The Cornell Photocathode Gun
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
Bruce Dunham, for the ERL Injector Team
We have not done much development in the last year, during injector commissioning.
Now, we are ready to design a second gun system.
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
Now, we are ready to design a second gun system.
What do we want in a next gen gun?
-750 kV
Insulator
16.5 inch
flangeCathode Preparation and Load Lock
750 kV, 100 mA HVPS
DC Photocathode Gun
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
Laser input
Electron
beam
Focusing electrode,
Cathode support
Drive Laser
750 kV, 100 mA HVPS
HV Power Supply
Kaiser
-750kV, 100mAGlassman
-500kV, 10mA
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
•We typically run well below the maximum power, thus the controls are not very reliable.
•Added a load resistor in parallel with the gun so that we always draw current to help with control stability
•Processing – another supply would be better
•Gas processing
DC Gun - Insulator
•Large size to keep field gradients low
•Field emitted electrons can build up on the insulator and punch thru
•External SF6
•High mechanical
-750 kV
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
e-e-
12 MV/m at
750kV
•High mechanical stresses due to SF6
pressure and bakeouts
•Difficult to find suppliers
•Braze difficulties due to large size
Braze and punch-thru problems limit us to 250kV for now
‘New’ Insulator
Multi-segmented
(KEK, Cornell)
DC Gun - Insulators
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
‘New’ Insulator design
collaboration
Conductive alumina with improved braze joint
(Daresbury, Jlab, Cornell)
Adapting Industrial tube designs (‘inverted’)
(Jlab, Cornell)
Insulator resistance
5
10
15
20
25
I (n
A)
Morgan AL970CD sample
Rectangular piece 0.12x0.14x2.3 inches
Resistivity of 6.5e10 Ohm-cm
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
0
0 10 20 30 40 50 60
kV
Resistivity of 6.5e10 Ohm-cm
In line with Daresbury measurement of 20 uA at 500kV -> resistivity is roughly linear with voltage
The total voltage effect
Our desired operating point
Field emission test stand results
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
Due to insulator problems, we have not probed this problem yet
-SRF-like cleaning techniques seem to give the best results. Need to follow new developments, like tumbling? Perhaps use all niobium parts?
-symmetry considerations for new guns. Jlab/Daresbury guns have good cylindrical symmetry, whereas our symmetry is broken -> often not a good idea for HV
-reduce electrode areas to reduce the probability of FE
Field Emission
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
-reduce electrode areas to reduce the probability of FE
-What is the best electrode material? Does it really matter, or is it just the cleaning method.
-Would 2 stage accelerating gaps help or hurt?
-Need to simulate A/C gap physics
0 to -125 kV
Test Electrode
Field Emission
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
3-4 mm
Pico-ammeter
anode
To get a better understanding of what happens at real voltages, we are upgrading this system to 300kV, and eventually 500kV
200
250
300
350
400
450
500
I (n
A)
Hand polished SS (pink)
Electropolished 316LN SS, high
pressure water rinse (yellow)
Hand polished 316LN SS, high
pressure water rinse (blue)
High Pressure Water Rinsing
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
0
50
100
150
200
0 10 20 30 40
Field (MV/m)
High pressure (1000 psi) water rinser
We use 16 NEG arrays and a large ion pump to reach 5e-12 Torr. With the gate valve to the beamline open, it increases to 8e-12 Torr and is normally < 1e-11 Torr during operations.
•Reduce electrode surface area to reduce out-gassing.
•Thick material has higher H2 desorption rates, want to
Vacuum
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
•Thick material has higher H2 desorption rates, want to use thin wall chambers. Thin wall stainless steel baked at 400C for 100 hours has extremely low H2 outgassing rates
•Maybe use materials other than stainless?
•Particle generation concerns– RF sealed gate valves, NEG and ion pumps, RF sealed viewers and buncher tuners, cathode puck seating mechanism
Vacuum
View from the top –NEG arrays
•Any better way to do this?
•Other pumping schemes?
•Requires a mesh shield (not shown) which is difficult to
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
•Requires a mesh shield (not shown) which is difficult to work with
•NEG’s need to be cleaned thoroughly – full of particles
Load lock
Works well, but in the next version . . .•Separated function chambers with isolation valves
•Heat pucks horizontally (to reduce indium solder drip)
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
indium solder drip)
•More storage locations
•Extra ‘arm’ for adding other types of cathode preparation (‘CsKSb’)
•A smaller footprint would help
New System Wish List
Load Lock
•More storage locations
•Better isolation
•Horizontal puck heating
•Smaller pucks
•Facility for different cathodes
Electrodes
•Minimize surface area
•Continue improving cleaning
•Material choices
•A/C gap simulations
•Cathode mounting to minimize particle generation
Other
•Biased anode for ion rejection
•Easier installation and alignment methods
•Method to change anode/cathode shapes or gap
•Symmetry
June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University
cathodes
HV Power Supply and Processing
•Improved stability and ripple
•Pulsed power supply for processing
•More manageable SF6 tank
•New diagnostics for processing
Vacuum
•Minimize surface area
•Thin wall chambers to reduce H2
•Dual accelerating gap?
•Particles from NEG’s, Ion pumps?
•Better RF seals
•Symmetry