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ISIS-II Working Group Report

Contents

1. Introduction .................................................................................................................................... 4

2. Stand-Alone Facility ........................................................................................................................ 6

3. Re-use of ISIS Infrastructure ......................................................................................................... 12

4. Compact Neutron Source .............................................................................................................. 24

5. References .................................................................................................................................... 26

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Executive Summary

At the start of 2016 a new ISIS-II feasibility study was launched in order to refocus on facility

upgrades in light of the advent of ESS and new forecast scenarios for neutron provision in Europe.

An ISIS-II Working Group was set up consisting of ISIS experts on accelerators, targets, neutronics,

instrument science, detectors and engineering, in acknowledgement of the reality that any upgrade

must be envisaged as a facility upgrade, not simply an accelerator upgrade. The working group was

charged with looking at ‘short-pulse’ (< ~1 µs proton pulse) options for a new stand-alone facility, a

facility upgrade which reuses as much existing ISIS infrastructure as possible and compact neutron

sources.

Stand-Alone Facility

Assuming a green field site, full funding and two target stations from day one, the working group

was unanimous that the most attractive option would be something similar to what SNS will look like

after the proposed Second Target Station (STS) upgrade:

• 1.3 GeV proton beam at ~2.5 MW after Proton Power Upgrade (PPU).

• First Target Station (FTS) at 50 pps (nominal frame length 16.7 ms), ~2 MW.

• STS at 10 pps (nominal frame length 100 ms) , ~0.5 MW.

However, 40 pps (nominal frame length 20 ms) was felt to be better optimised for ISIS-II, and we

would expect to add a muon facility at the end of the linac and/or before the higher repetition rate

neutron target station.

Working group recommendations regarding a stand-alone facility are given at the end of Section 2.

Re-use of ISIS Infrastructure

The working group found that:

• It should be possible to upgrade ISIS TS-2 (still at 10 pps) to ~0.25 MW with a plate target

similar to that proposed for the TS-1 upgrade which is planned to go ahead in ~2020. All

flight lines would remain the same.

• A new TS-3 at 40 pps (eventually replacing TS-1) with a compact TRAM could operate

effectively as a high resolution target station and complement an upgraded TS-2. If the

nominal 1 MW proves to be too much power for a TRAM fully optimised for useful neutron

output we could operate at lower frequency or reduced proton pulse intensity – we should

design for operability rather than raw power.

• 1.2 GeV is the maximum beam energy that would allow re-use of the majority of the

components in the present extracted proton beam lines (EPB1 and EPB2).

• It should be possible to fit a suitable 1.2 GeV accelerator running at ~1.2 MW in the present

synchrotron hall, based on a rapid cycling synchrotron, an accumulator ring or an FFAG.

• A staged approach should allow us to keep the ISIS science programme running as much as

possible during ISIS-II build and minimises beam off time to any one target.

• The overall energy use, efficiency and environmental impact of the facility will be of very

high importance.

• Highly optimised muon production should be possible at ~550 MeV by extracting a beam

from part way along the linac.

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The proposed accelerator specification is 1.2 GeV, ~1.25 MW, 50 Hz (but flexible frequency may

present some advantages) with a < ~1 µs pulse train.

Working group recommendations regarding re-use of ISIS infrastructure are given at the end of

Section 3.

Compact Neutron Sources

Pulse compression to produce a ‘short pulse’ proton or deuteron compact accelerator-driven

neutron source (CANS) is impracticable at ~10 MeV. Hence current ‘short-pulse’ CANS are typically

driven by electron linacs, but these produce relatively low neutron fluxes. Laser driven sources

(being developed at the Central Laser Facility at RAL and elsewhere) produce short pulses, but

currently repetition rates are very low and the quality of the neutron pulses is nowhere near good

enough to do useful science.

Working group recommendations on compact neutron sources are given at the end of Section 4.

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1. Introduction

ISIS has been studying accelerator upgrades for many years, but at the start of 2016 a new ISIS-II

feasibility study was launched in order to refocus on facility upgrades in light of the advent of ESS

and new forecast scenarios for neutron provision in Europe. The recent ESFRI Physical Sciences and

Engineering Strategy Working Group – Neutron Landscape Group report ‘Neutron scattering facilities

in Europe: Present status and future perspectives’ clearly shows that, even with ESS working at full

capacity, neutron provision in Europe will be considerably less than at present, largely due to the

closure of a number of reactor-based facilities (figure 1.1). Evidently now is the right time to be

thinking about an ISIS facility upgrade which would be complementary to ESS and provide enhanced

neutron capacity in Europe beyond 2030.

Figure 1.1: Scenarios for neutron provision in Europe from 2015 to 2035 (ESFRI report p.76).

In January 2016 an ISIS-II Working Group was set up consisting of ISIS experts on accelerators,

targets, neutronics, instrument science, detectors and engineering, in acknowledgement of the

reality that any upgrade must be envisaged as a facility upgrade, not simply an accelerator upgrade.

Working group membership is shown in Table 1.1.

Accelerator

Alan Letchford

Shinji Machida

John Thomason (Chair)

Chris Warsop

Instruments1

Rob Bewley

Mario Campana (Secretary)

Adrian Hillier

Ron Smith

Target

David Jenkins

Detectors

Davide Raspino

Neutronics

Steve Lilley

Engineering

Steve Jago

Table 1.1: Working group membership.

1 Rob Dalgliesh also attended two meetings, providing updates on laser driven neutron sources and

perspective on the proposed recommendations.

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The working group was charged with looking at ‘short-pulse’ (< ~1 µs proton pulse) options for:

• A new stand-alone facility.

• A facility upgrade which reuses as much existing ISIS infrastructure as possible.

• Compact neutron sources.

Multiple day-one target stations, variety of repetition rates, Fixed Field Alternating Gradient (FFAG)

accelerator options and muon production were all important topics of discussion. Ten working group

meetings were held during 2016, each addressing a specific topic (table 1.2). Minutes, presentations

and other supporting material from all the meetings can be accessed here.

Meeting date Meeting topic

03/02/16 Introduction from ISIS Director, scene setting, review

of previous work.

01/03/16 Short presentations from each working group

member.

04/04/16 ‘Ideal instrument suite’.

28/04/16 Moderators for the ‘ideal instrument suite’.

24/05/16 Accelerator options using present ISIS infrastructure.

25/07/16 Accelerator and target options at 1.2 GeV, FFAG

magnets.

22/08/16 1.2 GeV in present ISIS synchrotron hall, TS-2 upgrade

power limit, muon production at end of linac.

