Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB3

ISIS-II UPDATE
D. W. Posthuma de Boer∗ , D. J. Adams, H. V. Cavanagh, C. Jolly, D. Kelliher,
B. Kyle, J. B. Lagrange, S. Machida, H. Rafique, C. Rogers, C. M. Warsop,
R. E. Williamson, E. Yamakawa at ISIS Neutron and Muon Source, Harwell, United Kingdom
Y. Ishi, T. Uesugi at Kyoto University Research Reactor Institute, Osaka, Japan
Abstract
ISIS, the spallation Neutron and Muon source at the
Rutherford Appleton Laboratory in the UK, uses a 50 Hz,
800 MeV proton rapid cycling synchrotron (RCS) to provide
an average beam power of 0.2 MW to its two fixed targets.
Whilst demand to answer fundamental questions in materials science is growing, access to neutron research methods
is expected to become more limited in Europe as reactorbased sources close [1]. In answer to this, detailed studies
for a new major pulsed neutron facility, ISIS-II, are now
under way [2, 3]. Fixed-Field Alternating gradient accelerators (FFAs), accumulator rings (ARs) and rapid cycling
synchrotrons are being considered to provide the required
MW beam power for this new facility. Progress on all three
accelerator options will be summarised, and relevant experimental results demonstrating beam stacking at the KURNS
FFA will be presented.

INTRODUCTION
The ISIS Neutron and Muon Source at the Rutherford
Appleton Laboratory has produced world-leading sciencefor close to 40 years [4]. Despite the anticipated capacity ofa new ESS facility [5], the number of neutron instrumentdays available in Europe is expected to fall in the coming
decade as reactor-based sources close [1]. This will create
an increased demand at the remaining sources such as ISIS,and a supply gap in the number of instrument-days available [1, 3]. A new short pulsed facility, “ISIS-II”, has been
proposed to address this gap [3], and a roadmap establishedwhich includes an R&D, feasibility and design stage thatwill conclude in 2027 [6]. The new facility will be designed
in collaboration with neutron users, but is expected to be
driven by a MW-class proton accelerator.
RCS and AR designs build on the experience of existing
facilities, whilst the FFA option would be the world’s firsthigh-intensity FFA and requires a demonstrator. For thispurpose, the FETS-FFA is currently being developed at the
Rutherford Appleton Laboratory and will serve as a testbed
for techniques such as low-loss injection and extraction froman FFA [7].

CONVENTIONAL RING DESIGNSThe outline design specification is to provide a 1.25 MW
proton beam at 1.2 GeV in short, ∼1 µs pulses to two targets,one at 40 Hz (1 MW) and a second at 10 Hz (0.25 MW),although these headline figures are yet to be finalised. The∗ david.posthuma-de-boer@stfc.ac.uk

Figure 1: Possible placement of ISIS-II on the RAL site.
key challenge of such a high power machine is in achieving
low beam loss levels of ∼0.1% such that activation is reduced
to a level where hands-on maintenance of components ispossible.
Physics designs of conventional rings have been produced [8] for both siting the new ring in the existing ISIS
synchrotron hall or building a stand-alone facility (Fig. 1).
However, reuse of the ISIS hall, despite potentially providing
significant cost savings, imposes constraints on the designand significantly a long break in neutron production at RAL.
Therefore, a stand-alone facility is the preferred choice. A
further important choice is between a full energy (1.2 GeV)
linac and AR or a lower energy (and cheaper) linac and amore complicated (0.4 - 1.2 GeV) RCS. To help understandthe feasibility and challenges of these choices, accelerator
designs have been studied of an RCS and an AR as a new
stand-alone facility at RAL.

