performance
as we incorporate damping and detuning in the design.

The proposed C3 beam format consists of trains of the electron bunches
recurring at 120 Hz. Each train will have 133 bunches with the bunch charges
of 1 nC separated by 5 ns for C3-250, and by 3 ns for C3-550. This short bunch
separation requires sophisticated wakefield control to prevent emittance
growth. Besides, the small diameter of the iris aperture of a/$\lambda$=0.05
has the potential to result in significant short range wake effects. The
emittance growth in the structure was analyzed for various beam parameters by
varying the offset in phase for longitudinal wake suppression and varying the
bunch length for transverse wake suppression. The goal of this simulation was
to achieve a residual energy spread in the range of $\sigma_{\delta 0}$ =
2-5x10-3 and a maximum of 1$\%$ correlated energy spread for Balakin-
Novokhatski-Smirnov (BNS) damping, which was analyzed with transport
simulations in the main linac[6].

Ongoing R&D for the structure development is focused on the design of the
structure’s damping and detuning to mitigate the effects of long range
wakefields. The kick factor must be suppressed below 1 V/pC/mm/m to provide
stable transport of the electron and positron beams. Detuning of higher order
modes is achieved by modifying the geometry of each cavity while maintaining
constant frequency of the fundamental mode. The frequencies of higher order
modes vary as a result of geometrical changes. Simulations have shown that
$4\sigma$ Gaussian detuning with a $\Delta f/f_{c}$ of 6$\%$ is sufficient to
suppress the longitudinal wake for the first subsequent bunch for the first
dipole band [6]. Detuned cavity designs exist already and they can cover the
necessary range in frequency ($\Delta f/f_{c}=6\%$). For longer range damping
we are designing a structure with the use of longitudinal damping slots in
quadrature that meets the required reduction in quality factor for higher
order modes.

The currently existing prototype accelerating structures are sufficiently
advanced to determine the rf power requirements, thermal loading, geometrical
constraints and suitability of manufacturing techniques. Still, significant
optimization of the accelerating structure within these limits is possible and
will be the focus of early stage R&D in this demonstration plan. Remaining
within these limits will allow improvements to the accelerator structure
without requiring a redesign of the cryomodule internals. These improvements
to the accelerating structure are in large part enabled by the rf design of
the distributed coupling topology which allows for fabrication of the
structure in halves, thereby allowing for simplified incorporation of rf
distribution and damping.

#### Accelerator Structure R$\&$D Topics:

  * •

Manufacturing \- Minimizing part count, minimizing raw material, simplifying
assembly and automating quality control and tuning.

  * •

Bonding Techniques \- Optimization of brazing with laser-cut shims, and
exploration of alternative low temperature techniques with electron beam
welding or diffusion bonding to increase throughput.

  * •

RF Design \- Optimization of rf phase advance to $3\pi/8$ phase advance per
cell, reduced cell length and proper power and phase rf manifold design for
distributed coupling.

  * •

Damping \- Advanced modeling and engineering are required to finalize the
design of wakefield damping features in the cavity. Material loss and
performance in UHV and high field environment must also be confirmed. Leading
material candidates are: NiCr, FeCrAl with a special technology of coat,
SiGraSiC-group composits, and doped SiC.

  * •

Thermomechanical Analysis \- Vibration, alignment, tolerancing, and thermal
analysis of accelerating structure.

  * •

Enhancement of heat transfer to LN2 \- Optimization of the accelerator
structure surface profile to minimize N2 gas bubble trapping under the
accelerator structure; Exploring surface treatment for example, special
texturing and coatings that enhance nucleate boiling heat transfer; surface
passivation techniques that prevent surface oxidation (oxidation is known to
reduce nucleate boiling heat transfer over time and so should be prevented).

####  3.1.2 Cryomodule Cryogenics

The accelerating structure operates immersed in a liquid bath of LN2 at
$\sim$80 K. The thermal load of the accelerator is cooled by the boiling of
liquid nitrogen. The cryomodule transports both the liquid and gaseous
nitrogen in the same vacuum jacketed enclosure that houses the accelerator.
The accelerator is in cryomodules that house 4 rafts; each raft supports 2
accelerator structures, a permanent magnet quadrupole, and beam position
monitors (BPM). A rendering of the C3 cryomodule is shown in Fig. 2. The
improvement in Cu conductivity at $\sim$80 K improves the accelerator
efficiency by a factor of 2.5-2.7X to more than recover the capital and
operating expenses of the refrigeration plants. For the C3 collider it is
envisioned that the main linac sectors consist of 10 cryomodules, and super-
sectors consist of 10 sectors111The actual main linacs may contain partial
super-sectors.. A super-sector is supported by a single entry and exit point
for liquid and gaseous nitrogen. It is anticipated that there will be either 2
or 3 super-sectors per main linac for C3 .

The cryomodule is a vacuum insulated tube with an inner radius of 30 cm. The
accelerator components are under slowly flowing LN2 at a pressure of $\sim$1.1
bar, with the LN2 $\sim$25 cm above the cryostat low point. The LN2 is
introduced at the super-sector boundaries or the ends, and flows in both
directions (one if at an end) for at most 500 m. Gaseous nitrogen counter-
flows, is removed at the boundaries or ends, is re-liquified, and then re-
injected. The LN2 for one direction of flow enters at 7.2 l/s, and flows at a
velocity (at the entry) of 0.06 m/s (0.2 km/hr). The counterflow gas flow has
an area equivalent to a 23 cm radius pipe, and has a pressure drop over the
full length of $\leq$10 mbar.

The LN2 flow is driven by gravity. For the C3 collider, the spans between
super-sectors are laser straight, the mid-span is normal to the earth radius
there, and thus the LN2 is deeper at the center of a super-sector by $\sim$7
cm. The beam will be bent in the vertical plane at the super-sector boundary
by $\sim$80 micro-radians to go into the next super-sector. For a 100 GeV
beam, this requires a dipole with a kick of 0.05 T-m. For C3-250 and C3-550,
the power dissipated in one accelerator section is 2500 W, or 0.4 W/cm2. This
thermal load is constant because of the reduced flat top of the rf pulse. With
this thermal load, the accelerator will operate in the nucleate boiling
regime, and the expected temperature rise is $\sim$2 K. The temperature rise
in the copper block in a 1D approximation is 0.6 K.

The open channel (“river”) of LN2 flow must be driven by a pressure head,
which comes from a liquid depth difference. The various features providing
liquid flow area changes, like vertical waveguides and cryomodule
interconnects, result in a resistance to liquid flow and could potentially
create a liquid flow problem due to a large pressure head required. This will
be carefully evaluated to provide the assured margin (through the cryomodule
diameter) as the design progresses.

Very early in the conceptual design phase, it is important to consider failure
modes and the potential impact on design features. One of those is loss of
insulating vacuum, which is not unusual and often results from a leaking
electrical feedthrough. The result will be cooling and thermal contraction of
the vacuum vessels, so one needs allowance for that. Thus, not only for
alignment and assembly, but also for thermal contraction, we will investigate
the inclusion of vacuum bellows periodically in the vacuum vessel string.

The C3 Demonstrator will run in two modes:

  * •

Accelerator development where there will be minimal LN2 flow. The LN2 will be
kept at nominal height above the accelerator sections, and nitrogen vapor will
be vented.

  * •

Vapor and liquid flow testing: LN2 flow and vapor return between 2.5 and 10
kg/sec. The actual flow rate will be determined by the power into the LN2
boiler at the end of the cryomodule string opposite to the input cross, and
the input LN2 flow will be adjusted to maintain the LN2 height in the
cryomodules.

The conceptual design is shown in Figure 1 and consists of an electron source
and injector that operates at 100 MeV followed by three cryomodules. The entry
point for the cryogenic liquid is between the injector and the three
cryomodules. After the three cryomodules a liquid nitrogen boiler is used to
simulate the thermal load of a full super-sector with resistive heating. The
boiler will consume power only during vapor and liquid flow testing.

LN2 enters and vapor exits through a cross that will be a full prototype of a
super-sector cross, see Fig. 6. LN2 enters the bottom of the cross through a
metering valve from a $\sim$50,000 liter LN2 storage dewar kept at $\sim$80 K.
The valve is controlled by the liquid level in the cryomodules, and feed
forward control based on power dissipated in the liquid can be tested. Vapor
exits from the top of the cross through a metering valve that is controlled to
maintain $\sim$1.1 bar in the cryomodules.

Figure 6: Rendering of the cross for inserting liquid nitrogen and removing
nitrogen gas from the cryomodule. Liquid nitrogen comes in from bottom and is
directed laterally in two counter propagating directions. The inlet control
valve regulates on liquid level and is capable of supplying the full liquid
flow rate envisioned for the C3 main linac. The outlet valve regulates on the
pressure of vapor in the cryomodules.

LN2 fills the boiler at the downstream end of the cryomodules to the level in
the cryomodules. During the first mode of operation, the heaters are off. For
the second mode of operation, the heaters can be powered up to $\sim$2 MW,
which will produce $\sim$10 kg/sec of vapor. This will provide adequate margin
for testing the system beyond expected flow rates.

We will pursue optical studies of nitrogen two-phase flow in the cryomodule to
probe probe two-phase instabilities in the vessel. $N_{2}$ gas phase
velocities corresponding to 10 kg/s mass flow can be high enough to initiate a
surface wave instability on the LN2 underneath the vapor. An optical setup
such as a transparent windows on the demonstrator vacuum and LN2 vessels will
help probe two-phase instabilities vessel. Alternately, cryogenic cameras can
be housed inside the LN2 vessel to capture images of LN2 surface and determine
gas velocities that lead to the instability. During a cold rf test, this
optical setup will also provide information on nucleate boiling, for instance
whether gas bubbles actually trap under the accelerator structure.

The S-band accelerator for the the electron source of the demonstrator will
also be in a cryomodule, and will be linked to the super-sector cross by a
cryomodule containing the normal conducting bunch compressor. This will allow
for the electron source linac topology to also be tested on the timeline of
the C3 Demonstration R&D Plan.

####  3.1.3 Cryomodule Internals Design

The Cryomodule functionality is:

  * •

Provision of stable support for the accelerator sections and quadrupoles.

  * •

Provision of the “cool” environment of $\sim$80 K, including heat removal from
the accelerators.

