Fabry-Perot cavities at a few selected wavelengths between $1550$ nm and $517$
nm.

###### Flux Performance

The flux requirements in Table 5.1 can be realized readily. A $\gamma$-ray
beam with a total flux of about $3.4\times 10^{11}$ $\gamma$/s (in the $4\pi$
solid angle) can be produced in a wide energy range. For example, a $350$ MeV
$\gamma$-ray beam of this intensity with a $95.4$ MHz repetition rate can be
produced using a $500$ mA electron beam ($3.27$ GeV) and a modest $20$ kW
laser power inside a $517$ nm FP cavity (see the detailed beam parameters in
the table in Fig. 5.3). The corresponding collimated flux for a beam with a
FWHM energy resolution of $2$% is about $9\times 10^{9}$ $\gamma$/s. For those
experiments requiring a lower repetition rate, $6$ out of $64$ rf buckets can
be filled with the same bunch charge to produce a roughly $9.5$ MHz beam with
a collimated flux of about $8\times 10^{8}$ $\gamma$/s, and higher flux is
possible by increasing the bunch charge. Furthermore, by increasing the
intracavity laser power to $100$ kW to match the capability of a high charge,
high repetition rate booster injector ($32$ nC/cycle, $10$ Hz), the total
$\gamma$-ray flux can be increased to about $2\times 10^{12}$ $\gamma$/s.
Ultimately, the $\gamma$-ray flux for this medium energy CGS will be limited
by the electron injection rate, not by the laser power in the FP cavity or the
stored beam current in the storage ring.

E-beam | $E_{\gamma}$ |  $\lambda_{1}$ (nm) |  $\lambda_{2}$ (nm) | $\lambda_{2}/3$
---|---|---|---|---
(GeV) | (MeV) | $1064$ | $1550$ | $517$
$1.2$ | $E_{\gamma,{\rm min}}$ | $25$ | $17$ | $51$
$3.5$ | $E_{\gamma,{\rm max}}$ | $205$ | $144$ | $398$

Electron beam |
---|---
Beam energy |  $3.27$ GeV
Stored currents |  $500$ mA
Bunch filled | $64$
Hori./Vert. emittance |  $1.94/0.16$ nm-rad
Hori./Vert. size (rms) |  $100/26$ $\mu\mathrm{m}$
Bunch length (rms) |  $60$ ps
Laser beam |
Wavelength |  $517$ nm
Intracavity power |  $20$ kW
Pulse length (rms) |  $20$ ps
Hori./Vert. size (rms) |  $40/40$ $\mu\mathrm{m}$
Gamma-ray beam |
Max. energy |  $350$ MeV
Collision rate |  $95.42$ MHz
Collision angle | $6^{\circ}$
Luminosity | $5.7\times 10^{35}\ \mathrm{cm}^{-2}\mathrm{s}^{-1}$
Total flux (in $4\pi$ solid angle) |  $3.4\times 10^{11}$ $\gamma/\mathrm{s}$

E-beam: | Laser beam: |  $\lambda=$$517$ nm;
---|---|---
$E=3.27$ GeV, $I=0.5$ A | Beam size: |  $40/40$ $\mu\mathrm{m}$
FP cavity power (kW) |  $20$ (kW) |  $100$ (kW)
Tot. flux ($\gamma/\mathrm{s}$): $\mbox{$\theta$=}6^{\circ}$ | $3.4\times 10^{11}$ | $1.7\times 10^{12}$

Figure 5.3: (Left) Energy of the $\gamma$-ray beam as a function of electron
beam energy ($1.2$–$3.5$ MeV) for lasers at $1064$ nm, $1550$ nm and its third
harmonic at $517$ nm with a $6^{\circ}$ collision angle. (Upper-Right) A table
listing the main parameters for producing a $350$ MeV $\gamma$-ray beam using
a $3.27$ GeV electron beam and a $517$ nm laser beam. (Lower-Right) A table
showing the total flux for two levels of intracavity laser powers.

###### Electron Beam

To achieve a high average current, the electron beam will be stored in
multiple bunches. The bunch pattern should be selectable as demanded for
specific user experiments for either high flux operation at a high repetition
rate, or a modest flux operation at a lower rate.

###### Laser Beam

The Fabry-Perot cavity will be powered by a commercial laser (fiber or solid
state) operated at tens to hundreds of MHz repetition rate with a picosecond
pulse duration. The commonly available wavelengths are around $1064$ and 1550
nm; their 2nd and/or 3rd harmonics can also be used with a lower power.

###### Fabry-Perot Cavity and Collision Scheme

For this medium energy gamma-ray source, the required intracavity laser power
is rather modest, reducing the need to push the FP cavity power limit. Both
head-on collision and small cross-angle collision can be arranged using either
a two-mirror or four-mirror cavity. The entire $\gamma$-ray energy range can
be covered using one Fabry-Perot cavity specially designed to build up the
laser power at both the fundamental wavelength at $1550$ nm and its third
harmonic at $517$ nm (see the table in Fig. 5.3). To cover a wide intermediate
energy range (from $30$ to $200$ MeV), a $1064$ nm Fabry-Perot cavity is well
suited and highly practical because the FP cavity technology at $1064$ nm is
well established and more mature than that for $1550$ nm.

###### Energy Resolution

The required $\gamma$-ray beam energy resolution ($1.5$–$2$%, FWHM) can be met
by using a narrow-band laser beam, and an electron beam with a small energy
spread and reasonably small transverse emittance [165, 166]. The electron beam
performance is readily achievable using the state-of-the-art storage ring
magnetic optics design, taking advantage of recent advances in developing low-
emittance storage ring based synchrotron radiation sources [167, 168].

###### Beam Polarization

A well-collimated Compton $\gamma$-ray beam driven by a polarized laser beam
can have a very high degree of polarization, either linear or circular. The
relatively slow polarization manipulation (up to $10$ Hz), switching between
two polarization states (the horizontal and vertical, or left and right
circular), can be realized using conventional polarizing optics (including
Pockels cells) outside the FP cavity where the laser power is much lower.

###### Production Modes

The medium energy gamma-ray source is likely to be operated as a single- or
few-user facility. Several Compton sources can be installed in one or more
straight sections as shown in Fig. 5.4. When operated simultaneously, the sum
of $\gamma$-ray flux from all sources will be limited by the electron
injection rate. For each gamma-ray source, a dedicated user target room with
proper shielding and a beam dump should be constructed. Multiple target rooms
are desirable as complex mdeium energy experiments will need a dedicated
staging area to develop, test, and integrate experimental equipment while the
other target room(s) is/are being used for production data-taking.

Several challenges and important issues for this medium energy source are
summarized here:

  * •

Electron injection rate

The cost-effective choice for the injection is a full-energy, top-off booster
injector. The booster injector needs to be designed to achieve high
reliability, a fast injection cycle (up to $10$ Hz), and a large average
injection rate (few $10^{11}$ to few $10^{12}$ $e^{-}$/s).

  * •

Dual wavelength Fabry-Perot cavity

A dual wavelength Fabry-Perot cavity system will need to be developed to
operate at both $1550$ nm and $517$ nm (the 3rd harmonic). The cavity mirrors
should have very high reflectivity at both wavelengths. A new type of optical
bench with additional third-harmonic generation optics, two sets of laser
optics, feedbacks, and diagnostics for both wavelengths should be developed
and then integrated with a shared FP cavity.

  * •

Low beam background

To reduce the high energy radiation background, a short interaction region can
be arranged to fully separate the $\gamma$-ray beam from the residual gas
bremsstrahlung radiation, with two possible schemes illustrated in Fig. 5.2.
This, together with ultra-high vacuum realized in advanced storage rings
[169], will reduce the radiation background by at least two orders of
magnitude compared to the already very low background achieved at the HIGS
facility.

The following is a prioritized list of important R&D topics for such a medium-
energy CGS:

  * •

High priority

    * –

To develop a dual-wavelength high-finesse FP-cavity for $1550$ nm and $517$
nm;

    * –

To design a reliable injector for single- and multi-bunch injection with high
charge;

  * •

Medium priority

    * –

To design an energy-varying storage ring with small emittance and large
dynamic aperture;

    * –

To design a specialized interaction region with ultra-high vacuum either in an
arc section or in a chicane to minimize radiation background;

    * –

To develop laser beam optics for polarization switch (circular and linear),
and for dual-wavelength operation;

    * –

To design radiation shielding to handle high local electron loss.

