collective effects in the clic damping rings g rumolo in clic workshop 09 14 october 2009 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
New parameter table and 500GeV option Comments on the electron cloud Known results PowerPoint Presentation
Download Presentation
New parameter table and 500GeV option Comments on the electron cloud Known results

Loading in 2 Seconds...

play fullscreen
1 / 21

New parameter table and 500GeV option Comments on the electron cloud Known results - PowerPoint PPT Presentation


  • 85 Views
  • Uploaded on

Collective effects in the CLIC-Damping Rings G. Rumolo in CLIC Workshop 09 , 14 October 2009. New parameter table and 500GeV option Comments on the electron cloud Known results Mitigation techniques Other collective effects Single bunch effects Space charge

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'New parameter table and 500GeV option Comments on the electron cloud Known results' - abiola


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
collective effects in the clic damping rings g rumolo in clic workshop 09 14 october 2009
Collectiveeffects in theCLIC-Damping RingsG. Rumoloin CLIC Workshop 09, 14 October 2009
  • New parametertable and 500GeV option
  • Comments on theelectroncloud
    • Knownresults
    • Mitigationtechniques
  • Othercollectiveeffects
    • Single buncheffects
      • Spacecharge
      • Instabilitiesdue to broad band impedance
    • Multi buncheffects
      • Resistive wall
      • Fast ioninstability
  • Summary
slide2

Updated list of parameterswiththenewlatticedesign at 3 TeV

With combined function magnets

  • Advantages: DA increased, magnetstrengthreduced to reasonable, reduced IBS
  • Relative to collectiveeffects (mainchanges):
    • Higherenergy, larger horizontal emittance (good)
    • Longercircumference (bad)

From Y. Papaphilippou

slide3

Updated list of parametersforthe 500 GeVoption

  • Relative to collectiveeffects (mainchanges):
    • Higherenergy, larger horizontal emittance (good)
    • Longercircumference (bad)
    • Shorter total wigglerlength (good foraperture and fore-cloud)

From Y. Papaphilippou

slide4

Vacuumchambersizes and photoemissionyields

D. Schulte, R. Wanzenberg, F. Zimmermann, in Proceed. ECLOUD‘04

Design of thevacuumchamberwithantechamber in thearcs (it has double sidedante-chamber in thewigglers)

Photoemission yields

Theantechamberabsorbsmost of thesynchrotronradiation andcanbereplacedby a an alternative schemewithdedcicatedabsorbersections. Theneteffectscales down thephotoemissionyield

slide5

Electroncloud (positron ring): howitaffectsthebeam

  • Tune shift
  • The tune increasesalong a train of positivelychargedparticlebunchesbecausethebunches at thetail of thetrainfeelthestrongfocusingeffect of theelectroncloudformedbythepreviousbunches.
  • Electroncloudinstability in rings
  • Coupledbunchphenomenon: themotion of subsequentbunchesiscoupledthroughtheelectroncloud and theamplitude of thecentroidmotioncangrow.
  • Single bunchphenomenon: themotion of head and tail of a singlebunchcanbecoupledthrough an electroncloud and giverise to an instability

Theelectroncloudbuild up cruciallydepends on bunchlength, spacing, current and on thedimensions of thebeamchamber. Itismarginallyinfluencedbythebeamtransversesizes. Therefore, no substantial changeisexpected in thepreviouselectroncloudbuild up

slide6

Electron cloud build up in the wigglers (simulations with Faktor2)

Central densities for different PEYs and SEYs

3

3

  • Theelectroncloud in thewigglerscanhave high densityvaluesif
    • The PEY is high enough (i.e., morethan 0.01% of theproducedradiationisnotabsorbedby an antechamberorbyspecialabsorbers), evenifthe SEY islow
    • The SEY isabove 1.3, independently of the PEY

3

slide7

Instability simulations to check beam stability (simulations done with HEADTAIL)

  • In case of electroncloudbuild up, weassumethesedensityvaluesin arcs and wigglers:

rwig= 1.8 x 1013 m-3 rdip = 3 x 1011 m-3

  • Thebeamisaffectedby a strong and fast instability
  • However, withthenewparameterswemaygain a smallfactorfromthehigherenergy and the larger transverseemittance (assumingthesame total wiggler and dipolelength). Besides, at 500GeV therearefewerwigglers and theintegratedeffect will bemuchlower

* Verticalcentroidmotion

* Verticalemittanceevolution

slide8

Against the electron cloud.....

  • Ifthereiselectroncloud in the CLIC-DR, thebeambecomesunstable!
      • Conventionalfeedbacksystemscannotdampthisinstability (wider band needed)
      • Itisnecessary to find techniquesagainsttheformation of theelectroncloud
  • Severalmitigationtechniquesarepresentlyunderstudy:
    • Low impedanceclearingelectrodes
    • Solenoids (KEKB, RHIC) -howeveronlyusable in fieldfreeregions!
    • Low SEY surfaces -> see M. Taborelli‘stalk on the AEC‘09 Workshop
      • Groovedsurfaces (SLAC)
      • NEG and TiNcoating
      • New coatingspresentlyunderinvestigation (SPS and Cesr-TA)

Carboncoatings,intensivelystudiedbythe SPS Upgrade Working Team, seemverypromising and a possiblesolution.....

slide9

PEY of a C-coatedsurface

  • Run with positrons at 5 GeV, example of intensity scan at Cesr-TA
  • Comparing data with two bunch spacings and train lengths (45 x 14ns, 75 x 28ns). The total electron current is displayed as a function of the beam current.