03/10/16 Specification of TS-3, compact neutron sources.

27/10/16 Power limit for upgraded TS-2, laser driven neutron

sources.

15/11/16 TS-3 frequency, review of recommendations.

Table 1.2: Working group meeting topics.

The working group presented its recommendations to the ISIS Scientific Advisory Committee and ISIS

Facility Board at the end of November 2016. These were well received and the working group is now

tasked with production of an R&D plan which will lead to a full Technical Design Report by ~2025, in

preparation for a new facility which could be delivered by ~2030. This R&D plan will be presented to

the ISIS Facility Board, and will also provide input to the current STFC Accelerator Strategy Review.

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2. Stand-Alone Facility

Overview

The model of two target stations optimised for different classes of instrumentation pioneered by ISIS

is now a recognised model for advanced spallation sources. The SNS, JPARC and CSNS are all

planning to build second target stations at lower repetition rates to provide optimised

instrumentation across the broadest possible range of science. The model is based on two general

features of neutron instrumentation:

• High resolution diffraction, total scattering, quasi-elastic scattering and high energy inelastic

scattering require high wavelength resolution and high peak intensity at shorter neutron

wavelengths (<0.6 nm). This is best supported by a high frequency target (~50 pulses per

second (pps)) with poisoned moderators.

• Other techniques such as SANS, reflectometry, magnetic diffraction, imaging, low energy

spectroscopy and broad bandwidth total scattering all require large simultaneous bandwidth

but can tolerate lower wavelength resolution. These techniques are naturally suited to lower

rep rate sources (~10 - 20 pps) with fully coupled moderators to maximise flux. In addition

the lower rep rate allows the more extensive use of guides and other optics and allows for

more shielding, thus significantly increasing signal to noise.

For muon production JPARC already have an operating muon facility and both the SNS and CSNS

(along with FERMILAB and RAON, South Korea) are considering possible muon facilities.

Figure 2.1: Schematic of the SNS with the proposed Second Target Station in the right foreground.

Assuming a green field site, full funding and two target stations from day one, the working group

was unanimous that the most attractive option would be something similar to what SNS will look like

after the proposed Second Target Station (STS) upgrade (figure 2.1):

• 1.3 GeV proton beam at ~2.5 MW after Proton Power Upgrade (PPU).

• First Target Station (FTS) at 50 pps (nominal frame length 16.7 ms), ~2 MW.

• STS at 10 pps (nominal frame length 100 ms) , ~0.5 MW.

However, 40 pps (nominal frame length 20 ms) was felt to be better optimised for ISIS-II, and we

would expect to add a muon facility at the end of the linac and/or before the higher repetition rate

neutron target station.

Accelerator Considerations

The key accelerator issues for the optimal, updated design of a new stand-alone facility will be

addressed in the extensive studies for the options re-using existing ISIS infrastructure outlined in

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Section 3 below. However, once the key options and best designs for the ‘re-use’ scenario have been

established, the same ideas should be applied to the less constrained stand-alone case – in particular

in evaluating the benefits of removing limitations on ring size. It is expected that the main design

choices will be very similar for both cases, e.g. in choice of linac energy and ring configuration as

deciding factors of beam loss, cost, fulfilment user demands, etc.

Neutron Target, Reflector and Moderator Considerations

The maximum facility power will probably be determined by target capability, operability and useful

neutron output rather than accelerator design and could be scaled up or down depending on

operational experience running the SNS FTS at 2 MW post PPU and/or overall cost envelope.

In reality, of course, maximum facility power must consider not just the target, but the complete

package of the target, reflector and moderator (TRAM), and these key elements need to be

considered in conjunction with wider facility constraints such as extracted proton beam windows,

TRAM containment vessels and the surrounding biological shielding monolith. These wider

constraints will often play a major defining role, particularly when existing facilities are being

upgraded.

A stand-alone, short-pulse facility at > 2.5 MW is probably feasible, but the scale and cost of the

engineering solutions required for a robust and reliable TRAM and supporting infrastructure may

well outweigh any advantage from increased neutron flux. The handling of TRAM components which

will have relatively short operational lifetimes due to radiation damage in the beam will inevitably

impact heavily on instrument operational cycles. The additional infrastructure and the impact on the

moderator design will affect the ability to couple the target and moderators closely, which will

reduce the efficiency of turning protons into ‘useful neutrons’ and will also generate additional

backgrounds.

Figure 2.2: Inputs and drivers for TRAM specification.

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When considering the maximum power of a new stand-alone facility we will assume the TRAM to be

the defining factor and therefore it is useful to review some of the major inputs and drivers in its

specification. Figure 2.2 and the following commentary go some way towards showing the

interaction of these inputs in delivering an efficient and robust neutron producing TRAM.

a) Beam power and intensity

Beam power and intensity drive heat load extraction and stresses directly. They also dictate the size

and shape of the beam profile at the target and at a fundamental level the stopping distance

required by the target. In addition, pulsed beams of low repetition rate bring additional issues of

cyclic loading and also potential early life fatigue in systems.

b) Geometry

A solid target gives optimal neutron production, but we must remember that cooling and stress in

the target material will ultimately limit capability. Beyond this we must consider moving towards

segmented plate targets, ‘rotating wheel’ solid or liquid mercury targets. Optimal neutron

production demands moderators and reflector as close in to the target as possible and current

experience suggests an optimal target size of 60 to 100 mm thickness or diameter for typical beam

profiles, moderator size and geometry. Equally, the moderators, generally in ‘wing geometry’ and

with water pre-moderators where needed, need to be closely coupled to the target to gain optimal

neutron capture and moderation. The reflector, generally fabricated from solid edge cooled

beryllium, must also fit snuggly around the whole assembly to give optimal efficiency.

c) Cooling

TRAM cooling requirements are often intense and even at the relatively modest levels currently

required for ISIS (10s to 100s of kW) effective cooling is vital. In higher powered facilities operating

now or in the construction phase several MW of cooling will be required.

ISIS has a well-developed set of operating limits based on sound experience and experiments and

generally current water cooled systems can readily cope with this type of loading and are based on

well-established industry standards for this type of system. In addition, surface heat transfer is

critical and is well understood for current levels of operation with, for instance, tantalum-clad

tungsten targets. We particularly watch critical heat flux and burn out, especially with water. The key

design and operational parameter here is to operate well below the regime where coolant ‘burn out’

can occur. We also need to be aware of target decay heat. Even without beam to target radioactive

decay heat, especially in tantalum cladding, can be a major issue if cooling fails suddenly. Robust

safety measures are required to mitigate against this rare, but potentially severe, problem.