  * •

Provision of alignment capability within a cryomodule, and to adjacent
cryomodules.

  * •

Provide feedthroughs for waveguides and other cables.

  * •

Enabling of the above safely and reliably.

The basic mechanical building block is the “raft”, see Fig. 7. A raft consists
of two accelerator sections and a permanent magnet quadrupole. The quadrupole
has an integrated cavity beam position monitor (BPM). The quadrupole design
shown in Fig. 7 is based on a prototype developed by Electron Energy
Corporation (EEC) with an innovative field adjustment mechanism using
dynamically tunable magnets. EEC’s design is engineered for immersion in LN2
and enables a compact and uniquely flexible permanent magnet quadrupole that
achieves the field requirements, superior to current designs, for C3 . There
are 4 rafts per cryomodule which is $\sim$9 m long. There are 10 cryomodules
per sector, which is $\sim$100 m long. Finally there are 10 sectors per super-
sector, which is $\sim$1 km long. The C3 Demonstration R&D Plan calls for
three cryomodules to be tested in series. The cryomodule will fit in a
standard ISO shipping container (40 feet long).

Figure 7: Rendering of the 2 m C3 support raft with 2 one meter structures and
one permanent magnetic quadrupole.

The raft components are held by a channel which is a very open truss
structure. The two accelerators and the quadrupole are pre-aligned on the
raft. These components have 5 degrees of freedom (d.o.f.), with $Z$
constrained by a ball in a groove at each midpoint. The raft is suspended from
piezoelectric transducer stacks with a range of 200 microns and a bandwidth of
$\sim$100 Hz. These actuators are for beam based alignment and dynamic
feedback. The transducers are an intermediate stage suspended from high
resolution cryogenic electric motor jacks, with a range of $\sim$2 mm. These
jacks are primarily for inter-raft alignment, including the cryomodule
transitions. It is expected that they would rarely be used after initial
alignment unless there are upsets such as earthquakes. The motors are unusual
in that they can be dirty in the sense of throwing off particulates, and only
need a minimal bearing lifetime. A cross section of the cryomodule showing the
accelerating structure, the waveguide feed, support raft, quadrupole,
mechanical and piezoelectric movers, and stretched wire aligners is shown in
Fig. 8.

Figure 8: Rendering of the cryomodule cross section. In this view the the
accelerating structure, the waveguide feed, support raft, quadrupole,
mechanical and piezoelectric movers, and stretched wire aligners are visible.

The cryostat is a vacuum insulated pair of concentric cylinders, with a 30 cm
inner radius of the cold cryostat. The cold cryostat will (probably) be made
of 304 stainless steel, which closely matches the integrated thermal
contraction of Cu. The outer cryostat can be soft steel. The inner cryostat
will be wrapped with Multi-Layer-Insulation (MLI). Each accelerator section is
fed by one waveguide, and the two waveguides from one raft are routed together
through a port. That port also accommodates cables for the alignment motors,
transducers, alignment sensors, the BPM signals, and the quadrupole controls.

A pre-conceptual plan for the assembly of the waveguide ports exists. It is
hoped that all connections associated with cryomodule are welded, avoiding all
bolted flanges. The weld preps would be designed for cutting (“can opener”) or
grinding in case access were needed.

The integrated thermal contraction from room temperature to $\sim$80 K is
-0.31% for Cu and -0.29% for SS304. Therefore the $\Delta L$ between 1 m of Cu
accelerator and 1 m of SS304 raft is $\sim$200 $\mu$m. Thus the accelerator
midpoints are pinned to the stainless and the accelerator sections separate by
200 $\mu$m. This $\Delta L$ is taken by bellows on the beam pipes between
accelrating structures.

The 9 m SS304 inner cryostat contracts $\sim$3 cm. Since the outer steel shell
stays warm, sliding support is needed. The adjacent cryomodule inner cryostats
separate by 3 cm. This $\Delta L$ is taken by bellows between the the two
shells. The outer cryostats do not require bellows.

Gradient | Power diss. | rf flat top | Pulse | Comments | Power/area |  $\Delta T$ Cu-bulk
---|---|---|---|---|---|---
(MV/m) | (W) | (ns) | compr. |  | (W/cm2) | to LN2 (K)
70 | 2500 | 700 | N | C3-250 | 0.393 | 2.3
120 | 2500 | 250 | N | C3-550 | 0.393 | 2.3
155 | 3900 | 250 | N | C3-550 in 7 km | 0.614 | 2.5
120 | 1650 | 250 | Y | C3-550 | 0.259 | 2.1
Table 5: rf power losses in the accelerator structures for 4 considered C3
scenarios.

The rf thermal loads are shown in Table 5. For C3-250 and C3-550, the loads
are 2500 W per accelerator section. If C3-550 is forced into 7 km, then the
load will go up to 3900 W per section. If rf pulse compression is added, the
thermal load can be reduced to 1650 W per section. A goal of the Demonstrator
is to show that this range of loads can be handled without causing vibration
problems at the flow rates required for full super-sector flows, i.e. going up
to 10 kg/sec of Nitrogen. These power dissipation values correspond to loads
between 0.26 and 0.61 W/cm2, which are in the nucleate boiling regime. The
expected $\Delta T$ is below 2.5 K.

####  3.1.4 Cryomodule Assembly

It seems likely that the cryostat consisting of cold and warm tubes plus the
wave guide ports and and inter-module connections can be commercially procured
for the Demonstrator. The cryostat would also be fitted with the sliding
supports for contraction of the cold tube. This cryostat would be leak tested
warm and shipped to the assembly site in an ISO container fitted with suitable
restraints.

Rafts will be assembled and aligned on a 5 m granite table placed so the rafts
can be coupled together and pulled into the cryostat along with their
alignment wires. It will be possible to adjust the alignment wires by reaching
through the waveguide ports. All the remote alignment systems (motor driven
jacks and piezo transducers) will be tested before raft insertion. Each raft
will have a locking feature to the cold cryostat for transport that is
accessible from the cryostat ends. These locks will be released before final
placement of the cryomodules on the beamline.

####  3.1.5 Beam Dynamics

There are some key beam dynamics studies required to fully validate the new
style of distributed coupled structures. Following a program similar to NLC in
the past, the impact of short and long range wakefields on the accelerated
electron and positron beams need to be studied in comprehensive start-to-end
tracking simulations, with associated experimental tests at the demonstration
facility to qualify the expected wakefield kick factors. Such an experimental
demonstration would closely follow measurements made for the NLC program at
the NLCTA test facility [7]. A magnetic spectrometer following the C3
accelerating structures can be used to determine the beam energy and energy
spread and bunch-to-bunch offsets. A kicker in the out-of-bend plane can
provide the ability to separate the bunches to allow independent measurement
along the bunch train. Optics before the chicane will be implemented, along
with profile measurement devices, to infer the emittance of the beam.

Vibration and alignment tolerances are also a key consideration. Again,
tolerance calculations will follow from start-to-end simulations, including
the beam delivery system, which include such dynamic error sources and
associated beam-based feedback systems. Experimental validation of the
alignment of structures and magnets, together with direct measurement of
structure vibrations and the direct impact on the measured beam properties at
the demonstration facility will directly feed into these simulations. The
demonstration facility beam measurement suite will also be utilized to
directly measure the impact of observed breakdown events in the structures.

####  3.1.6 DC Polarized Electron Gun and Injector

The baseline electron (polarized) and positron (unpolarized) sources for C3
are conventional linear collider designs. For the electron source, this
consists of a polarized DC gun, buncher and accelerator. Extensive development
has been undertaken for a DC polarized electron source that is able to meet
the requirements for a linear collider (including the parameters required C3
)[8, 9]. The baseline plan for the C3 demonstration does not incorporate the
testing of a DC electron source with the cryomodules. However, as the design
of the C3 collider progresses, the DC guns that have been designed will need
to be revisited for the specific parameters of C3 . If it is deemed necessary
to test the acceleration of the beam to a few GeV, the C3 demonstration
facility would be a suitable facility for doing so. In addition to a DC gun as
the electron source for C3 , we are also exploring the possibility of a
polarized rf photo-injector and it would be the target for this demonstration
plan to incorporate such a (unpolarized) photo-injector first.

####  3.1.7 RF High-Brightness Photo-Injector for the Demonstrator

The significant increase in peak surface fields, upwards to 250 MV/m, with
cryogenic copper accelerating structures also opens a new frontier in the
development of high brightness polarized electron sources [10]. The achieved
surface fields in cryogenic copper structures are in principal sufficient to
produce an asymmetric emittance low enough that it would negate the need for
the electron damping ring [11]. Indeed this source is under development at
UCLA as is discussed in Section 4.2. Early stage R&D for this cryo-RF electron
gun is underway, so that a source of this type could be utilized for the
demonstration facility. The rf gun for the C3 demonstrator would operate at
S-band to support high charge bunches. The injector for the demonstrator linac
will consist of an rf photo-injector with a removable CsTe cathode [3]. The
goal will be to extract 133 one nC bunches from the photo-injector. The photo-
injector will operate at S-band (a sub-harmonic of the C-band accelerator
frequency). The photo-injector will produce 3-5 MeV electron bunches. For the
demonstrator a symmetric emittance electron bunch will be sufficient. We will
not implement a polarized photo-cathode in the initial operation of the rf gun
at the demonstrator. The development of this electron photo-injector has
direct implications for high-energy XFELs and $\gamma\gamma$ collider
concepts[5] as discussed in 3.5.3.

####  3.1.8 RF High-Brightness Polarized Photo-Injector

Several co-authors of this white paper (Patterson, Liepe, Maxson, Musumeci,
Rosenzweig) also participate in the Center for Bright Beams (CBB), a multi-
institutional National Science Foundation Science and Technology Center
focused on high brightness electron beam science. The center is composed of
three research thrusts: one focused on advancing the brightness of
photocathode sources, another on improving the performance of superconducting
RF cavities, and a third on developing methods for advanced beam control and
fundamental understanding of high brightness beam dynamics. Several aspects of
the Center’s research, as well as other related sponsored projects of Center
PIs, are directly relevant to C3 R&D, as described below; significant
infrastructure and expertise in these areas are already in place.