15W is a C-coated chamber

15E is an Al chamber

Factor 4 less electron flux, to be multiplied by a factor 2 difference of photoelectron in 15W wrt 15E

slide10

Evolution of CNe9 over 1 month

SEY of a C-coatedsurface

Courtesy M. Taborellifrom SPSU-WT

2h

3d

8d

15d

23d

The maximum SEY starts from below 1 and gradually grows to slightly more than 1.1 after 23 days of air exposure.

The peak of the SEY moves to lower energy.

slide11

SINGLE BUNCH: SPACE CHARGE

    • Tune spreadinducedbyspacechargecanbeestimatedanalyticallyintegratingthespacechargeinducedgradient all aroundthelattice to takeintoaccount of thebeamsizechangedue to thebetatronmodulation
    • The horizontal tune spreadis in the order of 0.01 (new 3TeV) and 0.006 (500GeV)
    • Theeffectismuchstronger in thevertical plane becausethebeamissmaller in this plane

New 3TeV

500GeV

- 0.185

- 0.21

slide12

SINGLE BUNCH: INSTABILITIES

    • Longitudinal
      • TheBoussardcriterion (including in theformulathesuppressionfactor (b/sz)2) wouldgive a maximumimpedancevalue of ~5W
  • Transverse
    • The TMCI thresholdisgivenbytheformulabelow
    • TheCLIC-DRsare in shortbunchregime, and theformulatranslatesinto a tolerable impedancevalue of ~15 MW/mifwr=2p x 6 GHz
slide13

COUPLED BUNCH INSTABILITY FROM RESISTIVE WALL (I)

    • Transverse

~0.2ms (200 turns)

  • Pessimistic estimate because:
  • wigglers only cover half of the ring, which gives possibly a factor 2
  • instability rate has to be scaled by nb/M, because the formulae assume a uniformly filled ring.
slide14

COUPLED BUNCH INSTABILITY FROM RESISTIVE WALL (II)

    • Transverse
  • The lowest rise time does not change much with the new design
  • it is slightly higher for the new 3TeV design (~0.3ms or 200 turns)) and slightly lower for the 500GeV design (~0.1ms or 70 turns)
slide15

COUPLED BUNCH INSTABILITY FROM RESISTIVE WALL (III)

    • HEADTAIL simulations

z

L

W(z)

s

e

q

  • PresentsimulationmodelforHEADTAILmulti-bunch:
    • A bunchtrain (made of disk-likemacroparticlesets) istrackedthroughoneormoreinteractionpointschosenaroundthe ring
    • All particles in bunchessubsequent to thefirstonefeel a transverse kick in each point resultingfromthesum of theresistive wall contributions (integratedoverthe distance L betweenpoints) of all theprecedingbunches.
slide16

COUPLED BUNCH INSTABILITY FROM RESISTIVE WALL (IV)

    • HEADTAIL simulationswiththeparameters of thenew 3TeV design

Head of the bunch train

  • In the plot
  • Superposition of snapshots of the bunch by bunch vertical BPM signal taken every 50 turns during the first 5000 turns of evolution
  • The unstable wave develops at the tail of the train and propagates to the front
slide17

COUPLED BUNCH INSTABILITY FROM RESISTIVE WALL (V)

    • HEADTAIL simulations

Linear scale

Log scale

1/e

t=1ms

  • In the plot
  • The evolution of the vertical centroid of the train exihbits an exponential growth in both the horizontal (slow) and vertical (fast) plane
  • The rise time is larger than the calculated one because the simulation takes into account the real wiggler length and the train length
slide18

FAST ION (I)

Ions: residual gas ionization

CO molecule

CO+ ion

Nee- beam

l ≈ 4sz

The ions produced by gas ionization can be focused by the electric field of the following bunches and they accumulate in the vicinity of the beam (trapping condition)

Theioncloudcanaffectthemotion of thebunches and an unstableion-electroncoupledmotioncanbeexcited

Two-streaminstability

slide19

FAST ION (II)

Theionstrappedaroundthebeamarethosehaving a massnumberabove a criticalvalue, whichdepends on thelocation in the ring (due to the different betafunctions, and therefore different beamsizes)

CO, N2

H2O

Thismeansthatmoleculeslike N2, CO aretrappedaroundthebeamalmostalongthefull ring. H2O isbelowthethresholdfortrappingover a shorterlengthforthe 500 GeVdesign

slide20

FAST ION (III)

Thetrappedmolecules (likeN2, CO, H2O) can cause tune shift and instability

Trappedions cause tune spread (p=1 nTorr)

and a fast instabilityhaving a rise time offewturnsforbothdesigns, calculatedwiththefollowingformula.

conclusions
Conclusions
  • SomecollectiveeffectshavebeenestimatedwiththenewparametersfortheCLIC-DRs (also forthe 500GeV option)
  • Theelectroncloudbuild up isnotexpected to changemuch in thepositron ring and posesconstraints on PEY and SEY of thebeampipe.
    • Wigglersshouldbedesigned such as to beable to absorb99.9% of theproducedsynchrotronradiation
    • Themaximum SEY shouldbekeptbelow 1.3
    • Special chambercoatings(understudy) couldbe a viableoption
  • Spacechargecauses a verylarge tune spreadin thevertical plane (up to -0.2). Tolerable?
  • Instabilities (impedances)
    • Single bunch: theycanbeavoidedwith a smoothimpedancedesign
    • Resistive wall coupledbunch: risetimes of 100s of turns, thereforetheycanbecontrolledwithfeedback
  • Fast ioninstability:
    • Dangerous moleculesarelikely to betrapped all alongtheelectron ring
    • In absence of a wide band feedbacksystem, itposes a seriousconstraint on theacceptablevacuumpressure(0.1 nTorr)