1. Cesium Telluride Photocathode Preparation at Argonne
High QE Photocathodes for RF Guns Workshop
Manoel Conde

2. Staff: Personnel Argonne Wakefield Accelerator Group Sergey Antipov
Wei Gai
Manoel Conde
Felipe Franchini
Chinguang Jing
Richard Konecny
Wanming Liu
John Power
Zikri Yusof
Students & Visitors:
Sergey Antipov
Feng Gao
Haitao Wang
Jidong Long

3. Research at the Argonne Wakefield Accelerator Facility (AWA)
High Power Electron Beam (~ GW) Technologies
Operating a unique facility to study high current electron beam generation and propagation for efficient beam driven schemes.
Advanced Accelerating Structures
Current effort has lead to comprehensive knowledge on construction and testing of dielectric based accelerating structures.
Fundamental beam physics and advanced diagnostics
High brightness beam generation and propagation, and development of novel beam diagnostics.
Brief history:
The AWA Facility successfully demonstrated collinear wakefield acceleration and two-beam-acceleration in dielectric loaded structures. The initial accelerating gradients were limited to modest values (< 15 MV/m) due to the quality of the drive electron beam. The upgraded drive gun has led to increasingly higher gradients, recently reaching 86 MV/m.

4. AWA Drive Beamline 4.5 m Wakefield Structure Linac & Steering Coils
Drive Gun
Linac & Steering Coils
Quads
Wakefield Structure
Experimental
Chambers
4.5 m GV
YAG1
YAG2
Spectrometer
YAG5
Dump/
Faraday Cup
Slits
YAG4
YAG3
ICT1
ICT2
BPM
Single bunch operation
Q = nC
Energy = 14 MeV
High Current = 10 kAmp
Bunch train operation
4 bunches x 10 nC
32 bunches x 50 nC (future?!)

5. AWA Electron Beam Drive Gun 1 ½ cell, L-band (1.3 GHz)
12 MW yielding 80 MV/m on cathode
8 MeV electron bunches with nC
bunch length < 13 ps FWHM (with 35 nC)
Emittance < 300 mm mrad (with 35 nC)
Base pressure: 4x10-10 Torr
Mg photocathode (Cs2Te cathode under development)
Linac Structure
Boosts energy to 14 MeV
Large irises to minimize wakefields
100 nC at 8 MeV

6. AWA Sub-systems Laser System
Spectra Physics Tsunami oscillator, Spitfire regenerative amplifier, and two Ti:Sapphire amplifiers (TSA 50):
1.5 mJ at 248 nm
8 ps FWHM
If use Excimer amplifier:
15 mJ at 248 nm
RF System
Single klystron: 1.3 GHz, 24 MW, 8μs

7. Cesium Telluride Photocathode Fabrication
Anode A + hn
Cs2Te film
Mo plug
Evaporate Tellurium, followed by Cesium, onto the Molybdenum substrate.
Use Hg arc lamp to generate photoelectrons.

8. Deposition Chamber SYSTEM TOP VIEW SYSTEM SIDE VIEW 7. Heater
8. Anode/shutter 9.Thickness monitor
10. Temperature sensor
13. Hg light source 6
Cathode loadlock
Thermal evaporators 7
Cathode loadlock
Thermal
evaporators
7. Heater
9. Thickness monitor
10. Temperature sensor
13. Hg light source

9. Cs2Te Deposition 1. Starting base pressure ~ 5×10-10 Torr;
2. Heat Mo plug to C;
3. Lower Mo plug temperature to 120 C. Pressure never goes beyond Torr range;
4. Begin Te deposition. Pressure increases up to 3×10-8 Torr. Te deposition stops when thickness is ~10 nm (estimated using nearby thickness monitor);
5. Begin Cs deposition. DC current to Cs dispenser set to 6 A during deposition.
6. Photocurrent is monitored by applying +300VDC on an anode, and an Oriel 350 W Hg arc lamp source with a 250 nm filter.
7. Cs deposition is turned off once the photocurrent starts to level off.
8. Cathode is left at 120 C for another hour before heater is turned off.

10. Cs Deposition
End Cs deposition
Begin Cs deposition

11. Issues to be addressed:
QE is lower than expected.
4 out of 5 deposition runs produce measured current that is dominated by Cs deposition source (Cs ions?) and not the photocurrent (block UV light and still measure current).
Not repetitive.
Need better vacuum?