All proposed future $e^{+}e^{-}$ colliders including C3 require an electron
source with a high degree of spin polarization. The current state-of-the-art
polarized electron source remains GaAs activated to negative electron affinity
with a sub-monolayer of Cs and O2 or NF3. Several critical advancements have
improved performance beyond that of bulk GaAs: superlattice structures have
been shown to both dramatically enhance electron spin polarization[12], and
the inclusion of a Fabry-Perot-style optical resonator via additional layered
structures significantly enhances quantum efficiency by increasing
absorption[13]. However, perhaps the most challenging aspect using GaAs
photocathodes still remains: the Cs-containing activating layer is extremely
sensitive to oxidation, preventing use in all but extreme high vacuum (P<1e-10
torr).

CBB research in photocathodes devotes significant effort to the growth and
characterization of high quantum efficiency semiconductor materials. Among
these, both Cs3Sb and Cs2Te have been shown to be able to activate GaAs to
negative electron affinity. When grown thin enough, this activating layer
preserves the high electron spin polarization of superlattice GaAs[14].
Critically, while both Cs3Sb and Cs2Te must also be operated in pure vacuum,
they are far more robust than the previous monolayer Cs-based coatings; in
low-voltage test chambers, these spin-preserving activating layers have been
shown to have significantly longer lifetimes[15].

The cryocooled copper technology that underpins C3 enables the possibility of
a very high field photocathode rf gun (photocathode field in excess of 240
MV/m), with a brightness that potentially eliminates the need for an electron
damping ring. As the lifetime of polarized GaAs photocathodes is largely
determined by the integrity of the activating layer, it will be critical to
test these longer-lifetime activating materials in high extraction field for
lifetime, dark current, and intrinsic emittance. CBB researchers are actively
collaborating to test photocathodes grown at Cornell in the high field
photocathode rf guns at UCLA. CBB also actively investigates other novel
photocathode protection coatings, such as 2D materials, which may further
improve lifetime in extreme accelerator environments.

CBB researchers also actively investigate the performance of cryogenic rf
cavities. The primary cryogenic testing infrastructure for CBB is housed at
Cornell University. Beyond CBB, the Cornell SRF group has decades of
experience with in-house design, testing, and construction of superconducting
RF cavities and full cryomodule cooled to 2 K. The Cornell SRF group has also
pioneered several novel material approaches to cryogenic rf cavities,
including demonstrating superconducting Nb3Sb cavity coatings with very high
quality factor which can operate at significantly higher temperatures than Nb-
only SRF cavities. Dark current is a critical limiting factor for accelerating
structures operating at the extreme gradient limit. Cornell is well-suited for
the investigation of surface preparation techniques that mitigate dark current
at cryogenic temperatures, as well as surface coatings that passivate field
emission or increase the quality factor of the structure, thereby limiting the
energy dissipated in the cavity thus potentially the breakdown rate. There are
also CBB-sponsored activities at UCLA, embracing mainly ultra-high field and
cryogenic cavity issues, as is discussed in Section 3.1.7 and 4.2.

###  3.2 High Power Klystron Upgrades

The high power rf system for the initial demonstrator will consist of
commercially available 50 MW C-band klystrons and modulators; 18 klystrons are
required. To provide a path for full scale implementation, it is necessary to
adopt the latest innovations for rf source design leading to increased rf
source efficiency and power at equivalent or reduced construction costs.
Indeed, for a full-scale collider, the fabrication and installation costs of
the high power rf system can be comparable to that of the tunnel and
accelerating structure itself. In the near term, efficiency improvements can
be had by designing an optimized rf circuit. Techniques for maximizing
klystron efficiency, such as the Core Oscillation Method (COM), were explored
in depth through the High Efficiency International Klystron Activity (HEIKA),
yielding klystron efficiencies up to 80$\%$ at X-band; these same methods
could be deployed at C-band.[16] Fabrication and operation costs may be
reduced further by replacing the conventional solenoid focusing with a
periodic permanent magnet (PPM) focusing structure that leverages existing
mass-produced magnets. Efficiency can be increased further by incorporating
PPM focusing within a multi-beam device to reduce the effective perveance of
the device. Finally, advances in commercially available additive manufacturing
will be evaluated for rf sources, and may be incorporated in low-risk
assemblies (for example, in “dressing” assemblies outside the vacuum envelope)
when such an approach can lead to substantial fabrication cost reductions.

####  3.2.1 Low Level RF and Klystron Controls

The rf phase requirements of 0.1% and 0.3°C-band phase (150 femtoseconds) are
comparable to the requirements of 4th generation light sources. A recently
developed Low Level rf (LLRF) system at SLAC provided <20 femtosecond RMS
drive noise and <5 femtosecond RMS readback noise in a 1 MHz bandwidth,
considerably better than required for C3 . The klystron modulator interlocks
are comparable to those on existing accelerators.

Neither the LLRF or rf controls require any new technology in order to meet
the C3 requirements, and any development would be directed at reducing cost.
As the underlying technology is rapidly evolving, detailed designs for both
the C3 demonstration and the collider should be delayed until there is a firm
project schedule in order to allow use of the most recent technology. Its is
likely that these systems will be based on highly integrated RF / FPGA / CPU
parts such as the Xilinx RFSOC.

####  3.2.2 Precision Timing / Phase Distribution

This is required in order to control the phase of the rf stations. Again the
requirements are similar to those for 4th generation light sources and long
distance phase stabilised rf over fiber systems are available commercially.
The long distance fiber systems can be combined with local reflectometer based
rf phase distribution similar to that used on LCLS-II. As with LLRF, this is a
rapidly evolving technology and the design should be deferred until there is a
firm project schedule. Systems to phase lock rf stations and mode locked
lasers to rf phase references meeting these requirements have been used in
accelerator / FEL systems at various laboratories.

####  3.2.3 Beam Diagnostics

The beam diagnostics resolution requirements for C3 are similar to those for
4th generation light sources, but with the additional requirement of closely
spaced (few nanosecond) bunches.

Cavity BPMs are able to provide both high spatial resolution and with high
bandwidth electronics can provide independent measurement of bunches in the
train. Typically cavity BPM measurements are limited by mechanical stability
rather than by the readout electronics. Note that the readout electronics
could be identical to the LLRF electronics if high bandwidth RFSOC devices are
used. Multi-bunch cavity BPMs operating under conditions where the bunch
spacing is < cavity decay time, have been tested, but not used in production.
The algorithm is straight forward: Measure the change in vector field
amplitude caused by each additional bunch in the train. There are tradeoffs
between using cavities that are resonant with bunch rate which provide better
train average position resolution, and using off-resonance cavities which
provide better single bunch resolution.

Beam profile monitor technology will be driven by details of the accelerator
design. Optical Transition Radiation monitors (OTR) provide high spatial
resolution images, but cannot be used with beams with high longitudinal
brightness due to coherent effects. In general they will function after a
damping ring but not with a beam from an rf gun. The alternative of wire
scanners can be applied anywhere beam densities are not too large, but they
provide only integrated, multi-bunch scans and are very slow. In areas where
the charge density will destroy any type of intercepting diagnostic, laser
wire scanners are the only known option.

Bunch arrival timing can be determined with rf pickup cavities that operate
similarly to a BPM cavity. If a precision phase reference is provided, each
BPM reference cavity can also serve as an arrival time monitor. This type of
system has demonstrated <10 femtosecond single pulse noise, easily sufficient
for this application.

####  3.2.4 Accelerator Raft Alignment

The C3 accelerating structures are supported by rafts. The rafts are
responsible for providing a mechanism for alignment in warm and cold states
with mechanical actuators and with piezoelectric feedback for beam based
alignment. Each cryomodule contains 4 rafts; each raft supports two 1 m long
accelerator sections and a quadrupole with mechanically integrated BPM. The
accelerators and quads are attached to the raft with 5 d.o.f. ball joint
adjustable tie rods, and are aligned on the bench before insertion into the
cryostat. The $Z$ position of the midpoint is mechanically fixed.

The rafts will have coarse and fine alignment, with each raft having 5 d.o.f.
ball joint adjustable tie rods. The $Z$ position of the midpoint is fixed.
Each tie rod will have both a 200 micron travel piezoelectric actuator as
shown in Fig. 9, and a magnetically coupled manual screw adjustment from
outside the cryostat. The fine alignment of the rafts will be beam-based and
dynamic with a bandwidth of $\sim$100 Hz.

Figure 9: A 200 micron travel piezoelectric actuator. Actuators operate at
cryogenic temperatures and high mechanical loading. The devices were developed
for LCLS-II.

The coarse alignment of the rafts is done with stretched wires. Each wire is
$\sim$20 m long and spans two cryomodules, with the wires starting on
successive cryomodules. Each raft has two wire position sensors for each wire,
one on the first accelerator and the other on the quad. The sensors measure
$X$ and $Y$, with a precision of $\leq$100 microns. The rafts are slightly
over-constrained with 8 measurements for 5 d.o.f.

The stretched wire method[17] is chosen since it must operate in air, vacuum,
and under liquid nitrogen (rf off). Early-on in the demonstrator R&D plan
stretched wire prototypes must be designed and tested for operation in liquid
nitrogen, and to confirm that their performance is not degraded in this
environment.

####  3.2.5 Vibrations

As noted, three stages of alignment are foreseen: initial alignment of the
accelerators and quadrupole on the raft; alignment of the rafts with respect
to each other by stretched wires; and finally beam based alignment. It is also
required that structure vibrations be kept to acceptable levels. Vibrations
can be driven by seismic disturbances[18]. Prospective sites should be
adequately quiet, and this has been studied by NLC, ILC, and CLIC. A new
vibration source needs to be studied that arises from the nucleate boiling of
liquid nitrogen and possibly the flow of nitrogen vapor. Preliminary
measurements were started with a dummy accelerator fitted with 3 cryogenic
accelerometers [19] along the 3 axes, with everything under LN2, and a small
heater in the Cu slab. The test bed for experimenting with vibrations induced
from thermal loads is shown in Fig. 10.

Figure 10: Test bed for measuring vibrations from the boiling of liquid
nitrogen due to applied thermal load in copper structure.

The continuation of this study should be done before substantially more design
work is completed. Bubble formation and particularly bubble aggregation under
the accelerator at high thermal loads should be studied to understand margins.
This should utilize direct optical observation of the accelerator surfaces as
well as accelerometer measurements.

