M. Poelker, SPIN 2008, October 10, 2008 Workshop on Polarized Electron Sources and Polarimeters PESP 2008

M. Poelker, SPIN 2008, October 10, 2008 A tradition since 1983…but see unpleasant trend Facility Bonn-ELSA CEBAF-JLab Mainz Microtron MIT Bates Nagoya/JLC SLAC SLC NIKHEV Reduced Scope Closed …

M. Poelker, SPIN 2008, October 10, 2008 PESP2008 – Enhanced Scope  Idea: Invite participants from unpolarized e-beam programs…. note shared technology  More than 80 registered attendees!!  37 talks!!

M. Poelker, SPIN 2008, October 10, 2008 PESP 2008 Agenda Polarized e-Beam Programs – with focus on NEW and proposed machines Photocathodes: High Polarization, novel structure, obtaining longer lifetime Very High Voltage DC Guns (voltage >>100kV) RF and SRF guns – with focus on polarized and CW Drive lasers Vacuum Polarimetry

M. Poelker, SPIN 2008, October 10, 2008 NEW Polarized e-Beam Facility With help from Mainz…. From Y. Poltoratska, Darmstadt Univ.

M. Poelker, SPIN 2008, October 10, 2008 ILC and CLIC: Source Comparison ParameterCEBAFJLab/FELJLab 100mA FEL SLCCLICILC Number electrons/microbunch8.3 x x x x x Number of microbunchesCW Width of microbunch35 ps 2 ns~ 100 ps~ 1 ns Time between microbunches0.667 ns13 ns1.3 ns61.6 ns ns337 ns Microbunch rep rate1497 MHz75 MHz750 MHz16 MHz1999 MHz3 MHz Width of macropulse---64 ns156 ns1 ms Macropulse repetition rate Hz50 Hz5 Hz Charge per micropulse0.13 pC0.133 nC 16 nC0.96 nC4.8 nC Charge per macropulse---32 nC300 nC14420 nC Average current from gun200uA10mA100mA2 uA15 uA72 uA Average current in macropulse A1.9 A A Duty Factor: beam ON/beam OFF (during macropulse for pulsed machines) 5x x x x x10 -3 Peak current of micropulse3.8 mA3.8 A 8 A9.6 A4.8 A Current density (for spot size below)1.9 A/cm 2 19 A/cm 2 10 A/cm A/cm 2 6 A/cm 2 Laser Spot Size0.05 cm0.5 cm 1 cm Bulk GaAs

M. Poelker, SPIN 2008, October 10, 2008 Proposed Facilities: ILC and CLIC From F. Zhou, SLAC ILC and CLIC: High Bunch Charge, High Peak Current and “new” time structure compared to SLC. R&D Issues: Space Charge Induced Emittance Growth and Surface Charge Limit.

M. Poelker, SPIN 2008, October 10, 2008 Proposed Facilities: Electron Ion Collider Slide from E. Tsentalovich, MIT-Bates Residual Gas leads to Ion Back- Bombardment and QE decay and short lifetime:  Improve Vacuum  Large laser spot  Higher gun voltage  More rugged photocathodes  Better beam management

M. Poelker, SPIN 2008, October 10, 2008 Photocathodes From L. Gerchikov. State Polytechnic Univ. St. Petersburg Samples grown at Ioffe Technical Institute, St.Petersburg  Exceptional modeling helps identify depolarization mechanisms  Working on reflective back surface to boost QE

M. Poelker, SPIN 2008, October 10, 2008 Photocathodes Excellent work reported by T. Ujihara, X. Jin, N. Yamamoto, T. Nakanishi, Nagoya University Back-illuminated High Polarization (~ 90%) and High Brightness Photocathode, built for materials science, but could be used for accelerator applications with extremely low emittance requirement

M. Poelker, SPIN 2008, October 10, 2008 Photocathodes Recall SBIR breakthrough reported at PESP2002: MBE-grown strained superlattice with Polarization > 80% and QE 1% SVT now working to add Internal Gradient to boost QE further From A. Moy, SVT Associates AlGaAs/GaAs with graded Al content

M. Poelker, SPIN 2008, October 10, 2008 Very High Voltage DC Guns From M. Yamamoto, Nagoya Univ. 200kV Gun, load lock gun with segmented insulator and high polarization GaAs photocathode