####  5.3.2 Storage Ring Based Low Energy CGS

The required $\gamma$-ray beam performance at lower energies (Table 5.2) can
be realized with a storage ring based Compton gamma-ray source using a low
energy electron beam ($100$s of MeV) and multiple high power Fabry-Perot
cavities. One possible design of such storage rings is shown in Fig. 5.4 with
related machine design parameters listed. This $60$-meter-long storage ring is
comprised of four 5-bend achromatic arcs with a small emittance and a
reasonably large dynamic aperture.

Parameters | Values
---|---
Energy range |  $350$–$750$ MeV
Circumference |  $59.14$ m
Long straight section length |  $4.04$ m
Nominal beam current |  $1000$ mA
Hori. natural emittance, $\varepsilon_{x}$ |  $3.37$ nm
IBS induced hori. emittance |  $7.5$ nm
Coupling, $\varepsilon_{y}/\varepsilon_{x}$ | $0.1$
Betatron tune, $\left(\nu_{x},\nu_{y}\right)$ | $(6.21,4.21)$
Main radio frequency |  $121.66$ MHz
Main rf cavity voltage |  $250$ kV
Natural chromaticity, $\left(\xi_{x},\xi_{y}\right)$ | $\left(-7.6,-9.5\right)$
Momentum compaction | $1.35\times 10^{-2}$
Damping time $\left(\tau_{x},\tau_{y},\tau_{s}\right)$ |  $\left(27,68,126\right)$ ms
Natural bunch duration (rms) |  $50$ ps
Bunch length with harm. cavity |  $150$ ps

Figure 5.4:  (Left) Layout and parameters of a low energy electron storage
ring ($350$–$750$ MeV). Four gamma-ray sources are shown in two different
collision configurations: one for the head-on collision using a two-mirror
Fabry-Perot cavity in the arc and the other with a small crossing angle using
a four-mirror cavity in the straight section. (Right) Main parameters for the
storage ring, where all energy-dependent parameters are shown for $500$ MeV
operation.

For this low energy storage ring design, the intrabeam scattering (IBS) effect
needs to be properly controlled/mitigated. Intrabeam scattering is a process
in which lossless collisions between electrons in the same bunch lead to an
increase of the electron beam distribution in all three dimensions, which will
result in the transverse beam size growth, especially for low energy and
small-emittance storage rings. Because of a strong IBS effect, there is no
need to push the low emittance limit. One such an example storage ring design
is shown in Fig. 5.4. At $500$ MeV, this ring has a horizontal natural
emittance about $3.4$ nm-rad and the IBS effect increases the beam emittance
to about $7.5$ nm-rad with $1000$ mA beam current in $24$ bunches, and with
the help of a third harmonic rf cavity. The third-order harmonic cavity is
used to lengthen the electron bunch by a factor of three in order to increase
the beam lifetime which is mainly limited by large angle electron-electron
collisions (the so-called Touschek effect). A more complete design of a low
energy storage ring based CGS with a modest intracavity power ($20$ kW) can be
found in [170].

E-beam | $E_{\gamma}$ |  $\lambda_{1}$ (nm) | $\lambda_{1}/3$ |  $\lambda_{2}$ (nm) | $\lambda_{2}/3$
---|---|---|---|---|---
(MeV) | (MeV) | $1064$ | $355$ | $1550$ | $517$
$350$ | $E_{\gamma,{\rm min}}$ | $2.2$ | $6.4$ | $1.5$ | $4.4$
$750$ | $E_{\gamma,{\rm max}}$ | $9.9$ | $29$ | $6.8$ | $20$

Electron beam |
---|---
Beam energy |  $500$ MeV
Stored currents |  $1000$ mA
Bunch filled | $24$
Hori./Vert. emittance |  $7.5/0.75$ nm-rad
Hori./Vert. size (rms) |  $212/39$ $\mu\mathrm{m}$
Bunch length (rms) |  $150$ ps
Laser beam |
Wavelength |  $1064$ nm
Intracavity power |  $100$ kW
Pulse length (rms) |  $20$ ps
Hori./Vert. size (rms) |  $40/40$ $\mu\mathrm{m}$
Gamma-ray beam |
Max. energy |  $4.43$ MeV
Collision rate |  $121.66$ MHz
Collision angle | $6^{\circ}$
Luminosity | $3.3\times 10^{36}\ \mathrm{cm}^{-2}\mathrm{s}^{-1}$
Total flux (in $4\pi$ solid angle) |  $2.2\times 10^{12}$ $\gamma/\mathrm{s}$

E-beam: | FP cavity: |  $100$ kW
---|---|---
$E=500$ MeV, $I=1$ A | Beam size: |  $40/40$ $\mu$m
Laser wavelength (nm) | $\lambda{}_{1}=1064$ | $\lambda_{2}=1550$
Tot. flux ($\gamma/\mathrm{s}$): $\mbox{$\theta$=}6^{\circ}$ | $2.2\times 10^{12}$ | $2.8\times 10^{12}$
Tot. flux ($\gamma/\mathrm{s}$): head-on | $2.4\times 10^{13}$ | $3.1\times 10^{13}$

Figure 5.5:  (Left) Energy of the $\gamma$-ray beam as a function of electron
beam energy ($350$–$750$ MeV) for lasers at $1064$ and $1550$ nm and their
third harmonics. (Upper-Right) A table listing the key operational parameters
for producing a $4.4$ MeV $\gamma$-ray beam using a $500$ MeV electron beam
and a $1064$ nm laser beam with a $6^{\circ}$ cross angle. (Lower-Right) A
table showing the total flux for $6^{\circ}$ cross-angle and head-on collision
configurations with the same electron and laser beam parameters.

###### Energy Range

This storage ring based Compton source is a versatile gamma-ray factory. It
can produce $\gamma$ rays in a wide range of energies from $1.5$ to about $30$
MeV (see the table in Fig. 5.5). This is done by changing the electron beam
energy from $350$ to $750$ MeV, and by using FP cavities driven by commercial
lasers at multiple wavelengths.

###### Flux Performance

At the low energy end, the $\gamma$-ray flux is determined by the electron
beam current stored in the storage ring, intracavity laser power, and
collision configuration. Below $5$ to $8$ MeV, very high flux performance is
available using a high power Fabry-Perot cavity driven by lasers operating at
their fundamental wavelengths (e.g. $1064$ or $1550$ nm). For example, for
production of $4.4$ MeV $\gamma$ rays in a small cross-angle collision
configuration ($\theta=6^{\circ}$) using a $1000$ mA, $500$ MeV electron beam
and a $100$ kW laser beam inside a $1064$ nm FP cavity, the total $\gamma$-ray
flux is estimated to be $2.2\times 10^{12}$ $\gamma$/s, and the corresponding
flux on target with $2\%$ energy resolution (FWHM) is about $6.5\times
10^{10}$ $\gamma$/s (see a list of consistent operational parameters in the
table in Fig. 5.5). Using the same electron and laser beam parameters, the
total flux can be increased to $2$–$3\times 10^{13}$ $\gamma$/s, assuming
head-on collision can be realized. Between $15$ and $30$ MeV using an FP
cavity driven by either a 2nd or 3rd harmonic laser beam, the gamma-ray source
will be operated in the electron-loss mode, producing a lower total flux, on
the order of few $10^{11}$ $\gamma$/s as limited by the electron injection
rate. Between $5$ and $15$ MeV, the total $\gamma$-ray flux will range from
few $10^{11}$ to few $10^{12}$ $\gamma$/s (or higher), determined by specific
choices of operational parameters.

###### Electron Beam

To achieve a high average current, the electron beam will be stored in
multiple bunches, typically with a vacuum clearing gap. This gap in the
$\gamma$-ray beam can be used as the start of the time trigger for many
experiments. In the example storage ring (Fig. 5.4), when fully filled, the
electron bunch rate is $122$ MHz.

###### Laser Beam

The Fabry-Perot cavities will be powered by commercial MHz lasers (fiber or
solid state) with commonly available wavelengths around $1064$ and 1550 nm.
The 2nd and/or 3rd harmonic laser beam can also be used. The lasers with high
average power and excellent beam quality will be essential for achieving high
flux.

###### Fabry-Perot Cavity and Collision Scheme

Both head-on collision and small cross-angle collision can be arranged. For
the head-on collision, a simple and compact two-mirror cavity can be used; it
may be possible to install such a cavity in an arc section between two
adjacent dipole magnets as shown in Fig. 5.4. For the cross-angle collision, a
four-mirror cavity can be implemented in a dedicated straight section.

