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STUDY OF SINGLE WIRE SCANNER MONITORFOR FETS-FFA TEST RING
E. Yamakawa∗ , D. W. Posthuma de Boer, S. Machida, A. Pertica, A. Letchford
Rutherford Appleton Laboratory, Didcot, United Kingdom
Y. Ishi, T. Uesugi
Kyoto University Institute for Integrated Radiation and Nuclear Science, Osaka, Japan
Abstract
To confirm the use of Fixed Field Alternating gradientaccelerator (FFA) as a high power pulsed neutron spallation
source, a prototype called FETS-FFA is studied at Rutherford
Laboratory (RAL). A single Wire Scanner Monitor (WSM)
is planned to be used to measure beam position and beamprofile in the ring. One of the concerns of this monitor is
the thermal damage on the Carbon Nano Tube (CNT) wire
due to high energy deposition of low energy proton beam in
FETS-FFA (3–12 MeV). Furthermore, to measure a beamprofile during beam acceleration in the ring, a diameter of
CNT wire needs to be smaller than the orbit displacements
in turns. To confirm whether a single WSM is suitable for
FETS-FFA ring, two different beam tests were performed at
RAL and at the Institute for Integrated Radiation and Nuclear
Science, Kyoto University (KURNS). Both measurements
demonstrated that the single WSM is applicable for FETSFFA ring if the diameter of CNT is smaller than the orbit
separation between two consecutive turns. In this paper,the detail of the design study of single WSM as well as theperformance tests are presented.

INTRODUCTION
The ISIS-II project [1] is currently considering a 1.25 MW
proton driver for a new pulsed spallation neutron source
in Europe. A Fixed Field Alternating Gradient (FFA)ring [2, 3] is one option for the main ring of ISIS-II. In
order to demonstrate that FFAs can provide the reliability
required for user operations in ISIS-II, the small-scale Front
End Test Stand and Fixed Field Alternating gradient (FETSFFA) ring will be built by 2029. The baseline optics of thehorizontal excursion FETS-FFA (hFFA) ring were recently
finalised, and its main parameters are presented in Table 1.
Based on the planned commissioning scenario, a single
WSM will be used to identify the beam profile and thebeam position of the injected beam to optimize any injection mismatch with the design orbit and the Twiss parameters. Destructive monitors are extremely useful when anon-destructive monitor cannot detect the beam induced signal in the case of low intensity beam at the beginning of
the beam commissioning. According to the measurement of
the accelerating multi-turn beam profile in the FETS-FFA
ring, the single WSM will be placed on the orbit of a certain
beam energy. As the orbit radius changes with beam energy,
a single stationary wire can measure the profile by intercepting the beam over several turns during an acceleration cycle.
∗ emi.yamakawa@stfc.ac.uk

Table 1: Parameters of 4-fold Symmetry FD-spiral FETSFFA Ring in 2023
Parameter

Value

Beam energy range
3–12 MeV
Bunch intensity
3 × 1011 ppp
Repetition rate
100 Hz (50 pps)
Injection rate
50 Hz
RF frequency bandwidth
2–4 MHz
Harmonic number2
Normalised Beam Core10 𝜋mm mrad
Beam Size±20 mmOrbit Excursion
600 mm
Vertical Physical Acceptance ±32 mm
As the beam energy changes during a profile measurement,
adiabatic damping will cause the transverse beam size toreduce on each successive turn. Furthermore, beam loss due
to scattering at the wire might distort the profile during ameasurement. To investigate whether the monitor can measure the beam profile in the FETS-FFA ring, a feasibility
study of the single WSM is performed in this paper. This
will include the relationship between wire thickness and turnseparation, as well as predictions for wire heating. Profile
measurements made at the FETS Linac and KURNS hFFA
ring will then be shown and used to inform conclusions.

FEASIBILITY STUDY
Carbon Nano Tube (CNT) [4–6] wires are used for the single WSM in the FETS-FFA ring owing to their high melting
temperature (3000 °C) and high tensile strength (1 GPa).

