# Beam Test Facilities for R&D in Accelerator Science and Technologies

Christine Clarke  SLAC National Accelerator Laboratory, Stanford University,
Stanford, CA 94309 USA  Michael Downer  University of Texas, Austin, TX 78705,
USA  Eric Esarey  Lawrence Berkeley National Laboratory, Berkeley, CA 94720,
USA  Cameron Geddes  Lawrence Berkeley National Laboratory, Berkeley, CA
94720, USA  Mark J. Hogan  SLAC National Accelerator Laboratory, Stanford
University, Stanford, CA 94309 USA  Georg Heinz Hoffstaetter  Cornell
University, Ithaca, NY 14853, USA  Chunguang Jing  Euclid Techlabs LLC, Solon,
OH 44139, USA  Argonne National Laboratory, Argonne, IL 60439, USA  Steven M.
Lund  Michigan State University, East Lansing, MI 48824, USA  Sergei Nagaitsev
Fermi National Accelerator Laboratory, Batavia, IL 60510, USA  The University
of Chicago, Chicago, IL 60637, USA  Mark Palmer  Brookhaven National
Laboratory, Upton, NY 11973, USA  Philippe Piot  Northern Illinois University,
DeKalb, IL 60115, USA  Argonne National Laboratory, Argonne, IL 60439, USA
John Power  Argonne National Laboratory, Argonne, IL 60439, USA  Carl
Schroeder  Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Donald Umstadter  University of Nebraska-Lincoln, Lincoln, NE 68588, USA
Navid Vafaei-Najafabadi  Stony Brook University, Stony Brook, NY 11794, USA
Brookhaven National Laboratory, Upton, NY 11973, USA  Alexander Valishev
Fermi National Accelerator Laboratory, Batavia, IL 60510, USA  Louise
Willingale  University of Michigan, Ann Arbor, MI 48109, USA  Vitaly Yakimenko
SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94309
USA

###### Abstract

This is the Snowmass Whitepaper on _Beam Test Facilities for R &D in
Accelerator Science and Technologies_ and it is submitted to two topical
groups: AF1 and AF6.

###### Contents

  1. I Executive Summary
  2. II Introduction
  3. III Accelerator Beam Test Facilities Mission
  4. IV Overview of Existing Accelerator Beam Test Facilities
    1. IV.1 National Laboratories
      1. IV.1.1 The Accelerator Test Facility (ATF) at Brookhaven National Laboratory
      2. IV.1.2 The Argonne Wakefield Accelerator (AWA) at Argonne National Laboratory
      3. IV.1.3 The Berkeley Lab Laser Accelerator (BELLA) Center at Lawrence Berkeley National Laboratory
      4. IV.1.4 The Fermilab Accelerator Science and Technology facility (FAST) at Fermi National Accelerator Laboratory
      5. IV.1.5 The Facility for Advanced Accelerator Experimental Tests II (FACET-II) at SLAC National Accelerator Laboratory
    2. IV.2 Universities
      1. IV.2.1 The Cornell-BNL ERL Test Accelerator (CBETA)
      2. IV.2.2 Extreme light laboratory, Lincoln, Nebraska
      3. IV.2.3 Texas Petawatt Laser, University of Texas at Austin
      4. IV.2.4 ZEUS user facility, University of Michigan
    3. IV.3 International Landscape
      1. IV.3.1 European Facilities
      2. IV.3.2 Asian Facilities
  5. V Research Thrusts
  6. VI Past Accomplishments
    1. VI.0.1 Accelerator S&T: Progress Made since the Previous Snowmass Process
    2. VI.0.2 Community-Developed Roadmaps
    3. VI.0.3 Spinoffs from DOE GARD Beam Test Facilities
  7. VII Capabilities
  8. VIII ANA Beam Test Facility upgrades
    1. VIII.1 ATF
    2. VIII.2 AWA
    3. VIII.3 BELLA
    4. VIII.4 FAST
    5. VIII.5 FACET-II

##  I Executive Summary

Demonstrating the viability of emerging accelerator science ultimately relies
on experimental validation. A portfolio of beam test facilities at US National
Laboratories and Universities, as well as international facilities in Europe
and Asia, are used to perform research critical to advancing accelerator
science and technology (S&T). These facilities have enabled the pioneering
accelerator research necessary to develop the next generation of energy-
frontier and intensity-frontier User Facilities. This report provides an
overview of the current portfolio of beam test facilities outlining: the
research mission, the recent achievements, and the upgrades required to keep
the US competitive in light of the large investments in accelerator research
around the world.

_The mission of the US beam test facilities_ is carried out by a broad
community of accelerator scientists, drawn from Universities, Industry, and
National Laboratories and has three core aims,

  1. 1.

Provide the experimental testbeds where (1) exploratory accelerator research
can be conducted (2) emerging accelerator research can be validated and (3)
promising accelerator technology can be programmatically developed and undergo
integration testing.

  2. 2.

Develop accelerator S&T needed to enable the next generation of HEP User
Facilities for the energy- and intensity frontiers as well as other SC
facilities such as light sources.

  3. 3.

Educate and train future scientists and engineers.

