Beam Loss Mechanisms and Related Design Choices in Hadron Rings

1. Beam Loss Mechanisms and Related Design Choices in Hadron Rings Chris Warsop Nuria Catalan Lasheras

2. Purpose and Scope of Talk • Loss is expected to be a main factor limiting performance: • Activation, Risk of Damage • Detector Background Levels, Quenching of SC Magnets • Main Content 1. Summarise Loss Mechanisms 2. Implementation of Low Loss Design 3. Key Design Factors and Choices 4. Summary • Scope • Focus on Low-Medium Energy HI Proton Rings: ISIS, ESS, SNS, JPARC, … • Less on LHC, RHIC, SIS100 ~ the subject of later talks ~ Beam Loss Mechanisms and Related Design Choices

3. 1. Loss Mechanisms 1.1 Space Charge: Transverse (i) • Space charge shifts beam into resonant condition driven by Magnet Errors • Incoherent Space Charge Limit: • Overestimate! Must Consider Coherent modes • For the Non Coupled Case • EG: m=2, 2D round beam, non split • Cm=1/2, 3/4 • Breathing Mode, Quad Mode • Higher orders, coupling … more modes … • Avoid resonant conditions, correct errors! A Fedotov, I Hofmann Beam Loss Mechanisms and Related Design Choices

4. 1. Loss Mechanisms 1.1 Space Charge: Transverse (ii) • Space charge also drives loss • Space Charge Resonances (4th order, coupling) • Image Effects • Time varying distributions drive transverse halo creation see later … • Key Measures  Higher Energy, Large Transverse Emittance/Acceptance, Bunching Factor • Working Point (Qx,Qy) Selection, Magnet Error Correction  Optimised Injection Painting Beam Loss Mechanisms and Related Design Choices

5. 1. Loss Mechanisms 1.1 Space Charge: Longitudinal • Space Charge perturbs longitudinal motion • Need fine control of Longitudinal Motion • To prevent halo creation and bunch broadening • To optimise the momentum distribution & bunching factor Transverse tune shifts and stability • Key Measures  Optimised longitudinal injection painting including space charge, … • Inductive Inserts, Dual Harmonic RF Systems, … Beam Loss Mechanisms and Related Design Choices

6. 1. Loss Mechanisms 1.2 Instabilities: Longitudinal (i) • Longitudinal Microwave "coasting beam" • Keil-Schnell-Boussard • Key Measures  Minimise Z//: RF Shields, Smooth Transitions, Resistivity  Momentum Spread Distribution, Peak intensity • For High Space Charge KSB pessimistic: exceed by factor ~ 5 - 10 • Stability Under Capacitative Z// • Inductive Insert in PSR: • Compensate Reactive • Increase Resistive K Ng et al Beam Loss Mechanisms and Related Design Choices

7. 1. Loss Mechanisms 1.2 Instabilities: Longitudinal (ii) • Longitudinal Single Bunch • Robinson Stability & Beam Loading • Feed-forward compensation, compensation by de-tuning etc. • Multiple control loops • In addition to previous precautions  Powerful, Optimised (complicated) RF Systems • Longitudinal Coupled Bunch (nb≥3) • Narrow Band Impedances of cavities: damp High Order Modes Beam Loss Mechanisms and Related Design Choices

8. 1. Loss Mechanisms 1.2 Instabilities: Transverse (i) • Transverse Microwave "coasting beam" • Stability Criterion • Key Measures  Minimise Z┴: RF Shields, Smooth Transitions, Resistivity, Extraction Kickers  Momentum Spread Distribution, Peak intensity  Chromaticity sign (above or below transition), change Q  Landau Damping Octupoles  Damping Systems Beam Loss Mechanisms and Related Design Choices

9. 1. Loss Mechanisms 1.2 Instabilities: Transverse (ii) • Transverse Single Bunch: Head Tail • Effects of : transverse impedance, betatron and synchrotron motion • Key Measures ~ similar to above  Chromaticity sign: above or below transition (for "normal" impedance)  Select Q above integer, minimise resistivity (for resistive wall)  Landau Damping with Octupoles, Active Damping • Observation of Head Tail ~ Resistive Wall • ISIS Synchrotron single ~200 ns bunch, ~1013 protons, 200 MeV (γ< γt) • At Natural Chromaticity (ξ = -1.3), m=1 • Cured by Ramping Qy Monitor difference signal Beam Loss Mechanisms and Related Design Choices