####  3.2.6 High Power RF Distribution

  * •

Specification of waveguide network

This section describes the high power rf distribution system at C-band (5.712
GHz). The system transmits rf pulse power from the klystrons to accelerating
structure. The rf distribution system will provide for monitoring the pulse
power level of the klystron output. The major specifications of the waveguide
network are shown in Table 6.

Operating frequency (MHz) 5712
---
Waveguide Scattering Coefficient (S11) less 0.07
RF peak power (MW) 80
RF average power (kW) 20
Phase deviations of output RF power relative to an input reference phase (deg)
less than 3
Vacuum operating level (Torr) 10x10-7 at full rf power and 10x10-8 at zero rf
power
Gas burst rate (Rate) Less than 1 burst per hour average at full RF peak power
after conditioning
Table 6: Specifications for the Waveguide Power Distribution Network

An initial condition for the design of the waveguide network is based on the
decision to feed each standing wave structure raft from one klystron. In this
case, the first step of the rf phasing can be made at a low level rf drive
signal and less accuracy is needed for the waveguide temperature stability.
The precision adjustment of the accelerating rf phase is performed by a phase
shifter installed in each waveguide branch. A simplified rf network layout is
shown in Figure 11. The rf network layout is indicated with flanges connecting
hardware components. Evaluating which of these we can replace with welds will
be one of the activities for the C3 Demonstration R&D Plan.

Figure 11: Schematic of the high-power rf waveguide distribution network. The
configuration shown is for one klystron feeding a 2 m raft.

The waveguide system will be designed to operate in high vacuum. This decision
dictates developing of reliable vacuum directional couplers, power splitters,
vacuum pumpout orifices with a high conductance and negligible rf coupling.
The primary elements affecting the waveguide layout will also include the
mechanical sub-system allowing rapid replacement of the klystron. The overall
C3 rf distribution design philosophy is targeted at allowing the replacement
of the klystron without significant work at the waveguide network.

Three standard waveguides can be used to transmit RF power from the klystron
to the accelerating section. They are WR137, WR159, and WR187.

Specification of their inside dimensions is as follows.

WR137 inside dimensions: 1.372” x 0.622”

WR159 inside dimensions: 1.59” x 0.795”

WR187 inside dimensions: 1.872” x 0.872”

Maximum rf power transmitted via oxygen-free high conductivity (OFHC) copper
WR187 waveguide may be up to 350 MW peak in vacuum [20]. High-power
experiments have shown that the main reason for the rf breakdown is a poor-
quality inner waveguide surface. The waveguide thickness of 0.157” is enough
for easy machining and assembling for the brazing process. The 0.157”
waveguide thickness may be chosen to allow using a common jig to join each
element in the brazing process. Typically, a dimensional accuracy of less
$\pm$40 mils is acceptable.

The theoretical attenuation constant in WR187 is 3.635 mNp/m. It corresponds
to the 0.032 dB/m transmission losses at room temperature. A preliminary
length of each waveguide branch from the klystron to the cryomodule input is
30 ft (approximately 10 m). An average RF power loss in the copper WR187
waveguide will be less than 1.5 kW in this case.

  * •

RF Flange

An important issue to address is the design and adoption of a standardized
waveguide vacuum flange. This important component must serve in two roles as
both an rf seal and a vacuum seal. The VSWR of the joint must be less than
1.015 at a frequency 5712 $\pm$ 50 MHz. The flange pair should be capable of
transmitting 160 MW peak RF power. A leak rate should be less than 2x10-10 std
cm3 He/sec. The completed seal should be leak tight when the assembly is
subject to a bake-out cycle consisting of heating at rate of 100oC/hr to a
peak of 560oC, holding at this temperature for 48 hours and cooling at a rate
of 100oC/hr.

Further, the most commonly occurring problems such as discharge breakdown and
vacuum leak often happen near the flange gasket.

A new type unisex waveguide flange has been developed at KEK [20] to increase
reliability and reduce cost; it comes from the RIKEN-DESY WR187 flange as
shown in Figure 12.

Figure 12: Schematic of the face and profile of a RIKEN-DESY WR187 flange.

  * •

RF Window

The average high-power klystron lifetimes are typically $\sim$50k hours. The
klystrons must be replaced in a case of its failure. The replacement work will
be performed when a part of the vacuumed rf feeder is opened for disassembly
of the klystron. The rest of the rf feeder will be under the vacuum. The rf
window with the ceramic barrier is needed as an interface between an
atmosphere and the vacuum envelope.

The rf windows must reliably operate under 80 MW peak power levels. However,
in the case full reflection from the accelerating section, the rf window must
be working even with double rf power levels of the normal operating mode. A MW
peak rf power produces an electrical stress inside the vacuum envelope and
ceramic interface. RF breakdowns in these megawatt power environments could
damage the rf window if the design is not done correctly. There are several
innovations in the present rf window designs for S- and X- Band frequency
ranges. Many of these innovations are practically realised. For example, the
X-Band travelling wave mixed-mode rf window [21, 22, 23] tested in the pulse
mode up to 100 MW. The rf window for C-Band will be designed, tested, and
optimized under ideas and innovations discussed in [24], Figure 13.

Figure 13: X-Band travelling wave mixed-mode rf window[24].

  * •

Power Divider

One power divider will integrate into the waveguide network to split equally
the output klystron power for two branches. A short-slot, narrow wall, coupled
junction rf power divider is a tradition rf device that has been used in the
high power rf systems. It has high power handling capacity and each of its
ports lies in the same plane. The high power tests indicates that a short-slot
divider can operate without rf breakdown at more than 70 % of the power
capacity of the terminal waveguides.

The basic construction of the short-slot divider uses two rectangular WR187
waveguides that share a narrow wall. Between them a single slot couples half
of the incident power from one waveguide into the second guide. The two
resulting waves are 90 degrees out of rf phase. A simplified sketch is shown
in Figure 14. The electrical phase length matching of both waveguide arms will
be performed on the regular straight sections.

Figure 14: Schematic of a short-slot divider using two rectangular waveguides
that share a narrow wall.

The C3 accelerating structures operate in a standing wave. There will be a
reflection of power from the structure back to the klystron. Part of the
reflection power (ideally all) will propagate into Port 4. The high power
vacuum dry rf load will absorb this power.

  * •

RF Load

There are several concepts for high power load designs. Concepts may be
classified by the class of rf absorption material to be employed. The most
popular rf absorber that is used in the high power loads is water or other
liquid lossy dielectrics. This load concept requires a solid dielectric
interface separating the accelerating vacuum envelope and the liquid absorber.
Such a concept is not acceptable for the C3 demo because a catastrophic
failure of one interface could destroy the long linac vacuum system; recovery
from this failure would be expensive. As a result, the high power load
concepts based on the liquid rf absorbers are not being considered. Our
analysis will be focused on high power load concepts that are based on a
multipactor-free concept with an advanced rf absorption materials capable of
working in an UHV environment.

  * •

RF Monitors

Modified Bethe hole direction couplers are a typical components that have been
integrated into the waveguide network for rf power monitoring. The high power
level in the waveguide requires the use of more than 60 dB coupling ratios.
Three directional couples are shown in the rf feeder in Figure 11. This figure
represents a typical layout of one klystron station. Many couplers will be
needed for the whole installation. A modified Bethe hole directional coupler
is optimised to reduce their cost. The designs used allow for the insertion of
a waveguide pump-out and vacuum gauge in the same space where a coaxial strip-
line loop is placed. There are two types of designs of the couplers in the
SLAC linac. These designs will be evaluated in the C3 demo. An employment of a
dielectric window on the wide wave guide wall is used in the firs coupler.
This dielectric window is placed above the coupling hole and separates a
vacuum atmosphere regions. The strip line loop is placed from the atmosphere
side of the dielectric. Two vacuum rf feedthroughs are employed in the second
concept. Both concepts are a subject for evaluation in the C3 demo
installation.

  * •

Phase Shifter

One klystron feeds two accelerating sections in the present rf feeder layout.
If the two waveguide branches of waveguide network are tuned properly, the rf
power from the klystron into the input port of the power divider will arrive
in phase at each of the two output ports. There is a transition located a the
cryomodule for the waveguide to go into the LN2 cryostat where the temperature
will drop from room temperature to $]sim$80 K. The temperature decoupling in
the waveguide is needed to reduce the thermal losses in the cryogenic vessel.
There are several approaches for realizing this thermal break. One of them is
to employ the thin-wall waveguide fabricated from a material with low thermal
conductivity and low rf losses on the inner waveguide surfaces (for example
with a thin copper coating). Inclusion of a phase shifter based on waveguide
wall deformations will be evaluated in this space.

####  3.2.7 Start-to-end simulation

High fidelity start-to-end beam dynamics simulations were used in x-ray free
electron laser light source accelerator design to verify and optimize the
accelerator design concept [25, 26, 27, 28]. This type of simulation should be
carried out for the C3 accelerator design too. Important physical effects such
as three-dimensional space-charge, longitudinal and transverse wakefields,
coherent and incoherent synchrotron radiation, and intrabeam scattering should
be included in the simulation model. The effects of long-range wakefields on
beam quality should also be studied. Machine imperfections such as
misalignment errors and field amplitude and phase errors should be studied
using the start-to-end simulation.

To provide the required luminosity for a linear collider, it is critical to
reduce emittance dilution in the main linac from machine tolerances such as
cavity misalignments and imperfections [29, 30]. For the C3 linac,
misalignments can arise from individual cavity misalignments and changes in
the properties of coupled HOMs in a cryomodule with misaligned and deformed C3
cavities. The rf parameters and fields of the HOMs can be evaluated for random
distributions of cavity offsets (in a cryomodule) and cavity deformations
along the full linac. The fields are then used for beam emittance dilution
evaluation. A statistical analysis using the constraints form realistic
fabrication and component placement tolerances will be facilitated by the HPC
capabilities of advanced simulation codes [31].

Dark current and capture may have deleterious effects on the particle detector
in the form of unwanted backgrounds. Integrated simulations using rf and beam
dynamics codes will provide the needed tool for start-to-end simulation for
linear colliders. The simulation starts with electrons emitted from cavity
walls governed by field emission and their subsequent movements are tracked
under the influence of the cavity accelerating mode operating at a specified
phase. Secondary emissions will be generated if some of the electrons hit the
cavity wall. The exit electrons from the cavity will be used for beam tracking
along the linac with magnet components from the lattice until they reach the
next cavity. A pipeline workflow involving rf and beam dynamics simulations
will be developed to facilitate start-to-end simulations.