_The capabilities of the US beam test facilities_ considered in this report
possess one or more of the following capabilities: (O(100) MeV energy drive
beams, O(10) Petawatt drive lasers, O(1) Gigawatt RF power sources, high-
quality charged particle sources (e.g. low emittance electron beams), advanced
beam manipulation systems (e.g. nonlinear integrable optics, optical
stochastic cooling, emittance exchange) and capabilities to develop AI/ML for
accelerator science.

_Rapid progress was made by US beam test facilities_ since the previous
Snowmass Process. In 2015, the P5 strategic plan [1] made multiple
recommendations to the HEP General Accelerator R&D (GARD) program and the beam
test facilities were instrumental in addressing many of these. Most
importantly, various roadmaps were successfully formulated by the GARD
community in response to P5 to clearly articulate the long-term research
needed to develop the next generation of energy-frontier and intensity-
frontier machines. For example, the 2016 Advanced Acceleration Concepts (AAC)
Roadmap identified a series of milestones, many of which were demonstrated at
the US beam test facilities. Milestones achieved include multi-GeV/m
accelerating gradients, record-setting transformer ratios, the first
demonstration of optical stochastic cooling, ion acceleration, etc. In
addition to HEP milestones, multiple spin offs from the beam test facilities
have impacted mid-term applications, such as BES light-sources, with its role
in the development of the RF photoinjector, demonstration of SASE FEL, etc. as
well as near-term spinoffs impacting societal applications in the medical,
security, and industrial fields with the development of compact accelerators.

_Upgrades to US beam test facilities_ will require significant investment by
USDOE-HEP to ensure the US GARD program remains internationally competitive.
In turn, upgraded beam test facilities are needed to enable DOE-HEP to build a
cutting-edge program in the energy and intensity frontiers. For example, the
European Strategy for Particle Physics has committed considerable funds to
accelerator R&D in Europe. All USDOE GARD beam test facilities have near-term
upgrades underway and have developed proposals for mid-term upgrades to
continue progress on the previously developed roadmaps and support from DOE-
HEP is needed to realize these plans. In the long-term, several options are
under consideration ranging from a greenfield beam test facility to re-using
the infrastructure of next-generation facilities such as ILC, CLIC, FNAL site
linac, C3… etc with Advanced Acceleration technology. Once again, long-term
plans require serious study and support. In the long term, the US community is
exploring a possible greenfield beam test facility to enable a large-scale
demo of the application to AAC and possibly support medium-energy research in
elementary particle physics.

##  II Introduction

In the United States, accelerator research and development (R&D) is sponsored
by the General Accelerator R&D (GARD) Program of the Department of Energy
(USDOE) Office of Science [2], National Science Foundation (NSF) programs [3]
and the Office of Accelerator Research & Development (R&D) and Production
(ARDAP) [4]. These programs support fundamental accelerator research and
development of accelerator technology and methods to increase beam brightness,
accelerating gradient, average power, etc., to dramatically improve the cost
effectiveness and performance of accelerators [1]. These programs are the key
to creating accelerator User Facilities in the mid- and far-term for discovery
science (particle physics, neutron sources, light sources, etc.) and to create
near-term, low-cost, compact accelerator technology for future societal
applications in the medical, security, and industrial fields. US beam test
facilities are crucial components for advancing accelerator science and
technology (S&T).

##  III Accelerator Beam Test Facilities Mission

The mission of the beam test facilities are to provide the experimental test
beds where fundamental accelerator research can be explored and advanced
accelerator technology can be developed and undergo integration testing. While
limited R&D can be done in User Facilities dedicated to non-accelerator
science (e.g. LHC), typically these facilities have a dedicated user base that
cannot tolerate interruption. Their limited beam availability is used for
near-term accelerator technology to improve operations. These facilities are
ill-suited for exploring and developing advanced accelerator science. This is
where beam test facilities, dedicated to accelerator science, play an
essential role. In addition to the S&T mission, beam test facilities are where
a large number of future scientists and engineers needed by the accelerator
community are educated and trained [5, 6]. At the highest level, the mission
of the facilities can be broken down as:

  1. 1.

Providing experimental test beds to carry out basic research in advanced
accelerators and beam physics.

  2. 2.

Developing the S&T needed to enable the next generation of energy-frontier and
intensity-frontier science facilities and societal accelerator applications.

  3. 3.

Educating and training future scientists and engineers.

##  IV Overview of Existing Accelerator Beam Test Facilities

The above mission is carried out at several beam test facilities located both
domestically, at US National Laboratories and Universities and international
facilities in Europe and Asia, are used to perform research critical to
advancing accelerator science and technology (S&T). In this section, a list of
major beam test facilities that possess one or more of the following
capabilities is presented: (O(100) MeV energy drive beams, O(10) Petawatt
drive lasers, O(1) Gigawatt RF power sources, high-quality charged particle
sources (low emittance electron beams and positron beams), advanced beam
manipulation systems (e.g. nonlinear integrable optics, optical stochastic
cooling, emittance exchange) and emerging capabilities to demonstrate AI/ML.
Each facility description is accompanied by an overview of its research
program. Note, this list is not exhaustive as it leaves out many university-
scale beam test facilities due to space limitations. We note that university-
based beam test facilities are vital to accelerator research since this is
where many ideas are first developed before being tested on a larger scale.

