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
Parameter CEBAF JLab/FEL JLab 100mA FEL
SLC CLIC ILC
Number electrons/microbunch 8.3 x 105 8.3 x 108 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 35 ps 35 ps 2 ns ~ 100 ps ~ 1 ns
Time between microbunches 0.667 ns 13 ns 1.3 ns 61.6 ns 0.5002 ns 337 ns
Microbunch rep rate 1497 MHz 75 MHz 750 MHz 16 MHz 1999 MHz 3 MHz
Width of macropulse - - - 64 ns 156 ns 1 ms
Macropulse repetition rate - - - 120 Hz 50 Hz 5 Hz
Charge per micropulse 0.13 pC 0.133 nC 0.133 nC 16 nC 0.96 nC 4.8 nC
Charge per macropulse - - - 32 nC 300 nC 14420 nC
Average current from gun 200uA 10mA 100mA 2 uA 15 uA 72 uA
Average current in macropulse - - - 0.064 A 1.9 A 0.0144 A
Duty Factor: beam ON/beam OFF (during macropulse for pulsed machines)
5x10-2 2.6x10-3 2.6x10-2 2.8x10-7 0.2 3x10-3
Peak current of micropulse 3.8 mA 3.8 A 3.8 A 8 A 9.6 A 4.8 A
Current density (for spot size below) 1.9 A/cm2 19 A/cm2 19 A/cm2 10 A/cm2 12.1 A/cm2 6 A/cm2
Laser Spot Size 0.05 cm 0.5 cm 0.5 cm 1 cm 1 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. PetersburgSamples 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 330kVField 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 DevelopmentExploit Telecom Fiber Laser/Amplifier Technology
15 Watts at 532nm! RF structure and short pulsesFiber 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 VacuumImperfect vacuum and ion back bombardment limits photocathode lifetime…
NEG and Ion Pumps provide 10-11 Torr vacuumAccurate pressure measurement difficultOutgassing rate can be reduced via 400C bake in airChamber can then be vented and rebaked at 150CThick walls (e.g., flanges) are a problem…limit
outgassing rate to 10-13 TL/scm2
NEG pumps have revolutionized e-gun business, not aware of limitations
Ion pumps might be limiting our vacuum…
Pressure = Outgassing Rate x Surface AreaPump 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, 10-12 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…
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