RAL Template

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

3
1.1 Space Charge Transverse (i)
1. Loss Mechanisms

• 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 m2, 2D round beam, non split
• Cm1/2, 3/4
• Breathing Mode, Quad Mode
• Higher orders, coupling more modes
• Avoid resonant conditions, correct errors!

A Fedotov, I Hofmann
4
1.1 Space Charge Transverse (ii)
1. Loss Mechanisms

• 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

5
1.1 Space Charge Longitudinal
1. Loss Mechanisms

• 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,

6
1.2 Instabilities Longitudinal (i)
1. Loss Mechanisms

• 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
7
1.2 Instabilities Longitudinal (ii)
1. Loss Mechanisms

• 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 (nb3)
• Narrow Band Impedances of cavities damp High
  Order Modes

8
1.2 Instabilities Transverse (i)
1. Loss Mechanisms

• 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

9
1.2 Instabilities Transverse (ii)
1. Loss Mechanisms

• 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 (?lt ?t)
• At Natural Chromaticity (? -1.3), m1
• Cured by Ramping Qy

Monitor difference signal
10
1.2 Instabilities Electron Cloud and Related
Losses (i)
1. Loss Mechanisms

• Current RD 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?

11
1.2 Instabilities Electron Cloud and Related
Losses (ii)
1. Loss Mechanisms

• 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

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

13
2.1 Stability and Control of Injected Beam
2. Low Loss Designs

• 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

14
2.1 ESS Injection Achromat
2. Low Loss Designs
HEBT
LINAC
EC
MR
BR
MS2
HS1
HS2
HS3
HS4
MS1

• Collimation in three planes
• Exploits Foil Stripping of H-
• Achromaticarc r42.5 m
• Normalised dispersion 5.5 m1/2
• Low field pre stripping

EC Energy Enhancement Cavity MR Momentum Ramping
Cavity BR Bunch Rotation Cavity HS Horizontal
Foil Scrapers MS Momentum Foil Scrapers VS Vertica
l Foil Scrapers
42.5 m
MS3
VS1
ACHROMAT
VS2
VS3
VS4
Rings
15
2.2 (i) Multi-Turn Charge-Exchange Injection
2. Low Loss Designs

• 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

16
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 HV
• Energy Spreader for P
• Includes
• Removal of H, e-
• Flexible!

17
2.2 (iii) Optimised Transverse Painting - SNS
2. Low Loss Designs

• What is Best Transversely Correlated or Anti
  correlated

J Beebe-Wang et al
Correlated
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
18
2.2 (iv) Simulation Results
2. Low Loss Designs
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
19
2.3 Storage, Acceleration, Extraction,
2. Low Loss Designs

• 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

20
3.1 Major Systems and Lattice Considerations
3. Key Factors

• 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

21
3.2 ESS Accumulator Lattice
3. Key Factors

• 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
Sp3 frf1.24 MHz, h1 (h2)
22
3.3 Other Important Features
3. Key Factors

• 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

23
4. Summary and Thoughts (i)
4. Summary

• Have given an outline of major considerations for
  low loss design
• New machines depend on a very large body of
  knowledge
• Important RD 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!

24
4. Summary and Thoughts (ii)
4. Summary

• 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

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.