###  3.3 Parallel Research and Development

####  3.3.1 Damping Rings

One important issue in advanceing the damping ring R&D is the
injection/extraction system for the train of bunches which are separated a few
tens of nanoseconds. Powerful and fast solid-state switches driving a
transmission line kicker are needed. They must work in a pulse mode and be
capable of generating power from MW to GW and rise/fall time from 10x ps to
100 ns. There are practical physical effects that can overcome the speed
limitation for commercially available off self-solid-state switches. For
example:

  * •

nonlinear ferromagnetic media

  * •

solid-state electron-hole plasma in semiconductors

In both approaches the nonlinear characteristics of the switching media
(ferromagnetic and semiconductor) are employed in the final stages of the
pulser to form multi- MW level nanosecond pulses. The pulse dynamic processes
in ferromagnetic and semiconducting materials can support a 1 MW/ns switching
speed stably, reliable, and efficiently. This fact is confirmed by our pilot
experiments. For example, the effect of the high-power switching speed in
semiconductors diodes is shown in Figure 15.

In this case diodes were fabricated with special doping gradients in Si-based
wafers via a deep diffusion process.

The waveform shown in Figure 15 demonstrates a 1.3 MW/ns switching speed
continually running at 25 kHz repetition rate with less than 20 ps RMS time
jitter. A development of physics basis of semiconductor behaviour for a
generation of GW levels of nano- and sub-nano power swings would be useful for
the development of the damping ring injection/extraction system in the C3
machine.

We will explore a variety of ultra-wide band-gap (UWBG) semiconductors
(diamond, SiC, etc.) which are of great interest for their efficiency, speed,
thermal management and radiation resistance.

Figure 15: Demonstration of a 1 MW/ns switching speed.

####  3.3.2 Beam Delivery System and Final Focus (BDS/FF)

Mature designs exist for beam delivery systems with an electron or positron
beam energy in the range of 125 GeV – 1.5 TeV as part of the ILC and CLIC
programs. C3 will continue to leverage the significant effort in simulations
and hardware tests that have been developed and carried out over the past 30
years in preparation for a future linear collider. Trade-offs exist in design
length and aggressiveness of beam parameters (e.g. $\beta$*,L*). These
manifest as tighter, more complex diagnostics/correction hardware (higher-
order magnets, $<\mu m$ accuracy movers, BPMs etc) and more complex, slower
online beam size tuning systems, limiting achievable luminosity. The design
for the BDS and FF is complicated and time consuming to evaluate. To date, a
final focus system with ILC levels of demagnification has been experimentally
demonstrated [32], but not beyond. We will use caution when designing the BDS
and FF for C3 with the main goal of simplifying and reducing the length of the
BDS and FF. Pushing the design beyond key ILC FF parameters (principally,
energy spread and final focusing doublet chromaticity $\sim L^{*}/\beta^{*}$)
would require further experimental verification of performance.

The backbone for assessing the realistic luminosity potential of any linear
collider is that of the start-to-end simulation (S2E). This should include a
full description of the beam transport from the exit of the source (damping
ring or rf photo-injector) through to the BDS and FF and to the collisions at
the interaction point (IP). All relevant physics processes should be included:
e.g., chromatic effects of magnets, short and long-range longitudinal and
transverse wakefield kicks in accelerating structures, synchrotron radiation
in bends, magnetic field errors and placement errors of all accelerator
components. Both “static” misalignments and other errors as well as dynamic
effects (errors which are fast on the timescale of corrective actions being
applied) need to be considered. The BDS is especially sensitive to errors,
containing diagnostics systems required by the physics detectors experimental
program, and strongly non-linear beam dynamics with large multi-km beta
functions, for example. Careful simulation with accurate descriptions of the
various errors internally and input conditions from the linac are critical to
assess the final performance of the collider. Extensive S2E simulation
programs have been implemented in the past for the ILC and CLIC communities,
with a strong focus on the FF tuning system [33]. It is envisioned to re-
create such a community of beam dynamics simulation experts to generate a S2E
simulation environment to assess the C3 performance and form the basis for
tolerance specifications.

The use of Machine Learning (ML) and Artifical Intelligence algorithms (AI) in
the design and operation of particle accelerators is somewhat new and has not
yet been fully explored for the field of next-generation linear colliders. The
C3 program should benefit from this rapidly advancing area of research. The
BDS and FF are strongly non-linear systems which require complex tuning
procedures to achieve full luminosity [33]: Application of modern, multi-
objective physics-informed Bayesian optimizers could yield powerful
improvements in tuning performance and speed, possibly increasing confidence
in an ability to push on the FF parameters. This would manifest in either an
increased luminosity performance or operation at a similar luminosity with
reduced beam power requirement (cost). Another example application of ML/AI
technology could be applied to the problem of reconstructing beam aberrations
at the IP collision using a combination of “event shapes” reconstructed using
“beamstrahlung” monitors situated close to the IP [34]. Again, such technology
could help with the accuracy of the beam tuning process yielding increased
tuning speed, whilst also providing useful information to the detector
community.

As exhaustive experimental tests have shown, at ATF2 [32], and FFTB [35]
before it, despite the highly non-linear design aspects and complex tuning
procedures, modern optics design and beam dynamics software is well capable of
accurately describing the BDS and FF systems. Whilst there is always value
added in test facilities, a test final focus system demonstration for C3 is
likely unnecessary as it does not strongly deviate from existing designs for
the BDS/FF. A possible exception is the particular case of the FF design for
the $\gamma$-$\gamma$ design (XCC) [5]. This option requires the focusing of a
round-beam configuration, necessitating a final triplet configuration of
magnets which has not yet been designed or studied for small-beta focusing
optics required at a linear collider. One possibility to demonstrate this
configuration for the FF might be a future upgrade to the FACET-II [36] Sector
20 experimental region which already operates with round-beams (E=10 GeV,
$\beta$*$>$5cm, sz $<$1um, Q$<$2 nC). Further novel aspects of the XCC concept
which demand a fresh look at the BDS design include the integration of keV
energy x-ray focusing optics to overlap with the electron beams close to the
IP and generate the colliding gamma beams. Consideration of FF optics which
pushes substantially beyond that already demonstrated can be beneficial in
terms of cost savings and/or luminosity performance. The premier facility in
the world for such experimental studies is ATF2 at KEK: whilst the accelerator
is currently still operating, serious further investment would be required to
enable a program to experimentally verify such pushed parameters are feasible.

####  3.3.3 Levitated Positron Target - Radiatively Cooled

Positron sources are based on electron or photon beam striking a high-Z
target, often Ti. Because both the instantaneous and average power are high,
the concept of a rotating Ti hoop in the accelerator vacuum is accepted. The
the ILC baseline, water cooling introduced through rotating seals, and
rotating seals are used for an axial drive shaft. A concept has been
proposed[37] using radiation cooling of high performance Ti alloy blades to a
water cooled vacuum can, completely eliminating water channels in the vacuum
space has been proposed. In addition, a concept for magnetic suspension and
drive of the hoop, eliminating shaft seals, that does not require permanent
magnets, and so should survive in the high radiation field of the target is
also being explored. Increasing the reliability and performance of the
positron target would have significant benefits for C3 operation and advancing
this concept will be explored.

####  3.3.4 Advanced RF Source Research and Development

With the development of this highly efficient linac topology, we expanded our
vision for a future high gradient linac architecture that comprises a much
more simplified RF system with a great deal of optimization for the cost of
the peak power generation. A central tenant of the C3 concept is a rf only
upgrade to increase the energy from 250 GeV to 550 GeV CoM. This upgrade
affords us the opportunity to implement novel concepts for the rf sources that
will be used for the upgrade. To this end, we are proposing a very simple,
modular system with an order of magnitude cost reduction for the accelerator
complex. The idea starts with developing low voltage RF sources with extremely
high efficiency, which translates to a low current and lower power. To get to
high power, we utilize the narrow bandwidth of the system, which allows us to
design an efficient combining network for a large number of low voltage
systems. This, in turn, leads to inexpensive, relatively low voltage
modulators and hence fast rise and fall times, eliminating the need for pulse
compression. With a large number of small tubes, mass production cost
reduction could result in substantial savings.

To realize this vision, we need to have a comprehensive program for these
advanced microwave rf sources[38] with the end goal of an experimental
demonstration of C-band modular multi-beam klystrons with efficiencies $\geq$
65$\%$, with periodic permanent magnet (PPM) focusing and high power. This
should be followed by a demonstration of the combining mechanism; many
methodologies are possible, but there is a new invention for combining with a
two-dimensional Flouquet network; a 16 tube combiner has already been
demonstrated[39]. Finally, a demonstration of a fully integrated tube with
suitable power combination operating with efficiency $\geq$65$\%$ should
follow.

####  3.3.5 RF Pulse Compression

Another possible upgrade to the C3 concept is the implementation of rf pulse
compression. RF pulse compression can decrease the fill time and thermal load
into the cryogenic vessel. Given the efficiency of cooling at cryogenic
temperatures of $\sim$80 K (approximately 15%) this can have a significant
impact on the cost and operational efficiency for the complex. A compression
of 3X for the filling portion of the rf pulse has been shown to have
tremendous benefits[6]. The development of pulse compressors for cryogenic
linacs that meet this level of compression is an ongoing topic of R&D, as
noted in the discussion of Very High Energy Electron therapy systems in a
concurrent submission to Snowmass 2021 on FLASH Radiation Therapy [40]. RF
pulse compression has been used extensively for large scale accelerator
applications to supply short high peak power pulses with cost efficient long
pulse, low power sources[41]. Recent innovations in RF compressor cavity
design have enabled a dramatic reduction in the system footprint while
maintaining the capability to produce 6-fold pulse compression and isolate the
source from the reflected RF signal from the cavities.[42, 43] For cryogenic
copper structures, cavity designs will need to achieve high intrinsic quality
factors, Q0 up to 400,000, and high coupling factors, $\beta$ up to 10. Active
research in this area will continue to benefit the broader accelerator
community that relies on pulse compressors to supply the peak powers needed
for high gradient operation. Indeed, the development of accelerator technology
needed for C3 , from the distributed-coupling linac design at cryogenic
temperatures to potential pulse compression for high-efficiency operation,
will have a cross-cutting impact in support of efforts to design compact,
cost-efficient accelerators for medical and security applications.

