1. LLGUN2 Biased Anode Commissioning
(B-Team Meeting, 10/30/07, Grames)
Based on work presented at PST’07…
A Biased Anode to Suppress Ion Back Bombardment in a DC High Voltage GaAs Photoelectron Gun
J. Grames, P. Adderley, J. Brittian, J Clark, J. Hansknecht,
D. Machie, M. Poelker, E. Pozdeyev, M. Stutzman, K. Surles-Law
Jefferson Lab
September 10, 2007

2. Ion Back-Bombardment We don’t run beam from electrostatic center
Laser spot size is ~0.5 mm and can be moved to different locations on the photocathode.
electron beam
OUT
laser light IN
Ions create QE trough to electrostatic center

3. Modifying the HV Chamber Anode Structure
Anode Electrode (Grounded)
Cathode Electrode (-100 kV)
Anode Support (Grounded)
Ion Trapping ???
Beloved Photocathode
Evil Accelerated Ions

4. Limiting the Dominant Species
Ionization cross section for H2
Wtot= σ(E)∙dE ~ 4∙10-18 cm2 ∫
Lgap ~ 5 cm
Pgun ~ 5∙10-12 Torr
Ygun ~ 1.5∙107 ions/C
W(100keV) = 4∙10-19 cm2
Lbeamline ~ 100∙Lgap
Pbeamline ~ 20∙Pgun
Ybeamline ~ 200∙Ygun

5. Limiting Ionized Gas from HV Chamber
Beamline
Ions
Anode
E. Pozdeyev,
Phys. Rev. ST Accel. Beams 10, (2007)

6. H2: Reduce (vacuum) or Limit (ions) ?
Experimental Setup
Faraday Cup
Laser
(1 532 nm)
High Voltage
(-100 kV)
NEG pipe
Activation
(Cs/NF3, 5 mm)
350 mm
Load lock
(GaAs on puck)
Biased Anode
Connections

7. Unbiased vs. Biased Running at 5mA
LL Biased Anode Running April ‘07
Anode = 0 V, Support = 0 V
gas
Gathode
Anode
QE trough to
electrostatic center
Anode = 2 kV, Support= 300 V

8. Photocathode Lifetime at “EC” at 2mA

9. Biased Anode Commissioning 10/24/07
Electrode (+2000 V)
Support (+300 V)

10. Beam Orbit Didn’t Change…
Grounded
Biased

11. Transverse Profile Didn’t Change…
Grounded
Biased

12. Longitudinal Profile Didn’t Change…
(but bunch arrives ~1 deg sooner)
Grounded
Biased

13. Summary
The electrostatic center is the most susceptible location of a photocathode to be damaged in conventional (cylindrical symmetric) DC guns.
Although electron gun vacuum and design continues to improve the bombarding ion contribution of the beam line can be (likely is) significant.
Positively biasing the DC gun anode successfully limited ion damage at the “EC”, increasing photocathode lifetime there and improving QE uniformity.
Biased anode has implication for other DC gun designs, e.g., where spot size is large or uniformity critical.