###  IV.1 National Laboratories

####  IV.1.1 The Accelerator Test Facility (ATF) at Brookhaven National
Laboratory

The Accelerator Test Facility (ATF) at Brookhaven National Laboratory serves
the US DOE Accelerator Stewardship program and develops advanced acceleration
methods for leptons and ions as well as high-energy photon sources. The ATF
provides access to three classes of experimental facilities: a long-wave
infrared high-power laser at 9.2 µm, a high-brightness linac-driven 75-MeV
electron beam, and near-IR laser sources. This combination of capabilities
enables programs on particle and photon source development, wakefield
acceleration, inverse Compton scattering, and laser-driven plasma ion
acceleration.

Figure 1:  Overview of the ATF facility. User experiments can be configured to
utilize the near-IR laser beams, long-wave IR beam, and electron beam singly
or in any combination.

####  IV.1.2 The Argonne Wakefield Accelerator (AWA) at Argonne National
Laboratory

The Argonne Wakefield Accelerator (AWA) Facility [7] is dedicated to the
investigation of Structure Wakefield Acceleration (SFWA) including both
collinear wakefield acceleration (CWA) and short-pulse two-beam acceleration
(TBA). The AWA facility (Fig. 2) consists of three accelerators: the Drive
Photoinjector (producing up to 70-MeV high-charge electrons), the Main
Photoinjector (which forms 15-MeV electron bunches), and a few MeV cathode
test-stand. AWA research focuses on underlying beam-dynamics challenges (e.g.,
production of bright- and high-charge-beams), beam manipulation (e.g., beam-
current shaping) and beam control (e.g., mitigation of the BBU instability),
and the development of advanced accelerating structures (e.g., dielectric
waveguides, planar structures, metamaterials, rapid filling metallic
structures, etc.) for efficient, high-peak-power RF generation and high-
gradient acceleration. AWA has demonstrated unprecedented beam-transformer
ratios ($>5$) in both SWFA and Plasma Wakefield Accelerator (PWFA) setups.
Likewise, it has generated GW-scale peak-power RF-pulses and demonstrated
staging in a TBA configuration in the pursuit of a linear collider. AWA
develops these technologies for other applications (FELs and compact
accelerators) and synergistically supports other advanced-accelerator
concepts.

Figure 2:  Overview of the AWA facility consists of a 70-MeV drive-beam
photoinjector, a 15-MeV main-beam photoinjector, and the Argonne cathode test-
stand (ACT).

####  IV.1.3 The Berkeley Lab Laser Accelerator (BELLA) Center at Lawrence
Berkeley National Laboratory

The BELLA Center at LBNL has been performing research on laser-plasma
accelerators (LPAs) for over two decades. The main research objectives are the
development of LPA modules at the 10 GeV level and the staging (coupling) of
LPA modules, which are two essential R&D components for a future plasma-based
linear collider. The current laser systems at the BELLA Center include the
BELLA Petawatt (PW) laser and two independent 100 Terawatt (TW) laser systems.
Upgrades are underway to the BELLA PW beamline to allow the delivery of two
synchronized pulses, with independent compressors, on target, enabling
staging, and a short focal length capability, enabling experiments at
ultrahigh intensity (Fig. 3). The BELLA Center is also pursuing a new
facility, kBELLA, consisting of a 1 kHz, few J, 30 fs, high average power
laser for the demonstration of a high rep-rate, precision LPA and subsequent
applications. The BELLA Center functions as a collaborative research center
and is part of LaserNetUS [8].

Figure 3:  Schematic of the two beamline configuration on the BELLA PW laser.

####  IV.1.4 The Fermilab Accelerator Science and Technology facility (FAST)
at Fermi National Accelerator Laboratory

The facility (Fig. 4) consists of a storage ring, IOTA, capable of operating
with both protons and electrons with beam momentum range 50-150 MeV/c and two
injector linacs (electrons and protons). The IOTA research goals are mostly
focused on the challenges, posed by future high-intensity machines, such as
beam instabilities and losses. The IOTA storage ring is unique in its
flexibility and performance. It has a circumference of 40 m and a relatively
large aperture (50 mm). It is easily reconfigurable to accommodate the
installation of 1-3 concurrent experiments. The focusing lattice was designed
to have significant flexibility to enable a wide variety of studies. The IOTA
proton injector produces beam at a fixed energy of 2.5 MeV (kinetic), while
the energy output of the electron linac can be varied up to 300 MeV. Research
with electron beams into optical stochastic cooling is underway at both IOTA
(Fermilab) and CESR (Cornell).

More specifically, the IOTA research program aims to push the maximum beam
intensity and brightness of future ring accelerators while minimizing the
accelerator scale and cost. Along this direction, the key research areas are
i) suppression of coherent beam instabilities by Landau damping; ii)
mitigation of space-charge effects, and iii) beam cooling. The main candidate
technologies being pursued at IOTA are the Nonlinear Integrable Optics,
Electron Lenses, and Optical Stochastic Cooling.

Figure 4:  Layout of the FAST facility.