Lattice Design
The same lattice design has been utilised for both AR
and RCS and is similar to that of SNS [9], but with drift
spaces in the arc sections to accommodate the RCS RF requirements. It incorporates 4 superperiods (see Fig. 2), eachcomprising a four cell FODO achromatic arc and a twocell reverse doublet for the straights. The nominal working point is (𝑄𝑥 , 𝑄𝑦 )=(6.40, 6.32) and the circumference is
282 m. Charge exchange injection at 400 MeV accumulates
1.3 × 1014 protons over a 600 µs pulse, with beam chopped
at the dominant (ℎ = 2) ring RF frequency to allow efficient
capture.
The beam is painted in both the transverse and longitudinal planes, with the design carefully optimised to form
distributions that minimise beam loss and make best use of
the acceptances (600 𝜋 and 350 𝜋 mm mrad for collimated- 40 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB3

Further Work

Figure 2: Ring schematic (left) and lattice Twiss parameters,
dispersion and phase advance (right).
acceptances of the RCS and AR respectively). Table 1 summarises the main parameters of the stand-alone RCS andAR ISIS-II designs.
Table 1: Key Parameters for Conventional Ring ISIS-II Designs
Kinetic energy (GeV)
Intensity (protons per pulse)
Repetition rate (Hz)Circumference (m)Number of superperiods
Magnet Excitation
Dipole fields (T)Nominal betatron tunes (𝑄𝑥 , 𝑄𝑦 )Gamma transition, 𝛾𝑡
Peak RF volts ℎ =(2,4) (kV/turn)RF frequency, ℎ = 2 (MHz)Acceptances: collimation, aperture(𝜋 mm mrad) (Δ𝑝/𝑝 ± 0.01)
Natural chromaticity 𝜉𝑥 , 𝜉𝑦Momentum compaction, 𝛼𝑐
Number of injected turns

RCS

AR

0.4 - 1.2
1.3 × 1014
50
282
4
Sinusoidal
0.42 - 0.89
(6.40, 6.32)
5.2(300, 150)
1.52 - 1.91
600, 750

1.2
1.3 × 1014
50
282
4
DC
0.89
(6.40, 6.32)
5.2(50, 28)
1.91
350, 500-11.49, -9.62
0.036
455

-11.49, -9.62
0.036
573

The ISIS-II RCS and AR feasibility designs have identified good solutions within conventional technology limits fora stand-alone facility. However, more detailed simulations
investigating beam loss below the 0.1% level are essential forreaching, optimising and exceeding current state-of-the-art
designs. Studies are ongoing into loss mechanisms and the
production of more realistic models with appropriate magnet
errors and impedances, benchmarked on the current ISIS
synchrotron [11]. Furthermore, operating in a sustainable
manner is increasingly a priority on ISIS and for the designof ISIS-II. Incorporating full lifetime carbon costs is now
under consideration and will inform the next iteration ofdesigns.

FFA DESIGNFor a future accelerator based facility, sustainability andstable operation are factors that will be given more weight.
An FFA is expected to have an advantage in that respect.
A proposal of an FFA accelerator as a high intensity proton driver or as a spallation neutron driver is not new [12].
Long before conventional ring options were established, an
FFA was considered as a credible option. Unfortunately, it
was hard to convince people to choose this option because
there was no FFA constructed other than a small proof ofprinciple machine. Nowadays a couple of FFA accelerators
are running for a variety of applications. Flexibility of theoutput energy and wide momentum acceptance are among
unique features an FFA can provide easily. The applications
the existing FFAs are dedicated to, however, do not require
high intensity, either in average or at peak. The experimental
validation of high intensity acceleration in an FFA, more
specifically the same level of space charge tune shift as in
RCS or AR has not been achieved.

FETS-FFA

Design Elements
Both RCS and AR designs include detailed studies into
longitudinal and transverse beam dynamics, combined 3D
beam dynamics, charge-exchange foils and injection straight,
correction magnets, collimation, extraction system and instabilities [8]. Each machine has its own challenges in meetingthe demanding physics specification. For example, the RCS
injects at lower energy and has larger space charge effects
whilst the AR, with injection at higher energy, requires a
thicker injection foil leading to more foil heating.
Detailed PIC simulations using PyORBIT [10] have been
performed of injection and trapping with full 6D dynamics, space charge, magnet multipole errors and chromatic
correction. Probing the beam extremes with virtual scrapers shows the beam acceptance for 0.1% loss is 585 𝜋 and
250 𝜋 mm mrad for RCS and AR respectively, in close
agreement to initial estimates. Longitudinal distributions
are well-controlled through acceleration, however transverse
emittance growth continues longer than expected and is the
subject of further study.

