ISIS-II Update
D. W. Posthuma de Boer
On behalf of:D. J. Adams, H. V. Cavanagh, B. Kyle, H. Rafique, C. M. Warsop, R. E. Williamson,S. Machida, E. Yamakawa, J. B. Lagrange, D. Kelliher, C. Jolly, J. Pasternak, C. Rogers,
Y. Ishi, T. Uesugi

20th Annual Meeting of the Accelerator Society of Japan2023-08-30

Contents• ISIS Neutron and Muon Source• Future of neutron science in Europe• ISIS-II facility proposal• Beam stacking and experimental results• Conclusion

2ISIS Neutron and Muon Source• ISIS is a Neutron and Muon source at theRutherford Appleton Laboratory in the UK [1].
• ISIS started operations in 1985 and continues toproduce world-leading science in many fields, such as
materials, medicine, magnetism etc.
• The primary constituents are:• Penning H- ion source• RFQ• 70 MeV H- drift-tube linac• 800 MeV, 50 Hz rapid cycling proton synchrotron• Two, tungsten fixed targets + instruments• One, graphite target + instruments

ISIS Today• Research and development is ongoing• Some current and recent upgrade projects
include:• All RF drive systems upgraded, with all parts
except cavities being replaced. [2]• RF ion source development [4]• TS1 upgrade [3]• All linac wire scanners upgraded including
new control and DAQ systems [5]• Linac tank-4 was replaced in 2021
• Beam dynamics and model development [6]• Instrument upgrades4

Neutron Sources in Europe[7, pp.75]• Europe is a leader in neutron-based research[7, pp.66].
• The eventual closure of ILL will significantly
reduce the number of available instrument-days;
even with contribution from ESS.
• Long-term plans are needed for Europe tomaintain its leading position.
• For the UK, STFC Strategic Review[8,9]recommendations:• “[…] maintaining and developing ISIS should be thehighest priority for the UK and ISIS should continue tobe fully supported over at least the next 10 to 15 years.”• “Support continued development of the ISIS II concept.”5ISIS-II• ISIS-II is a proposed short-pulsed neutron source toaddress the gap in instrument day capacity.
• £5.1e6 UKRI infrastructure funding has been allocatedfor Feasibility, Design Studies and R&D.
• ISIS II Working Group is optimising for neutrons andconsidering a range of options. Here, I will concentrateon accelerator studies.

• Headline Specifications (subject to change)• 1.25-2.50 MW power on-target• 1.2 GeV beam energy on-target• 0.1% beam loss during operations• Environmental impact is fundamental consideration

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ISIS-II Accelerator Options• Machines that could be capable of meeting theserequirements are:RCS• Rapid Cycling Synchrotron (RCS)“Conventional Rings”• Accumulator Ring (AR)• Fixed Field Alternating Gradient Acc. (FFA)• Two site options• Reuse existing ISIS accelerator hall
• New accelerator halls

AR• Existing RCS and AR designs have demonstrated
similar specifications.
• FFA has not yet demonstrated high intensity
operation.

FFA

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Conventional Rings

RCS

AR

Energy Range

0.4 -1.2 GeV

1.2 GeV

Intensity

1.3e14 ppp

1.3e14 ppp

Repetition Rate

50 Hz

50 Hz

Mean Power1.25 MW

1.25 MW• RCS design will be influenced by JParc

Circumference

282 m (45 m)

282 m• AR design will be influenced by SNS

No Super Periods

44• Achromatic Arc (4x90deg FODO cells)

Magnet ExcitationSinusoidal

DC

Dipole Fields

0.42-0.84 T

0.84 T

Betatron Tunes (Qx, Qy)(6.40, 6.32) (±~0.2)(6.40, 6.32) (±~0.2)

Gamma Transition

5.2

5.2

Peak RF h=(2,4)(300, 150) kV/turn(50, 28) kV/turn

RF Frequency (h=2)

1.52-1.91 MHz

1.91 MHz

Number of Bunches

22

Acceptance (∆p/p ±0.01)(painted, collimator, aperture)(π mm mr)

400, 600, 750

300, 350, 500• Designs are in active development,
all numbers subject to change.
Lead by D. J. Adams• Straight: Reverse doublet• Tune control in straight, 4 trim quads

