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

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 ~

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 A Fedotov, I Hofmann For the Non Coupled Case EG: m=2, 2D round beam, non split –C m =1/2, 3/4 –Breathing Mode, Quad Mode Higher orders, coupling … more modes … Avoid resonant conditions, correct errors! 1. Loss Mechanisms

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 1.1 Space Charge: Transverse (ii) Space charge also drives loss –Space Charge Resonances (4 th 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 (Q x,Q y ) Selection, Magnet Error Correction  Optimised Injection Painting 1. Loss Mechanisms

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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, … 1. Loss Mechanisms

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 ~ –Stability Under Capacitative Z // Inductive Insert in PSR: –Compensate Reactive –Increase Resistive 1. Loss Mechanisms K Ng et al

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 (n b ≥3) –Narrow Band Impedances of cavities: damp High Order Modes 1. Loss Mechanisms

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

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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, ~10 13 protons, 200 MeV (γ< γ t ) –At Natural Chromaticity (ξ = -1.3), m=1 –Cured by Ramping Q y 1. Loss Mechanisms Monitor difference signal

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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? 1. Loss Mechanisms

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 1.2 Instabilities: Electron Cloud and Related Losses (ii) 1. Loss Mechanisms R Macek 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

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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, H 0 Excited States Intrabeam Scattering 1. Loss Mechanisms

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 2. Low Loss Designs

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 2.1 ESS Injection Achromat ACHROMAT HEBTLINAC MR BR EC 42.5 m 2. Low Loss Designs Collimation in three planes Exploits Foil Stripping of H - Achromaticarc r=42.5 m Normalised dispersion 5.5 m 1/2 Low field: pre stripping MS1 MS2 MS3VS1 VS2 VS3 VS4 HS1HS2HS3HS4 ECEnergy Enhancement Cavity MRMomentum Ramping Cavity BRBunch Rotation Cavity HSHorizontal Foil Scrapers MSMomentum Foil Scrapers VSVertical Foil Scrapers Rings

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 (H 0, H -, e - ) Practical Factors –Foil support and exchange, material –Apertures, realistic layout of injection region –Optimised magnet fields to avoid pre stripping 2. Low Loss Designs

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 2.2 (ii) SNS Injection 2. Low Loss Designs 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!

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 2.2 (iii) Optimised Transverse Painting - SNS What is Best: Transversely Correlated or Anti correlated 2. Low Loss Designs Correlated J Beebe-Wang et al y x foil y x 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

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices Correlated Anti-Correlated 2.2 (iv) Simulation Results 2. Low Loss Designs Simpsons code 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? J Beebe-Wang et al

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 2. Low Loss Designs

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 3. Key Factors

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices Key features –Triplet Structure –Long Dispersionless Straights –Two Rings 3. Key Factors 3.2 ESS Accumulator Lattice Collimation RF Injection Extraction Parameters Energy GeV Rep Rate = 50 Hz Circumference =219.9 m Intensity 2.34x10 14 ppp Power 2.5 MW per ring Q=(4.19,4.31), No Sp=3 f rf =1.24 MHz, h=1 (+h=2)

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 … 3. Key Factors

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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! 4. Summary

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 4. Summary

Chris Warsop Nuria Catalan Lasheras Beam Loss Mechanisms and Related Design Choices 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 R Macek, E-P WG Summary AIP CP642, PAC 2001, etc G Rees, C Prior, ESS Technical Reports etc. …