Two beam instabilities in low PowerPoint Presentation, PPT - DocSlides

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Slide1

Two beam instabilities in low emittance rings

Lotta Mether, G.Rumolo, G.Iadarola, H.Bartosik

Low

Emittance

Rings Workshop

INFN-LNF,

Frascati

September 17

th

, 2014

Slide2

Two beam instabilities

Collective effects induced by electromagnetic field of second charge distribution, in addition to primary beamBeam-beam effects in collidersElectron cloud in positron machinesTrapped ions in electron machinesSecond “beam” may be produced by primary beamSynchrotron radiation on wall: electrons, ionsResidual gas ionization: electrons, ionsDesorption from wall due to losses: electrons, ions, neutralsMay cause instabilities, tune shift, emittance growth, beam and energy losses

17 September 2014

Low emittance rings 2014, L. Mether

2

Slide3

Outline

IntroductionElectron cloud in positron machinesElectron cloud formation Effect on beamObservations ModellingIon effects in electron machinesTrapping of ions Effect on beamObservations Modelling

17 September 2014

3

LER2014, L.

Mether

Slide4

Electron cloud formation

Primary (seed) electrons are generated inside beam chamber

17 September 2014

Low emittance rings 2014, L. Mether

4

Ionization of residual gas

Photoelectrons

from

synchrotron radiation

Desorption due to losses on wall

Slide5

Electron cloud formation

Primary (seed) electrons are generated inside beam chamberSeed electrons are accelerated by beam field, and may produce secondary electrons when hitting the wall

17 September 2014

Low emittance rings 2014, L. Mether

5

Slide6

Electron cloud formation

Primary (seed) electrons are generated inside beam chamberSeed electrons are accelerated by beam field, and may produce secondary electrons when hitting the wallUnder suitable conditions, avalanche electron multiplication (multipacting) occurs

17 September 2014

Low emittance rings 2014, L. Mether

6

Slide7

Electron cloud formation

Primary (seed) electrons are generated inside beam chamberSeed electrons are accelerated by beam field, and produces secondary electrons when hitting the wallEventually a stationary state - the electron cloud - is reached, when space charge limits further growth of electron density

17 September 2014

Low emittance rings 2014, L. Mether

7

CLIC-DR

wiggler

CLIC-DR

quad

Slide8

Electron cloud induced instability

The electron density gives rise to a single bunch head-tail instability, due to the beam focusing (pinching) the electron distributionIf e.g. the head of the bunch is displaced, an asymmetric pinch will take place, resulting into a net kick felt by the bunch tailAfter several turns, the offset in head motion can be transferred to the tailAfter a sufficient number of turns, the unstable coherent motion has propagated to the whole bunch

17 September 2014

Low emittance rings 2014, L. Mether

8

Slide9

Electron cloud effects & observations

Beam degradationCoherent instabilitySingle bunch, affecting the last bunches of a trainCoupled bunchBeam size blow-up and emittance growthTune shift along the bunch trainEnergy loss measured through synchronous phase shiftMachine observablesFast pressure rise, outgassingAdditional heat loadObserved in several machines KEK-LER, DaFne, CesrTA (see following presentation)…

17 September 2014

Low emittance rings 2014, L. Mether

9

Slide10

Electron cloud simulations

Several numerical codes for modelling electron cloud formation and/or instabilities existAt CERN, partly coupled simulation tools for modelling electron cloud formation and instabilityPyECLOUDMacroparticle code for simulation of electron cloud build-up(Py)HEADTAILMacroparticle code for simulation of single bunch instability, based on electron distribution from PyECLOUD

17 September 2014

Low emittance rings 2014, L. Mether

10

Slide11

Electron cloud simulations

Electron build-up and single bunch instability in CLIC-DR wigglersElectron cloud builds up for SEY > 1.4Threshold electron density for instability ~ 1.2 x 1013 / m3 Emittance growth rate fast compared to damping times ~ 2 ms

17 September 2014

Low emittance rings 2014, L. Mether

11

PyECLOUD

rise time

τ

≈ 0.7 ms

τ

≈ 0.5 ms

τ ≈ 0.4 ms

Vertical emittance

(

Py

)HEADTAIL

Slide12

Maximum central density along train

Electron cloud simulations

Electron build-up and single bunch instability in CLIC-DR wigglers

Electron cloud builds up for SEY > 1.4

Threshold electron density for instability ≈ 1.2 x 10

13 / m3  Beam is unstable for all SEY values above build-up threshold!

