Space Charge Studies on the ISIS Ring

Space Charge Studies on the ISIS Ring C M Warsop, D J Adams, B Jones, S J Payne, B G Pine, H V Smith, C C Wilcox, R E Williamson, ISIS, RAL, UK with contributions from S Machida, C R Prior, G H Rees & members of ASTeC/IB, RAL, UK V Kornilov, GSI, Germany Space Charge 15, Oxford University, UK, March 2015

Contents 1. Introduction to ISIS 2. Reasons for Space Charge Study Developments and Upgrades 3. What we can learn from ISIS 4. Present Space Charge R&D Topics 5. Summary 6. Acknowledgements 7. ISIS tour

1. The ISIS Facility Outline of ISIS The ISIS Facility Injector H - Penning Ion Source 665 kev RFQ 70 MeV DTL Linac Ring 70-800 MeV RCS Target Stations TS1 40 Hz TS2 10 Hz Mean beam power ~ 200 kw

1. The ISIS Synchrotron Intensity and Loss Through Cycle Injection 2.8 x10 13 ppp Circumference: 163 m Energy Range: 70-800 MeV Rep Rate: 50 Hz Intensity: 2.5-3.0 x10 13 ppp Beam Power: 160-200 kw Losses: Inj: 2%, Trap: <3%, Acc/Ext <0.5% Injection: 130 turn, H - charge-exchange Acceptances: Collimated ~350 π mm mr RF System: h=2, f 2 =1.3-3.1 MHz, V 2 ~160kV/turn (2 bunches) h=4, f 4 =2.6-6.2 MHz, V 4 ~80 kv/turn Extraction: Single turn, vertical Tunes: (Q x, Q y )=(4.31, 3.83) (programmable) Extraction

2. Reasons for Space Charge Study 2 Why do we care about space charge at ISIS? To identify the best upgrade routes and development ideas More specifically studies for: Improved operations Increase running intensity from 210 μa to consistent 240 μa Upgrades to the existing machine ~ 0.5 MW Upgrade injector from 70 to 180 MeV Develop next generation short pulse spallation source ~ 10 MW ISIS II idea: A flexible 1-10 MW, multi target design Main schemes presently based on RCS, FFAG accelerators

2. Reasons for Space Charge Study 2.1 Upgrades to the existing ISIS machine Main route: New 180 MeV injector Injects into existing 800 MeV ring 0.5 MW New injection system ~ 8E13 ppp, 50 Hz New 180 MeV Linac Existing 70 MeV Linac Injection Upgrade Design Study Intensity limits losses Space charge, instabilities Related, smaller, piece-wise upgrades also possible Ideas: superconducting linac section, injection upgrades,

2. Reasons for Space Charge Study 2.1 Upgrades to the existing ISIS machine Main route: New 180 MeV injector Injects into existing 800 MeV ring 0.5 MW New injection system ~ 8E13 ppp, 50 Hz New 180 MeV Linac Injection Upgrade Design Study Intensity limits losses Space charge, instabilities Related, smaller, piece-wise upgrades also possible Ideas: superconducting linac section, injection upgrades, New Target?

2. Reasons for Space Charge Study D J Adams, R E Williamson, G H Rees, C R Prior, C M Warsop, et al 0.8 3.2 GeV RCS cavities 2.2 ISIS II: 1-10 MW machine RCS route Starting point: 5 MW RCS (G H Rees) Beginning detailed simulation study Beam dynamics of painting, capture triplet triplet cavities collimators cavities dipoles 8 dipole cavities dipoles extraction to 2 MW target 0.8 3.2 GeV RCS 5 Super Period, 370 m, RF (h=4) Optimised for low loss multi turn H - injection Operation at 30 Hz, 1.3 10 14 ppp (2MW), Upgrade to 50 Hz, 2.0 10 14 (5 MW) Two stacked rings @50Hz produce 10 MW. H, Hˉ beams 800 MeV Hˉ Initial injection simulations (ORBIT) (1.3E13 ppp, h=4, 30 Hz) Intensity limits losses Injection, 3D painting, Space charge, instabilities Injection region Trial painting scheme

2. Reasons for Space Charge Study 0.8 3.2 GeV FFAG 2.3 ISIS II: 1-10 MW machine - FFAG route FFAGs now a serious option Studies of ASTeC/IB at RAL Designs now being developed EG G H Rees 0.8-3 GeV FFAG design Intensity limits losses? New R&D into intensity limits of FFAG Experimental work KURRI with ASTeC/IB SPOD at RAL (plasma trap) Ideas for new research ring on FETS Understand relative merits FFAG & RCS Important overlap in RCS-FFAG studies KURRI FFAG D Kelliher, S Machida, C R Prior, G H Rees, S Sheehy, et al ASTeC/IB

