09/10/2007 Polarized Source for eRHIC Evgeni Tsentalovich MIT

09/10/2007 OUTLINE Introduction Existing gun review Gun for eRHIC Linac-ring version: –Peak current –Average current –Heat load Work plan Conclusion

09/10/2007 Ring – ring version AGS BOOSTER TANDEM S RHIC 2 – 10 GeV e- ring e-cooling 2 -10GeV Injector LINAC Proven technology. No major R&D required. Luminosity limited to

09/10/2007 Linac – ring version PHENIX STAR e-cooling Two e-beam passes: 6.8 and 10 GeV e + storage ring 5 GeV 1/4 RHIC circumference Main ERL (3.2 GeV per pass) Allows higher luminosity. Requires development of Energy Recovery Linac (ERL) and high intensity Polarized Electron Source (PES)

09/10/2007 Bicycle Space flights ERL MP3 Lasers Birth control Popcorn TV PES for eRHIC Perpetuum Mobile Immortality Human cloning Interstellar travel Invisibility Antigravitation Computers MODERN TECHNOLOGY

09/10/2007 eRHIC gun (linac-ring) Extremely high current demand !!! Average laser power ~ 80 W (fresh crystal) Hundreds Watts might be needed as crystal loses QE Luminosity ~  I(average) ~ 250 mA I(peak) ~ 100 A High polarization → strained GaAs → QE ~ 0.5%

09/10/2007 Existing guns: SLAC V = 120 kV Active spot 15 mm

09/10/2007 Existing guns : Nagoya V = 200 kV Active spot 18 mm

09/10/2007 Existing guns : Cornell DESIGN: V = 750 kV, I =100 mA Achieved: V = 300 kV, I =5 mA No polarization, operated with blue =525 nm light

09/10/2007 Existing guns : Bates V = 60 kV Active spot  12 mm

09/10/2007 Existing guns : JLAB V = 100 kV Active spot  0.2 mm Gun for FEL V=350 kV I=9 mA (green light, no polarization) Tests with green light (no polarization) I > 1 mA

09/10/2007 Existing guns : Mainz V = 100 kV Active spot .25 mm Recent results on a test bench: Bulk GaAs (P~ 40%) Active spot ~  2 mm I=0.5  2 mA (up to 11 mA) Lifetime ~ 70 C (20 hours at 1 mA)

09/10/2007 Existing polarized guns I(peak)I(average) Beam  Polarization SLAC10 A 5  A 15 mmHigh Bates30 mA 120  A 12 mmHigh JLAB 120  A.2 mmHigh Mainz 50  A.25 mmHigh Mainz2 mA2 mmLow

09/10/2007 Main challenges High average current – cathode damage by ion bombardment High peak current – surface charge saturation (QE drops at high light intensity); space charge saturation High heat load on the cathode – tens or hundreds of Watts of laser power Solution: Cathode with very large area

09/10/2007 Photocathodes degradation Poisoning by residual gases Ion bombardment Oxygen- and carbon-containing species are more harmful Hydrogen and noble gases are more tolerable This degradation can be healed by heat-cleaning at moderate temperatures (<550  C) Most harmful Only high-temperature (~600  C) heat cleaning restores QE, and only partially Effect is proportional to pressure in the chamber and to average current

09/10/2007 Average current (~250 mA) Current of ~ 1mA with lifetime ~ 20 hours has been achieved with the active spot of  ~ 2 mm. If we increase spot to  2 cm, will we get 100 mA with the same lifetime ? residual gas cathode Ionized residual gas strikes photocathode anode Ion damage distributed over larger area

09/10/2007 Damage location Electrons follow electrical field lines, but massive ions have different trajectory. Usually, they tend to damage central area of the cathode. Laser spot Cathode Damage groove JLAB data Ring-like cathodes ? Emittance ?? Beam losses? Attractive option, but requires serious investigation

09/10/2007 Additional opportunity Multiple gun approach (BNL idea) RF combiner This approach may reduce the average current requirements by order of magnitude. Perhaps average current of ~ 50 mA will be sufficient to satisfy luminosity requirements.

09/10/2007 Peak current (~100 A) (Not absolutely necessary, duty factor could be increased with RF pulse compressor after the gun… But for the price of the emittance growth) For DC gun : Space charge saturation Surface charge saturation

09/10/2007 Charge saturation Vacuum level E x surface

09/10/2007 Charge saturation High doping →low polarization ! (SLAC data)

09/10/2007 High gradient doping Substrate Buffer Superlattice High ( )doped layer ~ 5 nm Works very well The high-doped layer is thin enough to preserve high polarization Charge saturation is highly suppressed (at least for fresh crystals) The top layer can survive only few high-temperature (~600 C) activations Might be problematic for high-current guns Large area cathode solves the problem

09/10/2007 Heat load (80 W on the cathode) With a conventional cathode stalk system, the cathode would heat up to stellar temperatures, but, fortunately, melt first. HEAT t=1 mm ACTIVE COOLING GaAs ( for average current of 250 mA ) Large area cathode improves the situation

09/10/2007 Tasks In order to design a gun with large cathode area very detailed calculations must be performed (especially beam losses) Even more complicated calculations are needed for ring-like cathode geometry Experimental measurements require cathode with active cooling

09/10/2007 Work plan Phase I: oGun simulation (including ring geometry) oDesign of the cathode with active cooling Phase II: o Design and construction of the gun o Design and construction of the beam line o Lifetime measurements at different currents

09/10/2007 Phase I - calculations Achieve the required current of at least 50 mA, up to 250 mA Calculate ion trajectories and optimize gun geometry to minimize ion damage Ensure that no beam scraping takes place in the gun vicinity Estimate and optimize the emittance of the beam after the gun Design the electron optics of the beam line following the gun to transport the beam from the gun to a beam dump The calculations will be conducted with a 3-D code that includes space charge effects.

09/10/2007 Phase I – cooling system design Cooling system attached to the rear side of the photocathode Design should allow crystal replacements without braking vacuum Good thermal connection to the crystal UHV compatibility High (~100 kV) voltage compatibility Temperature monitoring Test chamber will be built, and the cathode will be heated by diode laser with cathode temperature monitored. HV and UHV compatibilities will be tested.

09/10/2007 Phase II – prototype gun and beam line design Prototype gun will be designed and built according to the results of the simulations in Phase I Cathode cooling system designed in Phase I will be incorporated Load lock and preparation chamber Beam line: two 90  turns, beam dump, lenses, steering coils Pumps: ion pumps and NEGs Diagnostics: flip screens, toroidal pickups, later – wire scanners for emittance measurements

09/10/2007 Gun with beamline Prep. Ch. Tr. Vessel Man. Manipulator Sol Dipole Sol Dump Sol Dipole GUN Sol

09/10/2007 Conclusion MIT-Bates in collaboration with BNL will study the possibility to build a very high intensity polarized electron gun Large area cathode will be implemented Ring-like cathode geometry will be investigated Active cooling of the cathode will be used