10. 1. Loss Mechanisms 1.2 Instabilities: Electron Cloud and Related Losses (i) • Current R&D Topic: understanding incomplete • Key observations • PSR: strong vertical instability at thresh hold, fast loss • ISIS: no e-p effects seen (yet!) • CERN PS, SPS – large No. of electrons under LHC conditions • RHIC – pressure rise with halved normal bunch spacing • Problems • E-P instability threshold limits intensity, or causes emittance growth. • Vacuum pressure rise • Heating effects (SC Magnets) • Effects of Neutralisation: tune shifts, resonance crossing, loss?, diagnostics? Beam Loss Mechanisms and Related Design Choices

11. 1. Loss Mechanisms 1.2 Instabilities: Electron Cloud and Related Losses (ii) • Electron Production • Stripping Foil, Residual Gas Ionisation, Loss Induced, Multipacting, (SR) • Much work into Measurement & Simulation of electron production • PSR Solutions: Combined measures raised stable beam threshold • PSR RFA Signal: Trailing Edge Multipacting • Use of Skew Quads, Sextupoles, Octupoles (Landau Damping) • RF Buncher, Inductive Inserts (beam in gap) • Solutions  TiN Coating, Surface Scrubbing  Longitudinal Magnetic field  Clearing Electrodes  Damping R Macek Beam Loss Mechanisms and Related Design Choices

12. 1. Loss Mechanisms 1.3 Other Loss Mechanisms • Magnet Errors, Transverse Resonances, General Optimisation • closed orbit errors, alignment ~ correction dipoles • gradient error correction, Q setting ~ trim quadrupoles • chromaticity control, correction ~ sextupole families • Landau damping ~ octupole families • Interactions with Residual Gas • Interactions with the Stripping Foil • Inelastic/Elastic Scattering, Ionisation Energy Loss, H0 Excited States • Intrabeam Scattering Beam Loss Mechanisms and Related Design Choices

13. 2. Low Loss Designs 2.1 Stability and Control of Injected Beam • For consistent low loss in ring need stable well defined injection beam • Examples • LHC "Injector Chain" • Injection Line Collimation for ESS, SNS, JPARC, … • Remove Linac beam variations in the Injection Line • Transverse Collimation • Momentum Control Beam Loss Mechanisms and Related Design Choices

14. 2. Low Loss Designs 2.1 ESS Injection Achromat HEBT LINAC EC MR • Collimation in three planes • Exploits Foil Stripping of H- • Achromaticarc r=42.5 m • Normalised dispersion 5.5 m1/2 • Low field: pre stripping BR MS2 HS4 HS3 HS2 HS1 MS1 EC Energy Enhancement Cavity MR Momentum Ramping Cavity BR Bunch Rotation Cavity HS Horizontal Foil Scrapers MS Momentum Foil Scrapers VS Vertical Foil Scrapers 42.5 m MS3 VS1 ACHROMAT VS2 VS3 VS4 Rings Beam Loss Mechanisms and Related Design Choices

15. 2. Low Loss Designs 2.2 (i) Multi-Turn Charge-Exchange Injection • Main Considerations • Paint optimal distributions for stability • Transverse: Closed Orbit and Injection Point Manipulation • Longitudinal: Chopping, Injected Momentum & Ring RF Manipulation • Minimise Foil Traversals: Loss, Foil Lifetime • Small Cross Section, Optimised Optics - mis-match • Thickness: heating & stress, efficiency • Remove Stripping Products (H0, H-, e-) • Practical Factors • Foil support and exchange, material • Apertures, realistic layout of injection region • Optimised magnet fields to avoid pre stripping Beam Loss Mechanisms and Related Design Choices

16. 2. Low Loss Designs 2.2 (ii) SNS Injection • Zero Dispersion at Injection Point • In Chicane Magnet • Independent H, V and P • Correlated or anti-correlated H&V • Energy Spreader for P • Includes • Removal of H*, e- • Flexible! Beam Loss Mechanisms and Related Design Choices