###  3.4 Industrialization

As with any linear collider designs the volume of hardware production that
will be required is a daunting task for C3 . In an effort to render the
required production practical we will strive to engage with the existing and
emerging accelerator technology sector and related technological sectors that
will be required for C3 . Fortunately, the production of hardware directly
applicable to a normal conducting rf accelerator is already quite significant.
This is in large part driven by the demand for medical and industrial
accelerators that produce high-flux x-rays. For example, in less than a decade
from the launch of Varian’s flagship radiation therapy model, one company
built and installed the equivalent of a C3 main linac in hospitals all over
the world.[44] If C3 technology is successfully transitioned to the industrial
sector not only will C3 benefit from industrial production, but commercial
vendors will be able to offer more compact and higher efficiency accelerator
and rf sources.

This potential exists beyond the direct accelerator technology sector. For
example, one of the key advantages of the C3 accelerating structure is the
minimization of part count, a robustness of manufacturing through CNC end
milling of split block structures, and use of bonding techniques that minimize
the possible impacts on structure performance. While commercial vendors are
presently integrated in the production of linac components, increasing the
rate of fabrication, reducing part-to-part variability, automating tuning and
diversifying the supply chain are all topics to be explored during the
demonstrator R&D plan. The production of accelerating structures for three
cryomodules, individual structure tests and the injector, will allow for a
broader engagement with manufacturing industry (in particular small-business
machine shops with aerospace experience).

The RF sources (klystrons) are expected to be significant cost drivers for any
future accelerator facility, and C3 is no different in this regard. To drive
down klystron procurement costs, commercial partners should be engaged as soon
as possible in the development effort. There should be robust support for a
public-private collaboration to evaluate the most cost-effective path forward.
A few options could be considered, depending on the available infrastructure,
tradeoff between lab-run or supplier-run operations, and the capabilities of
industrial partners:

  * •

Klystron design, fabrication, and test entirely by multiple commercial
partners. It is important to ensure simple interchangeability of sources in
this situation.

  * •

Klystron design completed by government labs; klystrons built-to-print,
tested, installed, and maintained by commercial partners.

  * •

Klystron design completed by government labs; built-to-print by commercial
partners; but tested, installed, and maintained by the C3 facility.

Industrial R&D is driven by immediate and long-term market potential. From
this viewpoint, a proposed future collider presents a significant risk that
may not merit substantial investment in cost reduction and process
improvements by industry. Therefore, in any partnership scenario, the
industrialization effort should require – and financially support – a
collaboration with government labs to evaluate and improve upon manufacturing
processes. When the most economical approach is to use existing fabrication
methods, those methods should be leveraged. When process improvements can be
made (for example, using additive manufacturing to reduce part counts, or
automation of testing processes), the investment required for industry to
implement these changes should be supported directly. This provides a “win-
win” scenario where the klystron costs for the future C3 klystrons are
reduced, and commercial partners can benefit from cost reduction measures
which are broadly applicable and attractive to industry participants
regardless of the timetable for the C3 machine itself.

With this in mind, during the C3 Demonstration R&D timeline (and in many cases
presently ongoing) we will directly engage with industry in many technological
areas: accelerator fabrication, beamline components, permanent magnets, vacuum
vessel production, rf sources (klystron and modulator), LLRF, tunnelling,
large scale cryogenics, etc.

###  3.5 Additional HEP Research and Development

####  3.5.1 Artificial Intelligence and Machine Learning (AI/ML)

Modern Artificial Intelligence (AI) and Machine Learning (ML) techniques can
be utilized with great efficacy during accelerator design in the CDR/TDR
efforts and for machine control during operation of the collider. The
demonstration accelerator also provides interesting opportunities for co-
design of components and methods for ML-based modeling and control.

During the design work, a number of accelerator parameters that are correlated
need to be studied, and optimized combinations of parameters will need to be
found prior to performing detailed simulations. We plan to use and potentially
expand upon ML-based optimization techniques that can identify the most
important combinations of parameters and parameter ranges that need to be
explored in detail for optimal setups, likely resulting in a faster design
process, reduced burden on computational and personnel resources, and superior
optimization results.ML-based optimization methods are able to perform high-
dimensional multi-objective optimization efficiently [45], allowing for many
tradeoffs between output beam parameters of interest to be examined
simultaneously. Bayesian methods aimed at automatically exploring accelerator
parameter spaces in a sample-efficient way have also been tested in simulation
[46] and experimentally. These have been incorporated into open-source
software tools for accelerator simulations and optimization that are readily
available for use in the design process [47, 48].

These methods can also be used for characterizing and optimizing the
accelerator during startup and commissioning, and we plan to integrate ML
heavily into the commissioning process. Bayesian exploration and optimization
can be used without extensive previous data, and can be conducted in a way
that incorporates learning to avoid constraint violations (such as a beam
going off of a screen, or a beam parameter moving too far away from an optimal
band) [46]. This makes these methods ideal for optimizing the accelerator
incrementally during early commissioning. Information from physics simulations
generated during the design process can also be used to help inform Bayesian
optimization and speed up its convergence [49, 50]. Data gathered during
commissioning can then be used for automatic calibration of physics models to
better match machine measurements and potentially identify early sources of
systematic mis-match between the two that can be corrected either on the
machine or in the physics simulation [51], as well as adapted over time to
make model predictions more robust to time-varying sources of error [52].
During regular operation, ML-based models can be used for making fast online
predictions of beam behavior and fast feed-forward corrections (e.g. see [53,
54, 55, 56]). Feed-forward corrections can potentially be used to improve beam
stability and incorporated into standard continuous control tasks (such as
trajectory control and low level rf control) for improved performance.

There is also a relatively unprecedented opportunity for co-design of the
accelerator diagnostics and control methods. The placement and types of
diagnostics can be optimized to maximize useful information from the machine
and reduce redundancies. This could be paired with running simulations of the
anticipated online modeling and control techniques that will be used once the
machine is built. For example, several ML approaches have been demonstrated
for inferring unseen aspects of the beam behavior based on a subset of
available measurements (e.g. [57, 52, 58]). The expected performance of these
methods could be taken into account directly by integrating simulated versions
of them in the design of diagnostics placements. Several other examples of
possible applications of AI/ML are discussed in Sections 3.2.7 and 3.3.2.

####  3.5.2 Plasma Lens

Plasma lenses can focus electron beams with strengths several orders of
magnitude stronger than quadrupole magnets [59, 60, 61]. In passive plasma
lenses, the transverse focusing force in the underdense, nonlinear blowout
plasma wake regime is due to the presence of the stationary plasma ions. If
the transverse density profile of this ion column is uniform, then the
focusing force experienced by the electrons in a relativistic beam is both
axisymmetric and linear. These properties lead to an aberration-free focus of
the beam that can achieve unprecedented small beam spots. The first order beam
dynamics have been described in Ref. [59].

The underdense plasma lens (UPL) operates in a two-bunch configuration where a
“driver” electron bunch drives the nonlinear wake and a second, “witness”
electron bunch is subsequently focused in the nonlinear blowout wake. It may
also be possible to operate in a single-bunch configuration where the head of
the electron bunch drives the nonlinear wake and the bulk of the electron
bunch is focused. In this regime, the linear focusing force from the ions
results in a focal length given by the focusing strength

$f\equiv\frac{1}{KL}=\frac{1}{2\pi r_{e}}\frac{\gamma_{b}}{n_{p}L}.$ (1)

Here, $K$ is the focusing strength, $L$ is the lens’s longitudinal thickness,
$r_{e}$ is the classical electron radius, $\gamma_{b}$ is the Lorentz factor
of the electron beam, and $n_{p}$ is the plasma density of the lens. A UPL
with a plasma density of $n_{p}=1\times 10^{17}\mathrm{\ cm^{-3}}$ can have a
focusing strength $K=88400\mathrm{\ m^{-2}}$ and focal length $f=3.3\mathrm{\
cm}$. For comparison, a conventional quadrupole with field gradient
$G=1\mathrm{\ T/m}$ would have $K=0.3\mathrm{\ m^{-2}}$ and focal length
$f=1000\mathrm{\ cm}$ and a permanent magnetic quadrupole with $G=500\mathrm{\
T/m}$ can expect $K=150\mathrm{\ m^{-2}}$ and focal length $f=81\mathrm{\
cm}$.

The UPL can be generated by laser ionization of gas, e.g. by focusing a
femtosecond laser pulse into a gas jet. To minimize the footprint, the laser
pulse can propagate transverse to the electron beam axis, reducing the
required space along the beam line to millimeters. The longitudinal density
profile of the plasma lens is then given by the laser parameters and focusing
optics for gas ionization. Something as simple as a Gaussian focus from a
spherical lens with a variable offset from the laser focus to the beamline can
give a plasma lens thickness $10^{\prime}s-100^{\prime}s\mathrm{\ \mu m}$.

There has been discussion about the potential use of a plasma lens for the
final focusing element of a collider for decades [60, 61]. The strong, linear,
axisymmetric focusing of the UPL in combination with its ultra-compactness and
“self-aligning” characteristics may show it to be a viable candidate, although
further study is still required. It should be noted that the functionality of
the UPL described above applies only to negatively charged relativistic
leptons. Relativistic positrons would experience a linear defocusing force in
this scheme. It may be possible to achieve similarly strong focusing with the
UPL if the positrons are sent through the lens in a different phase of the
nonlinear wake, but this is speculative. Another important consideration is
the scattering of the beam off of the plasma ions, which may produce both an
increase in the beam emittance and a forward-directed shower of secondary
particles. The former could result in a reduced luminosity and the latter
could result in an increased background for the particle detectors.

The possible application of such a technology to C3 would require experimental
investigation with the bunch structure and beam format of C3 . Experiments
targeted at understanding this technology and associated issues such as plasma
recovery time [62] could utilize the C3 demonstrator facility.

