PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 1 Polarized Electron Sources for Future Electron Ion Colliders M. Farkhondeh, Bill Franklin and E. Tsentalovich MIT-Bates accelerator Center Ilan Ben-Zvi, V. Litvinenko, Brookhaven National Laboratory PST05 Workshop, Tokyo, Japan, November 14-17, 2005

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 2 OUTLINE  Electron Ion Colliders (EIC)  Current EIC designs (eRHIC and ELIC)  Polarized source for eRHIC  ring-ring design  linac-ring concept  Polarized RF gun  Polarized source for ELIC  Summary

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh EIC Kinematics range, a unique region EIC Gluon momentum distribution measured Nucleon spin structure studied High Ecm  large range of x, Q2 Lepton probe provides precision but requires high luminosity to be effective Fixed target experiment Collider Electron Ion Collider

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 4 EIC in USA Currently two designs are under considerations: 1.At BNL and MIT: eRHIC an electron –Ion collider based on the existing RHIC accelerator at BNL. In 2004 produced a zeroth design report (ZDR). Electron ring design and polarized source by MIT-Bates. _start.pdf 2.At Jefferson Lab: ELIC an Electron Light Ion Collider based on a 3-7 GeV ERL linac electron linac, a new electron circulator ring and a new light ion ring.

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 5 eRHIC Polarized electron and positron sources for storage mode of modest peak intensities required

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 6 Very high intensity polarized source is required, I>100 mA Linac Ring Concept

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 7 One accelerating & one decelerating pass through CEBAF ELIC Layout Ion Linac and pre - booster IR Beam Dump Snake CEBAF with Energy Recovery 3-7 GeVelectrons GeV light ions Solenoid Ion Linac and pre - booster IR Beam Dump Snake CEBAF with Energy Recovery 3-7 GeVelectrons GeV light ions Solenoid Ion Linac and pre - booster IR Beam Dump Snake CEBAF with Energy Recovery 3 -7 GeVelectrons GeV (light) ions Solenoid Electron Injector Electron Cooling (A=1-40) Operated by the Southeastern Universities Research Association for the U.S. Department Of Energy Thomas Jefferson National Accelerator Facility Electron circulator ring Source requirements for ELIC less demanding with circulator ring.

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 8 Macroscopic Time structure for eRHIC 120 Bunches 480 mA Collider storage ring (eRHIC) Ion ring 360 Bunches

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 9 QuantityValueUnit Collider Ring (stacking pulses) Stored current Frequency 480 (20ncx120/4.3us) 28. mA MHz Ring circumference Number of bunches in the ring Charge per macroscopic bunch ms nC stacking: pulse train rep. rate Duration Total pulse train from injector Charge per bunch e per bunch ,000 (25x10x60) 1.3 8x10 +6 Hz minutes pC Photocathode Bunch duration Bunch charge Peak current ~ ps pC mA Linac Microscopic duty cycle (within 4.3 us) Macroscopic duty cycle during fill Macropulse average current Average current during fill 2x x mA nA Electron beam parameters for the ring-ring eRHIC

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 10 Peak current requirements for eRHIC (ring-ring ) 450 mA average current in the ring and 120 bunches 10 minutes fill time at 25 Hz injection  pulse trains stacked bunches from the injector each 1.3 pC and ~ ps wide (I=dQ/dt) mA peak current in linac (instantaneous current within each bunch ) With QE of 5x10 -4, =800nm, would need ≥ 50 Watts of peak laser power.

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 11 Polarized source options for eRHIC (ring-ring) Option 1: Modelocked laser Option 2: High power diode array laser Ring timing F_bunch~2-10, I_inj ~ 2-20 mA Need work on the 102 MHz bunching. Time Bandwidth laser J-lab G0 laser: P peak =150 W Need > 50 W Bates laser system >120 W peak

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 12 Linac-Ring specification for the electron beam (based on an ERL-CW electron linac and RHIC) Beam rep-rate[MHz]28.15 RMS normalized emittance [  m] Bunch length at cathode [ps] Electrons per bunch 1-10x·10 10 Charge per bunch [nC] Average e-beam current [A] 0.45 Peak current [A]135 Linac-ring

