MEIC Electron Cooler Design Concept

EC potential impact to colliders Reaching a high start luminosity Very short i-bunches achieved by longitudinal cooling in combination with SRF (cannot be attained with stochastic cooling!) make sense to design a super-strong focusing (low beta) at IP Short bunches allow one to employ the crab-crossing beams, thus avoiding the parasitic b-binteractions Low transverse emittance + high rep. rate allow one to minimize charge/bunch Extending the luminosity lifetime EC suppresses beam heating and luminosity loss caused by multiple and Touschek IBS

ion bunch electron bunch Cooling section solenoid HEEC basics Magnetized e-gun Injector SRF linac Cooling time grows with Therefore: staged cooling Cooling conditions: Co-moving “cold” electron beam serves as thermostat for a hot ion beam (i – e Coulomb collision exchange)

Parameter (p/e) UnitValue Beam energy GeV150/7 Energy of cooling beam MeV75 Bunch rep rate GHz1.5 Particles/bunch10 0.2/1 Beam current A0.5/2.5 Cooling current A2.5 Horizontal emittance* mm 1/100 Vertical emittance* mm 0.01/1 Number of interaction points 4 Total beam-beam tune shift 0.04/0.16 Laslett’s tune shift in p-beam 0.02 Luminosity overall IP (10 35) cm -2 s -1 2 Cooling/IBS time in p-beam core min5 Luminosity Touschek’s lifetime h20 High luminosity colliding beams Parameter (p/e) UnitValue EnergyGeV/MeV20/10 Cooling length/ circumf.%1 Particles/bunch10 0.2/1 Energy spread** /1 Bunch length**cm20/3 Proton emittance, norm** mm 4 Cooling timemin10 Equilibrium emittance, * mm 1 Equilibrium bunch length*cm2 Laslett’s tune shift0.1 Initial electron cooling ** max. amplitude * norm. rms Staged EC

Staged Cooling in Ion Collider Ring Initialafter boostColliding Mode EnergyGeV/MeV15 / / proton/electron beam currentA0.5 / 1.5 Particles/Bunch / 2 Bunch lengthmm(coasted)10 / 20~30 Momentum spread / 25 / 23 / 2 Hori. & vert. emittance, norm.µm4 / / 0.07 Laslett’s tune shift (proton) Initial cooling after ions injected into the collider ring for reduction of 3d emittance before acceleration After boost & re-bunching, cooling for reaching design values of beam parameters in colliding mode Continuous cooling during collision for suppressing IBS, maintaining luminosity lifetime

High Energy e-Cooler for Collider Ring Design Requirements: up to 10.8 MeV for cooling at injection energy (20 GeV/c) up to 54 MeV for cooling top proton energy (100 GeV/c) Cooling e-beam current : up to 1.5 A CW beam at 750 MHz repetition rate About 2 nC bunch charge (possible space charge issue at low energy) Solution: ERL Based Circulator Cooler (ERL-CCR) Must be an SRF Linac for accelerating electron beam Must be Energy Recovery (ERL) to solve RF power problem Must be Circulator -cooler ring (CCR) for reducing current from source/ERL ERL-CCR is considered to provide the required high cooling current while consuming fairly low RF power and reasonable current from injector

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 (Layout A)

ERL Circulator Electron Cooler ion bunch electron bunch Cooling section solenoid (Fast) kicker SRF Linac dump injector (Layout B)

Optimized Location of Cooling Channel 10 m Solenoid (7.5 m) SRF injector dumper Eliminating a long circulating beam-line could cut cooling time by half, or reduce the cooling electron current by half, or Center of Figure-8 (Layout C)

Cooler Design Parameters Max/min energy of e-beamMeV54/11 Electrons/bunch bunch revolutions in CCR~100 Current in CCR/ERLA1.5/0.015 Bunch repetition in CCR/ERLMHz750/7.5 CCR circumferencem~80 Cooling section lengthm15x2 Circulation duration ss 27 RMS Bunch lengthcm1-3 Energy spread Solenoid field in cooling sectionT2 Beam radius in solenoidmm~1 Beta-functionm0.5 Thermal cyclotron radius mm 2 Beam radius at cathodemm3 Solenoid field at cathodeKG2 Laslett’s tune MeV0.07 Longitudinal inter/intra beam heating ss 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.

Issues Space charge limitations in CCR: Coulomb interaction (non-linear Laslett detune) CSR Intra- and Inter-Beam Scattering in CCR Source/Injector/ERL/CCR beam matching gymnastics Magnetized cathode Matching with cooling solenoids, straights and arcs Beam size at cathode and related canonical emittance Other agendas? ( space charge dominated beam in axial optics …) Fast kicker (beam-beam or other) And more…

Backup slides

Parameter UnitValue Max/min energy of e-beamMeV75/10 Electrons/bunch10 1 Number of bunch revolutions in CR1001 Current in CR/current in ERLA2.5/0.025 Bunch rep. rate in CRGHz1.5 CR circumferencem60 Cooling section lengthm15 Circulation duration ss 20 Bunch lengthcm1 Energy spread Solenoid field in cooling sectionT2 Beam radius in solenoidmm1 Cyclotron beta-functionm0.6 Thermal cyclotron radius mm 2 Beam radius at cathodemm3 Solenoid field at cathodeKG2 Laslett’s tune shift in CR at 10 MeV0.03 Time of longitudinal inter/intrabeam heating ss 200 ERL-based EC with circulator ring

Technology: Ultra-Fast 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 Beam-beam kicker

Electron source e-gun V 500 KeV Pulse duration 0.33 ns Bunch charge 2 nC Peak current 0.65 A Emittance, norm 1 mm.mrad Rep.rate 15 MHz Average current 30 mA 1 st compressor Prebuncher frequency 500 MHz Voltage 0.2 MV Energy gradient after prebuncher 2x 10% 1 st drift 2 m Bunch length after 1 st compression 1 cm Beam radius (assumed value) 2 mm Coulomb defocusing length 30 cm 1 st accellerator cavity Voltage 2 MV Frequency 500 MHz Beam energy 2.5 MeV 2 nd compressor Buncher frequency 1.5 GHz Energy gradient 2 x 10% 2 nd drift 1.8 m Bunch length, final 0.5mm Beam radius 2 mm Coulomb defocusing length 35 cm Estimates for Injector to ERL