Experimental investigation of an optimum configuration for a high-voltage photoemission gun for operation at≥500  kV

Abstract

We demonstrated the generation of a 500-keV electron beam from a high dc voltage photoemission gun for an energy recovery linac light source [N. Nishimori et al., Appl. Phys. Lett. 102, 234103 (2013)]. This demonstration was achieved by addressing two discharge problems that lead to vacuum breakdown in the dc gun. One is field emission generated from a central stem electrode. We employed a segmented insulator to protect the ceramic insulator surface from the field emission. The other is microdischarge at an anode electrode or a vacuum chamber, which is triggered by microparticle transfer or field emission from a cathode electrode. An experimental investigation revealed that a larger acceleration gap, optimized mainly to reduce the surface electric field of the anode electrode, suppresses the microdischarge events that accompany gas desorption. It was also found that nonevaporable getter pumps placed around the acceleration gap greatly help to suppress those microdischarge events. The applied voltage as a function of the total gas desorption is shown to be a good measure for finding the optimum dc gun configuration.

Compact −300 kV dc inverted insulator photogun with biased anode and alkali-antimonide photocathode

“…A number of dc high voltage photoemission guns (photoguns hereafter) rely on large cylindrical ceramic insulators to electrically isolate the cathode electrode [13][14][15], which must be supported on a long coaxial metal support tube. A symmetric-field vent-and-bake version of this type of photogun operating at −350 kV demonstrated 500 Coulomb quantum efficiency (QE) 1=e charge lifetime while delivering up to 8 mA from a bulk GaAs photocathode for the Jefferson Lab free electron laser (FEL), which set many records including the highest optical output power from an FEL at millimeter, infrared and ultraviolet wavelengths, and achieving at the time the highest average beam current in an energy recovery linac from a dc high voltage photogun [16].…”

Operational experience of a 500 kV photoemission gun

“…HV conditioning up to 539 kV with the stem electrode was achieved in 2009 [12], and generation of a 500 keV beam with current up to 1.8 mA was demonstrated for short duration less than a minute in 2012 [11]. The cathode-anode gap length was changed from the original design of 100 mm to 160 mm [13] because field emitters that could not be pacified were created repeatedly with a 100 mm gap, which prevented stable gun operation at 500 kV.…”

“…The other problem is high-voltage (HV) prebreakdown between the cathode and the gun vacuum chamber wall, including an anode. We have solved this problem by optimization of the accelerator gap length for 500 kV operation [13].…”

Development of a Multialkali Photocathode Dc Gun for a Smith-Purcell Terahertz Free-Electron Laser

“…The wafer is attached to a Mo puck with an In seal. The puck is the same as that used for the GaAs photocathode for the 500 kV photoemission dc gun in the compact energy recovery linac (cERL) at the High Energy Accelerator Research Organization (KEK) [18,19]. Figure 2 shows a photograph of the inside of the alkali antimonide photocathode preparation chamber.…”