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 13 Polarized Source for eRHIC linac-ring design Polarized source Using scaling law from current J-lab charge/cm2 and an FEL laser, need to use a large area photocathode. No provision for positron source. ERL-FEL to produce KW of IR laser for Polarized source. A conceptual consideration by BNL

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 14 With kW laser power illuminating a 3 cm 2 surface. k= 0.75 W/cm.C  for GaAs., 0.1 cm thick.,  T = . Too much for an NEA surface with mono layer of Cs atoms. With a molybdenum cathode stock, L=30 cm, S=0.5 cm 2,  T will be too high across the stock without active cooling. Must have active cooling (flowing liquid or cold gas) to remove heat from photocathode and the cathode stock. Photocathode heat dissipation: (For Linac –ring photocathodes) R&D for heat removal from cathode: Design and construct actively cooled cathode Test cathode assembly with cooling using high power diode lasers while monitoring the UHV conditions. 1 mm

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 15 Super conducting RF Gun, BNL and AES Cathode installation cart Cathode installed location Cathode retracted location Vacuum vessel Linear rail system Rail system adjustmen t Under construction at AES for the RHIC electron cooling. GaAs based photocathode tests in this SRF gun may begin in 2 years (BNL, AES and MIT-Bates). Ilan Ben-Zvi, V. Litvinenko BNL Also considered for ILC polarized source (FermiLab meeting, Nov 7, Main issues: 1.base vacuum: UHV? 2.electron back- bombardment in an RF field

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 16 Continuing Trend Towards Higher Average Beam Current ELIC with circulator ring JLab FEL program with unpolarized beam Year Ave. Beam Current (mA) First polarized beam from GaAs photogun First low polarization, then high polarization at CEBAF Source requirements for ELIC less demanding with circulator ring. Few mA’s versus >> 100 mA for required for linac-ring. M. Poelker, EIC2 Workshop, 2004

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 17 Summary  EIC is required within a decade to maintain progress in the study of the fundamental structure of matter  partonic basis of atomic nuclei  spin structure of nucleon  An eRHIC ring-ring accelerator design has been developed based on realistic considerations with luminosity close to cm -2 s -1. Storage ring reduces polarized source requirement for this option. Some R&D for polarized source is required and will be pursued at MIT-Bates. Polarized positrons based on self polarization in the ring is considered.  A more futuristic linac-ring eRHIC concept is also under consideration that requires very high intensity CW polarized source > 100 mA. An FEL based laser system is envisioned. Also, a superconducting polarized Rf gun may be considered for long term.  An ELIC design at J-Lab is under consideration based on a CW ERL-linac and a circulating ring. The circulating ring reduces the current requirement compared to a linac-ring design but complicates the time structure requirements. R&D in laser systems for this design at J-Lab is need.

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 18 Bates polarized Source facility

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 19 Additional slides

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 20 Gun Issues for ELIC Need 80% polarized e-beam. Use SVT superlattice photocathode. 1% QE at 780 nm; ~ 1 W provides 1/e operation at 2.5 mA (if CW) Commercial Ti-Sapp lasers with CW rep rates to 500 MHz provide 0.5 W. Homemade lasers provide ~ 2W. Injector micropulse/macropulse time structure demands laser R&D. 25 mA operation requires more laser power and/or QE. Charge Limit? Yes, at 1.6 nC/bunch and low QE wafers. Lifetime? Can benefit from further vacuum improvement Gun HV ~ 500 kV to mitigate emittance growth. Must limit field emission. M. Poelker, EIC2004 Workshop R&D needed on all of the above areas.

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 21 Option 2: F_bunch~2-10,  _cap~0.5 at Linac frequency I_inj ~ 2-20 mA Need work on the 102 MHz bunching. P peak =150 W CW-1 kHz Linac frequency

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 22 Option 1: P peak =50 W For ring I av =450 mA, need I peak =18 mA from injector Seems there is enough laser power. But need some R&D to test such lasers for this application. Surface charge limit and lifetime. Time Bandwidth laser J-lab G0 laser: P peak =150 W

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh 23 MIT-Bates 60 keV test beam setup Source R&D at Bates using test beam setup R&D on the eRHIC polarized source at Bates Laser R&D for the two concepts of eRHIC injector