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

M. Poelker, SPIN 2008, October 10, 2008

Workshop on Polarized Electron Sources and Polarimeters

PESP 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…


PESP2008 – Enhanced Scope

Idea: Invite participants from unpolarized e-beam programs….note shared technology

More than 80 registered attendees!!

37 talks!!


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


NEW Polarized e-Beam Facility

With help from Mainz….

From Y. Poltoratska, Darmstadt Univ.


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


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.


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


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


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


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


Very High Voltage DC Guns

From M. Yamamoto, Nagoya Univ.

200kV Gun, load lock gun with segmented insulator and high polarization GaAs photocathode



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”



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



From L. Jones, K. Middleman, J. McKenzie, ASTeC Daresbury

Working on load-lock design: rapid photocathode replacement without bakeout



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…



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…


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?)


SRF Gun

From D. Holmes, AES and J. Kewisch, BNL

Polarized SRF Gun at BNL: for ILC, eRHIC and ERLs


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.


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


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.


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…

Documents

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