PESP 2008 Workshop on Polarized Electron Sources and

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

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

PESP 2008 – 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 >>100 k. V) • 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 Parameter CEBAF JLab/FEL JLab 100 m. A FEL SLC CLIC ILC Number electrons/microbunch 8. 3 x 105 8. 3 x 108 1 x 1011 6 x 109 3 x 1010 Number of microbunches CW CW CW 2 312 3000 Width of microbunch 35 ps 2 ns ~ 100 ps ~ 1 ns 0. 667 ns 13 ns 1. 3 ns 61. 6 ns 0. 5002 ns 337 ns Microbunch rep rate 1497 MHz 750 MHz 16 MHz 1999 MHz 3 MHz Width of macropulse - - - 64 ns 156 ns 1 ms Macropulse repetition rate - - - 120 Hz 5 Hz Charge per micropulse 0. 13 p. C 0. 133 n. C 16 n. C 0. 96 n. C 4. 8 n. C Charge per macropulse - - - 32 n. C 300 n. C 14420 n. C Average current from gun 200 u. A 10 m. A 100 m. A 2 u. A 15 u. A 72 u. A - - - 0. 064 A 1. 9 A 0. 0144 A Duty Factor: beam ON/beam OFF (during macropulse for pulsed machines) 5 x 10 -2 2. 6 x 10 -3 2. 6 x 10 -2 2. 8 x 10 -7 0. 2 3 x 10 -3 Peak current of micropulse 3. 8 m. A 3. 8 A 8 A 9. 6 A 4. 8 A 1. 9 A/cm 2 19 A/cm 2 10 A/cm 2 12. 1 A/cm 2 6 A/cm 2 0. 05 cm 0. 5 cm 1 cm Time between microbunches Average current in macropulse Current density (for spot size below) Laser Spot Size Bulk Ga. As M. Poelker, SPIN 2008, October 10, 2008

Proposed Facilities: 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. From F. Zhou, SLAC M. Poelker, SPIN 2008, October 10, 2008

Proposed Facilities: Electron Ion Collider 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 Slide from E. Tsentalovich, MIT-Bates M. Poelker, SPIN 2008, October 10, 2008

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

Photocathodes 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 Excellent work reported by T. Ujihara, X. Jin, N. Yamamoto, T. Nakanishi, Nagoya University M. Poelker, SPIN 2008, October 10, 2008

Photocathodes Recall SBIR breakthrough reported at PESP 2002: MBE-grown strained superlattice with Polarization > 80% and QE 1% Al. Ga. As/Ga. As with graded Al content SVT now working to add Internal Gradient to boost QE further From A. Moy, SVT Associates M. Poelker, SPIN 2008, October 10, 2008

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

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

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

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

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

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

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

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

Drive Laser Development Exploit Telecom Fiber Laser/Amplifier Technology 15 Watts at 532 nm! RF structure and short pulses Fiber components exist for 780 nm 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… Pressure = Outgassing Rate x Surface Area Pump Speed § NEG and Ion Pumps provide 10 -11 Torr vacuum § Accurate pressure measurement difficult § Outgassing rate can be reduced via 400 C bake in air § Chamber can then be vented and rebaked at 150 C § Thick walls (e. g. , flanges) are a problem…limit outgassing rate to 10 -13 TL/scm 2 § NEG pumps have revolutionized e-gun business, not aware of limitations § Ion pumps might be limiting our vacuum… 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-oflife) 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 300 k. V (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, 10 -12 Torr and below • SRF gun technology – terrific promise. BNL (and Rossendorf? ) to demonstrate operation with Ga. As… • Develop accurate beam transport modeling, benchmark against real beam data… M. Poelker, SPIN 2008, October 10, 2008