Charge and fluence lifetime measurements of a dc high voltage GaAs photogun at high average current

Abstract

GaAs-based dc high voltage photoguns used at accelerators with extensive user programs must exhibit long photocathode operating lifetime. Achieving this goal represents a significant challenge for proposed high average current facilities that must operate at tens of milliamperes or more. This paper describes techniques to maintain good vacuum while delivering beam, and techniques that minimize the ill effects of ion bombardment, the dominant mechanism that reduces photocathode yield of a GaAs-based dc high voltage photogun. Experimental results presented here demonstrate enhanced lifetime at high beam currents by: (a) operating with the drive laser beam positioned away from the electrostatic center of the photocathode, (b) limiting the photocathode active area to eliminate photoemission from regions of the photocathode that do not support efficient beam delivery, (c) using a large drive laser beam to distribute ion damage over a larger area, and (d) by applying a relatively low bias voltage to the anode to repel ions created within the downstream beam line. A combination of these techniques provided the best total charge extracted lifetimes in excess of 1000 C at dc beam currents up to 9.5 mA, using green light illumination of bulk GaAs inside a 100 kV photogun.

Charge lifetime measurements at high average current using aK2CsSbphotocathode inside a dc high voltage photogun

“…Previous tests with GaAs indicated that photocathode charge lifetime could be enhanced at the electrostatic center using this technique [11]. As shown in Fig.…”

“…7 as a function of radial position of the laser beam relative to the electrostatic center of the photocathode. It was both surprising and disappointing that the charge lifetime of photocathode #1 was comparable to that of GaAs [11]. Furthermore, the charge lifetime was observed to improve as the laser position was moved radially outward, which suggested ion bombardment played a role in QE decay.…”

“…In fact, initial runs at 1 mA and 100 and 200 kV gun voltages from the electrostatic center indicated QE only increased with time. Beam delivery from the electrostatic center using a GaAs photocathode is short lived due to ion bombardment [3,11].…”

“…The constituent sources consisted of high-purity Sb pellets, resistively heated in a tungsten crucible, while the K and Cs sources consisted of SAES alkali dispenser strips [12] for photocathode #1, and Alvatec source vials [13] for photocathode #2. The puck was similar to pucks used at the Continuous Electron Beam Accelerator Facility at JLab for securing GaAs photocathodes [11] but made of aluminum with a thin layer of stainless steel explosion bonded to the top which served as the coating substrate. The substrate must be heated during the photocathode fabrication and, because the heater inside the BNL deposition chamber had modest heating capability, aluminum was chosen for the puck body because it has relatively small mass and good thermal heat conduction.…”

“…The purpose of this work was to compare the performance of the K 2 CsSb photocathode inside a dc high voltage photogun that had been used extensively to characterize GaAs performance [11]. Under nominally identical conditions (gun and beam line vacuum, drive laser wavelength and laser spot size, etc.…”