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New parameter table and 500GeV option Comments on the electron cloud Known results

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

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New parameter table and 500GeV option Comments on the electron cloud Known results

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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.....

  9. 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

  10. 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.

  11. 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

  12. 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

  13. 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.

  14. 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)

  15. 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.

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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.

  21. 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)

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