####  IV.1.5 The Facility for Advanced Accelerator Experimental Tests II
(FACET-II) at SLAC National Accelerator Laboratory

This facility is an upgrade of the FACET facility and operates as an Office of
Science National User Facility. FACET-II will provide beams optimized for the
next generation of PWFA experiments and will be the only facility in the world
capable of providing 10 GeV electron beams in support of accelerator science
R&D. It allocates roughly half of the beam time towards investigating plasma
wakefield acceleration in support of the R&D roadmaps defined in the DOE
Advanced Accelerator Development Strategy Report [6]. Primary elements of this
program include demonstration of a 10 GeV plasma stage with preserved beam
quality, development of ultra-high brightness beams from plasma-based
injectors and developing high-intensity X-ray and Gamma-ray sources. The other
half is dedicated to a diverse set of research programs enabled by high-energy
high-intensity electron beams and their interaction with lasers, plasmas and
solids. Novel diagnostics to characterize the extreme beams are being
developed combining beam-physics, machine learning and artificial
intelligence. Future upgrades to deliver 10 GeV high intensity positron beams
and upgrades to the 10 TW experimental laser systems are under development to
exploit the full scientific potential of the facility.

Figure 5:  Overview of the FACET-II facility. The existing facility includes
the photoinjector (electron source), bunch compressors and experimental area
(blue) and accelerators in between to achieve the nominal 10 GeV Energy. The
facility has been designed to be compatible with upgrades to restore high-
energy high-current positron bunches including a refurbished positron target
and transfer line, a new compact damping ring and modified bunch compressors
(red). The additional compressor upstream of the experimental area will allow
simultaneous delivery of electrons and positrons to the experiments.

###  IV.2 Universities

####  IV.2.1 The Cornell-BNL ERL Test Accelerator (CBETA)

The Cornell-BNL ERL Test Accelerator (CBETA) consists of a 4-turn SRF Energy
Recovery Linac with a permanent magnet Fixed Field Alternating-gradient (FFA)
arc at Cornell University. The facility is equipped for studies of energy
recovery, including phenomena such as the beam-breakup instability, beam
timing and SRF phasing for simultaneous beam; for permanent magnet technology;
and for FFA optics with large energy aperture. An additional focus is the
saving of energy in accelerators.

####  IV.2.2 Extreme light laboratory, Lincoln, Nebraska

The Extreme Light Laboratory (ELL) is home to the Diocles laser, a petawatt-
class laser. It generates light with nearly a million-billion watts of power,
or more than 100 times the combined output of all the world’s power plants,
but compressed into the briefest of bursts. It can recreate some of the most
extreme conditions in the universe. Experiments using Diocles enhance the
basic understanding of physics and develop solutions for advanced
manufacturing, national security and medicine. The lab houses two separate
high-intensity femtosecond-duration lasers. One of them, the Diocles Laser, is
petawatt class, and has been in operation for over 14 years. It is
distinguished by exceptional laser beam quality, stability and parameter
control. This enables research with high laser light intensity interacting
with either plasmas, or GeV-energy electron beams from a coupled laser-driven
wakefield accelerator. The interactions can be studied with synchronized
pulses of laser light, electrons or tunable x-rays. A peak laser intensity of
$6\times 10^{20}$ $6\times 10^{20}$ W/cm2 was directly measured in an
experimental demonstration of high-order multi-photon inverse Compton
scattering. The laser and experimental research facility occupies
8,000-sq.-ft. three floors of the Behlen Lab building on the UNL City Campus.
The laboratories meet stringent requirements on temperature, humidity, and
vibration control, made possible by separate designated electrical, processed-
chilled-water, and air-handling systems, and offers flexible experimental
arrangements and diagnostics. A total of six large 72”x48”x24” vacuum chambers
are available, with three separate and independent beamlines: (1) 0.7-PW peak
power at 0.1-Hz repetition rate, (2) 100-TW at 10-Hz and (3) 6-TW at 10-Hz.
All use Ti:sapphire chirped-pulse amplification, and lase at 800-nm in 30-fs
duration pulses.

Figure 6:  The Diocles laser at the Extreme Light Laboratory delivers peak
intensity ranging from to $10^{18}-10^{21}$ W/cm2.

####  IV.2.3 Texas Petawatt Laser, University of Texas at Austin

The Texas Petawatt Laser is a Nd:glass CPA laser using a unique architecture.
It combines amplification in an series of Optical Parametric Amplifiers and
subsequent high energy amplification in a combination of silicate and
phosphate glass to enable recompression of the pulses to under 140 fs,
considerably shorter than most glass-based CPA lasers. It can be fired into
one of two target chambers, the first (TC1) focuses the beam tightly with an
$f/3$ optic to intensity of well over 2 x $10^{21}$ W/cm2. An $f/1.1$ optic
can be deployed in this chamber to yield intensity of over $10^{22}$ W/cm2.
The beam can also be directed into a chamber with a long focal length, $f/40$
focusing. This is optimal for gas target wakefield acceleration experiments.

Figure 7:  Overview of the Texas Petawatt facility.