Wire Thickness
For the case of profile measurement of an accelerating
beam over several turns in the FETS-FFA, separation between subsequent transverse profile samples will be fixedby the change in orbit displacement, as illustrated in Fig. 1.
This will depend on the RF program and is not expected
to be constant throughout an acceleration cycle. To investigate the impact of wire thickness on the measured beamprofile, a computer simulation was performed with the vertical excursion FFA (vFFA) lattice [7, 8]1 . In the simulation,
1 The lattice design of the FETS-FFA ring (Table 1) was changed from thevFFA to the horizontal excursion FFA (hFFA) in 2022. The lattice designof hFFA is different from that of vFFA, however the orbit displacements
and the RF program are similar

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1µm(σ

RMS

200

10µm(σ180

30µm(σ160

50µm(σ140=18.6mm)

RMS

RMS

RMS

=18.4mm)=18.3mm)=18.3mm)

Initial(σRMS=17.1mm)

120

1µm(σ

Hit particles

injected particles were generated with a Gaussian distribution and transported using 4×4 transfer matrices with PyTRACK [9]. Although the scattering angle and energy lossof particles at the wire were not taken into account in this calculation, the calculation stopped when a particle hit the wire,assuming 100 % beam-loss due to scatterings. The wire wasplaced at 3.41 MeV (averaged turn separation of 64.2 µm)
and 10.95 MeV (averaged turn separation of 25.3 µm).

Hit particles

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10µm(σ180

30µm(σ160

50µm(σ140

Initial(σRMS=12.7mm)

100

80

80

60

60

40

40

20

20

Orbit radius, mm=12.6mm)

RMS=12.4mm)

RMS=12.0mm)

RMS

120

100

0
−100−80−60−40−20 0 20 40 60 80 100=13.0mm)

RMS

200

0

−40

−20

0

20

40

Orbit radius, mm

Figure 2: Computed beam profile with several thicknessesof CNTs at around 3 MeV (left) and 12 MeV (right).

Figure 1: Schematic drawing of the profile measurement ofan accelerating beam over several turns (red circles) in the
FETS-FFA ring.
Figure 2 shows the computed beam profile measured bywires of differing thicknesses, at around 3 MeV and 12 MeV.
In the case of 12 MeV, the beam size measured by each thickness of wire is consistent to the initial beam size of 12.7 mm.
On the other hand, in the case of 3 MeV, a distortion can beseen when the wire thickness is larger than 𝜙30 µm. Furthermore, the Root Mean Square (RMS) beam size is overestimated compared to the initial beam size of 17.1 mm. Thisis thought to be caused by the change in particle population
throughout the simulation. Whilst the distortion is affectedby the assumed loss due to scattering, these results are indicative of a more general relationship between profile accuracy
and wire thickness. A beam which intercepts less material
will be scattered less, which is particularly significant forthe proposed multi-turn profile measurement. Ensuring thatthe wire thickness is less than the turn separation can help tolimit the number of consecutive scattering events, and thus
give a more accurate profile. The signal strength from thewire is expected to be proportional to the cross-section of thewire, and affects a Signal to Noise Ratio (SNR) and signal
sensitivity of the wire. The thickness of the wire should
be chosen with a reasonable balance between the SNR andprofile distortion.

Figure 3: Transient temperature in each CNT wire. A thermal emissivity of CNT in vacuum is estimated to be 0.2 inthis simulation.
time duration for the beam orbit to travel over the beam size
(2𝜎=8 mm) is 336 µs with harmonic number 2, corresponding to 0.143 mA of 3 × 1011 ppp, at injection beam energyof 3 MeV. As shown in Fig. 3, the steady temperature oneach wire is below the CNT melting temperature.

Heat Test on FETS
To confirm whether a thin CNT wire is applicable witha 3 MeV proton beam, tests were performed with a 3 MeV
H− beam on the FETS Linac [11], using the ISIS singlewire injector profile monitor (SWIP) [12] (Fig. 4). Several
diameters of CNT wires: 𝜙20, 𝜙30, 𝜙40 and 𝜙50 µm were
mounted on the monitor head. The monitor shaft is inclined
at 44.5 degrees, and travels nearly 150 mm from the park
position.