Eventually solid targets are no longer practicable and some segmentation of the target material is

required for sensible operation and inevitably with even higher beam powers or intensities a point

will be reached where fixed solid segmented targets are not practicable from a cooling point of view.

‘Rotating wheel’ solid targets or liquid metal targets (for instance mercury as currently run at SNS

and JPARC) must then be considered. These types of target bring their own additional issues, for

instance beam induced ‘shock’ causing cavitation in mercury leading to erosion of stainless steel

containment vessels as seen at both SNS and JPARC. Rotating targets and even fixed targets

operating at low repetition rate (< ~10 pps) bring another set of issues related to cyclic loading and

fatigue where efficient cooling can cause large thermally induced stresses. In such cases CW

operation for the target might be preferred, but this is unlikely to be optimal for neutron production.

A further constraint may be that a host country’s legal requirements preclude using materials such

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as mercury. This is the case in Sweden where a solid rotating target has been developed as a solution

to the high power requirements of the ESS facility and we would be wise to bear this in mind when

contemplating such a facility in the UK.

At increased beam powers different cooling methods may need to be considered, for instance gas

cooling as for the ESS bare tungsten target or even radiation cooling. Interestingly, tungsten

becomes more robust in operation at higher temperatures, but ISIS (along with many other similar

facilities) runs with it in its lower temperature brittle state for the very good reason that we must

always take into account the close proximity of low melting point materials like the aluminium alloys

in the moderators which are close to the target. The cooling of pre-moderators, moderators and

reflectors is relatively straightforward as the power levels are relatively low. However, here more

subtle neutronic requirements drive the cooling requirements, especially bearing in mind the need

to work with potentially explosive cryogenic fluids.

d) Stresses

Thermo-mechanically induced stresses are well established and understood for the un-irradiated

materials that ISIS [Fletcher 2013] and other facilities currently use (tungsten, tantalum, aluminium

alloys, beryllium etc.), but cyclic loading and fatigue effects are less well understood, especially for

important materials like tantalum cladding. However, operational experience with the current ISIS

TS-1 target shows that in our current stress regime this cladding works for many millions of beam

pulse cycles and also for trips, which are often the most significant stressing situation over a typical

target lifetime of several years. Again, perhaps a CW beam such as that at operated at the PSI SINQ

would be better than pulsed operation if stresses were the sole consideration.

In addition it is important to remember that over an extended operational life and with high beam

power or intensity the acceptable stress limits of TRAM components are potentially degraded by

radiation damage embrittlement too. Unfortunately this is an area where limited reliable

information on the mechanical properties of irradiated materials is available.

e) Materials

Acceptable material types are very well established for the present ISIS facility. For targets these are

commonly high Z materials like tungsten and tantalum, but low Z materials like beryllium could be

used where appropriate and perhaps other high Z materials like lead or molybdenum or even

mercury could be employed in the future. For moderators, aluminium and aluminium alloys are

generally required for optimal neutronic and cryogenic performance and where required, neutronic

poisoning with gadolinium and decoupling with cadmium and boron is also employed. For reflectors,

edge cooled beryllium is the first, albeit expensive, choice. However, nickel or lead or mixtures

might be considered, although care needs to be taken as issues of weight and creep at elevated

temperatures can be a problem.

f) Radiation damage

Radiation damage brings with it a reduction in the TRAM component equipment robustness and

longevity through embrittlement and transmutation. There is limited information available on these

effects in TRAM components, but ISIS and a few similar facilities do have some relevant operational

experience. There is also a reasonable body of information for aluminium alloys irradiated in

reactors which goes some way towards defining limits for moderators and a lesser set for beryllium

which helps with reflectors. The difficulty is always where we have to extrapolate and infer

outcomes from our current position of understanding. Ultimately we have to be somewhat

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conservative in our approach and make sensible decisions on adopting reduced stress levels or

lifetime expectations and prepare accordingly. All opportunities to carry out post irradiation

examination of spent ISIS TRAM materials should be grasped wherever possible, but unfortunately

this can be prohibitively expensive.

g) Manufacturability

Manufacturability of TRAM components is very important as, however good the concept or design

for a novel TRAM is, we have to ensure that its components can be made and made to a repeatable

quality and quantity by existing manufacturing routes. Often it is important to assure the ongoing

provision of specialist manufacturing techniques even to the extent of taking on those activities in-

house when necessary to maintain availability. The availability of materials of the right quality is an

extremely important factor and our limited ability to influence suppliers with our relatively small and

infrequent demand patterns must be recognised. These materials are often very expensive and

command a premium from suppliers.

We should work to develop sound manufacturing routes using established techniques wherever

possible. For instance at ISIS we have recently invested a significant amount of resource and money

in perfecting the EB welding of tantalum. Ultimately there is no substitute for having a dynamic R&D

programme for many of these components, especially targets, as this is vital to delivering sound

engineering solutions.

h) Operation

In the end this is what we are judged on and we must do this reliably and safely. Users expect the

reliable availability of a facility because they are constrained to limited and pre-booked ‘access

windows’ for their experiments. In turn we work towards long lifetimes for TRAM components to

minimise time spent by operations replacing them with new ones. An ideal time frame for TRAM

components is 5 years, but increasing beam power makes this goal extremely hard to achieve and so

it must always be sensibly reviewed and balanced against any drive to higher powers.

Remote handling must be straightforward as this is an expensive and time consuming operation and

eventual disposal routes for materials/components need to be understood, agreed and as well

established as possible before design and operation of any novel TRAM components. This is not

always straightforward as is evidenced by most facilities operating today having legacy materials and

components held pending the availability of suitable disposal routes (e.g. for beryllium from

reflectors or aluminium alloy moderator shells with hydrogenous moderating materials inside them).

The regulatory framework that we work with in this field is developing all of the time with strict and

limited accumulation times for active components and national environment agencies setting limits

on discharges of radioactive gases, fluids and solids from any facility.

Muon Target Considerations

The expectation is that in any facility development where the muon target remains situated in the

extracted proton beam line running from the accelerator to the higher repetition rate target station

the existing target arrangement of an edge and surface cooled graphite blade of an appropriate size

to suit the incident proton beam would be used. If there is a significant increase in beam power or

intensity then we may well need to consider a rotating target similar to those employed in facilities

such as PSI and JPARC.

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Existing edge cooling with demineralised water and radiative cooling to the surrounding target

vessel are well understood at the current levels of operation. However, any significant increase in

beam power would require the investment in engineering development of a suitably robust solution.