17 September 2014

Low emittance rings 2014, L. Mether

12

PyECLOUD

(

Py

)HEADTAIL

Slide13

Outline

IntroductionElectron cloud in positron machinesElectron cloud formation Effect on beamObservations ModellingIon effects in electron machinesTrapping of ions Effect on beamObservations Modelling

17 September 2014

13

LER2014, L.

Mether

Slide14

Fast beam ion instability mechanism

Generation of ions inside beam chamberScattering / field ionization of residual gas

17 September 2014

Low emittance rings 2014, L. Mether

14

Slide15

Fast beam ion instability mechanism

Generation of ions inside beam chamberIons are accelerated by beam field, and possibly trapped, depending on ion mass

17 September 2014

Low emittance rings 2014, L. Mether

15

T

b

=

L

sep

/c

Slide16

Fast beam ion instability mechanism

Generation of ions inside beam chamberIons are accelerated by beam field, and possibly trapped, depending on ion mass

17 September 2014

Low emittance rings 2014, L. Mether

16

CO, N

2

H

2

O

H

2

CLIC-DR

Slide17

Fast beam ion instability mechanism

Generation of ions inside beam chamberIons are accelerated by beam field, and possibly trapped, depending on ion massIons accumulate along bunch train, coupling train head and tail

17 September 2014

Low emittance rings 2014, L. Mether

17

Slide18

Fast beam ion instability mechanism

Generation of ions inside beam chamberIons are accelerated by beam field, and possibly trapped, depending on ion massIons accumulate along bunch train, coupling train head and tailOffset of each bunch is recorded into generated ion distribution, and transferred to the following bunches  coupled oscillations between electrons and ions

17 September 2014

Low emittance rings 2014, L. Mether

18

Slide19

Ion instability & observations

Ion trapping can be seen asCoherent multi-bunch instabilityPhase shift over bunch trainBeam size blow-up & emittance growthObservations in running machines usually under vacuum degradationDuring commissioningCaused by impedance heatingDeliberately, with injected gas, for study purposesFast Beam Ion Instabilities have been observed in several machinesAPS (with He injection), PLS (with H2 injection) SOLEIL, SSRF, BESSY II, ELETTRA, ALBA …Measurements at CesrTA (Dec. 2013, Apr 2014)Varying ion species and pressure, bunch charge, train structure, feedback etc.

17 September 2014

Low emittance rings 2014, L. Mether

19

Slide20

Observations at CesrTA

17 September 2014

Low emittance rings 2014, L. Mether

20

Vertical beam offset as function of bunch number at varying pressure, with vertical feedback on (dark blue) and off.

From A. Chatterjee

et al

. IPAC

2014

Slide21

Observations at CesrTA

17 September 2014

Low emittance rings 2014, L. Mether

21

Vertical beam size as function of bunch number at varying pressure, with vertical feedback on (dark blue) and off.

From A. Chatterjee

et al. IPAC 2014

Slide22

FBII Models

Analytical model Characteristic ion frequency Instability rise timeLimitations: linear regime, assumes bunch train as uniform line charge, assumes ion distribution trapped within bunch distributionNumerical model available (FASTION) Macroparticle simulation tool, including several ingredientsUsed for CLIC Main Linac, transfer lines, modified version used at CesrTAOptimization for CLIC-DR ongoingInstability typically seems to be less severe than predictions, probably stabilizing effects not included in existing models?Quantitative comparison between theoretical predictions, simulations and measurements (CesrTA) in progress

17 September 2014

Low emittance rings 2014, L. Mether

22

Raubenheimer et al. Phys. Rev. E 52, 5, 5487, Stupakov et al. Phys. Rev. E 52, 5, 5499

Slide23

FASTION simulations for CesrTA

Simulations of vertical beam offset and beam size as function of bunch number at varying pressure, with vertical feedback on (dark blue) and off

17 September 2014

Low emittance rings 2014, L. Mether

23

From A. Chatterjee

et al

. IPAC

2014

Slide24

FASTION simulations for CesrTA

Comparison of vertical beam offset and beam size

17 September 2014

Low emittance rings 2014, L. Mether

24

From A. Chatterjee

et al. IPAC 2014

Simulations

Measurements

Slide25

FASTION simulations for CesrTA

Comparison of vertical beam offset and beam size

17 September 2014

Low emittance rings 2014, L. Mether

25

From A. Chatterjee

et al. IPAC 2014

Simulations

Measurements

Slide26

Summary & conclusions

Two-beam effects are relevant for the performance of both running and future low-ε accelerators or damping ringsElectron cloud formation and instabilitiesDetailed models available for both processesObserved frequently in running machines  reliable estimates for future Ongoing research on techniques for mitigation or suppression (coating, clearing electrodes, scrubbing), to be applied to future machinesIon accumulation and instabilitiesTheories developed  formulae typically used to predict behaviorDetailed numerical model available  improvement & optimization ongoingObservations usually in presence of vacuum degradationImportant for vacuum specifications of future low-ε electron machinesmore sensitive to FBIILow-gap chambers and high intensity short bunches  More outgassingMay be controlled through feedback, but feedback can also be trigger