Heat Analysis
Temperature rise in the wire is an important consideration
for all such interceptive diagnostics, but especially so whenmeasuring the beam profile over multiple turns. The cumulative interaction period will be significantly longer thanthat of one-pass measurement. A heat analysis has therefore
performed by the analytical simulation code [10] which computes a temperature change in the material. According to thevFFA beam parameters and its RF acceleration scheme, theFigure 4: Left: The single WSM on FETS beam line. Right:
The monitor head with CNTs. The horizontal wires are
upstream of the vertical wires, which are 3 mm apart in
the beam direction. The 𝜙10 µm CNT was too thin to be
mounted on the monitor head.

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Figure 5 shows light emission at the wire when interactingwith the FETS beam. The smaller the diameter of the wire,
the weaker the light emission can be seen at the wire. Basedon the beam parameters when the pictures were taken, thesteady temperature on the 𝜙20 µm is predicted to be beyond
1000 °C, and the 𝜙20 µm CNT was not broken after at least
10 minutes of exposure to the FETS beam of 28 mA peakcurrent (3.5E13 ppp with a 200 µs pulse). This is a factorof 1 × 103 and 1 × 102 larger than the peak current of theinjection pulse (6 × 109 ppp) and the multi-turn accelerating
beam (3 × 1011 ppp) in the FETS-FFA ring. Therefore, theconsidered thickness of CNTs are not expected to be damaged rapidly by the injection beam in the FETS-FFA ring.
However, the heat damage over longer durations should also
be analysed in future.

Figure 6: The measured pulse length (left) and the integrated
pulse signal (right) of the FETS beam. The error bar is the
RMS pulse length and the RMS integrated wire signal over
5 pulses.

BEAM PROFILE MEASUREMENT
Profile Measurement on FETS

Figure 5: Light emission from the 𝜙50 µm (left) and the𝜙20 µm (right, in the circle) CNTs with the 3 MeV FETS H−
beam. The wire was aligned at the centre of the beam pipe
when the photo was taken.

Pulse Width Measurement on FETS
The pulse width of the FETS beam was measured by the
SWIP. The pulse width is determined by the time window
which is above 2 times of standard deviation of the noiselevel. The noise level is found by computing the RMS withina time window at the end of the data when there is no beam.
According to the SRIM [13] simulation, the proton range of3 MeV is expected to be about 120 µm in a Carbon material(1.4 g/cm3 ). Therefore, H− beam will not be fully stopped
at the CNTs mounted on the frame, and the electrons of H−
beam will be directly captured by the wire. The negative
charge signal is converted to the voltage signal by the operational amplifier and monitored by the oscilloscope. The
gain of amplifier is 1 × 103 V/A with 500 kHz frequency
bandwidth.
As shown in Fig. 6, the pulse width measured by each CNT
wire is within about 5 % different from the one measured by
the CT monitor (210 µm) located at about 535 mm upstreamof the SWIP. In the integrated signal within the pulse width,
the number of primary H− particles is degraded by about
18 % when it reaches the downstream wires (vertical profile
wires in Fig. 6), that agrees with the PHITS [14] simulation.
Although the integrated signal was not proportional to thecross-section of the wires, these data will be useful to predict
the signal strength when the required diameter of wire isused in the FETS-FFA ring.

The measurement was done by SWIP scanning the wire
positions over the beam size. The control units set the wire
positions with a stepper motor and acquire the signal at every
trigger timing. The control units have a signal amplifier witha gain of 4.7E3 V/A at bandwidth of 50 kHz after low passfiltering. At each monitor step, 10 samples were acquired
over 100 ms following a trigger, and averaged before being
saved to disk. As shown in Fig. 7 and Table 2, beam profiles
were measured by all CNT wires, and all measured horizontal beam sizes are consistent. The vertical beam sizes are
not all consistent, which could be due to scattering from the
upstream horizontal profile wires. Based on lattice simulations on the FETS, the RMS beam size at the position of theSWIP is about 4–5 mm. Although the distance between adjacent wires is bigger than the expected beam size, the protons
generated at the wires will come in the neighbouring wires,
decreasing the signal strength and the signal sensitivity.

Figure 7: Beam profiles measured by each CNT wire. The
scan range was 148.5 mm with a step size of 1 mm. The
peak pulse intensity measured by the CT monitor was about
13 mA in this measurement.