This would look at acceptable thermo-mechanical stresses for graphite in whatever new geometry

was required for the higher power operation. It would also look at the effect any increase in power

may have on radiation damage. It is fortunate that there is a reasonably well established body of

knowledge on this material available from existing muon facilities and in the nuclear fission world.

In any case this will need to be considered carefully to make sure we still maintain a robust

operations regime at least equivalent to that experienced with the current ISIS muon target.

Any novel design will also require a proven route to be established for manufacture and operational

considerations may well dictate the redesign of remote handling facilities and flasking to cope with a

higher radiation level on spent targets. If it is decided that a separate proton beam line is to be

extracted to the new muon target a similar engineering development programme would be

envisaged, but perhaps with broader scope to adopt technologies proven at other facilities such as

PSI or JPARC and to develop an appropriate end station and beam stop.

Recommendations

1. Keep accelerator design on ‘back burner’ as most of the issues and design choices are the

same as those for ‘re-use of ISIS infrastructure’ scenarios.

2. Keep a watching brief on SNS FTS mercury target performance post PPU and STS ‘rotating

wheel’ target development.

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3. Re-use of ISIS Infrastructure

Overview

The present ISIS facility is based around an 800 MeV accelerator system producing up to 0.2 MW of

beam power in intense pulses of protons 50 times a second. The two ISIS target stations (TS-1 and

TS-2) use the high energy protons produced by the ISIS accelerator to generate neutrons by the

spallation process and the neutrons are then moderated to make them usable for neutron scattering

experiments. Muons are also produced using a thin carbon target located directly in the proton

beam upstream of the TS-1 neutron target. In addition, ISIS is host to MICE, the Muon Ionisation

Cooling Experiment. A schematic layout of ISIS is shown in figure 3.1a.

Figure 3.1: a. Schematic layout of the ISIS Facility, b. Schematic of principal ISIS buildings.

ISIS is housed in a number of buildings, many of which have been built specifically for accelerator

and target components and include shielding and other appropriate infrastructure which could be

re-used in order to minimise the cost of a new facility. Support buildings provide space for

accommodation of staff, power supply systems, cooling plant, workshop facilities, spare component

storage, test stands and handling of active waste. Some of the principal ISIS buildings are shown in

figure 3.1b. In addition the RAL Site provides suitable electrical and other infrastructure necessary to

operate a large facility.

Some of the topics of particular interest for a feasibility study of re-use of ISIS infrastructure are

listed below (and clearly many of these are highly inter-dependent):

• The maximum beam power possible to an upgraded TS-2.

• What a TS-3 would look like and how this would eventually replace TS-1.

• The maximum beam energy that could be transported re-using the majority of the

components in the present EPB1 and EPB2 beamlines.

• Whether the present synchrotron hall (which was originally built for the old 7 GeV proton

synchrotron Nimrod) could be re-used for a new ring and whether the constraints of the hall

limit the type of accelerator that could be used.

• What the parameters of a new linac would be and where could it be built.

• How to maintain and optimise muon provision in the new facility.

• How a staged approach can be optimised to minimise cost and/or downtime.

a. b.

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In anticipation of some of the likely results of the feasibility study space has already been reserved

on the RAL Site Development Masterplan for a new high-power linac and a TS-3, as shown in figure

3.2.

Figure 3.2: RAL Site Development Masterplan with ISIS shown at the left. Grey denotes existing buildings

and red denotes plots reserved for future developments.

Findings from the Working Group

a) It should be possible to upgrade ISIS TS-2 (still at 10 pps) to ~0.25 MW with a plate target similar

to that proposed for the TS-1 upgrade which is planned to go ahead in ~2020. All flight lines would

remain the same.

We estimate that ~0.25 MW is possible based on recent work performed by SNS for their Second

Target Station with a plate target (as used for ISIS TS-1) designed for 0.5 MW at 20 pps. SNS chose

not to adopt this design because the calculated decay heat was considered to be higher than

acceptable at that power, particularly in the tantalum cladding. However, recent measurements of

decay heat at ISIS TS-1 suggest that it is significantly below the limit at the current power of 128 kW.

This gives some confidence that a 0.25 MW plate style target at 10 pps is feasible.

However, there are things we need to consider carefully if we upgrade TS-2. The existing proton

beam window (figure 3.3) is rated for 800 MeV and 60 µA operation and any increase in beam

current and power would require (all costs are estimates based on 2017 values):

• The redesign of the proton beam window to cope with this at an estimated cost of ~£500k.

• That any aperture size increase would be very limited by the existing monolith and void

vessel infra-structure.

• The addition of an active cooling system (which would not be easy to retro-fit) with cost

estimate of ~£500k.

• Some consideration of cooling systems other than water, e.g. helium gas.

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Figure 3.3: Images of the existing proton beam window and its situation

within the TS-2 void vessel.

The existing TS-2 TRAM (figure 3.4) is rated for 800 MeV and 40 µA (target limit) operation. Any

increase in beam current and power would require:

• A new target design with services, involving a design, procurement, build and commissioning

project of ~£1M:

– The target would in all probability still be cylindrical and the same diameter as the

current one although an oval cross-section might be considered too. This is driven by the

current instrument moderator views.

– The current instrument views onto the moderators are fixed by the existing ‘hard’

infrastructure.

– The target would probably be divided or segmented into plates to maintain our cooling

and thermo-mechanical stress limits.

• A new reflector to accommodate the new target and moderators:

− Beryllium machining and fabrication prices are rising, so the current estimate of ~£3M

may prove to be low.

− We probably have enough capacity in the existing water cooling services to cope with

the rise to 0.25 MW already.

− But certainly new pre-moderators to accommodate higher power and a new target

would be required at an estimate of ~£500k.

Figure 3.4: Image of the ISIS TS-2 TRAM illustrating the closeness of the existing moderators to the target.

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If the current moderator configuration is still in place before the upgrade to higher power then any

increase in beam current and power would require a new solid methane moderator design:

• The cooling and processing of current solid methane is already on the limit of viability, so a

new moderator heat exchanger to increase cooling power would be required at a cost

estimate of ~£500k.

• Alternatively we could develop a mesitylene based (1,3,5-trimethylbenzene) moderator at

an estimate of ~£1M.

• A third option would be to consider a liquid methane moderator as on TS-1. This would

require case a new cold box at an estimate of ~£2M. A similar cost would be incurred for a

switch to liquid hydrogen.

It is likely that in order to maintain the current instrument flight lines and to optimise the neutron

output the upgrade moderators may be pancake style moderators.