###### Energy Resolution

A beam energy resolution of about $2$% (FWHM) can be readily realized by
properly collimating the $\gamma$-ray beam. To achieve higher resolution
($\leq 1$%), a narrow-band drive laser will be required and the resolution
impact due to the electron beam energy spread and angular spread should be
minimized.

###### Beam Polarization

Highly polarized, well-collimated Compton $\gamma$-ray beams can be produced
using a polarized drive laser beam. For certain experiments requiring helicity
switch (as fast as $50$ Hz), polarizing optics (including Pockels cells) can
be utilized outside the FP cavity.

###### Production Modes

Like a storage ring based synchrotron radiation user facility, this low energy
gamma-ray factory can be operated as a multiple user facility with several
$\gamma$-ray beamlines. As shown in Fig. 5.4, several Compton sources can be
installed in straight sections and arc sections. For each source, a dedicated
user target room with proper shielding and a beam dump will be constructed.
The gamma-ray facility can be operated in the single- or multi-user mode. In
the single-user mode, the experiment on the floor can take full control of the
$\gamma$-ray beam production, including having the ability to change both
electron beam current and energy. In the multi-user mode, all gamma-ray
sources share the electron beam, therefore, individual beamlines do not have
the control of the electron beam current and energy. Beamlines can be
developed with the independent energy tuning capability—this can be realized
by constructing a special FP cavity which allows variation of the collision
angle. It is worth pointing out that the flux will be reduced as the collision
angle is increased. This gamma-ray factory provides a full energy coverage
($1.5$ to $30$ MeV) by operating multiple beamlines using lasers of several
different wavelengths and by running the storage ring at a few pre-determined
energies.

Several challenges and important issues for this low energy source are
summarized here:

  * •

High power FP cavity design

The main challenge for this low energy source is to develop a very high power
FP cavity ($\geq 100$ kW) operating at $50$ to $200$ MHz with a very small
beam size at the collision point (radius $\sim$$40$ $\mu$m or smaller). The
smallest beam size practically realizable will be limited by the high
intracavity power.

  * •

Dual wavelength FP cavity

Any gamma-ray source on this storage ring can greatly expand its energy
coverage by using a dual-wavelength Fabry-Perot cavity (e.g. at both $1550$ nm
and $517$ nm).

  * •

Electron injection rate

There are two viable choices for the injector: (1) a low-cost, full-energy,
top-off booster injector; and (2) a full-energy room-temperature linac
injector at a higher cost. At low energies, the $\gamma$-ray flux is not
limited by the injection rate. At higher energies in the electron-loss mode,
the $\gamma$-ray flux will be limited by the electron injection rate.

The following is a prioritized list of important R&D topics for such a low
energy CGS:

  * •

High priority

    * –

To develop a very high power FP cavity ($\geq 100$ kW) with a small beam size
($\leq 40$ $\mu$m);

    * –

To develop a dual-wavelength high-finesse FP-cavity (e.g. $1550$/$517$ nm);

  * •

Medium priority

    * –

To design an energy-varying storage ring with small emittance and large
dynamic aperture;

    * –

To design a specialized interaction region with ultra-high vacuum either in an
arc section or in a chicane to minimize radiation background;

    * –

To develop laser beam optics for fast polarization switch (circular and
linear), and for dual-wavelength operation;

    * –

To design radiation shielding to handle large local electron loss.

####  5.3.3 Energy Recovery Linac Based Low-Energy CGS

The required $\gamma$-ray beam performance at lower energies (Table 5.2) can
also be realized with a Compton gamma-ray source based on an Energy Recovery
Linac and one or more Fabry-Perot laser cavities. One possible design of such
gamma source is shown in Fig. 5.6 with related machine parameters listed. This
ERL is comprised of a double-sided linac with two-pass recirculation loop and
an additional loop for $\gamma$-ray beam generation.

The key components of ERL are similar to the Compact ERL [69], in which
generation of a narrow-band Compton scattered beam was recently demonstrated.
The main linac and the injector linac are both L-band ($1.3$ GHz)
superconducting structures. The electron gun is a high-voltage ($500$ kV) DC
gun with a multi-alkali photocathode, which can generate a small-emittance
beam, $\varepsilon_{n}\leq 1$ mm-mrad, at a bunch charge of $200$–$300$ pC
[171].

Parameters | Values
---|---
Energy range |  $100$–$750$ MeV
Bunch charge |  $246$ pC
Bunch repetition |  $81.25$ MHz
Ave. current |  $20$ mA
Norm. emittance |  $1$ mm-mrad
Bunch length |  $10$ ps (rms)
Energy spread |  $25$ keV (rms)
Collision spot |  $15$ $\mu$m (rms)

Figure 5.6:  (Left) Schematic layout and parameters of an ERL-based gamma
source. The insets show two possible collision schemes. (Right) Electron beam
parameters for the ERL.

###### Flux Performance

The performance of an ERL with an electron beam energy of 500 MeV and a laser
wavelength of $1064$ nm is shown in a table (see Fig. 5.7).

###### Energy Range

A wide $\gamma$-ray energy range can be achieved by changing the electron
energy and laser wavelength ($1064$ nm and $532$ nm). In this design, the
electron beam energy is varied from $100$ to $750$ MeV to cover $\gamma$-ray
energies from 0.18 to 20 MeV. An electron beam below $370$ MeV can be
delivered by bending the beam out after a single turn or single linac to
simplify operation.

###### Laser Beam

Because the energy range of the ERL is large, the laser system does not need
to adjust to wavelengths longer than 1064 nm. To get to higher $\gamma$-ray
energies it will be necessary to frequency-double the laser light to $532$ nm
using an LBO crystal.

###### Fabry-Perot Cavity and Collision Scheme

Two types of Fabry-Perot cavities can be used for the $\gamma$-ray generation
with a head-on or small cross-angle configuration as shown in Fig. 5.6. In
order to stack laser pulses synchronizing with the electron bunch, the cavity
frequency should be equal to the bunch repetition rate ($81.25$ MHz) or its
sub-harmonic. Parameters of laser beam and expected $\gamma$-ray flux are
summarized in a table (part of Fig. 5.7). The flux requirement, $10^{13}$
$\gamma$/s, can be obtained with head-on collision, and an intra-cavity laser
power of $70$ kW with a tight laser focus at IP (beam size $\sim 15$ $\mu$m).
Separate cavities are needed for $1064$ nm and $532$ nm operation due to the
need for very high reflectivity in the cavity.

###### Energy Resolution

The ERL provides excellent beam quality so that the energy resolution is
determined by the angular acceptance of the $\gamma$-ray beam collimation
aperture. The high brightness of the electron beam allows a very small
electron beam size even for a small beta function of $5$–$10$ cm while still
providing a small angular spread. The rms energy spread is typically much less
than $10^{-3}$. The laser must be as narrow band as possible. A $\gamma$-ray
beam energy spread of $1$–$2$% (FWHM) should be easily attainable.

###### Beam Polarization

The $\gamma$-ray beam polarization is determined by the laser system. As with
the storage ring design, polarization changes at a $50$ Hz rate are
attainable.

E-beam | $E_{\gamma}$ |  $\lambda_{1}$ (nm) | $\lambda_{1}/2$
---|---|---|---
(MeV) | (MeV) | $1064$ | $532$
$100$ | $E_{\gamma,{\rm min}}$ | $0.18$ | $0.36$
$750$ | $E_{\gamma,{\rm max}}$ | $9.9$ | $19.5$

Electron beam |
---|---
Beam energy |  $500$ MeV
Average current |  $20$ mA
Hori./Vert. emittance (norm.) |  $1$mm-mrad
Hori./Vert. size (rms) |  $15$ $\mu$m
Bunch length (rms) |  $3$ ps
Laser beam |
Wavelength |  $1064$ nm
Intracavity power |  $70$ kW
Pulse length (rms) |  $10$ ps
Hori./Vert. size (rms) |  $15/15$ $\mu\mathrm{m}$
Gamma-ray beam |
Max. energy |  $4.41$ MeV
Collision rate |  $81.25$ MHz
Collision angle | $6^{\circ}$
Luminosity | $2.6\times 10^{36}\ \mathrm{cm}^{-2}\mathrm{s}^{-1}$
Total flux (in $4\pi$ solid angle) |  $1.7\times 10^{12}$ $\gamma/\mathrm{s}$

E-beam current (mA) | $20$ | $20$ | $40$
---|---|---|---
FP cavity power ( kW) | $70$ | $70$ | $70$
Laser beam size ($\mu\mathrm{m}$) | $30/30$ | $15/15$ | $15/15$
Tot. flux ($\gamma/\mathrm{s}$): $\mbox{$\theta$=}6^{\circ}$ | $1.1\times 10^{12}$ | $1.7\times 10^{12}$ | $3.4\times 10^{12}$
Tot. flux ($\gamma/\mathrm{s}$): head-on | $5.4\times 10^{12}$ | $1.3\times 10^{13}$ | $2.7\times 10^{13}$

Figure 5.7:  (Left) Energy of the $\gamma$-ray beam as a function of electron
beam energy ($100$–$750$ MeV) for lasers at $1064$ and $532$ nm. (Upper-Right)
A table listing the key operational parameters for producing a $4.41$ MeV
$\gamma$-ray beam using a $500$ MeV electron beam and a $1064$ nm laser beam
with a $6^{\circ}$ cross angle. (Lower-Right) A table showing the total flux
for $6^{\circ}$ cross-angle and head-on collision configurations for two sets
of values for the electron beam current and laser beam size at IP.