####  IV.2.4 ZEUS user facility, University of Michigan

The National Science Foundation (NSF) Zettawatt Equivalent Ultrashort pulse
laser System (ZEUS) will be a user facility located at the University of
Michigan. Due to become fully operational in late 2023, ZEUS will offer 30
weeks per year to users through a scientific merit based proposal system.
There will be several modes of operation into three different target areas,
each with different, flexible geometries to support a wide range of
experiments. The full 3 PW power (75 J in 25 fs at 800 nm) will be delivered
as a single pulse at a repetition rate of 1 shot per minute and should enable
focused intensities exceeding $10^{22}~{}\rm{Wcm}^{-2}$. Alternatively, the
power can be split into one beam with 2.5 PW and the with 500 TW. A signature
configuration will use 2.5 PW to drive laser wakefield acceleration (LWFA) to
multi-GeV electron energies and the 500 TW pulse will be tightly focused to
counterpropagate to the electron beam. The apparent power of the focused laser
pulse is boosted proportional to the square of the Lorentz factor (by more
than a millionfold) from the Petawatt range to the Zettawatt range. Thus, the
wakefield electrons see a Zettawatt Equivalent Ultrashort pulsed laser System
– ZEUS. Target Area 1 (TA1) will accommodate a very long focal length optic
($\sim f/60$) primarily designed to drive LWFA with up to 3 PW pulses. TA1
will also have a short focal length optic to create high-intensity conditions,
that could be counter-propagating to the LWFA electron beam. Target Area 2
(TA2) will have a double plasma mirror capability, to create very high-
contrast conditions that are desirable for high-harmonic generation or ion
acceleration experiments using solid target interactions. Target Area 3 (TA3)
will be operational at 500 TW at have a 5 Hz burst mode with $f/20$ or $f/40$
focusing, ideal for many LWFA or secondary betatron x-ray source studies and
applications.

Figure 8:  Overview of the ZEUS facility.

###  IV.3 International Landscape

Advancing accelerator science and technology (S&T) is the subject of great
international interest as the international community seeks to develop the
next generation of energy-frontier and intensity-frontier user facilities.
This section provides a brief overview of beam test facilities around the
world.

####  IV.3.1 European Facilities

The European landscape currently maintains a large portfolio of Advanced and
Novel Accelerator (ANA) beam test facilities for the development of technology
which can be divided into two kinds: (i) beam-driven PWFA facilities and
laser-driven laser wakefield acceleration (LWFA) facilities. In addition, the
recent European Strategy for Particle Physics [9] has committed considerable
funds towards the commissioning of new ANA R&D facilities. Muggli et al.,
(ref. JINST 076P 0122 Facility European) have published an article surveying
this portfolio of current and future European ANA facilities. Beam-driven PWFA
facilities with the objective of generating high-energy (GeV or higher), high-
quality beams, with established timeline and structured collaboration,
typically beam driven facilities, stemming from the accelerator community
structuring include operating the facilities AWAKE [10], FLASHForward, and
SPARC-LAB. Large scale PW laser facilities include a number of PW-class laser
systems, although none are dedicated to R&D towards accelerators for HEP.
However, R&D towards accelerators for HEP is part of the user program at many
laser facilities, which significantly contribute to the development of high-
energy accelerators.

In addition to the current ANA facilities, the European community has recently
committed to the commissioning of a new large-scale accelerator project called
EuPRAXIA. In its first implementation phase, the EuPRAXIA consortium will
construct a PWFA facility, 479 named EuPRAXIA@SPARC-LAB at the LNF-INFN [11].
In its second phase, EuPRAXIA consortium will build a LWFA facility at a site
to be chosen within the next 2 years between several options in Europe. This
major commitment by the European accelerator community will have a high impact
on the future of accelerator technology.

####  IV.3.2 Asian Facilities

The Asian accelerator landscape includes a variety of facilities to study the
four ANA categories of LWFA, PWFA, structure wakefield acceleration (SWFA),
and dielectric laser acceleration (DLA). Kando et al., (ref. JINST 076P 0122
Facility European) have published an article surveying this portfolio of
current and future Asian ANA facilities. Most of the facilities are dedicated
to LWFA including RIKEN SPring-8 in Japan and CoReLS in South Korea. The
authors note that most of the Asian LWFA facilities possess high-peak-power
(¿10 TW) lasers with tens of femtosecond duration for laser wakefield
acceleration. There are also facilities at KEK and Tsinghua University in
China that are capable of developing beam-driven PWFA and SWFA with novel
structure development including dielectric lined circular waveguide (DLW),
dielectric wall acceleration (DWA), and dielectric assisted accelerator (DAA).

##  V Research Thrusts

Identifying the most promising accelerator research and development areas to
be conducted at the ANA beam test facilities is an ongoing process and subject
to continuous revision. For this reason, the US ANA beam test facilities are
designed to provide access to a suite of complementary and diverse
capabilities to serve a broad community of accelerator scientists drawn from
Universities, Industry, and National Laboratories to carry out the national
ANA mission.

DOE and NSF work with US accelerator scientists to identify the most promising
accelerator research thrusts and these findings have been published a number
of documents. In 2016, HEP published the Advanced Accelerator Concepts (AAC)
R&D Roadmap [6] for three AAC schemes, laser-driven plasma wakefield
acceleration (LWFA), beam-driven PWFA, and structure wakefield acceleration
(SWFA). The roadmaps describe a series of near-term, mid-term and long-term
research milestones with the final goal of constructing a multi-TeV e+e–
collider. Other important research thrusts include the Radiofrequency
Accelerator Roadmap [12], Accelerator and Beam Phyics Roadmap [13],
Accelerator Stewardship [4], Particle Sources, and Artificial Intelligence and
Machine Learning [14].

