f Cs 2 Te and NEA GaAs photocathode activities at Fermilab Raymond Fliller FNAL Workshop on High Quantum Efficiency Photocathodes for RF guns INFN Milano - LASA 4-6 October 2006 f Past and Current Activities A0 Photoinjector Time Dependant QE with Cs2Te Secondary Emission studies NML New photoinjector built to be an ILC cryomodule test stand and AARD machine Polarized RF Electron Gun New Preparation chamber PWT type gun Participation with BNL/AES/MIT on SRF gun f A0 Photoinjector Cs2Te photocathode 1.3GHz, 1.5cell normal conducting RF gun 35MV/m cathode field 9 cell Tesla type superconducting cavity, 12 MV/m accelerating gradient 15MeV beam energy 2.4ps laser pulse width 16 J/laser pulse s RF pulse width 1 Hz rep rate 1mA dark current f A0 Preparation Chamber Cathode plane f A0 Cathode Preparation Chamber The A0PI cathode preparation chamber is located inside of the cave. The future upgrade/move to NML will have the preparation chamber will be of the Milano design. Cesiation Chamber Transfer chamber Gun f Cathode Current cathode has been in operation since prior to October 2004 (nobody remembers when it was last inserted) Cathode plane shows groves that have been made since gun was installed. Images taken May 17,2006 HeNe spots 16 mm f Time Dependant QE and Dark Current A0PI has a QE that asymptotically increases or decreases with time depending on solenoid settings. With all solenoids approximately the same current, B z =0, the quantum efficiency rises about a factor of 2. Other solenoid configurations show an asymptotic decrease in QE. f Multipactoring in the A0 gun A multipactoring spike has been observed when the QE is increasing. The theory is that multipactoring cleans the cathode surface, increasing the QE and dark current. With multipactoring off, the contaminants settle back to cathode lowering them. Photoelectrons Dark Current Multipactoring spike f Secondary Emission Simulation Pink (no secondary emission) Green (secondary emission) Data Blue Studies at DESY show that secondary emission from the cathode can cause multipactoring in the RF gun (Han, Ph.D. thesis. We have decided to try to understand the secondary emission from our cathode. Below data taken at Ecath=15MV/m. Secondary emission is obvious. Data/simulations by summer student Rob Inzinga f Secondary Emission By comparing simulations of phase scans with data for a variety of Ecath and solenoid configurations we plan to characterize the secondary emission characteristics of our cathode MV/m Round Beam 35.4 MV/m Flat Beam f NML A 750 MeV test accelerator will be built in a building formerly known as the New Muon Lab (now just NML). 50 MeV injector Eventually 1 ILC RF Unit (1 st cryomodule to arrive at NML in Summer 07) 1 RF unit = 3 cryomodules w/ 8 cavities each driven from 1 klystron ILC Bunch charge/length/train Gun Testing planned in the future. Also AARD machine in future. f NML Injector Milano type cathode prep chamber (MOU is with the lawyers.) Modified FLASH type RF gun CC1 (presently in A0) and CC2 (presently at Meson lab) (Tesla type 9 cells) 3.9 GHz accelerating mode cavity for longitudinal Phase space linearization Bunch compressor 50 MeV dogleg for beam experiments (such as 3.9 GHz deflecting mode cavity). To fit third cryomodule either the building needs to be extended or the injector shortened. f NML Cathode Prep chamber Open questions Base design is for Milano system. Is there anything to be gained by moving the transverse stalk to the other side? Stalk limits minimum distance from beam centerline to wall. Need aisle on east side for servicing anyway, stalk is no problem there. Does it really gain you anything? A small aisle is needed on west side to insert cathodes and service croymodules on backside. Is this really any smaller than what is shown? f Polarized RF gun By using a strained GaAs semiconductor and using the correct laser, polarized electrons can be produced from the cathode. The QE is improve by coating the surface with a monolayer Cesium Fluoride Figure taken from R.L. Bell, Negative Electron Affinity Devices f Polarized RF gun Strained NEA GaAs photocathodes have been used at SLAC and JLab for a number of years in DC guns. Experiments at Novosibirsk (Alexandrov, EPAC98) showed that an NEA GaAs photocathode lasted in an RF gun only a few tens of RF pulses. At low gradient the cathode could be reactivated. At high gradient (>30MV/m) the cathode was damaged. We would like to make an RF gun that will support a GaAs photocathode f Cryogenically Cooled Normal conducting RF gun Using a spare 1.3 GHz gun, we cooled it using liquid N2 to attempt to improve the vacuum. Results show that this does not work because the gun is not cold enough. The gas migrates from the warm section to the cold producing a lower pressure, lower temperature, higher density gas than existed prior to cooling. Liquid He is necessary to reduce the vapor pressure of the residual gas to effectively remove the gas from the volume. RGA comparision of gun at 300K and 90 K. Little difference in spectrum. f Major Challenges Vacuum The NEA GaAs photocathode is susceptible to surface damage from carbon compounds. SLAC DC gun operates at a H2 pressure