Main areas of work are:

Transverse Space Charge

The action of transverse space charge and resulting beam loss is a key intensity limitation and research topic on lower energy hadron rings.

Experimental studies of beam loss as a function of location in the betatron tune plane, see Figure 1 [1], are giving valuable information on the field non-linearities present in the ISIS ring. These are allowing detailed studies of resonance effects as a function of intensity, for linear quadrupole, and non-linear sextupole and octupole terms. Detailed studies in coasting beam mode, with fast profile measurements [see ISIS Synchrotron Project and Development section] and comparison with simulations is helping an improved understanding of the relative importance of coherent and incoherent effects. Current work builds on previous studies of half integer loss [2], and detailed studies of image effects [3].

Understanding and optimising injection into high intensity rings is critical to upgrade designs. Simulation studies are under way to achieve the best multi-turn injection painting schemes that minimise loss and preserve foil lifetimes. Comparison of simulation results with theoretical predictions from single particle and collective models is helping guide optimal configurations of horizontal and vertical painting, and in understanding the influence of tunes and space charge coupling. [4]

Figure 1: Measurements of Loss vs Betatron Tune on the ISIS Ring, with Associated Resonance Lines

[1] P T Griffin-Hicks et al, Experimental Measurements of Resonances near to the ISIS Working Point,

http://accelconf.web.cern.ch/ipac2018/papers/tupal054.pdf

[2] C M Warsop et al, Simple Models for Beam Loss Near the Half Integer Resonance with Space Charge http://jacow.org/hb2016/papers/mopr030.pdf;

C M Warsop et al, Studies of Loss Mechanisms associated with the half integer limit on the ISIS Ring, http://jacow.org/HB2014/papers/mopab40.pdf

[3] Ben Pine, DPhil Thesis, Oxford University, 2016, "Space Charge Induced Beam Loss on a High Intensity Proton Synchrotron"

B G Pine et al, High Order Image Terms and Harmonic Closed Orbits at the ISIS Synchrotron http://jacow.org/ipac2018/papers/tupal056.pdf

B G Pine et al, Image fields in rectangular vacuum vessels of the ISIS Synchrotron, http://jacow.org/HB2014/papers/mopab39.pdf

[4] Status of the ISIS Upgrade: ISIS II, C M Warsop, Proc 4^th ICFA Mini-Workshop on Space Charge 2019, CERN, 4 November 2019

https://indico.cern.ch/event/828559/contributions/3528347/attachments/1937200/3210714/CMW_Space_Charge_2019_ISIS_II_F01.pdf

Coherent Beam Instabilities

The transverse head-tail instability is an intensity limiting loss mechanism on the ISIS ring and cause for concern on many accelerators around the world. This has been the subject of much study on ISIS, see Figure 1 [5,6], and is particularly interesting because of the high space charge levels, the possible influence of conformal, rectangular vacuum vessels and large transverse beam sizes. Extensive experimental campaigns are under way, studying effects of fixed RF frequency running, normal fast RCS ramping, single and dual harmonic RF running, low and high intensity as well as effects of tune and beam size. These are being used to guide simulation studies using in house codes, PyHEADTAIL, and help gain a better understanding of how to control this instability. Closely associated with this work is development of a damper system and impedance studies (see below). Studies also involve more general instability work, including coasting beam motion on ISIS, and as well as essential assessments of all possible instabilities for the ISIS II rings.

Figure 2: Measurements of Head-tail motion on the ISIS Ring; blue - BPM difference signal, green - BPM sum signal.

[5] R E Williamson et al, Measurements and damping of the ISIS Head-tail Instability, Proc of the ICFA mini-workshop on Mitigation of Coherent Beam Instabilities in Particle Accelerators, Zermatt, Switzerland, 23 September 2019 https://indico.cern.ch/event/775147/contributions/3439011/attachments/1917199/3170147/MCBI_Poster_final.pdf

[6] R E Williamson et al, Simulations of Head-tail instability on the ISIS Synchrotron, http://jacow.org/HB2014/papers/mopab38.pdf

Beam Impedance Studies

A critical part of understanding intensity limitations due to beam instabilities is determining and understanding the main sources of impedance driving them. To help understand the behaviour of the existing ISIS ring and future ISIS II designs, a programme of studies looking at bench and beam measurement of impedances, along with simulation and analytical calculations is being undertaken. Of particular interest on ISIS is the low frequency transverse impedance driving head-tail motion, including effects of RF shields and ceramic vacuum vessels in the fast cycling magnets Figure 1, kicker magnets and overall resistive wall effects. This work will give essential information to help understand present losses on ISIS and for guiding impedance budgets on ISIS II machines.

Figure 1: View of RF Shield Structures in the ISIS Fast Cycling Dipole Magnets.

Reliable and benchmarked modelling of beam behaviour and loss underpin the design of future machines. Comparisons of simulations of the ISIS ring have compared well with operational and experimental results [7,8], using both in-house codes and the ORBIT code. Work to build more detailed benchmarks continues, and installation of the new ISIS MEBT and chopper [9] will allow benchmarking of codes for injection designs of ISIS II.

[7] B Jones et al, Injection Optimisation on the ISIS Synchrotron. https://accelconf.web.cern.ch/e08/papers/thpp096.pdf

[8] D J Adams et al, Beam Loss Studies of the ISIS Synchrotron Using ORBIT, https://accelconf.web.cern.ch/IPAC2012/papers/thppp088.pdf

[9] S R Lawrie et al, A pre-injector upgrade for ISIS, including a medium energy beam transport line and an RF driven H- ion source. https://doi.org/10.1063/1.5127263

Beam Measurement and Experimental Methods

Understanding of high intensity beams ultimately depends on experimental observation. In order to probe the interesting beam physics experimental methods often require careful reconfiguring of machine parameters and rely in accurate instrumentation. Examples on ISIS include reconfiguring of the beam to measure coasting beams in storage ring mode (RF off) and bunched stored beams (fixed  frequency RF), as well as normal RCS mode. Development of key, non-destructive measurements of transverse profiles, allowing for space charge distortions have been studied in detail on ISIS.