beam loss mechanisms and related design choices in hadron rings chris warsop nuria catalan lasheras

Beam Loss Mechanisms and Related Design Choices in Hadron Rings

Chris Warsop

Nuria Catalan Lasheras

Beam Loss Mechanisms and Related Design Choices

Chris Warsop


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 ~


Chris Warsop


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


Chris Warsop


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


Chris Warsop


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


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


Chris Warsop


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


Chris Warsop


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


Chris Warsop


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


Chris Warsop


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


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


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


Chris Warsop


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


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


Chris Warsop


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


Chris Warsop


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


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


Chris Warsop


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


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


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


• 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)


Chris Warsop


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


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


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


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 in hadron rings chris warsop nuria catalan lasheras

Documents

transverse i space charge

transverse iispace charge

transverse impedance

longitudinalspace charge

transverse single bunch

beam loss mechanisms

lossspace charge resonances

rf shields