Status of GaAs Photoemission Guns at JLab M. Poelker, P. Adderley, M. Baylac, J. Brittian, D. Charles, J. Clark, J. Grames, J. Hansknecht, M. Stutzman, K. Surles-Law CEBAF: Gun lifetime Parity violation experiments Commercial Ti-sapphire lasers Superlattice Photocathodes Atomic H/D exposure Ion Pump supply with nA current monitoring JLab IRFEL: 10 kW output power 350 kV DC GaAs gun 9 mA CW ave current New Photoinjectors: 10’s of mA ave current at high polarization. Obstacles? Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility M. Poelker, PESP2004, Oct.7-9, Mainz, Germany

CEBAF 2-Gun Photoinjector New cleanroom for lasers

Gun Charge Lifetime Measured over Gun Charge Lifetime Steadily Decreasing NEG pump replacement Summer 2003 improves lifetime M. Baylac 10 NEG pumps surround cathode/anode gap

Parity Violation Experiments at CEBAF Experiment Physics Asymmetry Max run-average helicity correlated Position Asym. Max run-average helicity correlated Current Asym. HAPPEX-I 13 ppm10 nm1.0 ppm HAPPEX-II1.3 ppm2 nm0.6 ppm HAPPEX-He8 ppm3 nm0.6 ppm G02 to 50 ppm20 nm1.0 ppm Qweak0.3 ppm20 nm0.1 ppm P-REx1 nm0.1 ppm completepartially completeapproved

G0 Forward Angle was a Difficult Experiment Very long experiment!  “commissioning” run: August 2002 – January 2003  “engineering” run: October 2003 – February 2004  “production” run: March 2004 – May MHz time structure caused problems;  Homemade lasers could not generate 70 ps pulses  Homemade lasers were not long-term stable  High bunch charge caused beam handling problems Managing helicity correlated position asymmetry was painful, at least initially.

Injector acceptance 111 ps 1 uA 2 uA 8 uA 20 uA 40 uA 70 uA Significant bunchlength growth Beam profile distortion when laser spot tightly focused at photocathode Lots of beamloss at apertures Current from gun Current delivered to Hall Aperture loss 8 times Higher Bunchcharge; ~ 2 pC/pulse

G0 Exp used active feedback to control asymmetries - Pockel cell aligned using spinning linear polarizer -Rotating halfwaveplate sets asymmetries close to zero before turning feedback ON. - IA cell (waveplate+pockel cell+linear polarizer) used to correct charge asymmetry. - PZT mirror used to correct position asymmetries. - Helicity correlated charge and position asymmetry seen to converge to zero over the course of a few hours. Helicity correlated differences integrated over a few hours. Correlations: - Position differences induce charge asymmetry through scraping. - Charge asymmetry induces energy and position differences through rf cavity loading. - Position differences “appear” as energy differences at the dispersive point.

Helicity Correlated Beam Specs for g0 Forward Total of 744 hours (103 Coulombs) of parity quality beam with a 4  cut on parity quality. All parity quality specs have been achieved!! Beam Parameter Achieved (IN-OUT) “Specs” Charge asymmetry ± 0.28 ppm 1 ppm x position differences 6 ± 4 nm20 nm y position differences 8 ± 4 nm20 nm x angle differences 2 ± 0.3 nrad2 nrad y angle differences 3 ± 0.5 nrad2 nrad Energy differences 58 ± 4 eV75 eV

Commercial Laser was critical to success of experiment. Cavity length ~ 5 m Tunable over ~ 20 nm at 780 nm and 850 nm. Etalons provide range of pulsewidths from 10 p to 70 ps. More than 250 mW power

499 MHz modelocked Ti- sapphire lasers for high current polarized beam experiments (~ 100 uA ave.) 300 to 500 mW output power 770 nm or 850 nm operation Stable phase-locked pulse train Maintenance required

HAPPEx Approach In the Laboratory;  Compare pockels cells from different vendors  Choose cell with smallest birefringence gradient In the Tunnel;  Without cell, verify good optics and laser polarization.  Align insertable halfwaveplate for 90 degree rotation.  Align pockel cell  Minimize optical beam steering by positioning laser on geometric center of cell. With Beam;  Verify laser table measurements; plot e-beam HC position and intensity asymmetry vs rotating halfwave plate orientation for “ideal” cell voltages. Asymmetries should be small.  Adjust rotating waveplate orientation and cell voltages to minimize beam asymmetries. Use IA cell to apply charge asymmetry feedback. Minimize asymmetries at outset via careful choice and alignment of pockel cell. No position feedback.

