Page 1 Review 09/2010 MEIC ERL Based Circulator Electron Cooler Yaroslav Derbenev and Yuhong Zhang
Page 2 Review 09/2010 Outline Introduction Staged Electron Cooling ERL based Circulator Cooler Enabling Technologies Summary
Page 3 Review 09/2010 Introduction Electron cooling is a critical component of conceptual design of MEIC, responsible for achieving high luminosity (~10 34 cm -2 s -1 ) Assisting accumulation of low energy ion beams An order of magnitude reduction of emittances Achieving very short (cm or less) bunch Suppressing IBS induced beam heating MEIC design employs staged cooling scheme, i.e., cooling ions in pre-booster (low energy, DC, mature technology) in the collider ring (~ 60 GeV, beyond state-of-art, focusing of this talk) continuous cooling during collision An ERL based circulator cooler is proposed for delivering high average current high beam power cooling electron beam ERL, fast kicker are two key technologies required for this cooler
Page 4 Review 09/2010 Staged Cooling in MEIC Ion Collider Ring GeV/MeVInitial Cooling after boost and bunching Colliding Mode EnergyGeV/MeV15 / / Beam currentA1 / 3 Particles/Bunch / 3.75 Bunch lengthmm(coasted)10 / 20~30 Momentum spread / 25 / 23 / 2 Horizontal emittance, norm.µm40.35 Vertical emittance, norm.µm40.07 Laslett’s tune shift (proton) Cooling length/circumferencem/m15 / 1000 Initial electron cooling after ion beams injected into the collider ring for reduction of longitudinal emittance before acceleration After energy boost and RF debunching/rebunching, electron cooling for reaching final design values of beam parameters Must continuous cooling ion beam for suppressing IBS and maintain high beam quality and luminosity lifetime
Page 5 Review 09/2010 Cooling Time and IBS Growth Time formulaLongitudinalHorizontalVertical IBSPiwinskis (?) IBSDerbenevs IBSMartini (BetaCool) s CoolingDerbenevs~7.9 Assuming I e =3 A, 60 GeV/32.67 MeV Cooling section 30 m
Page 6 Review 09/2010 Design of Electron Cooler Requirements and Challenges Cooling beam Up to 3 A CW beam at GHz repetition rate, about 2 nC bunch charge About 260 kC per day from source/photo-injector (state-of-art is 0.2 kC per day) 1 st challenge: cathode life time Energy of cooling electron beam up to 33 MeV for cooling 60 GeV medium energy protons Beam power Up to 100 MW 2 nd challenge: RF power Design Choice: ERL Based Circulator Cooler (ERL-CCR) Energy Recovery Linac (ERL) solving 1 st challenge, RF power Circulator-cooler ring (CCR) solving 2 nd challenge, reducing average current from source/ERL
Page 7 Review 09/2010 Conceptual Design of Circulator e-Cooler ion bunch electron bunch Electron circulator ring Cooling section solenoid Fast beam kicker SRF Linac dump electron injector energy recovery path Circulator ring by-pass Path length adjustment (synchronization) Electron bunches recirculates 300+ times Reduction of current from photo-injector/ERL by a factor of 300+
Page 8 Review 09/2010 Location Does Matter ! 10 m Solenoid (7.5 m) SRF ERL injector dumper Eliminating return path of the circulator ring Double the cooling rate
Page 9 Review 09/2010 ELIC e-Cooler Design Parameters Max/min energy of e-beamMeV33/8 Electrons/bunch bunch revolutions in CCR~300 Current in CCR/ERLA3/0.01 Bunch repetition in CCR/ERLMHz500/1.67 CCR circumferencem80 Cooling section lengthm15x2 Circulation duration ss 27 Bunch lengthcm1-3 Energy spread Solenoid field in cooling sectionT2 Beam radius in solenoidmm~1 Beta-functionm0.5 Thermal cyclotron radius mm 2 Beam radius at cathodemm3 Solenoid field at cathodeKG2 Laslett’s tune MeV0.07 Longitudinal inter/intra beam heating ss 200 Number of turns in circulator cooler ring is determined by degradation of electron beam quality caused by inter/intra beam heating up and space charge effect. Space charge effect could be a leading issue when electron beam energy is low. It is estimated that beam quality (as well as cooling efficiency) is still good enough after 100 to 300 turns in circulator ring. This leads directly to a 100 to 300 times saving of electron currents from the source/injector and ERL.
