LLGUN2 Biased Anode Commissioning

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Presentation transcript:

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

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

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

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

Limiting Ionized Gas from HV Chamber Beamline Ions Anode E. Pozdeyev, Phys. Rev. ST Accel. Beams 10, 083501 (2007)

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

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

Photocathode Lifetime at “EC” at 2mA

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

Beam Orbit Didn’t Change… Grounded Biased

Transverse Profile Didn’t Change… Grounded Biased

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

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.