Cs 2 Te and NEA GaAs photocathode activities at Fermilab

1. Cs2Te and NEA GaAs photocathode activities at Fermilab Raymond Fliller FNAL Workshop on High Quantum Efficiency Photocathodes for RF guns INFN Milano - LASA4-6 October 2006

2. 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

3. 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 mJ/laser pulse • 25-35 ms RF pulse width • 1 Hz rep rate • <5nC bunch charge (recently) • >1mA dark current

4. A0 Preparation Chamber Cathode plane

5. 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 Gun Transfer chamber

6. 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. HeNe spots 16 mm Images taken May 17,2006

7. 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, Bz=0, the quantum efficiency rises about a factor of 2. • Other solenoid configurations show an asymptotic decrease in QE.

8. 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”

9. Secondary Emission • 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. Simulation Pink (no secondary emission) Green (secondary emission) Data Blue Data/simulations by summer student Rob Inzinga

10. 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. 35.4 MV/m Round Beam 35.4 MV/m Flat Beam

11. 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 (1st 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.

12. 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.

13. 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?

14. 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

15. 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

16. 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.

17. Major Challenges • Vacuum – The NEA GaAs photocathode is susceptible to surface damage from carbon compounds. • SLAC DC gun operates at a H2 pressure <1e-11torr. • The A0 RF gun operates at 1e-9 torr. • How do we achieve lower pressure?? • Ion backbombardment • Simulations done at AES in collaboration w/FNAL show that ions may not be such a large issue because the ion energy is limited to 14keV or so. This is much lower than in DC guns. • Electron backbombardment • This issue is nonexistent in DC guns, and needs study in RF guns.

18. PWT Gun Schematic of an L-band, 1+2/2 cell, PWT polarized electron injector Being designed by DULY Research Inc. Open RF structure allows use of NEG strips or SNEG coating outer vessel.

19. 220 0° rf phase at emission 200 90° rf phase at emission 180 160 140 Threshold Peak Field on Axis (MV/m) i 120 100 - 80 1.6 cell L band gun - 60 scaled from S band FNAL/TESLA 40 1.6 cell gun - 20 DULY L band PWT 0 0.11 0.22 0.33 0.44 0.55 0.66 0.77 0.88 0.99 1.1 1.21 1.32 Axial Distance from Iris Center (cm) PWT gun - - Operating the L-band PWT at a low peak field helps prevent backstreaming electrons emitted from the first PWT iris from reaching the photocathode.

20. BNL/AES SRF Gun • FNAL is collaborating with BNL/AES/MIT Bates on a half cell SRF gun. • The vacuum environment should be good enough to support at GaAs photocathode if sputtered Cs does not hurt the superconductor. • I’ll let the folks from BNL say more on this.

21. GaAs Preparation Chamber • We are in the process of building a cathode preparation chamber for GaAs. • The main vessel was designed and built by AES. • It will initially be a stand alone chamber to gain experience with making bulk GaAs cathodes and initial survivability tests. • With a stalk replacement it can be fit to an RF gun. • Currently on the cusp of being assembled, and awaiting safety review for H2 and NF3. H2 thermal gas cracker Cs getter and collector bellows Spare port for future load lock 500 L/s ion pump

22. GaAs stalk design • Mo cup with Mo cap • GaAs soldered to cup with indium • Ceramic break to measure charge emitted from cathode • Stalk heater (not shown) • Stalk is only for initial tests to gain experience. • Adding a bellows type actuator and additional stainless spool will allow for insertion into a gun

23. Conclusions • Cs2Te research at FNAL focuses on • Time dependant QE • Secondary Emission • Multipactoring • Starting a program to produce an RF gun to support an NEA GaAs photocathode • Prep chamber under construction • Collaborating with BNL and Duly Research Inc, to produce candidate guns with focus on • Producing a high vacuum environment during RF operation • Attention to electron backbombardment issues