Gun2 strained layer GaAsGun3 superlattice GaAs From HAPPEx-H 14 mm Gun3 lifetime was poor Frequent laser spot moves were necessary QE holes caused HC asymmetry variations Final beam quality numbers pending but preliminary results indicate HC specifications were met;

Superlattice Photocathode from SVT The highest polarization yet measured at CEBAF; ~ 85% QE 0.8% versus 0.15% Analyzing power 4 % versus 12% Preliminary photon electron From Hall A Compton Polarimeter Polarization QE (%) Analyzing Power Wavelength (nm) here

SVT Superlattice Summary Highest polarization ever measured at JLab: P = 85% Measurements of many samples at test stand indicates this is no fluke. 5 times higher QE than strained layer GaAs material (not yet demonstrated at tunnel). Smaller analyzing power should provide smaller inherent charge and position asymmetry. (Recent HAPPEx results do not support this claim.) We suffered surface charge limit. QE drops with increasing laser power. A concern for high current experiments like Qweak. QE not constant

QE vs hydrogen cleaning Hydrogen exposure time (min) QE (%) Drawback: Superlattice material is delicate Can’t clean with atomic hydrogen Makes it tough to anodize edge of photocathode Typical H-dose to clean anodized samples From M. Baylac et al., in press

Ion Pump Locations Pumps detect bad orbit and beamloss Gun chamber pump Y-chamber Laser chamber Wien filter Ion Pump Power Supplies with nanoA Current Monitoring Designed and constructed by J. Hansknecht “Free” pressure monitoring at 10^-11 Torr

Ave. Power (kW) 10 kW for 1 sec. (2.5 kW ave power at ¼ DF) Courtesy C. H. Garcia and JLab FEL team

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility FEL The JLAB FEL Injector is driven by a 350 kV DC GaAs Photocathode Gun 40 cm PHOTOCATHODE PERFORMANCE Photocathode QE~6% after initial activation at 532 nm Photocathode delivers ~200 C between re-cesiations Typical day of operations draws ~35 Coulombs About 96% of previous QE is recovered with each re-cesiation 12 activated cathodes and close to 40 re-cesiations performed on a single GaAs wafer in one year Photocathode Ball cathode Drive Laser Courtesy C. H. Garcia and JLab FEL team

Demonstrated DC Photocathode Gun performance at JLab IR-FEL Macropulse operation at 8 mA, 16 ms-long pulses at 2 Hz repetition rate. RF microstructure within macropulse. CW operation at 9.1 mA with 75 MHz microstructure and 122 pC/bunch. Gun routinely delivers 5 mA in pulsed and CW modes with 135 pC/microbunch. Vacuum environment: 4.0E-11 Torr, 99.9% H 2 Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility FEL Courtesy C. H. Garcia and JLab FEL team

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy M. Poelker, EIC Workshop, March 15-17, 2004 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. Big difference compared to past talks. Few mA’s versus >> 100 mA of highly polarized beam.

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

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy ELIC e-Beam Specifications Typical parameters; Ave injector gun current 2.5 mA (and then 25 mA) Micropulse bunch charge 1.6 nC Micropulse rep rate 150 MHz (and then 1.5 GHz) Macropulse rep rate ~ 2 kHz, 5 usec duration.

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy M. Poelker, EIC Workshop, March 15-17, 2004 Need 80% polarized e-beam. Use SVT superlattice photocathode. 1% QE at 780 nm; 6.3 mA/W/%QE ~ 1 W provides 1/e operation at 2.5 mA 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? Probably wise to improve vacuum (more later) Gun HV ~ 500 kV to mitigate emittance growth. Must limit field emission. Gun Issues for ELIC

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy M. Poelker, EIC Workshop, March 15-17, 2004 Gun Lifetime CEBAF enjoys good gun lifetime;  ~ 200 C charge lifetime (until QE reaches 1/e of initial value)  ~ 100,000 C/cm2 charge density lifetime (we operate with a ~ 0.5 mm dia. laser spot) Gun lifetime dominated by ion backbombardment. So it’s reasonable to assume lifetime proportional to current density. Use a large laser spot to drive ELIC gun. This keeps charge density small. Expect to enjoy the same charge density lifetime, despite higher ave. current operation, with existing vacuum technology.

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy M. Poelker, EIC Workshop, March 15-17, 2004 Use 1 cm diameter laser spot at photocathode. At 2.5 mA gun current, we deliver 9 C/hour, 216 C/week. Charge delivered until QE falls to 1/e of initial value; Need to test the scalability of charge lifetime with laser spot diameter. Measure charge lifetime versus laser spot diameter in lab. (J. Grames presentation this workshop) Gun Lifetime cont. 100,000 C/cm2 *1 Wk/216 C * 3.14(0.5 cm)^2 = 360 Wks! 36 Weeks lifetime at 25 mA. Lifetime Estimate;