In summary, if we were to upgrade the existing TS-2 to ~0.25MW, the following would be required:

• A full science and neutronic evaluation and design.

• A new proton beam window with active cooling.

• A new cylindrical plate target cooled with heavy water but of the same overall depth or

diameter as the current target (~60 mm).

• A new reflector and pre-moderators to suit the new target geometry but match the existing

instrument infra-structure.

• A new solid methane moderator or alternatively a moderating material like mesitylene or

liquid hydrogen if neutronically acceptable.

• Full services upgrade to match the new TRAM.

• Full programme to dispose of existing components when they are removed.

• Alignment of new TRAM components with existing or planned disposal routes.

This could be done at a cost estimated at between ~£15M and ~£25M and would be similar in some

ways to the current TS-1 project and with a one year installation and commissioning timescale to

match. A longer timescale could be beneficial, but currently one year of downtime is seen as the

maximum acceptable for this type of project.

It should be noted that with a significant step change in the neutron flux existing TS-2 instruments

may need to be upgraded or replaced to optimise for the new power and configuration. This work

could include; new choppers (thicker discs may be needed and additional choppers may be needed

such as frame overlap choppers), additional shielding (beam stops and beam line shielding are all

designed based on current power levels) and changing the source to sample position.

b) A new TS-3 at 40 pps (eventually replacing TS-1) with a compact TRAM could operate effectively as

a high resolution target station and complement an upgraded TS-2. If the nominal 1 MW proves to

be too much power for a TRAM fully optimised for useful neutron output we could operate at lower

frequency or reduced proton pulse intensity – we should design for operability rather than raw

power.

The option of building an ISIS TS-3 and re-using the existing ISIS infrastructure would require the

continuous running of all 3 target stations for a reasonably long period in order to avoid irreparably

damaging the user program. TS-1 currently supports 20 instruments on which the vast majority of

experiments are short (2-4) days – the science carried out is generally high throughput as well as

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high impact. Furthermore this community is very large and international and is responsible for a

large proportion of ISIS output. The loss of the facility for an extended period (3-4 years) will result in

a significant loss in the continuous transfer of expertise and training within the community. It would

take significantly longer than 3-4 years to recover, rebuild and retrain the user base if this were to

happen.

Whether this is possible will be critically determined by the choice of accelerator technology. Two of

these target stations will take 40/50 pps while the existing TS-2 receives 10 pps, requiring an

accelerator capable of operating at ~100 Hz. An accumulator ring or FFAG solution will allow higher

rep rates while an RCS solution is unlikely to be able to accommodate a high enough frequency to

support sufficient pulses per second.

c) 1.2 GeV is the maximum beam energy that would allow re-use of the majority of the components

in the present extracted proton beam lines (EPB1 and EPB2).

If the ISIS facility is upgraded with a higher energy accelerator to provide greater power to new

targets, it would be advantageous to have to change as little as possible of the current facility. It is

easy to imagine the target stations would remain in the same locations as they are currently with

any new accelerator being housed either in, or near to, the current synchrotron hall. A short study

was carried out to look at how much of the two current transfer lines from the synchrotron hall to

the two target stations could be re-used if the energy of the new accelerator was chosen to be

1.2 GeV.

The top energy of the ISIS accelerator is currently 800 MeV. Increasing this energy to 1.2 GeV,

increases the momentum of the beam by around 30%. Simplistically, an identical optic in each

transfer line can be reproduced at 1.2 GeV by scaling the ampere turns of each magnet in the

beamline by the same 30%. Increasing the current in each magnet by 30% increases the power

dissipated in each magnet by around 70%. However, since the TS-2 magnets were originally designed

with a margin included and most of the magnets are operating some way below this maximum

current, it is felt that most of these magnets would perform adequately at this increased power

level. Similarly, most of the magnets in the TS-1 EPB are currently being used well below their

maximum design capabilities. The notable and obvious exceptions are the bending magnets. Some

of the dipoles currently operate with a field in the gap of around 1.4 T. Pushing the field in the gap

close to 2 T, although possible, would give a very non-linear and inefficient magnet.

At first cut, therefore, it looks possible to run a 1.2 GeV beam down both EPBs without changing the

magnets. However, a number of other issues should also be considered. Firstly, the location of the

extraction point in the new accelerator is likely to be different from that in the current accelerator.

In addition, it is unlikely that the beam Twiss parameters will be identical at the extraction point.

Therefore it is almost certain that the first part of the transfer line would need to be re-designed,

meaning it is less certain that magnets could be re-used and diminishing the benefit of re-using them

in the first place. The same argument could be made at the target end of each transfer line. Since it

is possible that new targets would have different requirements from the proton beam, there is a

good chance that the end of each transfer line would need to be re-designed. Secondly, the size of

the beam (a function of both the optic and the emittance of the extracted beam) is also an

important factor to consider. If it is too large it would be impossible to re-use the existing magnets.

If it is too small the system would be extremely inefficient. There are also many issues around the

peripheral magnet equipment. With the expected increase in power, water systems would need to

be upgraded, power supplies uprated, cabling changed, etc.

17

d) It should be possible to fit a suitable 1.2 GeV accelerator running at ~1.2 MW in the present

synchrotron hall, based on a rapid cycling synchrotron, an accumulator ring or an FFAG.

The existing ISIS synchrotron hall is large enough to accommodate a 1.2 GeV accelerator ring

providing beam powers of ~1.2 MW and probably considerably more. The ISIS hall, having been

designed originally for the much bigger Nimrod synchrotron, has considerable free space and allows

for options such as multiple stacked rings, or for overlap between operation of the existing ring and

installation of major upgrades to minimise down time.

The design of an accelerator ring providing these power levels is well within established technology –

the challenge is to determine the optimal design in terms of capital and operation costs as well as

performance and flexibility. Options under consideration are an accumulator ring, a rapid cycling

synchrotron or the novel new idea of a fixed field alternating gradient (FFAG) accelerator and will be

discussed in detail later in this chapter.

e) A staged approach should allow us to keep the ISIS science programme running as much as

possible during ISIS-II build and minimises beam off time to any one target.

This is however highly dependent on choices of accelerator architecture (and consideration of an

optimal staged approach may indeed act as a driver for some of the choices to be made).

f) The overall energy use, efficiency and environmental impact of the facility will be of very high

importance.

It is likely that funding for an upgraded facility will become increasingly dependent on our having

demonstrated that we have considered all the possible options.

