photocathodes for rossendorf srf gun

Rong Xiang I [email protected] I www.hzdr.de

Photocathodes for Rossendorf SRF gun

Rong Xiang

for HZDR SRF-gun team

1st Topical Workshop on laser based particle sources 20-22, February, 2013 CERN

Rong Xiang I [email protected] I www.hzdr.de Member of the Helmholtz Association

ELBE radiation source and SRF gun

Since 2007 in operation


OUTLINE

1. Rossendorf SRF gun and cathode system2. Running experience on Cs2Te in SRF gun3. Tests on new photocathode candidates4. GaAs photocathodes for the SRF gun


1. Rossendorf SRF gun and cathode system

HOM CouplerTESLA-CellsChoke-Filter

Cathode

Electron Beam9.4 MeV, 1 mA, 9.4 kW

77 pC - 1 nC

Laser1 W, 262 nm

Half-Cell Main Coupler

• Laser and photo cathode short and highly charged bunches • RF field low emittance and high peak brightness• Superconducting cavity CW operation and high average current

More info: A.Arnold, et al., NIM A 593(2008) J. Teichert, et al., J. Phys.: Conf. Ser. 298 012008

Jochen Teichert, Talk on Friday



Transportchamber

Load-lock

Cs2Te

Pb/Nb

Mo Mo

Cs2Te

SRF Gun

GaAs

Cathode and cooling system


Requirements for cathode transport:

• Load lock system with < 10-9 mbar

• Exchange without cavity warming up & in short time and low particle generation

Transport chamber



Preparation Lab

2. Cs2Te photocathodes for SRF gun

R. Xiang, PhysRevSpecialTopics_13_043501

TeCs

Monitor1

Monitor2

• UHV (10-10 - 10-9 mbar )• Halogen-light heating• Ar+ beam cleaning• Evaporation • Qualification



Typical preparation process:

1. Plug mechanical polish ( Ra=8-10nm)2. Clean3. Assemble in clean room4. Pump and bake5. Deposited in Prep. Chamber6. Stored in transport chamber7. Transport to SRF gun

Standard Te+Cs


Cathode #250310Moin gun from 2010.5.25 to 2011.7.26Q.E. 1%total beam time 1013 h extracted charge 35 C

Cs/Te=2.3

SEM / EDX for Cs2Te on Mo




Open questions:

• QE decrease during transport• Strong multipacting around cathode plug• Dark current• layer destroyed during operation

120nA

• 8 Cs2Te cathodes tested in gun• QE ~1 % in months• I dark~ 120 nA @ 15 MV/m• Cs2Te contributed 30% of dark current• ɛthermal ~ 0.7 mm·mrad/r(mm)

R. Xiang, et al., FEL2010, Sweden

https://www.hzdr.de/ELBE/logbook/SRF_GUN_2012/08.23/cathode%20170412Mo(TiN)%20compare%203.jpg


3. Tests new photocathode candidates :

• Use Cs2Te chamber, vacuum ~10-9 mbar• On Mo plug, @ 160oC, two-cycle “Yo-yo” process• fresh QE reached 1.5 % • SEM surface morphology

On Mo Substrate, Cs/Sb rate 2.7

Optical-polished Mo substratum

Cs3Sb

R.Xiang, et al., IPAC2012

Sb-source in boat

Cs3Sb out from chamber


GaN (Cs)

• GaN on sapphire with Au/Ni electrode

• Only cesium • 262 nm laser• ~ 10-9 mbar

• wide band gap (3.4 eV)• negative electron affinity (NEA)• Robust: 10-9 mbar

R.Xiang, et al., ERL 2011

3. Tests new photocathode candidates:


4. GaAs photocathodes for the SRF gun

preparation and transfer system for GaAs (Cs,O)

• Close to SRF gun• XHV chamber 10-11 mbar• Green light• First cathode planned in 2013

Interesting points: • QE / lifetime in SRF gun • dielectric rf loss on crystal• temporal response • thermal emittance• potential field emission dark current• polarized beam


Cathode 106 mm longplug ɸ10mm x 6mm (modified from HZB-plug)• Easier for transport• Easier for heat-cleaning


GaAs

• Transport only small plugs (up to 6 plugs)

• HZB-carrier for storage and transport

Plug on transport plate

Plug handle


original Etched sample

O content 2.5% 1.2%Ga/As 1.19 1.15

Etched GaAs sample


wafer treatment: 1. Wash in 4:1:1 acid solution (H2SO4, H2O2 and H2O)2. rinse with pure water and methanol3. Blow with dry N2

Result: reduce the Oxygen content on the surface

SEM / EDX

p-GaAs(100) crystal 400μm, 5 x1018- 5 x1019 /cm3 Zn-dopingResistivity of 0.0053 Ohm cmMobility of 209 cm2/V s


No detectable dark current from GaAs-crystal. But after activation?

time (hh:mm)

Gun with GaAs crystal (0.35mm) and holderWithout RF, PN2=34.52WWith RFmax=5.5MV/m on surface , PN2=35 W


R. Xiang, Task 10.7, EuCARD-SRF Annual Review 2012



• Preparation chamber for activation test: Ø300 SS chamber, with NEG pumps and ion getter pump. • First activation will be performed in the following monthssamples through load lock system directly to preparation chamber

Preparation chamber with load lock system

• Designed process: 1. Heat @ 640oC for 5min with

focused Halogen light, cool down to 100oC

2. Activate by using SAES cesium dispenser and computer controlled O2 leak valve

3. Green laser for photocurrent detect



New photocathode Lab. in accelerator building is in building. Cs2Te preparation system and the new GaAs chamber will be moved together.


AcknowledgementWe acknowledge the support of the European Community-Research Infrastructure Activity under the FP7 programme since 2009 (EuCARD, contract number 227579) as well as the support of the German Federal Ministry of Education and Research grant 05 ES4BR1/8.

Thank you!

photocathodes for rossendorf srf gun

Documents

srf gun rong xiang

cathode system rong

gun qe

operation rong xiang

helmholtz associationpage

outline rossendorf srf

rossendorf srf gunrong

sweden rong xiang