Page 10 Review 09/2010 Electron Source/Injector MEIC Circulator Cooler driving photo-injector, assuming 300 recirculations 10 MHz, up to 33 MeV energy, 2 nC bunch charge, magnetized Challenges Source life time: 0.86 kC/day (state-of-art is 0.2 kC/day) still a factor of 4 source R&D, & exploiting possibility of increasing evolutions in CCR Conceptual design High current/brightness source/injector is a key issue of ERL based light source applications, much R&D has been done We adopt light source injector as a baseline design of CCR driving injector Beam qualities should satisfy electron cooling requirements (based on previous computer simulations/optimization) Bunch compression may be needed. 300keV DC gun solenoids buncher SRF modules quads
Page 11 Review 09/2010 Circulator Ring & Synchronization Transverse focusing lattice Bunch In/out kicking An ultra fast kicker switches electron bunches in/out circulator ring. Deflecting angle should be large enough to separate outgoing bunches from circulating bunches and be further deflected by a dipole Duration of kicking should be less than bunch spacing (~ 0.67 ns) Synchronization Bunch spacing depends on beam energy, about 1.8 mm difference from 12 to 60 GeV/c energyMeV33 Kick angle0.04 Integrated BDLGM400 Frequency BWGHz2 Kicker aperturecm2 Repetition RateMHz5 PowerkW13 Kicker Parameter
Page 12 Review 09/2010 Energy Recovery Linac SRF ERL based FEL High average power, up to14 kW (world record) mid-infrared spectral region Extension to 250 nm in the UV is planned Photocathode DC injector, 10 mA class CW beam, sub-nC bunch charge Beam energy up to 200 MeV, energy recovery JLab FEL Program Energy Recovery EnergyMeV Charge/bunchpC135 Average currentmA10 Peak currentA270 Beam powerMW2 Energy spread%0.5 Emittance, normµm-rad<30 JLab is world leader in ERL technology !
Page 13 Review 09/2010 Ultra-Fast Beam-Beam Kicker h v0v0 v≈c surface charge density F L σcσc D kicking beam A short (1~ 3 cm) target electron bunch passes through a long (15 ~ 50 cm) low-energy flat bunch at a very close distance, receiving a transverse kick The kicking force is integrating it over whole kicking bunching gives the total transverse momentum kick Proof-of-principle test of this fast kicker idea can be planned. Simulation studies will be initiated. Circulating beam energyMeV33 Kicking beam energyMeV~0.3 Repetition frequencyMHz5 -15 Kicking anglemrad0.2 Kinking bunch lengthcm15~50 Kinking bunch widthcm0.5 Bunch chargenC2 An ultra-fast RF kicker is also under development. V. Shiltsev, NIM 1996
Page 14 Review 09/2010 Summary Electron cooling is essential for forming (through stacking & accumulating) and cooling of the high intensity ion beam for MEIC. Electron cooling is responsible for improving MEIC luminosity by an order of magnitude and more, and is considered the only show-stop for any EIC proposal. Conceptual design of an ERL circulator-ring based electron cooler has been proposed to provide high intensity (3 A) and high energy (up to 33 MeV) cooling electron beam. Estimation and initial simulations indicate that efficiency of electron cooling provided by this cooler is adequate for MEIC Key enabling technologies and critical RD on ERL, circulator ring, high bunch charge electron source and ultra-fast kicker are also discussed and planed.
Page 15 Review 09/2010 Backup Slides
Page 16 Review 09/2010 Advanced Concepts of Electron Cooling Staged cooling Start (longitudinal) electron cooling at injection energy in collider ring Continue electron cooling after acceleration to high energy Sweep cooling After transverse stochastic cooling, ion beam has a small transverse temperature but large longitudinal one. Use sweep cooling to gain a factor of longitudinal cooling time Dispersive cooling compensates for lack of transverse cooling rate at high energies due to large transverse velocity spread compared to the longitudinal (in rest frame) caused by IBS Flat beam cooling (for high energies) based on flattening ion beam by reduction of coupling around the ring IBS rate at equilibrium reduced compared to cooling rate Matched cooling (low energies) based on use of circular modes optics of ions matched with solenoid of cooling section separates cooling of beam temperature from cooling (round) beam area results in removal temperature limit due to space charge (strong reduction of achievable 4D emittance)
Page 17 Review 09/2010 Flat-to-Round Transform & Reduction of Space Charge Flat colliding ion beam and space charge Colliding ion beam should be flat at interaction point in order to match flat electron beam (due to synchrotron radiation) Space charge tune shift is a leading limiting factor for low energy ion beam, and it further effect luminosity of the collider Flat beam enhances space charge tune-shift. i.e., Laslett tune-shift is determined by smaller transverse dimension Luminosity optimization: flat-to-round transform if colliding ion beam can be arranged as flat at interaction point matching flat electron beam Round in the storage maintaining large transverse beam area for overcoming space charge Technical feasibility circular (100% coupled) optics (ring) under matched cooling Special adapters to converting round beam to flat beam and back to round beam at collision point