Muon Production

Currently at ISIS the muon production target works via transmission mode and the same primary

proton beam continues on to the main TS-1 neutron production target. Figure 3.1 shows the present

layout of muon and neutron instruments. The intermediate graphite target only intercepts some 3%

of the proton beam. Nevertheless, the production conditions are so favourable and the detector

arrays so efficient that the source serves 7 µSR instruments and experimental areas, giving some of

the highest muon polarisation detection rates currently available worldwide. This should soon be

challenged by JPARC.

The nature of the pion decay is such that muons emerging in a given direction have their spins fully

aligned – a spin polarised beam.

The pions are produced via the relation

p + p →π± + p + n

and the pion rapidly decays into a muon, via the two body decay π± → µ

± + ν

The lifetime of the muon is 2.2 µs and during an experiment we would typically measure out to

32 µs. Therefore, this leads us to the first preferred design parameter – ideally, the next muon

source at ISIS would have a higher repetition rate of 1/32 µs i.e. 30 kHz.

18

Now let us consider the pulse structure itself. In our experiments we are measuring the muon

precession which is clearly dependent on timing resolution. Therefore, pulsed sources, such as ISIS

and J-PARC, are excellent at measuring ‘slow’ relaxation and have no fundamental data rate limit.

Fast relaxation and/or high precession frequencies are difficult to measure and often require

additional experiments at a continuous source such as PSI or TRIUMF. As a result, the pulse structure

should be as short as possible – a few ns would be ideal.

Figure 3.5: Raw muon production for a given proton beam energy [Bungau 2013].

If we now consider the proton beam energy, we can see from figure 3.5 that the production yield is

proton energy dependent with a peak at 550 MeV and a minimum at around 1 GeV; once above

~1.75 GeV the rate exceeds that of 550 MeV, plateauing for energies greater than 2 GeV. Hence, the

second preferred design parameter is to access the proton beam at an optimum energy, such as

550 MeV or greater than 2 GeV.

Figure 3.6: The thickness dependence of the muon yield [Bungau 2013].

19

This leads to the third design parameter – figure 3.6 clearly shows the interaction length

dependence of muon production, so a thick target is desirable.

So considering all these design parameters, there are two options for the location of the muon

target at ISIS.

Firstly, we could continue with the current ISIS model of an intermediate target in front of the

neutron target. This would give a similar beam characteristic (with respect to frequency response

and data rate) to the current instrument suite, but given an increase in proton energy by a new

accelerator and the same repetition rate an increase in flux by a factor of 2.

Secondly, we could consider making use of the ‘spare’ capacity of the linac. If this linac were running

at 550 MeV (peak muon flux production) then we could make use of the proton beam when it is not

feeding the accelerator and divert the beam to a dedicated muon target area. This could have a

number of targets optimised for maximum muon production and has the advantage of increased

repetition rate (better suited to the timescales of the muon lifetime), short pulse structures (better

suited to the gyromagnetic ratio of the muon) and increased target thicknesses (increased muon

production). This would make a very impressive muon source. An additional benefit is that the

extracted proton beam to the neutron target would no longer be disturbed, minimising losses, and

crucially, the muon target would no longer present constraints on the pulse length to the neutron

target. Fermilab and the SNS are both considering this type of design.

Such a design would make a formidable target for a formidable suite of instruments and place ISIS-II

in a leading role in creation of the next generation of muon facilities.

Highly optimised muon production should be possible at ~550 MeV by extracting a beam from part

way along the 800 MeV linac, which is probably the most likely design energy. Because the linac duty

factor is only about 5% for neutron production, the other 95% of the time between pulses could be

dedicated to 'CW' muon production. This idea is illustrated in figure 3.7.

Figure 3.7: A close to CW beam for muon production interleaved with the neutron production pulses.

Neutron production only With CW muon production

Number of RF klystrons 30 x 3 MW 66 x 1 MW + 8 x 3 MW

Cooling water requirement 3.7 MW 30 MW

SC cavity cryogenic load 1.0 13.0

RF system electrical load 6 MW 33 MW

Table 3.1: The additional RF requirements of running the linac in CW mode for muon production.

20

Assuming the linac design remains unchanged and a beam current of 1 mA, with a suitable chopped

time structure, is accelerated to 550 MeV the cost would be significant as shown in table 3.1.

In reality it would be very difficult to operate the room temperature part of the linac (up to

180 MeV) in CW mode due to excessive cavity RF losses. It would be necessary to redesign the warm

linac, limiting the accelerating gradient to reduce cavity heating. Table 3.2 shows the cost for two RF

loss limits.

50 kW/m 25 kW/m

Linac length (from 244 m) 310 m 372 m

Number of RF klystrons 65 x 1 MW + 8 x 3 MW 60 x 1 MW + 8 x 3 MW

Cooling water requirement 18 MW 14 MW

SC cavity cryogenic load 13.0 13.0

RF system electrical load 21 MW 17 MW

Table 3.2: The additional RF requirements of a modified warm linac.

Significant savings in the RF power and non-cryogenic cooling requirements could be made by having

a superconducting linac from as low an energy as possible. This is the strategy adopted at PIP-II

[Holmes 2015] where the linac is superconducting from 3 MeV. Cryoplant requirements obviously

increase and it would require a complete redesign of the linac up to 180 MeV. Reduced running

costs are exchanged for increased construction costs but the increase is hard to quantify: the linac is

shorter, but the cost per metre is higher.

Accelerator Options

The proposed accelerator specification is 1.2 GeV, ~1.25 MW, 50 Hz (but flexible frequency may

present some advantages) with a < ~1 µs pulse train.

Figure 3.8: Schematic linac architecture.

Whichever upgrade path is pursued the linac options have been studied in some detail (figure 3.8).

The low energy part of the linac will be based on the Front End Test Stand already under

construction at RAL [Letchford 2015]. The 180 MeV room temperature and 800 MeV

superconducting linac designs are well developed [Plostinar 2015]. The feasibility of injection at

180 MeV into the existing RCS to achieve 0.5 MW has been demonstrated in several studies

including [Adams 2014], [Thomason 2013], [Jones 2012a], [Jones 2012b]. The superconducting linac

design is flexible enough that it could be used at 500 MeV to inject into a new FFAG, at 800 MeV to

inject into a new RCS or at 1.2 GeV for injection into an accumulator ring. The design is relatively

conservative and benefits from the work done on similar linacs at CERN, J-PARC and SNS making the

technological risk for any of the options relatively low.

21

The accelerator ring options are framed by reuse of the existing synchrotron hall and a final proton

energy of 1.2 GeV, with assumed beam powers of ~1.2 MW, and < ~1 μs beam pulse length at the

target.

