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Beam Loss Mechanisms and Related Design Choices in Hadron Rings
Chris Warsop
Nuria Catalan Lasheras
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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 Content1. 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
Chris Warsop
Nuria Catalan Lasheras
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– Cm=1/2, 3/4
– Breathing Mode, Quad Mode
• Higher orders, coupling … more modes …
• Avoid resonant conditions, correct errors!
mm mn 0
scmm mC
x
px
Nr
322
1. Loss Mechanisms
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
1. Loss Mechanisms
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
.
2220// /
instI
PP
e
cmF
n
Z
1. Loss Mechanisms
K Ng et al
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
1. Loss Mechanisms
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
cm
p
RI
QQQn
e
EFZ
000
04
1. Loss Mechanisms
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
1. Loss Mechanisms
Monitor difference signal
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
1. Loss Mechanisms
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
2.1 ESS Injection Achromat
ACHROMAT
HEBT LINACMR
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 m1/2
• Low field: pre stripping
MS1MS2
MS3 VS1
VS2
VS3
VS4
HS1HS2HS3HS4
EC Energy Enhancement Cavity
MR Momentum Ramping Cavity
BR Bunch Rotation Cavity
HS Horizontal Foil Scrapers
MS Momentum Foil Scrapers
VS Vertical Foil Scrapers
Rings
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
2. Low Loss Designs
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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!
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
foil
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
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
• Key features– Triplet Structure
– Long Dispersionless Straights
– Two Rings
3. Key Factors
3.2 ESS Accumulator Lattice
CollimationRF
RF
Injection
Extraction
ParametersEnergy 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
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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
Beam Loss Mechanisms and Related Design Choices
Chris Warsop
Nuria Catalan Lasheras
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.
…