Photocathode Femtosecond Electron Linac and Its Applications
Cs 2 Te and NEA GaAs photocathode activities at Fermilab
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Transcript of Cs 2 Te and NEA GaAs photocathode activities at Fermilab
fCsCs22Te and NEA GaAs Te and NEA GaAs
photocathode activities at photocathode activities at FermilabFermilab
Raymond FlillerFNAL
Workshop on High Quantum Efficiency Photocathodes for RF guns
INFN Milano - LASA4-6 October 2006
f Past and Current ActivitiesPast 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 PhotoinjectorA0 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 25-35 s RF pulse width 1 Hz rep rate
<5nC bunch charge (recently)
>1mA dark current
f A0 Preparation ChamberA0 Preparation ChamberCathode plane
f A0 Cathode Preparation ChamberA0 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 CathodeCathode 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 CurrentTime 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.
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off - 12.5 h
round - 12.5 h
flat - 12.5 h
f Multipactoring in the A0 gunMultipactoring 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 EmissionSecondary Emission
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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 EmissionSecondary 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.
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35.4 MV/m Round Beam 35.4 MV/m Flat Beam
f NMLNML 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.
f NML InjectorNML 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 questionsNML 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 gunPolarized 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 gunPolarized 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 Cryogenically Cooled Normal conducting RF gungun
Using a spare 1.3 GHz gun, we cooled it using liquid N2 to attempt to improve the vacuum.
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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 ChallengesMajor 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.
f PWT GunPWT Gun
Schematic of an L-band, 1+2/2 cell, PWT polarized electron injectorBeing designed by DULY Research Inc.
Open RF structure allows use of NEG strips or SNEG coating outer vessel.
f PWT gunPWT 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.
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FNAL/TESLA
1.6 cell gun
DULY L -band PWT
1.6 cell L-
band gun scaled from S
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f BNL/AES SRF GunBNL/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.
f GaAs Preparation ChamberGaAs 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
f GaAs stalk designGaAs 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
f ConclusionsConclusions 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