Plausible options include accumulator rings (AR), rapid cycling synchrotrons (RCS) or fixed field

alternating gradient accelerators (FFAG). The choice of ring and its injection energy also determines

the size and cost of the linac. Determining the optimal choice, including key factors of capital cost,

operational cost and exploitation of existing infrastructure, is the objective of the current accelerator

physics design studies. The AR and RCS options are based on well-established principles, but require

optimising for “next generation” levels of cost and performance. The FFAG designs potentially offer

major advances in power and flexibility in beam pulse structure, but require practical

demonstrations of the technology to verify that they are a realistic option. In addition to design aims

on cost and performance, flexibility and potential for future expansion are also important factors.

a) Accumulator Ring or Rapid Cycling Synchrotron Options

Figure 3.9: ESS accumulator ring (Rutherford design).

Accelerator options at appropriate power levels based on AR and RCS designs are well established

and demonstrated at operational facilities around the world: SNS [Cousineau 2016] and JPARC

[Hotchi 2016] respectively. The optimal configuration for exploiting the ISIS synchrotron hall and

infrastructure is yet to be established, but designs from existing studies (see figure 3.9, [ESS 1996])

indicate a 1.2 GeV linac and accumulator ring should provide beams in the 1.2 MW regime and

probably significantly higher (certainly with a stacked ring option). Suitable ring designs require a

mean radius of ~26 m, intensities of ~1×1014

protons per pulse, low loss (<0.1%), highly optimised

multi-turn injection, and ample space for RF systems, extraction and collimation. Making use of

experience and simulation models benchmarked on the existing ISIS ring, detailed studies are now

underway to identify a new ring design that optimises the performance of the upgraded facility.

The optimal choice between an AR and RCS is not yet clear; some key considerations are

summarised in Table 3.3 (red text in the table indicates the more challenging areas that require R&D

work). An AR benefits from a much simpler design, with less challenging DC magnets, reduced RF

requirements, and simplification of many beam dynamics issues due to relatively few circulating

turns and no requirement for a fast energy ramp. The downside is the requirement for a full energy,

full power linac, which may add considerably to capital and operational costs. An RCS on the other

hand is more challenging to design, with pulsed magnets, much more demanding RF requirements

and tighter constraints on space and beam dynamics, but could reduce costs with a smaller linac

(with, for example, an energy of 800 MeV or even lower). A novel option for the accumulator ring,

using permanent magnets in place of conventional electromagnets, may allow for significantly

reduced power requirements and help offset the disadvantages of the larger linac.

22

Previous studies [Thomason 2013] have shown that an intermediate upgrade providing 0.5 MW

would be possible using a modified injection straight in the existing ring and an 180 MeV linac (which

could be the lower energy end of a later upgraded 1.2 GeV linac).

b) Fixed Field Alternating Gradient Accelerator Options

As a proton driver for a muon and neutron source, Fixed Field Alternating Gradient (FFAG)

accelerators present significant potential advantages over both the AR and RCS options. The DC

magnets make it easy to operate at relatively low cost. The freedom from the magnet ramping cycle

allows much more flexible time structuring of beam pulses, which are only determined by the linac

and RF in the ring. Whilst the magnets are fixed field an FFAG still provides acceleration and so a full

energy linac is not necessary which reduces the construction cost and footprint of the facility.

As far as the beam power is concerned, in all three accelerator options, the number of particles per

bunch or pulse is limited by space charge effects at injection and it is inversely proportional to β2γ3,

where β and γ are the beam speed normalised by the speed of light and the Lorentz factor

respectively. The total beam power in an FFAG can be increased by higher repetition rates (e.g.

100 Hz), which is not possible with an RCS. Note that although a high repetition rate of the

accelerator (e.g. more than 50 Hz) is not preferred by the neutron users, delivering proton pulses to

multiple target stations pulse by pulse reduces the repetition rate users will see and may be crucial if

we need to run the present TS-1, an upgraded TS-2 and a new TS-3 simultaneously until the TS-3

instrument suite is sufficiently mature to allow TS-1 to be decommissioned. The FFAG has potential

to stack several beam pulses at the outer orbit before extracting them thus introducing many new

options in pulse structure and intensity. This scheme needs further development, but could provide

high beam power with low repetition rate if there is a demand in the future.

The Main drawback of the FFAG option is a lack of operational experience as an accelerator for a

user facility. Despite early developments starting more than 50 years ago, R&D work of the FFAG has

an operational history of only around 15 years, which is relatively short in the accelerator

community. Two very successful proton FFAGs are running in the world (both in Japan), but they are

based in a university which means accelerator builders and beam users are not completely

separated. Nevertheless, its potential is huge and it is essential to undertake dedicated R&D

required to establish the full benefits an FFAG could provide as a proton driver for muon and

neutron production.

Recommendations

1. Keep development of RCS, accumulator ring and FFAG based designs active to the point

where we can make a well informed decision on which option to pursue based on technical

merit and lifetime cost.

2. The FFAG option will require R&D, with the initial proposal being the development of a

prototype magnet (and later an RF system). If this is successful then we will aim to

incorporate these as part of a small FFAG on the end of FETS in order to explore the beam

dynamics fully.

3. Ensure that the upgrade is optimised for neutron production, but with careful consideration

of muon production as well.

4. Pursue an appropriate development programme for a compact TRAM for TS-3, including

definition of suitable figures of merit for moderator output.

5. Continue to reserve the space on the RAL site for a new linac, TS-3 and possibly a new muon

target/instrument building.

6. Continue to explore staged upgrade scenarios in order to minimise cost and downtime at

each stage, feeding this information into the technical decision making process.

23

RCS FFAG AR

Extraction energy

(GeV)

1.2 1.2 1.2

Overall status of

technology

Established but a few

components need

attention.

Least conservative. Most conservative.

Beam power May need stacked

rings to get much

beyond 1 MW levels.

High repetition rate

easily gives above 1.2

MW.

Less challenging than

RCS to achieve above

1.2 MW, but foil may

be limit.

Injector linac 400 to 800 MeV 400 to 800 MeV 1.2 GeV

Ring footprint Existing ISIS Hall. Existing ISIS Hall. Existing ISIS Hall.

Linac footprint ~170-250 m ~170-250 m Longest ~317 m

Repetition rate Fixed at design, e.g.

50 Hz.

Flexible, could be

100 Hz or more.

Flexible, could be

100 Hz or more.

RF Harmonic number 1 to ~4 1 to ~4 1 to ~4

Pulse structure Total length ~1µs,

composed of h sub

pulses.

Total length ~1µs,

composed of h sub

pulses.