To narrow the gap, a project was started to design and construct a prototype high intensity FFA at RAL downstream of
Front End Test Stand (FETS). We call it FETS-FFA. FETSFFA takes the 3 MeV H- beams from FETS linac and accelerates it up to 12 MeV. The momentum ratio of 2 is the sameas that of ISIS-II. We aim at achieving space charge tune
shift of -0.3 at injection to demonstrate the high intensity
operation.
There are three challenges in the physics design. Firstly,
the optics should have enough flexibility so that horizontal
and vertical tune will be almost equal and both are below
90 degree phase advance per unit cell. The operation of
SNS and J-PARC empirically proves that this operation point
gives the minimum beam loss. The first choice of tune in ahigh intensity FFA is set to satisfy the same condition. The
FD (or DF) spiral lattice has been proposed which has theflexibility of optics without sacrificing the ratio of averageand bending radius [13]. The spiral angle of the magnets
reduces the requirement of a D magnet (reverse bending) toincrease vertical tune.

- 41 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB3

Table 2: Parameters of 4-fold Symmetry FDspiral FFA
4

100

4

80

0

Qy

y [m]

2

Kinetic energy
Number of superperiod
Number of cell per superperiod
Spiral angle 𝜉Radius at injection
Geometry of 𝐵𝑓 𝐵𝑑 doublet magnet
𝐵𝑓 magnet
𝐵𝑑 magnet
Space between 𝐵𝑓 and 𝐵𝑑 magnets
Fringe field model𝑠
𝐺(𝑠) = 1/ [1 + exp (𝑐10 𝜆/ cos𝜉 )]defined at 4.0 m
s: path along azimuthal angle
𝑐10 : Enge coefficient
𝜆: corresponds to the full gap
Nominal superperiod tune
Nominal ring tune
Field index 𝑘

60
3.5
40

-2

20
-4

3
-4 -2 0 2
x [m]

4

3

3.5
Qx

4

0

Figure 3: (left) Top view of 4-fold symmetry lattice. Orbits
at injection and extraction energy are also shown. There areseveral orbits corresponding to different operation points
in the tune rage from 3 to 4 in H and V. (right) Dynamic
aperture of 4-fold symmetry lattice. Colour scale indicates
dynamic aperture in 𝜋 mm mrad (normalised). 𝑄𝑥 =4 and𝑄𝑦 =4 are the lines where 4𝑡ℎ order systematic multipole
exists.
Secondly, injection and extraction must be tightly controlled, and enough space must be provided for those systems. An FFA lattice usually has high symmetry consistingof a simple unit cell, but that divide the available straightsection into equal distances, which may not be long enough.
By introducing 8 different types of magnets or 4 familiesof FD doublet magnets, we introduce a superperiod structure with 4-fold symmetry. Four out of 16 straight sections
increase the length by about 50% where injection and extraction system are installed. The top view of the lattice with
injection and extraction orbits at different operating points
is shown in Fig. 3 (left).
Finally, transverse emittance must be reasonably large todecrease space charge tune shift with the fixed number ofprotons. Because all orders of multipoles intrinsically exist
in an FFA magnet, the dynamic aperture is a concern. Tune
shift with amplitude in the vertical direction due to the octupole component of the fringe field is a primary source of
aperture limitation. By optimising parameters such as spiralangle and the distance between 4 families of FD doublet,a normalised dynamic aperture of almost 100 𝜋 mm mrad
has been achieved (Fig. 3 (right)). Since the nominal emittance after painting is set to be 10 𝜋 mm mrad, collimator
acceptance is 20 𝜋 mm mrad and the physical aperture isset around 40 to 80 𝜋 mm mrad, a dynamics aperture of
100 𝜋 mm mrad is comfortable.
Table 2 shows the main parameters of 4-fold symmetryFETS-FFA design.