###### Production Modes

An operation of low repetition rate, $10$ MHz or less, is possible by simply
reducing electron bunch repetition or reducing the collision rate with a
kicker magnet at the expense of total flux. Multiple collision points can be
placed in the recirculation loop. Since the collision rate is small enough
($10^{-4}$ for this design), electron beam degradation due to multiple
collisions is negligible. As in the case of storage ring sources, simultaneous
experiments will be possible at some fixed $\gamma$-ray energies unless a
tunable laser or variable angle system is developed.

The proposed gamma-ray source can be realized with existing technologies in
principle. Technical risks remaining to be resolved are summarized as follows:

###### Electron Source Performance

A photocathode used for the generation of small-emittance electron beams has a
limited life time. The Cornell injector demonstrated extraction of a $40$ mA
beam for $4$ hours without significant reduction of quantum efficiency [172]
and, using the same gun design, the LEReC project [126] at Brookhaven National
Laboratory demonstrated multi-day operation at $30$ mA. The required emittance
from the injector has been demonstrated [171]. Operation of an $8$-mA,
$115$-MeV ERL was demonstrated at JLab [173] and a proof-of-principle
experiment of ERL-based Compton source was conducted at the Compact ERL [69].

###### High Current Linac Operation

The accelerator modules for this design will see up to $80$ mA in total with
accelerating and decelerating beams. This much current will produce copious
higher order modes that must be heavily damped. L-band cryomodules for high-
current beams have been designed and fabricated at Cornell and KEK [174, 175].

###### Fabry-Perot Cavity Performance

A Fabry-Perot cavity was developed at Lyncean Technologies to store laser
power of $70$ kW with a beam waist of $40$ $\mu$m [70] and a cavity at the
cERL was $10$ kW with $24$ $\mu$m (horizontal) and $32$ $\mu$m (vertical) spot
sizes [69]. In addition, a cavity was developed for non-accelerator
environment at Max-Planck Institute to store laser power of $670$ kW with a
reported rms beam size of $13$ $\mu m$$\,\times\,$$17$ $\mu m$ at focus [150].
A Fabry-Perot cavity to store intra-cavity laser power of $70$ kW with a beam
waist of $15$ $\mu$m must be proven. Such a cavity requires state-of-the-art
low-loss mirror coatings and thermally stable mirror substrates.

###### Preservation of Injector Beam Quality

Though the Cornell injector has demonstrated excellent performance, it is
necessary to preserve this beam quality through a large number of long beam
transport arcs. Each of these is subject to wakefields, longitudinal space
charge (LSC) forces, and coherent synchrotron radiation (CSR) induced
emittance growth. Designs have been developed to minimize any increase in the
transverse emittance in an isochronous arc. The emittance growth can be kept
to very small levels. A more serious problem is the longitudinal phase space
distortion leading to a growth in the projected energy spread. Some of this
may be compensated by rf cavities. A harmonic rf cavity might be required to
take out higher-order curvature.

###### Operational Issues

Some of the scattered electrons may be lost when operating at very high photon
energy. These losses must be controlled and understood. The levels of loss
look tolerable and many electrons will still be energy-recovered but it is
important to control where the electrons are lost. Continuous energy tuning in
ERLs has not been demonstrated, yet. Sophisticated algorithms must be
developed to tune up the beam transport at a new energy as the energy is
changed.

We recommend to conduct the following R&D items:

  * •

High priority

    * –

Laser stacker at $1064$ nm and $532$ nm to verify the power, beam size, and
intensity at the IP;

    * –

Endurance runs with CsKSb cathodes at $20$ mA;

    * –

Lattice designs for ERL with CGS with CSR and LSC compensation.

  * •

Medium priority

    * –

Beam breakup (BBU) and higher-order mode (HOM) loading simulations for ERL
with CGS (e.g. Cornell and KEK modules);

    * –

Diagnostic development for high current beams and backscattering setup.

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##  Chapter 6 Appendix

###  6.1 Workshop Agenda

Thursday, November 17, 2016 | Overview Sessions, Grand Ballroom A |  |
---|---|---
_Time_ | _Title_ | _Speaker_ | Affliation |
8:45 | Welcome and remarks | Calvin Howell | Duke U & TUNL |
Compton Gamma-Ray Sources |  |
9:00 |  a) Global Review of Compton $\gamma$-Ray Sources | Ying Wu | Duke U & TUNL |
9:06 | b) Facility Talks |  |  |
| b.1) HIGS and CGS Projects in the U.S. | Ying Wu | Duke U & TUNL |
| b.2) ELI-NP and CGS Projects in Europe | Calin Ur | ELI-NP |
|  b.3) Compton $\gamma$-Ray Sources in Japan | Ryoichi Hajima | NIRS, Japan |
|  b.4) Compton $\gamma$-Ray Sources in China | Chuanxiang Tang | Tsinghua U, China |
9:54 | c) Possible Technologies for Next- |  |  |
| Generation Sources |  |  |
| c.1) High Power Lasers and Optics | Fabian Zomer | LAL, France |
| c.2) Accelerator Technologies | John Byrd | LBNL |
| c.3) Compton Source Configuration Strategies | Ying Wu | Duke U & TUNL |
10:30 | Break, Salons A&B Foyer |  |  |

Thursday, November 17, 2016 | Overview Sessions, Grand Ballroom A |  |
---|---|---
Low-Energy QCD |  |
_Time_ | _Title_ | _Speaker_ | Affliation |
10:50 | What to Learn from | Harald Griesshammer | GWU |
| Nucleon Polarizabilities |  |  |
11:15 | Compton Experiments I | Phil Martel | U Mainz & Regina |
11:30 | Compton Experiments II | Gerald Feldman | GWU |
11:45 | Testing Confinement Scale QCD | Aron Berstein | MIT |
| With Low-Energy Electromagnetic |  |  |
| Pion Production |  |  |
12:35 | Lunch, Salons A&B Foyer |  |  |
14:00 | Hadronic Parity Violation | W. Michael Snow | Indiana U |
14:25 | Nuclear Structure | Deniz Savran | GSI, Germany |
14:55 | Nuclear Astrophysics | Carl Brune | Ohio U |
Applications |  |
_Time_ | _Title_ | _Speaker_ | Affliation |
15:20 | Homeland Security | Calvin Howell | Duke U & TUNL |
15:35 | National Nuclear Security | Matthew Durham | LANL |
15:50 | Medical Diagnosis | Anuj Kapadia | Duke U |
16:05 | Charge to Working Groups | Calvin Howell | Duke U & TUNL |
16:15 | Break, Grand Ballroom Salons A&B Foyer |  |  |
16:45 |  Working Group Meetings |
| _Group_ | _Location_ |  |
| 1) Gamma-Ray Source | Grand Ballroom B |  |
| 2.a) LE QCD: Nucleon Polarizabilities | Grand Ballroom A |  |
| 2.b) LE QCD: Photopion Physics | Grand Ballroom A |  |
| 2.c) LE QCD: Hadronic Partity Violation | Bethesda |  |
| 3) Nuclear Structure and Astrophysics | Chevy Chase |  |
| 4) Applications | Rockville |  |
18:30 | Banquet | Congressional Ballroom |  |