3. ISIS contribution to studies 3.1 What can the ISIS ring teach us? Generally Rare combination of high space charge & low loss Large tune shifts ( Q~0.5) low loss fast cycle Can also reconfigure beam to study different effects (e.g. SRM) The challenge to understand real operational loss Complicated 3D process not a convenient idealised case Detailed empirical optimisation required for operations What potential is there for better understanding and control? Is improved beam control the route to higher intensity? ISIS Tune Footprint (Q x, Q y ) ORBIT, I= 2.8E13 ppp Maintain links between key R&D and operational machines SPOD at RAL, high intensity FFAG s,

4.1.1 Space charge image studies Effects of Images in vacuum vessels ISIS vessels are rectangular and conformal May provide additional driving terms for loss Loss driven by orbit errors? (G H Rees, C R Prior) Developing Set code to model effects Aim to model realistic process 2.5D PIC model Essential to look at simpler models, understand effects Recent work studying details of image terms Use FFT and FEA solvers compare theory E field for KV beam No boundary Square boundary Difference: image effect ISIS vacuum vessels Electric field for KV beam Meshes for FFT, FEA solvers B G Pine

4.1.2 Space charge image studies Useful expansion for parallel plates (R Baartman) Good basis for understanding intensity dependent driving terms What is effect of transition parallel plate rectangular? Detailed simulation studies Different for ~ square aperture Give effective driving terms e.g. closed orbit driven terms Model to explain simulations Can include 2D coupling Image terms ε 1, ξ 1, κ 30 vs aspect ratio Image terms κ 12, κ 21, κ 03, vs aspect ratio Parallel plate values Parallel plate values B G Pine

4.1.3 Space charge image studies Simulations now under way Initial results promising Next steps 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 10.8 Intensity x 1.0E14 Simpler simulations: smooth focusing 2D PIC with closed orbit errors Extend to AG case, then 3D PIC with longitudinal motion Compare effects of different geometries (e.g. circular) Identify some experimentally observable behaviour Will give key information on ISIS losses Relative merits of different vacuum vessel geometries Sextupole Amplitude Sextupole Frequency 2000 1800 1600 1400 1200 1000 800 600 10.7 10.6 10.5 10.4 Previous results: coherent sextupole resonance probably due to images Sextupole Amplitude Sextupole strength 10.3 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Intensity x 1.0E14 (Y,Y ) B G Pine

4.2.1 Half integer resonance with space charge Key loss mechanism Can we understand, predict evolution of halo, loss? Experimental studies 2D coasting beam RF off, DC field, inject small beam εε xx = εε yy εε rrrrrr 20 π mm mr, 2Q y =7 driving term, Q y =3.6 Ramp intensity (1E13 ppp), push onto resonance Study evolution of profile Observations agree with ORBIT models Clear formation of core and lobes ORBIT results Loss & Tune vs time (intensity) Transverse profile Measured over 400 μs

4.2.2 Half integer resonance Previous work: agreement measurement-simulation Rotation of half integer lobes Control with driving term Δk(θ)=k 0 cos(2q y θ+φ) Dependence on driving term phase Measured ORBIT Expected motion around ring at half integer resonance (illustrative example) Phase ϕ 1 (Y, Y') s Phase ϕ 2 (Y, Y')

4.2.3 Half integer resonance Recent work: agreement measurement-simulation Measure as a function of tune and driving term Dependence on tune Dependence on driving term Measured ORBIT Measured ORBIT Q 1 =3.71 DT 1 =0.02 Q 2 =3.67 DT 2 =0.03 Q 3 =3.63 DT 3 =0.06

Recent work: Observation of stationary distributions Slower accumulation of beam formation of stable lobes Short lived lobes ~50 turns (I inj =22 ma) 4.2.4 Half integer resonance Measured transverse profiles over 1 ms Long lived lobes ~500 turns (I inj =11 ma) Initial experiments on stable halo (profiles now shown as colour contour) (i) Constant (as above) (ii) Ramp Q down (iii) Ramp Q down/up (iv) Rotate phase

Speculation & work in progress! Models to explain observations? Coherent model limited: coherent limit Approach from incoherent direction? Simplest 1D single particle model Observation 4.2.5 Half integer resonance Total radial force Focussing + space charge Radial force (Y, Y ) Here have space charge potential 1 st guess usually KV model Linear motion: cannot describe growth 2 nd guess WB model (non-stationary) Non-linear motion: predict halo? Radial force circular KV (Y, Y ) Radial force circular WB Rough example! Simple simulations Driving term, fixed potential Non-linear motion ~ edge of core Complicated ~ still studying Incoherent model of halo? Next add coherent motion? RMS envelope modify halo Coherent model of halo? May be a useful idea Different KV-WB coherent motion?