####  3.5.3 XCC

The XFEL Compton Collider (XCC)[5] is an X-ray free electron laser (XFEL)
based $\gamma\gamma$ collider Higgs factory concept in which $62.8$ GeV
electron beams collide with 1 keV X-ray beams to produce colliding beams of
62.5 GeV photons. The Higgs boson production rate is 32,000 Higgs bosons per
$10^{7}$ second year, roughly the same the ILC rate. The XCC takes advantage
of C3 technology in several ways, and its development could in turn help
further C3 technology and the C3 $e^{+}e^{-}$ collider.

The use of C3 technology in the XCC starts with the injector, which has the
same specification as the cryogenic RF high-brightness polarized photo-
injector described in 3.1.8, with the exception that asymmetric emittances are
not needed for $\gamma\gamma$ collisions. The XCC linac is based on C3
technology, operating at a bunch train rate of 240 Hz, with 76 bunches per
train in the first half of the 63 GeV linac and 38 bunches per train in the
2nd half. With a linac gradient of 70 MeV/m, the XCC footprint is about 2.5
km. Given the relatively short linac length and the absence of damping rings,
the XCC may provide significant cost savings over $e^{+}e^{-}$ Higgs
factories.

The XCC XFEL has a pulse energy of 0.7 J, which is 300 times the pulse energy
of current soft X-ray FEL’s. The production and focusing of such an X-ray beam
are the greatest technical challenges for the XCC. These challenges can be
addressed by building a 1 nC 120 nm-rad cryogenic RF injector for LCLS-I. The
construction of such an injector would provide the RF high brightness photo
injector demonstrator discussed in 3.1.7. When this injector is incorporated
into LCLS-I, the upgraded XFEL could be used to investigate the production and
focusing of 100 mJ per pulse soft X-ray beams and, simultaneously, open up
exciting new research opportunities in photon science. The photon science
applications of the injector could shift the time frame of its development and
construction forward.

####  3.5.4 Polarized Positrons

In the final collider scheme, the availability of polarization for the
positron beams can be very beneficial to enhance particular channels, reduce
background and facilitate searches for BSM chiral new physics. The current
scheme to generate polarized positrons in the ILC utilizes a very long short
period superconducting undulator to obtain a high flux of circularly polarized
gamma-rays that can be converted in e-e+ polarized pairs in a thin Ti-alloy
target. The scheme assumes the availability of very high energy (125 GeV or
more) e-beam and creates a significant dependency burden on the collider
complex. Alternative schemes based on Inverse Compton Scattering have been
proposed to generate the gamma-rays. The main advantages are the stand-alone
nature of the source (since only GeV-level e-beam energy is required) and the
possibility by increasing the energy of the gamma-rays to achieve higher
polarization purity ($>$ 60 %) for the e+ beam. One of the major challenges is
in the low interaction cross section and related high required average power
for the laser, but efficient energy extraction from high quality electron
beams could be used in an optical cavity to replenish the laser losses and
sustain high repetition rate interactions. One important aspect to consider
here is the particular C3 time-format which might create a challenge for the
cavity thermal management and the heat dissipation on the conversion target.
Conversely, the short bunch-to-bunch separation matches well the
characteristics of novel laser stacking cavity developments. All of these
aspects could/should be tested and evaluated in a demonstrator facility.

##  4 Infrastructure Requirements for Demonstrator R$\&$D

###  4.1 Basic Requirements

Full implementation of the C3 Demonstrator R$\&$D program has the following
facility requirements:

  * •

a radiation shielded bunker at least 50 m long, 3 m wide, and 3 m high.

  * •

a properly shielded dump for a 20 kW 2 GeV electron beam.

  * •

480 VAC 3 phase power for 21 modulators at 15 kW each, or $\sim$300 kW

  * •

Low Conductivity Water sufficient to handle $\sim$300 kW

  * •

the site must be suitable for a liquid nitrogen storage facility of at least
25,000 liters (preferably 50,000 liters), with access for liquid nitrogen
tanker trucks.

  * •

it must have the electrical, cryogenic, and radiation safety organization for
a facility of this scale.

###  4.2 Siting Options

Options for locating partial or full-fledged demo facility are:

####  4.2.1 ESB at SLAC

The Demo Facility could be located in SLAC’s End Station B (ESB), where an
appropriate shielded bunker is available. This bunker currently houses the
NLCTA, which would be removed. ESB has adequate power for the linac RF system.
This approach also provides the path for future PWFA based collider R&D such
as staging.

A C3 Demonstration Facility is envisioned to be located inside SLAC’s End
Station B (ESB), where a shielded bunker is available. This bunker currently
houses the NLCTA, which would be removed. With the removal of the NLCTA,
sufficient electrical power and LCW would be available for the C3
demonstration facility, see Table 7. ESB is in SLAC south research yard with
heavy equipment and semi-truck access, allowing for delivery and storage of
LN2.

3$\phi$ electrical | 480 V | 1200 A
---|---|---
30oC LCW | 325 gpm | 85 psig
Table 7: Electrical Power and LCW cooling capacity

ESB has an operational overhead bridge crane with a maximum lifting capacity
of 50 tons, which will be invaluable to the C3 demonstration facility
construction and operation.

The radiation enclosure that houses the NLCTA has the required safety system
for machine operation, these critical safety systems include: Personnel
Protection System (PPS), Machine Protection System (MPS), Beam Containment
System (BCS) and Laser Safety System (LSS). These safety system are supported
by SLAC Accelerator Directorate. The NLCTA is activity supporting several
experimental programs utilizing the exciting S- and X-Band RF power sources
along with a UV photocathode drive laser system. It is envisioned that the
S-Band modulator/klystron and UV photocathode laser would be retained and used
for the C3 demonstration facility photo-injector. To accommodate the 3 C3 cryo
modules, the NLCTA bunker is 51 meters long with an internal height of 3.05
meters. The interior of the NLCTA enclosure has the following dimensions: 51 m
x 2.75 m x 3.05 m (L,W,H). It should be noted that the NLCTA radiation
shielding and terminating beam stop were designed for up to 1170 MeV. Minor
modifications to the NLCTA West entrance will be required to allow for the
installation and maintenance of the cryo rafts/modules.

Figure 16: Top-down view of End Station B along with the NLCTA.

####  4.2.2 FAST at Fermilab

IOTA/FAST is an R&D Facility for Accelerator Science and Technology at
Fermilab. It has two components: an Integrable Optics Test Accelerator (IOTA),
150 MeV electron / 2.5 MeV proton storage ring, [63] with a dedicated proton
injector and an SRF linac. The 300-MeV FAST linac serves as an injector of
electrons for IOTA and also provides beam to dedicated experiments.

Originally, the ILC-style SRF linac was envisioned as a demonstration facility
to test and operate a full ILC “RF unit” with “ILC beam intensity.” The RF
unit consists of 2 cryomodules driven by a single 10 MW klystron. However,
only one cryomodule was installed at FAST and has successfully demonstrated
the ILC specification on accelerating gradient of 31.5 MeV/m [64]. The ILC
beam intensity is a $\sim$1 ms long train of $\sim$3,000 bunches (3 MHz bunch
repetition frequency) with a charge of 3.2 nC per bunch. The bunch train
repetition rate is 5 Hz, and the r.m.s. bunch length is 300 $\mu$m.

Beside the 8-cavity SRF cryomodule, the electron injector includes a 5-MeV RF
photo-injector of a DESY/PITZ design, a 25-m long low-energy ($\leq 50$ MeV)
beamline with two SRF capture cavities, and a $\sim$100-m long high-energy
($\leq 300$ MeV) beamline. Both beamlines are equipped with a suite of high-
precision beam instrumentation.

The FAST high-energy beamline (Fig. 17) has plenty of space to accommodate C3
demonstration cryomodules and associated high-power rf equipment. The facility
has a dedicated cryogenic system, which includes a 5,800 gallons ($>26,000$
liters) capacity LN2 tank. At first, FAST can be used for cryogenic RF testing
of C3 cryomodules with and without beam. Though the present FAST injector
cannot provide beam in the C3 beam format, a relatively straightforward
upgrade with S-band injector can be undertaken for a full-scale beam
demonstration.

Figure 17: Photo of the FAST high-energy beamline at Fermilab.

####  4.2.3 UCLA Research and Infrastructure

Figure 18: Schematic of UC-XFEL project spearheaded by UCLA.

UCLA is proceeding with a vigorous program in C-band cryogenic RF structures,
with the twin goals of developing a linear collider-capable injector and the
first steps towards the realization of an ultra-compact XFEL[65]. One of these
steps is the construction of injector with unprecedented brightness, as in the
asymmetric emittance beam case mentioned above. This source has been
evaluated, and is expected to be able to drive a soft-x-ray FEL into
saturation in 4 meters of very short period undulator. The use of a short
period undulator is uniquely enabled by the very high brightness electron beam
obtained from the same high-field cryogenic RF photo-injector that produces
the linear collider-oriented magnetized beam.

With a short period undulator one may operated the beam at much lower energy,
that is 1 GeV in soft-X-ray case. By using cryogenic C-band linac sections,
with C3 design, this energy is achieved in 8 m of active length with 125 MeV/m
gradient. As such, the overlap between the C3 and ultra-compact X-ray FEL (UC-
XFEL) initiatives is considerable. The layout envisioned for UC-XFEL is shown
in 18. This shows a compact XFEL system, including two-stage compression and
compact X-ray optics all located within a 40 m footprint, as shown in Figure
18. The estimated cost of the first prototype of this system is 32M dollars.

The UC-XFEL overlap with C3 testing needs is considerable. An ultra-high
brightness beam is available to test linear collider beam dynamics including
short-range BBU in a significant stretch of linac. The photoijector itself
serves as a test-bed for the linear collider electron source.

As noted, at UCLA, the C-band cryogenic photo-injector is already under
development at the UCLA MOTHRA laboratory, which also hosts a 0.5 cell
cryogenic photo-injector having a load-lock system which permits testing of a
variety of photocathodes at cryogenic temperatures. This system is being
commissioned now. These cryogenic C-band structures are powered by a C-band
klystron (5 MW, with a SLED system under development to reach 20 MW). After
initial testing, the high brightness photo-injector will be transferred to the
UCLA SAMURAI lab,

UCLA SAMURAI is a large (55 ft long) bunker and associated laser room and RF
power complex. It is currently being commissioned with S-band accelerator
components, with the goal of converting most of the beamline space to a C-band
cryogenic photo-injector/linac-based prototype of UC-XFEL. There is space at
SAMURAI for 3 linac sections and 3 m of undulator along with a single
compressor, to permit testing an shortened EUV FEL using UC-XFEL/C3
technologies. The layout of SAMURAI that will be realized in two years, with
S-band and C-band infrastructure in place, in shown in 19.