17 September 2014

Low emittance rings 2014, L. Mether

26

For

more details,

see

session “Two-Stream Instabilities” at TWIICE 2014

Slide27

FASTION code development

17 September 2014

Low emittance rings 2014, L. Mether

27

Slide28

Electron cloud formation

Primary (seed) electrons are generated inside beam chamberSeed electrons are accelerated by beam fieldProduce secondary electrons when hitting the wallAvalanche electron multiplication (multipacting)Eventually a stationary state - the electron cloud - is reached, when space charge limits further growth of electron densityElectron cloud density may be very high around beam location

17 September 2014

Low emittance rings 2014, L. Mether

28

Slide29

Electron cloud formation

Primary (seed) electrons are generated inside beam chamber

17 September 2014

Low emittance rings 2014, L. Mether

29

Ionization of residual gas

Photoelectrons

from

synchrotron radiation

Desorption due to losses on wall

Slide30

Electron cloud formation

Primary (seed) electrons are generated inside beam chamberSeed electrons are accelerated by beam field

17 September 2014

Low emittance rings 2014, L. Mether

30

Slide31

Secondary electron production

Electrons hitting the chamber wallat low energies are reflectedat higher energies produce secondary

17 September 2014

Low emittance rings 2014, L. Mether

31

Slide32

Ion trapping by electron beam

Generation of ions inside beam chamberIons are accelerated by beam field, and possibly trapped, depending on ion massTrapped ions oscillate around beam with characteristic frequencyAfter the passage of several bunches, ion density may grow sufficiently to affect beam

17 September 2014

Low emittance rings 2014, L. Mether

32

Slide33

Ion trapping (Gaussian beam)

17 September 2014

Low emittance rings 2014, L. Mether

33

Section

i

Section i+1

T

b

=

L

sep

/c

Ion of mass A

Slide34

Ion trapping in CLIC Damping Ring

17 September 2014

Low emittance rings 2014, L. Mether

34

CO, N

2

H

2

O

H

2

Slide35

Ion trapping (Gaussian beam)

17 September 2014

Low emittance rings 2014, L. Mether

35

Section

i

Section i+1

T

b

=

L

sep

/c

Ion of mass A

Slide36

Fast beam ion instability mechanism

Generation of ions inside beam chamberIons are accelerated by beam field, and possibly trapped, depending on ion massIons accumulate along bunch train, coupling train head and tailAfter several bunches, the ion distribution can affect the beam  coupled oscillations between electrons and ions additional phase shift over bunch train beam size blow-up & emittance growth

17 September 2014

Low emittance rings 2014, L. Mether

36

Slide37

Fast Beam Ion Instability

Ions accumulate along a train of bunches, coupling head and tail of trainOffset of each bunch is reflected in generated ion distribution, and thus transferred to the following bunches

17 September 2014

Low emittance rings 2014, L. Mether

37

Slide38

Fast Beam Ion Instability

Ions accumulate along a train of bunches, coupling head and tail of trainOffset of each bunch is reflected in generated ion distribution, and thus transferred to the following bunches

17 September 2014

Low emittance rings 2014, L. Mether

38

Coherent multi-bunch instability

Oscillation expected to be at a main frequency related to the ion oscillation frequency

Tune shift

towards end of bunch train

Slide39

FBII observations

Signs of trapped ions / FBIICoherent beam instability: beam motion & blow upAdditional phase shift over bunch trainObservations in running machines usually made in presence of vacuum degradationDuring commissioningCaused by impedance heatingDeliberately, with injected gas, for study purposesFast Beam Ion Instabilities have been observed in several machinesAPS (with He injection), PLS (with H2 injection) SOLEIL, SSRF, BESSY II, ELETTRA, ALBA …Measurements at CESR-TA (Dec. 2013, Apr 2014)Varying ion species and pressure, bunch charge, train structure, feedback etc.

17 September 2014

Low emittance rings 2014, L. Mether

39

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