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

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

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.

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 = 1-100 nC Energy = 14 MeV High Current = 10 kAmp Bunch train operation 4 bunches x 10 nC 32 bunches x 50 nC (future?!)

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

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

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.

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

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.

Cs Deposition End Cs deposition Begin Cs deposition

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?