Profile Measurement on hFFA
A beam experiment with the single WSM was performedin the hFFA ring at KURNS [15]. The wire position was
fixed at the orbit of about 13.5 MeV where the turn separation is about 29 µm. A CNT wire with diameter 𝜙10 µm
was mounted on the frame as shown in Fig. 8. As the rangeof 13.5 MeV protons was found to be 1.67 mm in a Carbon

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Table 2: The measured RMS beam size with design values.
The errors are RMS of three different beam sizes.
CNT

RMS Beam Size RMS Beam Size
Horizontal [mm] Vertical [mm]

𝜙20 µs 3.5±0.28
𝜙30 µs 3.4±0.35
𝜙40 µs 3.8±0.77
𝜙50 µs 3.8±0.25
Design 4-5

4.2±0.04
3.8±0.27
4.6±0.27
N.A.
4-5

material (1.4 g/cm3) by SRIM simulation, the protons were
expected to be fully stopped at the CNT wire. Therefore,
secondary electron emissions were read by the signal amplifier with a gain of 1 × 107 V(A) and a cutoff frequency of
4.8 kHz that was wide enough to detect the ≈300 µs beam
signal.

Stray magnetic fields from the main hFFA magnets is
close to 0.05 T near the middle of the straight section where
the single WSM was installed. Based on CST tracking simulations [16], some of the secondary electrons generated at
the wire will be confined by the magnetic fields and eventually return to the wire, decreasing the signal and resulting ina smaller measured beam size. Profile measurements with anegative bias voltage were planned, but not performed due
to time constraints.
Hit Particles

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500

Initial, RMS=5.0mm

450
400

φ10um, RMS=5.0mm

350
300
250
200
150
100
50
0

−30

−20

−10

0

10

20

30

Orbit radius, mm

Figure 10: The computed beam profile measured by 𝜙10 µm
wire. The initial beam is a Gaussian distribution with thedesign value of beam size (𝜎=5 mm). The scattering angle
and energy loss at the wire are computed by PHITS, which
are taken into account in this simulation.
Figure 8: The radial probe (left) and the monitor head of thesingle WSM with 𝜙10 µm CNT wire (right). The monitor
head was attached to the radial probe and controlled by the
motion control driver.

Figure 9: The accelerating multi-turn beam profile measured
by a 𝜙10 µm CNT wire. The dashed line indicates 2 times ofstandard deviation of the noise level, computed by the RMS
within a time window at the end of the data when there is
no beam.
Although the shape of beam profile is similar between
the measurement (Fig. 9) and the simulation (Fig. 10) basedon the KURNS ring optics and the beam parameters, themeasured beam size is smaller than that of design value.
The injector (11 MeV H− Linac) was not stable during theexperiment, and the ion source as well as following beamtransports were recommissioned prior to the tests. Discrepancies between measurement and expectation could possibly
be explained by differences between the actual and design
beam emittance.

CONCLUSION
The WSM with a single, stationary CNT wire has beendeveloped as a beam profile monitor for the FETS-FFA test
ring. Tracking simulations have motivated the study of wires
with a small diameter, to minimise scattering over manyturns. The durability of these wires has been demonstrated
by exposing them to the 3 MeV H− beam of the FETS Linac.
Whilst it has been demonstrated with simulations and measurements that heating of these wires is not expected to
produce short-term effects, heat damage over a longer time
periods should be also observed in future. It has also been
demonstrated with simulations and experimentally, that thesingle WSM will be able to measure the pulse length andprofile of low energy mono-energetic beams as well as theaccelerating beam over multiple turns in the FETS-FFA ring.
A beam scraper will also be installed in the FETS-FFA ring,and offers an alternative method to measure the beam sizeof accelerating beams over multiple turns. However, there
are challenges in using such monitors when distributions
such as hollow beams are possible. As the single WSM will
be able to identify the beam profile and the beam size evenfor a hollow beam, the single WSM will be an invaluable
diagnostic for commissioning the FETS-FFA test ring.

ACKNOWLEDGEMENTS
The author wishes to acknowledge the efforts by S. Kellard
and A. Chamberlain for the design and manufacture of thesingle WSM.

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