M. Poelker, SPIN 2008, October 10, 2008 Very High Voltage DC Guns From C. Hernandez-Garcia, JLab Jefferson Lab FEL: two “Vent/Bake” guns, GaAs photocathode and unpolarized beam, 5mA typical ave. current Design Goal: 500kV but typical operation at 330kV Field emission can lead to catastrophic insulator “punch- through”

M. Poelker, SPIN 2008, October 10, 2008 Very High Voltage DC Guns From L. Jones, K. Middleman, J. McKenzie, ASTeC Daresbury Field emission not problematic but repeated mechanical failures at ceramic-to-flange joint… 350kV “vent/bake” gun

M. Poelker, SPIN 2008, October 10, 2008 Very High Voltage DC Guns From L. Jones, K. Middleman, J. McKenzie, ASTeC Daresbury Working on load-lock design: rapid photocathode replacement without bakeout

M. Poelker, SPIN 2008, October 10, 2008 Very High Voltage DC Guns From K. Slomenski, Cornell Univ. Cornell ERL Program: 750kV gun, GaAs photocathode, 100mA unpolarized e-beam, load lock and vertical insulator design Field emission and “punch through” limits voltage to ~ 300kV…

M. Poelker, SPIN 2008, October 10, 2008 Very High Voltage DC Guns From N. Nishimori, JAEA JAEA ERL Program: 250kV gun, GaAs photocathode, load lock side insulator design….working on 500kV version with segmented insulator Internal guard rings shield insulator from field emission…

M. Poelker, SPIN 2008, October 10, 2008 SRF Gun From J. Teichert, A. Arnold, FZD Dresden-Rossendorf Holy Grail of e-guns: Perfect vacuum, High Average Current, High Brightness, Low RF Power So far, 100s hours operation at 1uA. Next: Higher current (and GaAs photocathode?)

M. Poelker, SPIN 2008, October 10, 2008 SRF Gun From D. Holmes, AES and J. Kewisch, BNL Polarized SRF Gun at BNL: for ILC, eRHIC and ERLs

M. Poelker, SPIN 2008, October 10, 2008 Drive Laser Development Exploit Telecom Fiber Laser/Amplifier Technology 15 Watts at 532nm! RF structure and short pulses Fiber components exist for 780nm light (i.e., polarized beam) From D. Ouzounov and H. Li, Cornell Univ.

M. Poelker, SPIN 2008, October 10, 2008 Improving Vacuum Imperfect vacuum and ion back bombardment limits photocathode lifetime…  NEG and Ion Pumps provide Torr vacuum  Accurate pressure measurement difficult  Outgassing rate can be reduced via 400C bake in air  Chamber can then be vented and rebaked at 150C  Thick walls (e.g., flanges) are a problem…limit outgassing rate to TL/scm 2  NEG pumps have revolutionized e-gun business, not aware of limitations  Ion pumps might be limiting our vacuum… Pressure = Outgassing Rate x Surface Area Pump Speed From C. Sinclair, Cornell Univ., P. Manini, SAES Getters

M. Poelker, SPIN 2008, October 10, 2008 PESP 2008 Summary  PESP 2008 was a success!!  Well attended with plenty of evidence of healthy and productive photogun programs worldwide.  Continued R&D will improve beam quality (and quality-of- life) at existing facilities  Important to push state of the art, to demonstrate beam requirements for NEW machines  We should continue to invite our friends from the unpolarized gun community – shared challenges, lots to learn from each other  Three talks from commercial sector: photocathode, HV power supply and NEG pump – it was nice to have time to explain our issues, concerns, etc.

M. Poelker, SPIN 2008, October 10, 2008 Some Exciting R&D Projects Reliable trouble-free DC high voltage gun operation beyond 300kV (eliminate field emission, improved insulator design) Understanding (and eliminating) Surface Charge Limit (the phenomenon of reduced QE at high laser power) Develop rugged photocathodes with improved lifetime Back-illuminated, transmission-style photocathodes Achieve better vacuum, Torr and below SRF gun technology – terrific promise. BNL (and Rossendorf?) to demonstrate operation with GaAs… Develop accurate beam transport modeling, benchmark against real beam data…