Figure 19: Schematic of UCLA SAMURAI infrastructure as currently under
development.

The UCLA infrastructure and its goals, as described above, are highly
synergistic with the goals of the C3 project. This synergy is incorporated
into proposals for future work, particularly aimed at an NSF STC, to jointly
pursue UC-XFELs and advanced accelerators for HEP, such a C3 .

####  4.2.4 CERF-NM and LEDA building at LANL

Los Alamos National Laboratory currently serves as the pioneer in developing
the US C-band high gradient testing program. As a part of the program, LANL
commissioned the high gradient test stand called C-band Engineering Research
Facility – New Mexico (CERF-NM). Photographs of the facility are shown in
Figure 20. The CERF-NM test stand is built around a 50 MW 5.712 GHz Canon
klystron. The klystron system produces 50 MW pulses with the pulse length
between 300 ns and 1 microsecond, repetition rate up to 200 Hz, and is tunable
within the frequency range of 5.707 GHz to 5.717 GHz. The RF power is being
output from the klystron in a WR187 rectangular waveguide. The waveguide was
conditioned to the full output klystron power of 50 MW, pulse length of 1
microsecond, and repetition rate up to 100 Hz while running the power through
the whole length of the line into a water-cooled load. After the conditioning
was complete, the breakdowns in the waveguide line became extremely rare.
Currently we observe no more than several breakdowns in the waveguide line
during the whole day of operation. The WR187 waveguide brings power into a 3
foot by 4 foot lead box designed to protect equipment and operators from
X-rays generated in cavities under high gradient testing. The lead box is
radiologically certified for dark currents with electron energy up to 5 MeV
and average current up to 10 µA. So far, two accelerating cavities were tested
for high gradient operation at CERF-NM and surface electric fields in excess
of 400 MV/m were demonstrated. For more details on the CERF-NM test stand and
cavity testing see [66, 67].

CERF-NM is located in the radiation protection tunnel of the Low Energy
Demonstration Accelerator (LEDA) building at Los Alamos Neutron Science Center
(LANSCE). The LEDA building and the tunnel offer all necessary infrastructure
for locating the proposed C3 demonstration facility. The dimensions of the
tunnel are 470 feet long and 30 feet wide. The tunnel provides adequate
radiation protection for up to 10 $\mu$A average electron beam current at the
beam energy of 1 GeV. A waveguide basement is located under the tunnel and a
laser room with the proper laser infrastructure is at the end of the tunnel.
The LEDA building has a large liquid nitrogen tank located next to the
building and the required cryogenic infrastructure (see Figure 21. The
building has multiple options for a control room and rooms for ancillary
equipment.

Figure 20: Existing C-band test capabilities at LANL are already playing a
crucial role in the development of C3 technology with rapid testing of
structures in excess of 200 MeV/m[68].  Figure 21: The LEDA building at LANL
has a large accelerator tunnel and the cryogen infrastructure needed for theC3
demonstration facility.

####  4.2.5 LEA at Argonne

The Linac Extension Area (LEA) at the Advanced Photon Source (APS) at Argonne
is a flexible accelerator beamline test area and enclosure for demonstrations
of new accelerator concepts and technologies [69]. The LEA enclosure was
originally housed the Low-Energy Undulator Test Line (LEUTL) [70, 71] that
made the first demonstration of saturation of the self-amplified spontaneous
emission (SASE) process in the visible. The LEA enclosure, together with the
APS linac, meet the main requirements of the C3 demonstration system as
outlined in Section 4.1. The LEA enclosure is illustrated in Fig. 22. LEA is
located downstream of the APS linac which can provide electron beam energies
up to ${\sim}$500 MeV into LEA. The APS linac also operates with two electron
sources: a thermionic RF gun which provides a train of S-band bunches and a
low emittance photocathode RF gun.

Figure 22: Schematic illustration of the LEA enclosure at Argonne. In this
figure, electron beam propagates from right to left. The enclosure length is
48.2 m. An end-station building is located downstream of the accelerator
enclosure.

The LEA tunnel has more than adequate space for the 3 cryomodule linac (27 m)
demonstration facility. Photographs of the LEA enclosure are illustrated in
Fig. 23. The tunnel dimensions are: 3.66 m (width), 2.74 m (height), 48.19 m
(length). At the upstream end of the enclosure, the LEA beamline, which has
focusing and diagnostics, is presently installed, occupying approximately 17 m
of length. The demonstration facility would not need the full specified 50 m
length since APS already has a photocathode electron gun source. A feature of
the present labyrinth configuration is a ‘movable wall’ as part of the
downstream labyrinth, allowing large accelerator equipment (${\sim}3$ m
length) to be wheeled in without removing and restacking shielding.

Figure 23: Photographs of the LEA enclosure. (a) LEA enclosure viewed from
downstream end. (b) LEA enclosure viewed from upstream end, with the LEA
beamline in the foreground.

The LEA beamline receives electron beams from the APS linac, which can
accelerate electron beams up to 450 MeV. The APS linac includes as an electron
beam source a high-brightness 2856 MHz photocathode RF gun [72, 73]. With an
appropriate laser, this could allow APS to easily generate the desired C-band
compatible pulse format with 5.26 ns spacing (every 15th S-band bunch), for a
full demonstration of C3.

In addition to demonstrating the 3 cryomodule linac, LEA could also be used to
test the C3 injector (e.g. positron capture linac and booster linacs described
in Section 3.1.1). The LEA enclosure presently has two S-band RF sources. A 30
MW modulator and klystron and 50 MW solid state modulator and klystron are
located in the LEA endstation (downstream end of beamline). These could be
configured to provide RF power to structures within the LEA enclosure, either
for experiments with electron beam or structure conditioning. These klystrons
could for instance be used to power S-band structures.

The LEA enclosure and beam dump were designed and rated for 700 MeV beams at 1
kW of beam power. A new suitably-dimensioned dump could be specified for a 2
GeV electron beam. An access control and interlock system is installed, and
gamma ray and neutron radiation monitors are installed at the upstream and
downstream ends of the enclosure.

A concrete pad is located adjacent to the end-station that could be used to
support a liquid nitrogen tank. Truck access to LEA in the APS infield (to
refill the nitrogen tank) is via the vehicle tunnel beneath the APS storage
ring. The APS receives regular bulk liquid nitrogen deliveries via a service
provider at multiple locations around the ring, principally to operate user
beamlines.

Utilities including conventional AC power, deionized cooling water and
temperature regulation via the building heating, ventilation, and air
conditioning (HVAC) system are also installed.

Accelerators at APS are routinely controlled via the Experimental Physics and
Industrial Control System (EPICS), and centrally managed from the APS Main
Control Room. A laser room has been configured within the LEA end-station.

###  4.3 Hardware

Although the 3 cryomodues contain 24 accelerator sections, we propose to
acquire only 18 commercial 50 MW modulator/klystron packages to lower costs.
This will permit testing a cryomodule at more than 150 MeV/m to determine
where breakdown may set in; to run at 120 MeV/m to demonstrate that breakdown
levels at this gradient are small, and to run at 70 MeV/m which we expect for
C3 phase 1.

The high power RF system for the initial C3 demonstrator will consist of
commercially available klystrons and modulators. The HPRF system deployed at
LANL for C-band high gradient structure testing is taken as the baseline
configuration.[68] The LANL test stand utilizes a 5.712 GHz, 50 MW pulsed
klystron (Canon E37212) powered by a Scandinova K2-2 modulator. Using this
same configuration (or a commercially available equivalent) will support the
fastest, lowest risk path to an initial demonstration. Future upgrades to the
high power RF system for improved efficiency or reduced fabrication costs,
resulting from the direct and parallel R$\&$D initiatives described
previously, may be incorporated as they become available.

The C3 accelerators operate under LN2. The C3 demonstrator will operate in two
modes: The first will be simple operation the linac, with LN2 being fed to one
end of the linac with both feed-forward control based on estimated power lost
in the copper and feedback based on LN2 level sensors. With full operation of
the 18 klystrons, this is expected to consume $\sim$25000 liters/day. This
will come from a local storage dewar and be replenished by trucks. The minimum
local storage tank would be 25,000 liters and would require two truck delivers
per day to fill. For continuity of operations, we will pursue at least double
that to have some buffer (weather, traffic, etc.). A 50,000 liter storage tank
is a fairly common size tank ( 13,000 gal). The second mode will demonstrate
the full C3 flow of LN2, $\sim$6 liters/sec, which will require taking LN2
from the far end at a controlled rate and pumping it back to the storage
dewar. Note that for C3 , each super-sector is fed LN2 from each end.

The source will be an S Band rf photo-injector that will capable of producing
a full C3 equivalent beam current. The individual bunch charge will be up to 1
nC. This will demonstrate the ability of the structures and sources to
maintain the proper gradient over the full train.

C3 draws heavily on the damping ring and beam delivery/final focus work of ILC
[74, 75], NLC [76], and CLIC [77]. Those systems and components are extremely
well studied and will not be part of the demonstrator hardware development
unless it is necessary.

##  5 Acknowledgements

The work of Mei Bai, Tim Barklow, Martin Breidenbach, Ankur Dhar, Joseph
Duris, Auralee Edelen, Claudio Emma, Josef Frisch, Annika Gabriel, Spencer
Gessner, John Lewellen, Carsten Hast, Emilio A. Nanni, Cho-Kuen Ng, Anatoly K.
Krasnykh, Mohamed A. K. Othman, Marco Oriunno, Dennis Palmer, Michael E.
Peskin, Thomas J. Peterson, Emma Snively, Caterina Vernieri, Sami Tantawi,
Brandon Weatherford, and Glen White is supported by Department of Energy
Contract DE-AC02-76SF00515.

Argonne National Laboratory’s work used the resources of the Advanced Photon
Source, a U.S. Department of Energy (DOE) Office of Science User Facility
operated for the DOE Office of Science by Argonne National Laboratory under
Contract No. DE-AC02-06CH11357.

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