17. y y foil foil x x 2. Low Loss Designs 2.2 (iii) Optimised Transverse Painting - SNS J Beebe-Wang et al Correlated Anti-Correlated • What is Best: Transversely Correlated or Anti correlated Non "ideal" ~ but paints over beam halo Rectangular x-y cross section Preserved? Ideally gives a uniform density Elliptical x-y cross section Halo generated during injection Beam Loss Mechanisms and Related Design Choices

18. 2. Low Loss Designs 2.2 (iv) Simulation Results J Beebe-Wang et al • Correlated Seems Better • Smaller Halo • Fewer Foil Hits • Better Distribution for Target • Improved Schemes with Oscillating Painting … • Power supplies, Aperture demands? • How much might these ideas help on existing machines/upgrades? Correlated Anti-Correlated Simpsons code Beam Loss Mechanisms and Related Design Choices

19. 2. Low Loss Designs 2.3 Storage, Acceleration, Extraction, … • Accumulator Ring: Stability until Extraction (ESS, SNS) • Loss Control & Collimation, BIG • Longitudinal/Transverse Halo Control: Extraction Loss • RCS: Stability through Acceleration: (ISIS, JPARC) • As Accumulator but more difficult! • Power supply tracking, programmable trim magnets ... • Other Machines: • Bunch Compression for Proton Drivers • Collision Beam Loss Mechanisms and Related Design Choices

20. 3. Key Factors 3.1 Major Systems and Lattice Considerations • Basic Choices • Accumulator or RCS, Beam Energy, Circumference, … • Optical and Spatial Requirements for Lattice • Injection: dispersion, matching, … • Extraction: straights for fast kickers and septum, (redundancy, fail safe) • Collimation: two stage betatron, momentum, beam in gap kicker, … • RF: space in straights • Working point: space charge, stability, … • Optics: acceptance • Special Requirements Beam Loss Mechanisms and Related Design Choices

21. Injection RF Collimation Extraction RF 3. Key Factors 3.2 ESS Accumulator Lattice • Key features • Triplet Structure • Long Dispersionless Straights • Two Rings Parameters Energy 1.334 GeV Rep Rate = 50 Hz Circumference =219.9 m Intensity 2.34x1014 ppp Power 2.5 MW per ring Q=(4.19,4.31), No Sp=3 frf=1.24 MHz, h=1 (+h=2) Beam Loss Mechanisms and Related Design Choices

22. 3. Key Factors 3.3 Other Important Features • Aperture • Acceptance of Machine, Collimators and Extraction Line. Painted Emittance. • Diagnostics • Ability to Control and Manipulate beam and halo (large dynamic range) • Protection • Combination of hardware, diagnostics (fast), interlocks, procedures … Beam Loss Mechanisms and Related Design Choices

23. 4. Summary 4. Summary and Thoughts (i) • Have given an outline of major considerations for low loss design • New machines depend on a very large body of knowledge • Important R&D areas: Instabilities (e-p effects), Space Charge • Optimised Design of Low Loss Machines • Now a well developed art … • How reliably can we predict loss levels and distributions? • Critical to final performance • Must continue to test Theories and Codes with Experiment • More Experiments! Beam Loss Mechanisms and Related Design Choices

24. 4. Summary 4. Summary and Thoughts (ii) • Many Machines being built and commissioned now … • What are the key issues? • Differences between simulation and reality • Diagnostics and Control limitations • Optimisation Methods: e.g. loss, collimation, injection • Protection Strategies: Faults, Accidents Beam Loss Mechanisms and Related Design Choices

25. Acknowledgements Material from many SNS, JPARC, CERN, ESS related publications, including J Wei, Synchrotrons & Accumulators for HI Proton Beams, RMP, Vol. 75, October 2003 I Hofmann et al, Space Charge Resonances and Instabilities in Rings, AIP CP 642, etc. R Baartman, Betatron Resonances with Space Charge, AIP CP 448 K Schindl, Instabilities, CAS Zeuthen 2003, A Chao, Physics of Collective Beam Instabilities …, Wiley K Ng, Physics of Intensity Dependant Beam Instabilities, Fermilab-FN-0713 A Hofmann, B Zotter, F Sacherer, Instabilities, CERN 77-13 R Macek, E-P WG Summary AIP CP642, PAC 2001, etc G Rees, C Prior, ESS Technical Reports etc. … Beam Loss Mechanisms and Related Design Choices