Friday, November 18, 2016 | Working Group Sessions |  |
---|---|---
09:00 |  Working Group Meetings |
| _Group_ | _Location_ |  |
| 1) Gamma-Ray Source | Grand Ballroom B |  |
| 2.a) LE QCD: Nucleon Polarizabilities | Grand Ballroom A |  |
| 2.b) LE QCD: Photopion Physics | Grand Ballroom A |  |
| 2.c) LE QCD: Hadronic Partity Violation | Bethesda |  |
| 3) Nuclear Structure and Astrophysics | Chevy Chase |  |
| 4) Applications | Rockville |  |
10:30 | Break | Salons A&B Foyer |  |
11:00 | Working Group Meetings | Same Locations |  |
12:30 | Lunch | Salons A&B Foyer |  |
14:00 | Working Group meetings | Same Locations |  |
15:30 | Break | Salons A&B Foyer |  |
16:00 | Information Exchange Session | Salon A |  |
17:30 | Dinner (on your own) |  |  |
Saturday, November 19, 2016 | Working Group & Closeout Sessions |  |
09:00 | Working Group Meetings | Same Locations |  |
10:00 | Break | Salons A&B Foyer |  |
Closeout Sessions |  |
10:15 | Working Group Summaries | Grand Ballroom |  |  |
| 1.a) LE QCD: Nucleon Polarizabilities |  |  |
| 1.b) LE QCD: Above the Pion-Production Threshold |  |  |
| 1.c) LE QCD: Hadronic Partity Violation |  |  |
| 2) Nuclear Structure |  |  |
| 3) Nuclear Astrophysics |  |  |
| 4.a) Applications: Homeland and Nuclear Security |  |  |
| 4.b) Applications: Medicine |  |  |
| 5) Gamma-Ray Source |  |  |
11:35 | Open discussion |  |  |
12:00 | Summary and Closing Remarks |  |  |
12:10 | Workshop Adjourned |  |  |

###  6.2 Working Group Conveners

General Editors |
---|---
_Members_ | _Institution_ | _Email_ |
Mohammad Ahmed | NCCU & TUNL |<EMAIL_ADDRESS>|
Harald Grießhammer | GWU |<EMAIL_ADDRESS>|
Calvin Howell | Duke U & TUNL |<EMAIL_ADDRESS>|
Robert Janssens | UNC-CH & TUNL |<EMAIL_ADDRESS>|
Daniel Phillips | Ohio U |<EMAIL_ADDRESS>|
Roxanne Springer | Duke U |<EMAIL_ADDRESS>|
Ying Wu | Duke U & TUNL |<EMAIL_ADDRESS>|
Nucleon spin polarizabilities |
_Members_ | _Institution_ | _Email_ |
Harald Grießhammer | GWU |<EMAIL_ADDRESS>|
Rory Miskimen | U Mass |<EMAIL_ADDRESS>|
Daniel Phillips | Ohio U |<EMAIL_ADDRESS>|
Meson EM polarizabilities and QCD origin of CSB |
_Members_ | _Institution_ | _Email_ |
Aron Bernstein | MIT |<EMAIL_ADDRESS>|
Ulf-G Meißner | U-Bonn |<EMAIL_ADDRESS>|
Hadronic Parity Violation |
_Members_ | _Institution_ | _Email_ |
Mike Snow | Indiana U |<EMAIL_ADDRESS>|
Roxanne Springer | Duke U |<EMAIL_ADDRESS>|
Matthias Schindler | U S Carolina |<EMAIL_ADDRESS>|
Nuclear Structure |
_Members_ | _Institution_ | _Email_ |
Ani Aprahamian | U Norte Dame |<EMAIL_ADDRESS>|
Deniz Savran | GSI |<EMAIL_ADDRESS>|

Nuclear Astrophysics |
---|---
_Members_ | _Institution_ | _Email_ |
Carl Brune | Ohio U |<EMAIL_ADDRESS>|
Art Champagne | UNC-CH & TUNL |<EMAIL_ADDRESS>|
Security Applications |
_Members_ | _Institution_ | _Email_ |
Calvin Howell | Duke U & TUNL |<EMAIL_ADDRESS>|
Anton Tonchev | LLNL |<EMAIL_ADDRESS>|
Medical Applications |
_Members_ | _Institution_ | _Email_ |
Anuj Kapadia | Duke U |<EMAIL_ADDRESS>|
Madan Rehani |  |<EMAIL_ADDRESS>|
Advanced Accelerator and Light-source Technologies |
_Members_ | _Institution_ | _Email_ |
John Byrd | LBNL |<EMAIL_ADDRESS>|
Swapan Chattopadhyay | FNAL |<EMAIL_ADDRESS>|
Ying Wu | Duke U & TUNL |<EMAIL_ADDRESS>|

###  6.3 Workshop Participants

_Last Name_ | _First Name_ | _Email_ | Working Group
---|---|---|---
Afanasev | Andrei |<EMAIL_ADDRESS>| Nuclear Structure
Ahmed | Mohammad |<EMAIL_ADDRESS>| QCD origin of CSB
Alesini | David |<EMAIL_ADDRESS>| Accelerator & Light Source
Annand | John |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Aprahamian | Ani |<EMAIL_ADDRESS>| Nuclear Structure
Balabanski | Dimiter |<EMAIL_ADDRESS>| Nuclear Structure
Barty | Christopher |<EMAIL_ADDRESS>| Accelerator & Light Source
Benson | Stephen |<EMAIL_ADDRESS>| Accelerator & Light Source
Bernstein | Aron |<EMAIL_ADDRESS>| QCD origin of CSB
Brune | Carl |<EMAIL_ADDRESS>| Nuclear Astrophysics
Byrd | John |<EMAIL_ADDRESS>| Accelerator & Light Source
Carlsten | Bruce |<EMAIL_ADDRESS>| Accelerator & Light Source
Champagne | Art |<EMAIL_ADDRESS>| Nuclear Astrophysics
Chattopadhyay | Swapan |<EMAIL_ADDRESS>| Accelerator & Light Source
Davis | David |<EMAIL_ADDRESS>| Hadronic Parity Violation
Downie | Evie |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Durham | J Matthew |<EMAIL_ADDRESS>| Applications
Feldman | Gerald |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Friesen | Forrest |<EMAIL_ADDRESS>| Nuclear Structure
Gao | Haiyan |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Geddes | Cameron |<EMAIL_ADDRESS>| Accelerator & Light Source
Griesshammer | Harald |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Hajima | Ryoichi |<EMAIL_ADDRESS>| Accelerator & Light Source
Hao | Hao |<EMAIL_ADDRESS>| Accelerator & Light Source
Holstein | Barry |<EMAIL_ADDRESS>| Hadronic Parity Violation
Hornidge | David |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Howell | Calvin |<EMAIL_ADDRESS>| Applications
Huffman | Paul |<EMAIL_ADDRESS>| Hadronic Parity Violation
Iliadis | Christian |<EMAIL_ADDRESS>| Nuclear Astrophysics
Isaak | Johann |<EMAIL_ADDRESS>| Nuclear Structure
_Last Name_ | _First Name_ | _Email_ | Working Group
---|---|---|---
Kapadia | Anuj |<EMAIL_ADDRESS>| Applications
Kendellen | David |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Krasznahorkay | Attila |<EMAIL_ADDRESS>| Nuclear Structure
Kovash | Micheal |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Lee | Dean |<EMAIL_ADDRESS>| Nuclear Structure
Lorant | Csige |<EMAIL_ADDRESS>| Nuclear Structure
Martel | Phil |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Meissner | Ulf |<EMAIL_ADDRESS>| QCD origin of CSB
Miskimen | Rory |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Moon | Namdoo |<EMAIL_ADDRESS>| Government Agency
Mueller | Jonathan |<EMAIL_ADDRESS>| Applications
Opper | Allena |<EMAIL_ADDRESS>| Government Agency
Pasquini | Barbara |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Phillips | Daniel |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Pietralla | Norbert |<EMAIL_ADDRESS>| Nuclear Structure
Rai | Gulshan |<EMAIL_ADDRESS>| Government Agency
Rehani | Madan |<EMAIL_ADDRESS>| Applications
Rhodes | William |<EMAIL_ADDRESS>| Government Agency
Savran | Deniz |<EMAIL_ADDRESS>| Accelerator & Light Source
Schindler | Matthias |<EMAIL_ADDRESS>| Hadronic Parity Violation
Sikora | Mark |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Snow | Mike |<EMAIL_ADDRESS>| Hadronic Parity Violation
Springer | Roxanne |<EMAIL_ADDRESS>| Hadronic Parity Violation
Stone | Terri |<EMAIL_ADDRESS>| Government Agency
Sun | Changchun |<EMAIL_ADDRESS>| Accelerator & Light Source
Tang | Chuanxiang |<EMAIL_ADDRESS>| Accelerator & Light Source
Tiburzi | Brian |<EMAIL_ADDRESS>| Nucleon Polarizabilities
Tonchev | Anton |<EMAIL_ADDRESS>| Applications
Tornow | Werner |<EMAIL_ADDRESS>| Nuclear Structure
Ur | Calin |<EMAIL_ADDRESS>| Accelerator & Light Source
Wang | Dong |<EMAIL_ADDRESS>| Accelerator & Light Source
Weller | Henry |<EMAIL_ADDRESS>| QCD origin of CSB
Werner | Volker |<EMAIL_ADDRESS>| Nuclear Structure
Wu | Ying |<EMAIL_ADDRESS>| Accelerator & Light Source
Yan | Jun |<EMAIL_ADDRESS>| Accelerator & Light Source
Zilges | Andreas |<EMAIL_ADDRESS>| Nuclear Structure
Zomer | Fabian |<EMAIL_ADDRESS>| Accelerator & Light Source