Table 1:  Research Thrusts of the US National Laboratory ANA Beam Test Facilities. Research thrust | ATF | AWA | BELLA | FACET-II | FAST
---|---|---|---|---|---
Advanced Acceleration |  |  |  |  |
LWFA | ✓(MeV) |  | ✓(GeV) |  |
PWFA | ✓(MeV) | ✓(MeV) |  | ✓(GeV) |
SWFA | ✓(MeV) | ✓(MeV) |  | ✓(GeV) |
IFEL | ✓ |  |  |  |
staging | ✓ | ✓ | ✓ |  |
positron acceleration |  |  |  | ✓ |
Particle Source Development |  |  |  |  |
plasma-based e- sources |  | ✓ | ✓ | ✓ |
ion acceleration w/ lasers | ✓ |  | ✓ |  |
Inverse Compton Scattering | ✓ |  | ✓ | ✓ |
$\gamma$ ray via filamentation |  |  |  | ✓ |
coherent X rays from plasmas |  | ✓ | ✓ | ✓ |
Beam Physics |  |  |  |  |
phase-space cooling |  | ✓ |  |  | ✓
single $e-$ & crystalline beams |  |  |  |  | ✓
integrable nonlinear optics |  |  |  |  | ✓
extreme bunch compression |  |  |  | ✓ |
Diagnostics & Beam Control |  |  |  |  |
novel diagnostics | ✓ | ✓ | ✓ | ✓ | ✓
ML/AI: virtual diagnostics |  | ✓ | ✓ | ✓ | ✓
ML/AI: improve efficiency |  | ✓ | ✓ | ✓ |
bunch-current shaping | ✓ | ✓ | ✓ | ✓ |
phase-space/emit. exchange | ✓ |  |  |  |

In Table 1, we list the main thrusts of the US ANA program as carried out by
the ANA facilities at the national laboratories: advanced acceleration
concepts, particle source development, beam physics, and diagnostics and beam
control. Note that this list is not exclusive of all accelerator S&T conducted
at the US ANA facilities.

##  VI Past Accomplishments

ANA beam test facilities have been instrumental in advancing accelerator S&T
throughout their history. In this section, we present short summaries of these
advances in two subsections (i) progress made since the previous Snowmass
process and (ii) spinoffs from the DOE GARD beam test facilities.

####  VI.0.1 Accelerator S&T: Progress Made since the Previous Snowmass
Process

The primary goal of the US DOE GARD program is to carry out accelerator-based
particle physics research in support of the priorities defined in the P5
strategic plan [1]. At the highest level, GARD supports three different lines
of accelerators: the Intensity-Frontier with intense protons, the Energy-
Frontier with a hadron collider and the Energy-Frontier with an e+e- linear
collider. To accomplish this, a set of recommendations were made to the HEP
GARD program by the HEPAP Accelerator R&D Subpanel Report published in April
2015 [15]. Specifically, the subpanel published 15 recommendations for the
GARD program and beam test facilities were instrumental in addressing 7 of
these recommendations.

An comprehensive update on all of the GARD subpanel recommendations was
presented at the 2019 November HEPAP meeting [16]. In this subsection, we
present selected S&T highlights of progress made on the subpanel
recommendations in which the GARD beam test facilities were crucial. Examples
of milestones accomplished covered here will include progress in both the
intensity and energy frontiers. Intensity-Frontier accelerator science saw the
first demonstration of Optical Stochastic Cooling [17]. Energy-Frontier linear
collider accelerator science delivered multi-GeV/m accelerating gradients and
multi-GeV energy gain in plasma accelerators using beam-drivers at FACET [18]
and laser-drivers at BELLA [19], multi-GeV/m positron acceleration at FACET
[20], staging of two plasma accelerator modules at BELLA [21], demonstration
of controlled injection in a plasma wakefield accelerator [22], record setting
transformer ratios at AWA in both structures [23] and plasmas [24], pioneering
work at ATF on shock wave monoenergetic ion acceleration [25], nonlinear
effects in inverse Compton scattering (red shift, higher harmonics) [26],
high-gain high-harmonic generation FEL [27], fundamental research of electron
acceleration in dielectric waveguides (e.g., using 3D woodpile structures
[28]), fundamental research on nonlinear Thomson scattering [29], and novel
methods for controlled optical injection of electrons into wakefields [30].

####  VI.0.2 Community-Developed Roadmaps

The development of community-driven research roadmaps was major organizational
success for the GARD program. Roadmaps have been published for all GARD
thrusts except the Accelerator and Beam Physics Roadmap [31] thrust area which
was delayed by COVID but is expected to be published shortly. These roadmaps
enable pressing challenges to be more easily identified and addressed to move
the field forward and insured a list of prioritized milestones that were
aligned to the most compelling HEP science. The GARD beam test facilities play
crucial roles in the following roadmaps. (i) In 2016, US DOE published the
Advanced Accelerator Roadmap [6] that laid out a series of research plans and
goals that would provide the foundation for a technical design report of a
multi-TeV linear collider. Significant progress along the AAC roadmap has been
made at the GARD beam test facilities over the last 5 years and they remain
critical for pursuing the continued advancements.

