Cesium Telluride Photocathode Preparation at Argonne

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

2. Personnel Argonne Wakefield Accelerator Group Staff: • 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 driveelectron beam. The upgraded drive gun has led to increasingly higher gradients, recentlyreaching 86 MV/m.

4. Wakefield Structure Linac & Steering Coils Drive Gun Spectrometer Quads Experimental Chambers 4.5 m YAG3 YAG4 YAG1 YAG2 YAG5 ICT1 Dump/ Faraday Cup ICT2 GV GV Slits BPM AWA Drive Beamline Single bunch operation • Q = 1-100 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 1 -100 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 A Anode + 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 160-180 C; 3. Lower Mo plug temperature to 120 C. Pressure never goes beyond 10-10 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?