# Hybrid Continuum-Eversion Robot: Precise Navigation and Decontamination in
Nuclear Environments using Vine Robot

Mohammed Al-Dubooni1, Cuebong Wong2, and Kaspar Althoefer1, Senior Member,
IEEE This work has been submitted to the IEEE for possible publication.
Copyright may be transferred without notice, after which this version may no
longer be accessible.*Purpose of open access, the author(s) has applied a
Creative Commons Attribution (CC BY) license to any Accepted Manuscript
version arising. 1Authors are with the Centre for Advanced Robotics @ Queen
Mary, School of Engineering and Materials Science.2Authors are with the
National Nuclear Laboratory.Research is funded by the Nuclear Decommissioning
Authority PhD Bursary programme.

###### Abstract

Soft growing vine robots show great potential for navigation and
decontamination tasks in the nuclear industry. This paper introduces a novel
hybrid continuum-eversion robot designed to address certain challenges in
relation to navigating and operating within pipe networks and enclosed remote
vessels. The hybrid robot combines the flexibility of a soft eversion robot
with the precision of a continuum robot at its tip, allowing for controlled
steering and movement in hard to access and/or complex environments. The
design enables the delivery of sensors, liquids, and aerosols to remote areas,
supporting remote decontamination activities.

This paper outlines the design and construction of the robot and the methods
by which it achieves selective steering. We also include a comprehensive
review of current related work in eversion robotics, as well as other steering
devices and actuators currently under research, which underpin this novel
active steering approach. This is followed by an experimental evaluation that
demonstrates the robot’s real-world capabilities in delivering liquids and
aerosols to remote locations. The experiments reveal successful outcomes, with
over 95% success in precision spraying tests. The paper concludes by
discussing future work alongside limitations in the current design, ultimately
showcasing its potential as a solution for remote decontamination operations
in the nuclear industry.

##  I Introduction

Soft eversion robotics is now a rapidly growing field that has been driven by
the need to develop devices that can operate safely and effectively in areas
that are hard to access using traditional rigid bodied robots. Soft compliant
robots can navigate through tight spaces, making them ideal for applications
in fields such as healthcare [1], [2], [3] manufacturing, and exploration [4],
[5]. Eversion robots are a specialized type of soft robot designed to invert
or unfold [6], [7], [11], in turn, exhibiting a unique form of locomotion.

The primary characteristic of eversion robots is their ability to undergo
continuous transformation, inverting their structure to adapt to various
environments or perform specific tasks when pressurised by an air supply [8].
This design is particularly advantageous when navigating through constrained
spaces. Most eversion robots are constructed from nylon ripstop fabric due to
its useful combination of properties, including flexibility, durability, and
tear resistance, which allow for a seamless inversion process while
withstanding the mechanical stresses of repeated deformations [9].

Looking at the nuclear industry [10] we identify three significant operational
challenges associated with nuclear decommissioning, these being navigation,
sensing and sealing. The first of these involves access into and operation
within different sized and different shaped spaces, ranging from chemical
piping to AC ducting and underground crawl spaces. The other two challenges go
hand in hand; the delivery of sensors such as radiation sensors [12], [13],
[14] and the delivery of a liquid such as an “antidote” to potentially harmful
matter, a sticky substance like fix-it, or an expanding foam used to
immobilise potentially hazardous materials (such as radioactive contaminants
in pipes).

Figure 1: Innovative Hybrid Continuum-Eversion Robot: Selective Tip Steering
Enabled by a Rigid-component robotic Structure, Demonstrated by a) and b).
Subfigure c) shows the tip design without the nylon sleeve covering. Figure 2:
Front view of the hybrid robot showing its sprayer and camera integrated in a
hard cap.

There is a high demand for robots that can navigate hard-to-access spaces. The
biggest challenges, for example, in relation to inspection of pipe networks
are navigating within the pipe network [15] and delivering a payload to a
distant target area. Current robots are limited by their ability to navigate
these environments due to their lack of mobility [16]. Generic ROVs are often
ineffective as they are only able to navigate on level surfaces. As a result,
they can get stuck easily and are difficult to recover. Drones may be viable
in some instances [17] but they too have their own disadvantages as they
overly disturb the environment in which they travel through, are more
difficult to control and require a stable connection with the operator which
further adds to their operational complexity.

There are only a handful of papers that look at active steering in eversion
robots, leaving a relatively empty playing field, ripe for novel methods [18],
[19].

In this work a new idea is introduced that combines a soft eversion robot with
a continuum robot at its tip, to create a hybrid robot whose movements are
precise and responsive while retaining soft, flexible, and compliant
characteristics. This approach offers several advantages over traditional
rigid or flexible robots. It enables the robot to navigate through complex
environments with greater ease, as it can choose a path to traverse whilst
being everted. This is in contrast to most traditional eversion robots which
require pneumatically inflated pouches or actuators to be sewn onto them in
order to navigate certain elements of their environment. Our hybrid robots can
be turned using a joystick which controls a decoupled steering mechanism at
the tip, that allows the user to actively change its path while in a pipe or
enclosed space. Altering the direction of the tip of the robot and maintaining
the pressure within the soft robot body enables it to move forward in
specified directions. Unlike with cPAMs [18] or with integrated pneumatic
pouches, this method does not enable the robot to hold its shape once it has
been steered and is therefore reliant on its environment to do that. This
steering capability however is crucial, especially at T-junctions, at which
point the end user can guide the robot. Extending the capabilities of this
hybrid robot, it enables the transportation of various payloads, including
sensors, liquids, and aerosols. The decoupled steering mechanism at the tip
facilitates precise control over the robot’s movements, making it a versatile
tool for applications requiring controlled transportation in challenging and
dynamic environments and provides a solid mounting point for sensors and
payloads that need to be delivered.

##  II Related Works

Several steering methods for eversion robots have been proposed by
researchers, including those that involve internal steering devices and others
that use pneumatic actuators. One idea proposed is that of asymmetric
lengthening of the tip which causes extension of either side to induce the
robot’s turning behaviour. This could be controlled by several stimuli such as
the use of a camera and a light source to steer the robot towards the light
[6]. These methods however cannot be sustained for longer lengths and do not
provide any form of active steering after several bends.

Another steering mechanism used pouches that were sewn into the side walls of
the eversion robot, allowing predetermined steering at specific stages along
its path. This meant that actuators caused the inflated backbone of the
eversion robot to bend in different directions [19]. This was later revised by
Kübler et al’s cPAM design [20] and then further developed by Kübler et al’s
selective steering cPAM design [18].

There are many advantages of using such methods to control eversion robots -
specifically, in the case of pneumatic pouches and cPAM, its ability to
increase the angle to which the eversion robot is able to bend - as
demonstrated by the 133% improvement in bending angle per Abrar et al’s paper
[19]. However, some of the most significant disadvantages of using pneumatic
pouches and cPAM’s are the added complexity of needing extra air supplies,
valves and regulators, the limited actuation range, and the potential multiple
failure points due to the number of stitched regions, which make the end-
product bulky, less manoeuvrable and less reliable. Relying on predetermined
pouches presents challenges in many nuclear decommissioning scenarios as there
is often a lack of a prior knowledge of legacy pipe structures that makes
identifying specific locations for bends and turns along the path length
challenging. A method of active steering must therefore be developed to enable
directed steering as the robot traverses along a pipe network.