####  VI.0.3 Spinoffs from DOE GARD Beam Test Facilities

A secondary, yet vital goal, is to apply this long-term, general R&D carried
out at the ANA facilities to generate spinoff technologies to benefit other
applications in science, medicine and industry. Not only does this help
validate the accelerator S&T underway but also to benefit other accelerator
applications (e.g. light sources, and neutron sources, etc.). Note that these
spinoffs often come many years down the road, sometimes decades later.
Sometimes they come from Universities, sometimes from National Laboratories,
sometimes from Industry, or a combination of two or three stakeholders. A
partial list of some of these spinoff technologies is given in Table 2.

Table 2: R&D spinoffs developed and/or demonstrated, at least in part, at US-
ANA facilities.  Photoinjector R&D that enabled high brightness sources for
X-ray Free Electron Lasers (XFELs)
---
SASE/FEL R&D demonstrating that the self-amplified spontaneous emission (SASE)
instability would work at progressively shorter wavelengths from optical to
X-rays
Inverse Free Electron Laser (IFEL) techniques that are basis for the laser
heaters used to mitigate instabilities in XFELs
LWFA research that gave rise to betatron X-ray probes at the LCLS Matter in
Extreme Conditions (MEC) Instrument
Laser scattering from plasma accelerated beams to generate X-ray and gamma ray
probes
Wakefield studies that then were applied to create de-chirpers for XFELs and
passive deflectors for UED facilities
Bunch compression studies that informed compressor design for higher peak
current and shorter gain length at XFELS
Thomson scattering to measure electron beam emittance and bunch length
Paricle (electron, proton and ion) beams from laser plasma accelerators for
probing matter

##  VII Capabilities

The five US ANA beam test facilities, supported by the DOE Office of Science,
HEP General Accelerator R&D (GARD), provide access to a suite of complementary
and diverse capabilities, as summarized in Table 3. Similarly, the
capabilities of the university-based facilities are summarized in Table 4.
Capabilities with high-quality beam parameters and diagnostic tools are
essential to support research into new beam generation, acceleration, and
transport techniques with the potential to mitigate technical risks of
upcoming projects and to reduce the cost of future accelerator facilities.

Table 3: Capabilities: National Laboratory ANA Beam Test Facilities. The “P” label indicates a planned capability.  Capabilities | ATF | AWA | BELLA | FACET-II | FAST
---|---|---|---|---|---
operation model |  |  |  |  |
National user facility | ✓ |  |  | ✓ |
Accelerator stewardship | ✓ |  |  | ✓ |
Collaboration |  | ✓ | ✓ |  | ✓
Beams & Accelerators |  |  |  |  |
$\sim 100$-MeV $e^{-}$ | ✓ | ✓ | ✓ |  | ✓
$\sim 10$-GeV $e^{-}$ |  |  | P | ✓ |
$\sim 10$-GeV $e^{+}$ |  |  |  | P |
high-charge ($\sim 100$ nC) $e^{-}$ bunches |  | ✓ |  |  |
proton beams |  |  | P |  | P
NCRF S-band and X-band | ✓ |  |  |  |
NCRF L-band and X-band |  | ✓ |  | ✓ |
SCRF L-band and X-band |  |  |  |  | ✓
storage ring |  |  |  |  | ✓
Lasers |  |  |  |  |
TW-class 800-nm laser (Ti:Sapphire) | ✓ | ✓ | ✓ | ✓ |
PW-class 800-nm laser (Ti:Sapphire) |  |  | ✓ |  |
TW-class 10 µm laser (CO2) | ✓ |  |  |  |
Plasmas |  |  |  |  |
plasma capilaries (length [cm]) | ✓(2) | ✓(2) | ✓(20) |  |
gas jets | ✓ |  | ✓ | ✓ |
heat-pipe oven | ✓ |  | ✓ | ✓ |
hollow channel |  | ✓ |  | ✓ |
Table 4: Capabilities: University Beam Test Facilities. The “P” label indicates a planned capability.  Capabilities | ELL | Texas | ZEUS
---|---|---|---
Beams & Accelerators |  |  |
$\sim 100$-MeV $e^{-}$ | ✓ | ✓ | P
$\sim 10$-GeV $e^{-}$ |  |  | P
proton beams | ✓ | ✓ | P
Lasers |  |  |
TW-class 800-nm laser (Ti:Sapphire) | ✓ |  | P
PW-class 800-nm laser (Ti:Sapphire) | ✓ |  | P
PW-class 1057-nm laser (Nd:glass) |  | ✓ |
Plasmas |  |  |
plasma capilaries (length [cm]) |  |  |
gas jets | ✓ | ✓ | P
heat-pipe oven |  |  |
hollow channel |  |  |

##  VIII ANA Beam Test Facility upgrades

In order to continue progress in accelerator science and technology, for
instance progress along the AAC Roadmap, major upgrades to the facilities are
required.