BEAM STACKING
The term beam stacking refers to a number of methods
aimed at producing high intensity beams composed of several lower intensity beams [14]. Momentum stacking is one
such technique, where previously accelerated bunches coast
whilst additional bunches are injected, accelerated and debunched [15]. The resulting stack of coasting beams could
then be captured with a suitable RF bucket.

value

unit

3 to 12
4
4
30
3.6

MeV

0.2*2𝜋/16
0.1*2𝜋/16
0.1*2𝜋/16

rad
rad
rad

3.91
0.140
(0.8525, 0.8475)(3.41, 3.39)
7.4561degreemm

Momentum stacking has several potential applications at
ISIS-II, and is being studied for the FFA and AR designs(momentum stacking is not possible in a standard RCS due
to the AC magnet excitation). In both cases, stacking would
allow bunches to be extracted at a slower rate than they
were injected; making customised user-cycle conditions a
possibility. In the FFA case, it could also overcome theintensity limitation due to space charge at injection, and isthus of considerable interest.

Experimental Demonstration at the KURNS FFA
To gain experience with momentum stacking in an FFA,an experiment to stack two beams in the KURNS FFA has
been carried out. The procedure was to inject and acceleratebeam-1 to its final energy before debunching it adiabatically,
then to repeat this process for beam-2 but with a different
final energy. The longitudinal Schottky signal was acquired
on an electrostatic pickup [16] to measure the revolution
frequency distribution of the two coasting beams. Beams
were circulated for more than 105 turns after the RF amplitude reached zero before coasting-beam data was acquired;
see approximate timings and RF amplitudes in Fig. 4. Signals were acquired using an SA-220F5 low-noise amplifier,an 80 MHz low-pass filter and an oscilloscope sampling at
500 MSa/s.
Figure 5 shows power spectral density (PSD) for threecases, around the 8th harmonic of the revolution frequency;
PSD was estimated with the Welch method [17]. With one
injected beam, a single Schottky peak was observed above
the noise of the beam-off signal. When a second beam wasstacked at a lower energy, an additional peak was observed
at a lower frequency and with a similar amplitude to that of
the beam-1 only case. This confirms that stacking has beenachieved in the KURNS FFA; however, with two beams injected, the amplitude of the beam-1 peak was approximately
halved, indicating beam-loss during the beam-2 acceleration
process. The driving mechanism is now under investiga-- 42 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB3

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Figure 4: Schematic of RF amplitude versus time, to demonstrate the intended acceleration and debunching procedures
and indicate approximate timings. Signal acquisition windows are also shown.

[4] J. Thomason, “The ISIS spallation neutron and muonsource—the first thirty-three years”, Nuclear Instruments andMethods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 917,
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Figure 5: PSD estimate of longitudinal Schottky signals
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CONCLUSION
ISIS-II is a proposed short pulse neutron source to address
the upcoming gap in available neutron instrument-days in
Europe. Extensive research into RCS, AR and FFA machines capable of providing the necessary beam power is
now under way. A lattice for a stand-alone conventionalring has been presented, whose design has been influenced
by existing high-intensity accelerator facilities. Since no
existing high-intensity FFA-based facilities are currently inoperation, the FETS-FFA will act as a demonstrator to guidethe development of an ISIS-II FFA design. Beam stacking
is being considered to enable a flexible extraction rate in the
AR and FFA machines and to alleviate space-charge tune
shifts at injection in an FFA. An experiment has successfully
demonstrated stacking at the KURNS FFA, and the observed
losses are now being investigated.

[14] K. R. Symon and A. M. Sessler, “Methods of radio frequency
acceleration in fixed field accelerators with applications tohigh current and intersecting beam accelerators”, 1956.
doi:10.5170/CERN-1956-025.44[15] E. Ciapala, “Stacking and phase displacement acceleration”,
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Fixed Field Alternating Gradient Synchrotron at KURNS”,
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Prentice Hall, 2005.
[18] K. M. Terwilliger, “Radio frequency knockout of stacked
beams”, 1956.
https://inspirehep.net/literature/39027- 43 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB3[19] D. W. Kerst, “Suppression of betatron oscillation excitation(RF knockout) by the RF accelerating system of a fixed field
accelerator”, 1957.
https://inspirehep.net/literature/39054[20] K. M. Terwilliger, “Suppression of radio frequency knockout in stacked beams by phase shifting the betatron oscillations”,1957.
https://inspirehep.net/literature/39104- 44 -