Figure 3: Illustration depicting the eversion robot in action in figure a),
showcasing its operational mechanism and dynamic expansion process. Figure b)
illustrates the operation of how tail extrusion causes no shift in the outer
lining of the robot.

##  III Contributions

The contributions of this work are:

  * •

A three degrees-of-freedom eversion-continuum hybrid robot that uses a series
of servos to selectively steer an eversion robot tip.

  * •

A method by which the robot can deliver sensors to remote areas and facilitate
the controlled application or spraying of liquids and aerosols in these remote
areas, showcasing the robot’s potential in decontaminating remote nuclear-
contaminated vessels and glove-boxes.

##  IV Design

###  IV-A Eversion Robot design

The ‘vine’ section of the robot is made of nylon ripstop material. The
material is cut into strips with a width of around 60 mm. The material is
folded over along the short edge and made to overlap. A seam line is drawn on
the material and a guide setup to measure 5 mm from the edge. The strips are
then sown together forming a seam. This is done using a Singer sewing machine
with nylon thread. The seam is then covered with vinyl that is heat pressed to
create an airtight seal. This is then tested by inflating the robot and
inspecting for leaks. Once this is done, the end is tied off and the robot
inverted. A PvC tube is inserted to allow for a 5 mm airline tube.

Figure 4: 60 mm Nylon ripstop material folded in half and stitched 5 mm from
the edge. The dotted line shows where the seam should be. Figure 5: Detailed
view of the Tip Structure: Illustrating how the robot turns when the servos
are actuated b) and not actuated a).

###  IV-B Cap Design

To enable the tip of the eversion robot to move in several distinct
directions, we attached a continuum robot to its tip. Consequently, the robot
can turn in both the X and Y axis providing 2 degrees of movement –
effectively introducing selective steering into the robot’s repertoire. The
body of the continuum section is made of 3D printed discs connected in the
centre by a u-joint, and by springs along the edges. The central u-joint
provides a solid backbone to the tip which means that it cannot be compressed
if the robot hits a wall. It also allows the robot to be controlled more
effectively as the desired angle can be determined given the fixed distance
between discs. Each axis of springs coils around a nylon wire that spans the
length of the robot to a servo wheel. Each of the 4 servos is responsible for
its own quadrant - their combined effect determining the direction of the
robot.

Figure 6: CAD Model Demonstrating Theoretical Bending Angle and length of
Nylon Wire when the robot is turned 90 degrees.

###  IV-C Modelling the angles in CAD

Using CAD modelling as shown in figure 6, it is possible to measure the length
of nylon thread from the base of the robot to the tip, both when the robot is
straight and when it is turned. The difference in length can then be
calculated. Calculating the angle between each disc was achieved by dividing
the angle (90 degrees) by the number of plates minus 1, in this case 90/5=18
degrees. By modelling this, it was possible to determine that the arc length
of the eversion robot on the compressed side is 85.5 mm whereas the
uncompressed length is 100 mm. This meant that the motors used would require
enough torque and angle to pull the nylon wire by 14.5 mm. It was assumed that
this is also the distance that each spring would be compressed by.

###  IV-D Computing the forces required to actuate the tip structure

To work out the maximum force that would be required by each spring it was
assumed that the maximum length of compression was not 2.9 mm but rather 5 mm.
This provided a 72% tolerance over the required length. Based on the 5 mm
compression, the forces required to compress each spring were subsequently
modelled. 20 mm long 304 grade stainless steel springs, 6mm in diameter and
made of 0.05 mm wire, were used in this application. Using the following
equation, the spring stiffness could be determined:

$k=\frac{G\times d^{4}}{8\times N\times D^{3}}$ (1)

where $G$ is the shear modulus of the material, $d$ is the diameter of the
wire (in metres), $N$ is the number of active coils and $D$ is the mean coil
diameter. The shear modulus of the material was calculated using the Young’s
modulus and Poisson’s ratio of 304 stainless steel, which are approximated as
190 GPa and 0.27 respectively. Using this formula:

$G=\frac{E}{2(1+v)}$ (2)

where $E$ is Young’s modulus and $v$ is the Poisson’s ratio, and substituting
for the above values, a shear modulus value of 74.8 GPa was obtained, which
was rounded to 75 GPa. Using (2), and a shear modulus of 75 GPa, a diameter of
0.05 mm, 6 active coils and a mean coil diameter of 5.5 mm, which was
determined by subtracting the outer diameter of the spring by the wire
diameter, the spring constant was calculated to be approximately 586 N/m.
Using Hooke’s law, it is possible to approximate the force exerted by each
spring, given that each spring is required to move 5 mm and has a spring
constant of 586 N/m.

$F=-kx=-587\times-0.005=2.935$ (3)

Where $k$ is the spring constant and $x$ is the length in metres. This meant
that the force required to compress each spring was approximately 2.9 N. It
was then possible to calculate the torque required by the motor to
successfully pull the nylon wire and compress the springs by the required
distance, which will be further examined in the following section.

###  IV-E Choosing the right motors

With such a small form factor required to successfully fit into a 2-inch
diameter pipe, cutting down on the size of the rigid structure is the most
significant challenge. The eversion robot has a unique feature of being able
to traverse through piping of variable diameter, as the soft compliant
material on the outside can expand and contract accordingly to fit the size of
the pipe. This however is not possible when rigid material is used at the tip
of the eversion robot such as a 3D printed structure – indeed the maximum
diameter of pipe that the eversion robot is able to travel along would be
determined by the maximum width of the solid structure within the tip. As a
result, reducing the diameter of the steering mechanism is the highest
priority. However, one must also carefully consider the size of the motor in
relation to the force it can exert, especially given the small form factor
constraints and the 45 mm disc diameter. Another key requirement of the motor
is to be capable of providing its rotational position to inform the operator
of the robot’s tip position. This requirement rules out the use of direct
drive DC motors, leaving stepper and servo motors.

Stepper motors are synchronous DC motors that do not spin continuously but
rather use a series of coils arranged in phases. These can be turned on or off
in quick succession to turn the rotor shaft by a fraction of a rotation at a
time, each one called a step. Suitably small stepper motors do exist, with
models such as the NEMA 6 stepper motor with a highly stackable body size of
just 15 mm x 15 mm x 30 mm. However, stepper motors have limited torque and
the NEMA 6, for example, is rated to 0.58 Ncm (0.0000058 kg/cm).

Having eliminated stepper motors, servo motors were identified as the most
appropriate option. Although limited in their motion range and not as small as
the NEMA 6 stepper, servo motors represent a good compromise between DC motors
and steppers as they offer high torque in relation to their size due to their
gear ratio, and can transmit the current position of the shaft through built-
in encoders. They do however come at a cost in terms of size, with the
smallest production servos, the SG90/MG90 family, having dimensions of 22.8 mm
x 12.2 mm x 28.5 mm. Because of this, the servos must be stacked on top of
each other and offset by 90 degrees to each other to allow each one to control
its own nylon wire. Different manufacturers of MG90 servos were examined, as
well as different variants of servo motors, to identify the most appropriate
option. The most important features of the servo to consider are their
operating angle and stall torques. Using the servo with the largest operating
angle allows the design to be smaller, as the servo horn, a cylindrical disc
that houses the nylon spool, has a smaller diameter. However, this negatively
impacts the servo’s ability to pull the wire as the torque decreases. The
torque diminishes as the distance from the centre of rotation, where the force
is applied, decreases. This is due to torque being the product of force and
distance from the axis of rotation as shown by this equation:

$\tau=F\times r$ (4)

where $\tau$ is the torque, $F$ is the force applied and $r$ is the distance
from the centre of rotation to the point where the force is applied. This
means that a compromise must be made between the torque that the servo is able
to provide and the operating angle. The servos examined in this study are
listed in Table I.