###  VIII.1 ATF

The Accelerator Test Facility (ATF) at Brookhaven National Laboratory. The
upgrade plan for ATF facilities reflects the priority set by the science
planning workshop [32] based on community input. At the core of this plan is
upgrading of the LWIR laser with the goal of delivering reliable CO2 laser
pulses at high repetition rate with a power of $>25$ TW and a sub-ps pulse
length to the users in the long-term. Such intense LWIR sources (reaching
$a_{0}$ as high as 8 with $F_{\\#}\sim 1$ focusing) fill a unique niche in
advanced acceleration research for generation of electron and ion beams [33,
34, 35]. While the facility is currently capable of producing a 5 TW CO2
pulse, only about half of that is delivered to users due to limitations of the
air transport system. An upgrade to a vacuum transport system as well as the
development of nonlinear pulse compression (NLPC) [36] technique is expected
to allow the delivery of ¿10 TW pulses over the next several years with pulse
lengths as short as hundreds of femtoseconds. The facility is also pursuing an
active R&D program on optical pumping of CO2 molecules as well as other
technologies that make efficient use of the CO2 gain profile and enable the
delivery of high-repetition rate, high-average power, reliable LWIR pulses.
The ATF plans also envision several thrusts for upgrading the linac-driven
e-beam, including beam energy and pulse compression. Together with the
expanded near IR capabilities, which encompasses the integration and the
delivery of a compressed ($\leq 100$ fs), terawatt-class Ti:Sapphire laser
pulse to the IP, simultaneous experiments using the LWIR laser, e-beam, and
the Ti:Sapphire enable the research and development of next generation of
particle and photon sources.

###  VIII.2 AWA

A proposal to impliment a major upgrade of the Argonne Wakefield Accelerator
(AWA) to the AWA-HE (HIGH ENERGY) Facility is under development . In several
years, after completing the 500-MeV demonstrator, the AWA will become limited
in its ability to continue advancing the SWFA Roadmap. Our vision is to
continue to advance the Roadmaps by enacting a phased upgrade of the AWA
facility so that by the end of the decade the SWFA method will be ready to
present an SWFA CDR for a 3-TeV e+e- linear collider. Bunker expansion. Step 1
is to expand the AWA bunker length so that it occupies all available space in
its current building. Energy upgrade. Step 2 is to add more RF stations and
accelerating structures to increase the energy from 65 to 150 MeV. This will
enable GV/m class acceleration gradients and a 3-GeV demonstrator.

###  VIII.3 BELLA

Upgrades are underway to the BELLA PW laser beamline (operating at 1 Hz) to
allow the delivery of two synchronized pulses on target, enabling staging at
the few GeV level, and a short focal length capability, enabling experiments
at ultrahigh intensity (e.g., laser ion acceleration). The BELLA Center is
also pursuing a new facility, kBELLA, consisting of a 1 kHz, few J, 30 fs,
high average power laser for the demonstration of a high rep-rate, precision
LPA and subsequent applications. This will function as a user facility with
the kHz, GeV class electron beams and the intrinsically synchronized photon
pulses being available for a wide variety of experiments in the basic and
applied sciences. One possible candidate laser technology for kBELLA is
coherent combining of fiber lasers, which holds promise for providing high
average power at high efficiency. To this end, the BELLA Center has a very
active R&D program on coherent combining (in both time and space, as well as
spectrally) of fiber lasers in collaboration with University of Michigan and
LLNL. Furthermore, fiber lasers have the potential to provide high average
power and high efficiency at 10 kHz and beyond, which is needed for a future
LPA-based collider. The development of high-average power, high efficiency
lasers is delineated as an important R&D topic on the Advanced Accelerator
Roadmap.

###  VIII.4 FAST

In the coming years, the IOTA ring will be upgraded with a proton injector and
an electron lens/cooler [37]. The research with protons and electrons will
focus on nonlinear beam optics studies [38], investigation of space-charge
effects [39], realization of Electron Lens with its broad research program
[37], continuation of the OSC program [40], and photon science opportunities
[41]. A significant required upgrade to the FAST facility is the addition of a
laser system to enable research into electron-based coherent radiation
generation and activities synergistic with advanced acceleration technologies.

###  VIII.5 FACET-II

As part of the current Snowmass process, the High Energy Physics community is
evaluating a e-e+ collider options for precision measurements of the Higgs
Boson. Advanced Accelerator technologies are being considered for the
possibility of extending the energy reach of such a facility through high-
gradient advanced accelerator-based energy boosters or afterburners.
Additional studies are looking at the opportunities for Advanced Accelerator
technologies to provide an order of magnitude increase in energy for
discovery-oriented colliders in the future. In both of these scenarios it is
critical to demonstrate a technique for accelerating high-intensity low-
emittance positron bunches in plasma. The only experimental studies of
positron acceleration in plasma have taken place at SLAC in the Final Focus
Test Beam facility (FFTB) and FACET facilities. Novel ideas for positron
acceleration using tailored beam drivers and/or plasma sources have been
proposed using beam parameters envisioned for FACET-II. A plan is currently
being developed to restore positron capabilities to FACET-II and, for the
first time, provide the capability to deliver electrons and positrons to the
experimental area simultaneously to enable studies of positron acceleration in
electron beam driven wakefields.

###### Acknowledgements.

We acknowledge support by the U.S. Department of Energy, Office of Science
under contracts DE-AC02-06CH11357 with Argonne National Laboratory, DE-
AC02-05CH11231 with Lawrence Berkeley National Laboratory, DE-AC02-07CH11359
with Fermi Research Alliance, DE-SC0018656 with Northern Illinois University,
and DE-AC02-76SF00515 with SLAC National Accelerator Laboratory.

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