8Conventional Ring Development
Lattice design(D. J. Adams)Longitudinal dynamics(R. E. Williamson)

Injection straight design (B. Kyle)

Transverse dynamics(C. M. Warsop)

Stripper foil development(H. V. Cavanagh)

Extraction (H. Rafique)
Collimation(H. V. Cavanagh)

Space charge(C. M. Warsop)

Correction systems(H. Rafique)

Instabilities(R. E. Williamson,D. W. Posthuma de Boer)

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FFA Option• Designs in active development. Numbers subject tochange. Lead by S. Machida.
• A vertical FFA was considered, but did not fit withinproject timelines. Horizontal now being designed [10].
• Several potential advantages:• Environmental impact may be smaller as DC or evenpermanent magnets can be used
• DC Magnets are more reliable than AC magnets• Flexible acceleration pattern as magnets not ramped• Beam stacking alleviates space charge at injection andallows flexible extract rate.

FFA
Energy Range

0.4 -1.2 GeV

Intensity

1.3e14 ppp

Mean Power
Circumference
Magnet Excitation

1.25 MW~250 m
DCAcceptance (∆p/p ±0.01)(painted, collimator, aperture)(π mm mr)100, 200, 400• It would be the first high intensity FFA and cannot bedirectly based on any existing accelerator.
• A demonstrator FFA must be built.

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FFA Demonstrator: FETS-FFA• FETS is a 3 MeV, 60 mA H- linac at RAL [11].
It will be used as an injector to a demonstrator FFA
called the “FETS-FFA”.
• Several aims for demonstrator:• Develop low-loss FFA injection techniques• Develop beam stacking techniques• Develop recapture and extraction methods• Verify low-loss acceleration to top energy• Gain experience in FFA designFETS-FFA
Energy Range

3 -12 MeV

Intensity

3e11 ppp

Repetition Rate
Mean Power (equiv.)
Mean Orbit Radius
Magnet Excitation

100 Hz60 W
3.6 – 4.2 m
DCNormalised Acceptance(painted, collimator, aperture)(π mm mr)

10, 20, 40• Whilst the FETS-FFA is lower power than ISIS-II, its
incoherent tune spread will be -0.25; similar to that of
ISIS-II [6].

11FETS-FFA Development
Lattice (S. Machida)

Dynamic aperture
study (S. Machida)

Collimation (E. Yamakawa)

Optics (S. Machida)

Longitudinal
Gymnastics(D. Kelliher)Diagnostics(E. Yamakawa, D. Posthuma de Boer)

Magnet design (J. B. Lagrange, I. Rodriguez)

Injection (C. Rogers)

Extraction(J. Pasternak)

RF cavity design(I. S. K. Gardner, R. J. Mathieson, B. Kirk)

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Beam Stacking• “Beam stacking” is where several lower-charge
bunches are combined.
• It allows extracted beam intensity to be varied, andfor extraction repetition rate to differ from injection
repetition rate.
• Stacking is being considered for AR and FFA options.
• In the FFA, stacking will also alleviate intensity limitsimposed by space-charge tune-spread at injection.

Simulation plots from D. Kelliher

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FFA Beam Stacking Experiment• Experiment to demonstrate momentum stacking
in an FFA was performed in March 2023, at theInstitute for Integrated Radiation and Nuclear
Science at Kyoto University (KURNS) [12].

KURNS-FFA Main Ring
Energy Range
Intensity

11 – 150 MeV
1e9 ppp

Mean Current

1 nA

Average orbit radius 4.58 – 5.32 m• Aim was to stack and recapture two bunches.
• Measured incoherent revolution frequency
distribution of both coasting beams withSchottky noise.
• Used electrostatic pickup(Full Aperture Bunch Monitor, “FAB”, [12])• Remote controlled oscilloscope ~4m from FAB• 80MHz low-pass filter, 500 MSa/s• 1 MΩ amplifier connected to vacuum feedthrough

FAB

KURNS FFA.

Experiment Layout
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FFA Beam Stacking Results• With a single injected beam, Schottky revolution
harmonics were visible after debunching.
• Plots shows power spectral density (PSD) around
8th harmonic, estimated with Welch method [13].

• With two injected beams, two revolution
harmonics were visible.

