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Two beam instabilities in low PowerPoint Presentation, PPT - DocSlides - emittance. rings. Lotta. . Mether. , . G.Rumolo. , . G.Iadarola. , . H.Bartosik. Low . Emittance. . Rings Workshop. INFN-LNF, . Frascati. September 17. th. , 2014. Two beam instabilities. Collective effects .

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Two beam instabilities in low PowerPoint Presentation, PPT - DocSlides

    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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