Total length ~1µs,

composed of h sub

pulses.

Bunch length A few 100 ns A few 100 ns A few 100 ns

Ring magnet size,

technology and

potential

Medium size, AC

ramped fields, normal

conducting, well

established.

Large size, DC fields,

could be permanent or

superconducting, more

complicated design.

Small size, DC fields,

could be permanent or

superconducting,

simple design options.

Ring magnet power

supply size and power

needs

Large powerful unit for

AC operation.

Small, lower power for

DC operation.

Smaller, lower power

for DC operation.

Ring radio frequency

systems

Larger RF system:

multiple cavities for

high voltage, with

variable frequency.

Larger RF system:

multiple cavities for

high voltage, with

variable frequency and

larger aperture.

Small RF system: fewer

cavities for lower

voltages, with fixed

frequency.

Vacuum vessel Medium sized

aperture, requires

ceramic vessel in

magnets.

Large aperture. Small aperture.

Collimation Established methods. Beam loss control

needs study.

Established methods,

simplest.

Beam dynamics Challenging at intensity

limit, but operationally

established. Loss

control required for

~10000 turns.

Challenging at intensity

limit and needs R&D.

Loss control required

for ~10000 turns.

Least challenging, loss

control required for

~1000 turns.

Table 3.3: Comparison of three accelerator options (red text in the table indicates the more challenging

areas that require R&D work).

24

4. Compact Neutron Source

Compact accelerator-driven neutron sources (CANS) produce neutrons using smaller and cheaper

accelerators below the spallation energy threshold. Such facilities exploit low-energy, neutron-

yielding reactions such as protons or deuterons on beryllium or lithium and can be considerably

more compact than spallation sources. This makes it feasible to use CANS in hospitals for e.g. cancer

therapy, in industry for specific material studies and imaging or in universities or smaller

laboratories.

A few decades ago CANS were all based on electrostatic accelerators. More recent CANS are typically

based on a proton or deuteron RFQ followed by a linac to ~10 MeV (but some also use cyclotrons)

and a low Z target to produce neutron pulses in the > 1 ms range, providing 1012

– 1015

neutrons per

second to the user. CANS can now satisfy the minimum requirements for broader user communities

in fields such as cancer therapy, neutron imaging, and even neutron scattering (where CANS could

be considered as an alternative to small reactor-based sources).

Pulse compression to produce a ‘short pulse’ proton or deuteron CANS is impracticable at ~10 MeV.

Hence current ‘short-pulse’ CANS are typically driven by electron linacs, but these produce relatively

low neutron fluxes. Laser driven sources (being developed at the Central Laser Facility at RAL and

elsewhere) produce short pulses, but currently repetition rates are very low and the quality of the

neutron pulses is nowhere near good enough to do useful science [Alejo 2016], [Kar 2016], [Brenner

2016].

There is already quite a large community in Europe and Japan under the umbrella of UCANS (Union

for Compact Accelerator-driven Neutron Sources), which held its sixth annual meeting in Xian 25-28

October 2016. Existing CANS are geographically well distributed but with very few existing in

European countries (figure 4.1). The importance of having such facilities readily available around

Europe is well recognized and several initiatives have already started to group user communities

geographically and assure easier access to funding.

Figure 4.1: Compact neutrons sources in Europe, both existing and under construction.

25

In Europe today, the RFQ based FRANZ in Frankfurt, Germany and LENOS in Legnaro, Italy are under

construction and other countries are planning for such facilities. Ambitious plans have been

proposed for the Jülich High Brilliance Neutron Source, with an RFQ and normal conducting linac

producing a deuteron beam at 25 MeV, 100 mA 4% duty cycle which delivers 100 kW to multiple

beryllium targets, each with one optimised moderator (figure 4.2). This will support up to 20

instruments and have a price tag of at least €200M.

Figure 4.2: Schematic of the proposed Jülich High Brilliance Neutron Source.

Recommendations

1. If ISIS has serious ambitions to become involved in the development of compact accelerator-

driven neutron sources a small working group should be set up to investigate current

worldwide capability and demand in order to determine how best to participate. Attendance

at the next UCANS meeting and other relevant conferences and workshops should be

ensured at an appropriate level.

2. Keep a watching brief on developments in laser driven neutron production in case of

anything game-changing.

26

5. References

[Fletcher 2013] M D Fletcher & D M Jenkins ‘ISIS TS1 Project Target Design Philosophy’ Ver. 3 (June

2013)

[Bungau 2013] A Bungau et al., ‘Simulations of Surface Muon Production in Graphite Targets’

Physical Review Special Topics - Accelerators and Beams, 014701 (2013)

[Holmes 2015] S Holmes et al., ‘PIP-II Status and Strategy’ IPAC 2015

[Letchford 2015] A Letchford et al., ‘Status of the RAL Front End Test Stand’ IPAC 2015

[Plostinar 2015] D. Plostinar, C. Prior & G. Rees, ‘Review of Linac Upgrade Options for the ISIS

Spallation Neutron Source’ IPAC 2015

[Adams 2014] D Adams et al., ‘Ring Simulation And Beam Dynamics Studies For ISIS Upgrades 0.5 to

10 MW’ HB2014

[Thomason 2013] J W G Thomason et al., ‘A 180 MeV Injection Upgrade Design for the ISIS

Synchrotron’ IPAC 2013

[Jones 2012a] B Jones et al., ‘A 180 MeV Injection System for the ISIS Synchrotron’ IPAC2012

[Jones 2012b] B Jones et al., ‘ Injection And Stripping Foil Studies For A 180 Mev Injection Upgrade

At ISIS’ HB2012

[Cousineau 2016] S M Cousineau, ‘Fifteen Year Perspective on the Design and Performance of the

SNS Accelerator’ HB2016

[Hotchi 2016] H Hotchi, et al., ‘The path to 1 MW: Beam Loss Control in the J-PARC 3-GeV RCS’

HB2016

[ESS 1996] The ESS Study Group, ESS Technical Study, Volume III, ESS-96-53-M, (November 1996)

[Alejo 2016] A Alejo et al., ‘Numerical Study of Neutron Divergence in a Beam-fusion Scenario

Employing Laser Driven Ions’ NIMA 829 (2016)

[Kar 2016] S Kar et al., ‘Beamed Neutron Emission Driven by Laser Accelerated Light Ions’ NJP 18

(2016)

[Brenner 2016] C M Brenner et al., ‘Laser-driven X-ray and Neutron Source Development for

Industrial Applications of Plasma Accelerators’ PPCF 58 (2016)