Beam-2Beam-1• Beam-1-losses were observed and are currently
being investigated.
• Believe this problem was previously investigated atMidwestern University Research Association(MURA) in the 1950’s [14]• Beam-stacking in an FFA has been demonstrated
experimentally at KURNS.

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Conclusion• To maintain its position as a world leader in neutron science, Europe needs to develop newfacilities over the coming decade.
• ISIS-II is a proposed short-pulse neutron science facility, driven by a MW-class accelerator.
Designs are currently centred on three types of accelerator: RCS, AR and FFA.
• FETS-FFA will act as a demonstrator for a high-intensity FFA.
• Beam-stacking is being considered for the FFA and AR designs.
• Stacking has been demonstrated experimentally at the KURNS FFA.
Beam-1 losses are currently under investigation.
• Development work on conventional rings and the FFA option are ongoing.
• Further updates to be given at HB 2023

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References[1] - J.W.G. Thomason, “The ISIS Spallation Neutron and Muon Source—The first thirty-three years”, Nuc. Instr. and Methods A. Volume 917, 2019.
https://doi.org/10.1016/j.nima.2018.11.129[2] – A. Seville, “Progress on the ISIS synchrotron digital low level RF system upgrade”, LLLRF Workshop 2019.
https://doi.org/10.48550/arXiv.1910.07302[3] – S. Gallimore, “ISIS TS1 Project Summary”, ICANNS 2018. http://dx.doi.org/10.1088/1742-6596/1021/1/012053[4] – S. Lawrie, “Plasma commissioning in a high power external RF-coil volume-type H- ion source”, International Conference on Ion Sources 2021.
https://doi.org/10.1088/1742-6596/2244/1/012033[5] – D. M. Harryman, “An upgraded Scanning Wire Beam Profile Monitoring System for the ISIS High Energy Drift Space”, IBIC 2017.
https://doi.org/10.18429/JACoW-IBIC2017-WEPCC18[6] – C. M. Warsop, “Understanding the High Intensity Behaviour of the ISIS RCS and the Lessons for ISIS II Designs.”,
ICFA Mini-Workshop on Space Charge 2022.
[7] - ESFRI, “Neutron scattering facilities in Europe. Present status and future perspectives”, 2016.
[8] – STFC, “Neutron Science and Facilities: An update to the 2017 Strategic Review”, June 2020,
https://www.ukri.org/publications/isis-neutron-and-muon-source-annual-review/[9] – STFC. “Neutron and Muon Science and Facilities: A Strategic Review and Future Vision Report from the Advisory Panel”,
November 2017, https://www.ukri.org/publications/isis-neutron-and-muon-source-annual-review/[10] – S. Machida, “ISIS upgrade with FFA”, FFA Workshop 2022.
https://indico.stfc.ac.uk/event/487/contributions/3908/attachments/1340/2416/machida20220929_v3.2.pdf[11] – D. Findlay, “The RAL Front End Test Stand”, Nuc. Phys B, 2004. https://doi.org/10.1016/j.nuclphysbps.2005.05.060[12] - T. Uesugi et al, “Study of Beam Injection Efficiency in the Fixed Field Alternating Gradient Synchrotron in KURNS”, IPAC 2019.
[13] - P. Stoica and R. L. Moses, "Spectral Analysis of Signals", Pearson Prentice Hall, 2005, ISBN: 9780131139565.
[14] - K.M. Terwilliger, ‘Radio frequency knockout of stacked beams’, 1956. https://inspirehep.net/literature/39027[15] - A.A. Kolomensky and A. N. Lebedev, “Theory of cyclic accelerators”.

17Thank You

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Extra Slides

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Schottky Signal Verification• Verified that peak in PSD was a Schottky signal
from two measurements with a single injectedbeam (no stacking).
• In first case, RF amplitude was reduced slowly.
• Ideally, this conserves phase space area.

• In second case, RF amplitude was suddenly
reduced, causing abrupt debunch.
• Phase space area is not conserved; frequency
spread is increased.

• Observed increased peak width when RF
amplitude was suddenly reduced.
• Conclude that this is a Schottky signal.

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FFA Beam Stacking Control• Momentum spread after stacking should
be minimised to reduce RF voltage
required for recapture.

Beam-2Beam-1• Optimum acceleration time was found bysweeping beam-2 acceleration time.
• Phase displacement of beam-1 was also
observed [15].

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