The Cornell Photocathode Gun - accelconf.web.cern.ch · The Cornell Photocathode Gun June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University Bruce Dunham, for the

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.


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


Laser input

Electron

beam

Focusing electrode,

Cathode support

Drive Laser

750 kV, 100 mA HVPS

HV Power Supply

Kaiser

-750kV, 100mAGlassman

-500kV, 10mA


•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


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


‘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


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

Different Design Paths


The total voltage effect

Our desired operating point

Field emission test stand results


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


-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


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


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


•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


•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)


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


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

Acknowledgements

This work is supported by NSF PHY-0131508


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The Cornell Photocathode Gun - accelconf.web.cern.ch · The Cornell Photocathode Gun June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University Bruce Dunham, for the