TABLE I: Servo Torque Performance Comparison of different models at varying voltages. Servo |  | Operating
---
Angle
| Torque at
---
4.8 V (kg/cm)
| Torque at
---
6 V (kg/cm)
SG90 | 180 | 1.2 | 1.6
MG90s | 180 | 1.8 | 2.2
DMS-MG90-A | 270 | 1.3 | 1.5
DS-S006L | 300 | 1.0 | 1.2
DM-S0090MD | 270 | 1.8 | 2.0

The two candidate servos considered were the DS-S006L and the DM-S0090MD on
account of their increased operating angle. The question remained as to
whether the torque provided would suffice to compress the springs. This was
determined by multiplying 2.95 N by 5 to find the total axial force required
to compress all 5 springs, which is 14.75 N. Connecting this to a spool of
radius 12.50 mm the required torque was determined by:

$\begin{gathered}\quad\text{ Required torque }=\text{ Force }\times\text{
Effective Radius }\\\ =14.75\mathrm{~{}N}\times
0.0125\mathrm{~{}m}=0.184\mathrm{Nm}\\\ \text{ Stall torque
}DS-S006\mathrm{~{}L}(\mathrm{Nm})\\\ =1.2\mathrm{~{}kg}/\mathrm{cm}\times
0.01\mathrm{~{}m}/\mathrm{cm}\times
9.81\mathrm{~{}m}/\mathrm{s}^{2}=0.1177\mathrm{Nm}\\\ \text{ Stall torque
}DM-S0090\mathrm{MD}(\mathrm{Nm})\\\ =2.0\mathrm{~{}kg}/\mathrm{cm}\times
0.01\mathrm{~{}m}/\mathrm{cm}\times
9.81\mathrm{~{}m}/\mathrm{s}^{2}=0.1962\mathrm{Nm}\\\ \text{ Stall torque
}DM-S0090\mathrm{MD}>\text{ Required Torque }\end{gathered}$ (5)

The torque requirement exceeds the stall torque of the DS-S006L but is within
the stall torque of S0090MD. Therefore, S0090MD with a stall torque of 2.0
kg/cm at 6 volts is deemed suitable for pulling the wire and compressing the
spring under the given conditions. However, it is also necessary to determine
if enough wire would be pulled to compress all the springs by approximately 15
mm, taking into account the size of the servo hook and the angle at which the
servo is used. Calculating the arc length of a circle with a radius of 12.5 mm
and an arc angle of 270 degrees:

$s=\frac{270}{360}\times 2\pi\times 12.5\mathrm{~{}mm}\approx
58.9\mathrm{~{}mm}$ (6)

This confirms that the candidate servo motor would be able to apply sufficient
pulling force onto the wire to fully compress the continuum robot before
reaching the servo motor’s limits.

##  V Experimental Evaluation

The robot is controlled using a control panel, facilitating operation through
buttons connected to an Arduino. The control signals are then passed down a
tether to the tip of the robot, controlling the servos and the axial movement
of the hybrid robot. The Arduino is also connected to a SMC pneumatic
regulator that in turn is connected to a compressor supplying pressure to the
robot. This allows the operator to control movement in both sections of the
hybrid robot from a single control panel.

The performance of the hybrid robot was tested by conducting several real-
world experiments to observe its effectiveness and limits. Attention was paid
to the ability of the robot to successfully deliver a set of sensors/camera to
a remote location and to then deploy a “decontaminating” agent or sprayable
expanding foam to the target area.

This ability would pave the way for its use in a variety of nuclear industry
applications – most obviously those involving remote access and
decontamination of hazardous material. The precise workings of the continuum
robot are key if they are to allow for effective decontamination. Most
importantly, the integration of the control panel enables human-safe remote
operation, with the operator being able to be based some distance away, or
even in an entirely separate location. This is clearly useful in relation to
areas that have toxic vapours or concerning levels of radiation. The three
test scenarios conducted were tailored to the liquids that were sprayed. For
the liquid decontaminating agent, a course through a pipe was set up for the
eversion robot to traverse, at the end of which it had to identify the problem
area, as in the case shown in Figure 7, and then deploy the agent (water) to
remedy the issue within a model glove box.

For the liquid agent, we also wanted to test the robot’s ability to precision
spray in a specified direction. For this a 6x10 grid was produced on A3 cold
press paper (watercolour paper) which has a high water absorbency rate. This
was mounted on the inside wall of one of the model glove boxes and used as a
target onto which the eversion robot had to spray aerosol paint. The robot’s
ability to individually spray each of the squares was tested within a 5-minute
time span. The results were recorded and the experiment repeated 10 times to
find the percentage of paint coverage on the piece of paper. The third
scenario is like the first, though without any need to extinguish fire. In
this scenario the robot is used to immobilize a certain object or material.
The eversion robot traverses the pipe into a remote vessel or glove-box and is
then required to spray material in all directions to successfully treat all
surfaces of the box, as shown in figure 8.

Figure 7: a) Hybrid robot demonstrating its versatility in simulating Scenario 1, effectively extinguishing a candle by employing a water-spraying mechanism. b) A POV view of the target, using the built in tip mounted micro camera. TABLE II: Success Percentage of Spraying Grids using Hybrid Robot Test number |  | Number of
---
squares
| Percentage of
---
grid sprayed
1 | 60 | 100
2 | 59 | 98.3
3 | 57 | 95
4 | 60 | 100
5 | 56 | 93.3
6 | 59 | 98.3
7 | 59 | 98.3
8 | 55 | 91.7
9 | 60 | 100
10 | 57 | 95
Average | 58.2 | 96.99
Figure 8: Hybrid robot showcasing adaptability in simulating Scenario 3,
employing an expanding foam application to effectively cover the walls of the
glove box.

These experiments verified that the robot’s ability to spray water, paint and
foam in specified directions is feasible, in each case the robot achieving its
intended purpose. The eversion robot was able to successfully lead the
continuum robot to the required area whereupon it successfully deployed the
water, paint and the foam. The results of the grid spray test are shown in
Table II, where it is observed that the robot achieved more than a 95% success
rate. Upon closer examination of the data, it becomes evident that the 5% lack
of spray in the empty squares was randomly distributed across the 10
experiments. This randomness suggests that limitations in the mechanics of the
system, combined with the inherent difficulty of operator control, contributed
to the observed shortcomings.

The uneven distribution of the spray pattern may be attributed to mechanical
factors such as variations in pressure. Additionally, operator control plays a
crucial role in the effectiveness of the system. However, this proof of
concept successfully demonstrates a technology readiness level of 4, with a
prototype successfully demonstrated in a lab environment.

##  VI Conclusion

We present a novel hybrid continuum-eversion robot that can grow and
decontaminate pipe structures and access hard to reach remote areas. The user
can operate the robot using a controller that commands a set of servos. These
servos control a continuum robot, steering its tip. Simultaneously, a
pneumatic soft eversion robot moves the robot forward, creating a three
degrees-of-freedom hybrid robot. This hybrid robot is capable of transporting
liquids, aerosols or sensors to remote areas on account of its modular tip
that can be interchanged between a cone structure, to transport a camera and
sprayer, and a platform structure to transport a sensor payload.

Possible applications include the exploration of confined spaces such as air
conditioning ducting, navigation and sensing of underground ducting,
navigation and decontamination of glove-boxes, generic piping, enclosed
vessels, water tanks and more. This hybrid robot shows significant promise in
relation to the nuclear industry.

There were some limitations with the robot, mainly due to the manufacturing
processes involved and the resulting air leakage. Future work will need to
focus on better sealing methods, such as heat sealing or ultrasonic sealing of
special TPU coated nylon ripstop material. Another limitation was that the cap
that held the soft and hard components of the robot was only friction fit.
This caused some problems when everting at different angles, especially if
there was a vertical drop. This would sometimes cause the continuum robot
section of the robot to slide off and end up irretrievable other than by using
the tether to pull it back. This suggests that future considerations should
address how to better retract the robot safely. Further research
considerations include investigating the feasibility of decreasing the
footprint of the robot to enable deployment in smaller diameter pipes.
Currently the most significant restriction is that the robot can only work
within piping that has a diameter greater than the maximum width of the hard
PLA material in the continuum robot.

Future research directions also include extending the design to use a camera
with AI/computer vision components to guide the robot towards or away from
certain stimuli. This, along with gyroscopic tracking of the tip of the robot
could also be used to create a 3D visualisation of the area that the robot is
spraying, providing real-time feedback on progress to the operator. The
gyroscopic tracking of the head could also be used to create a visual
representation of the robot itself, providing an estimation of its everted
length, the path it has taken and the angle at which the robot is pointing. A
live camera feed would also facilitate remote steering. A study on the effects
of radiation on the materials that are used should also be carried out to test
the viability of the solution for the intended purposes and environments.

## Acknowledgments

The authors thank Mish Toszeghi for his comments and review. The authors thank
Abu Bakar Dawood for his valuable help.

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