D J Adams, B Jones, V Kornilov, R E Williamson, C M Warsop, et al 4.3.1 Head-tail instability: ops Limits operational intensity With dual harmonic RF upgrade Previously cured with Q y ramp Driven by resistive-wall Operational observations Symmetric bunches unstable Plots show effect of θ variation Normal beam Low loss 1RF = 108 kv, 2RF = 52.8 kv Δ = 0.489, δθ = 0 o Normal beam + Θ shift Large loss! 1RF = 108 kv, 2RF = 52.8 kv Δ = 0.489, δθ = -10 o ISIS Beam Bunches at ~ 2 ms Sum signal Difference signal Damper in development R&D under way See below Beam Loss vs Time 0-5 ms Loss!

4.3.2 Head tail experiments: lower intensity Measurements: Monitor sum/difference Aim to understand simpler case Minimise effects of space charge Study single harmonic RF, low intensity Simpler case, compare simulations & theory Experiments: mode m=1; code and theory m=2 Measured growth rates faster than theory Clearly not understood yet Plans Better model of beam impedances (Measurements and simulations) Explore limits of Sacherer theory (G H Rees) Build kicker/damper system HEADTAIL Simulation: Mode motion and growth V Kornilov, R E Williamson, D J Adams, B Jones, C M Warsop, et al

4.3.3 Head tail experiments: high intensity Aim to study effects of space charge Effects of space charge and images Vladimir Kornilov talk on Wednesday two threshold behaviour Beam stabilises at higher intensity Landau damped: space charge + images Qualitative agreement: simulation, theory, experiment Plans Build up detailed simulations of process Improve measurements (bunched SRM) Better model of beam impedances (again!) V Kornilov, R E Williamson, D J Adams, B Jones, C M Warsop, et al

4.3.4 Head-tail work outline plans at RAL Simulation work Investigation of HEADTAIL code (in progress) Investigation of TRANFT* code (in progress) Development of RAL code (next) Possibly adapt Set 3D PIC Impedance modelling Experimental work Beam based impedance measurements Measurements of emittance at instability Bunched beam, storage ring mode TRANFT* Simulation: Mode motion and growth Measurement Difference Signal 150 turns horizontally along bunch vertically (frequency sweep removed) Collaboration Continue to compare results and ideas with GSI colleagues *M Blaskiewicz R E Williamson, C M Warsop, D J Adams, B Jones, et al

Loss Through ISIS Cycle 4.4 Understanding real operational loss R&D above studies single loss mechanisms What are real loss mechanisms for ISIS trapping? 3D trapping process complicated many effects? Longitudinal loss (non-adiabatic capture) Transverse loss (half-integer crossing?) Tune Footprint (Q x, Q y ) ORBIT ORBIT Results: (x, x') (y, y') (x, y) ( E, ϕ) 2.8E13 ppp 0.5 ms How can we find out? ORBIT models give ~ agreement on loss vs time How do we know what processes are acting? Working to improve understanding of models Effect of transverse in/coherent motion, driving terms Loss vs Time ORBIT & Measurement

C C Wilcox, R E Williamson, S J Payne, C M Warsop, et al 4.5 R&D for transverse profile measurements Good transverse profile measurements essential Detailed models of ISIS residual gas ionisation monitors CST fields solvers and in-house code tracks ion trajectories Allow for non-linearities and space charge. Recent results checking halo measurements Input distributions predicted by ORBIT Check behaviour as function of drift field and intensity Simulation of Ion tracks Intensity Drift field 15 kv Drift field 30 kv 0e13ppp 1e13ppp 2e13ppp Input profile Predicted measured profile

Q vs loss map 4.6 Other key work Profile and emittance measurements RGI checks vs harp monitors, scraper measurements, Detailed low intensity lattice measurements Optics parameters, non-linear lattice model, magnet measurements, Code development: ISIS 3D PIC Set Now being bench marked, injection, foils, smooth focusing options Longitudinal instability KS, KSB in bunched storage ring mode Diagnostics developments Kickers and damper systems, multipole monitors and deflectors Foils, activation and collimation modelling

5. Summary 5 Summary R&D is essential to identify best upgrade routes for ISIS Need to find best option for the next generation spallation source R&D on ISIS is improving the machine & our understanding On going development of computer models, benchmarking Studying key topics that have relevance for new machines Space charge, instabilities, injection, activation,. Methods of experimental verification and measurement Building simulation models and codes In house (SET3Di) and established codes as required for studies Still much to learn & much to gain from understanding more! We have a lot of work under way ~ results in the pipeline

6. Acknowledgements Many thanks to ISIS Diagnostics Section ISIS RF Section ISIS Operations ASTeC Intense Beams Group

7. ISIS Tour ISIS Tour on Friday afternoon FETS RFQ Linac tank build TS2 and more!