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.
Highlights
The photocathode lifetime of modern dc high voltage GaAs photoguns is limited primarily by ion bombardment [1], the mechanism where residual gas is ionized by the extracted electron beam and transported backward to the photocathode where the ions adversely affect photocathode yield commonly referred to as quantum efficiency (QE)
Video 1 shows an animation of ion bombardment within the dc high voltage continuous electron beam accelerator facility (CEBAF) photogun, with the electron beam originating at or away from the electrostatic center (EC), defined as the axis formed by the cathode and anode
One of the most significant breakthroughs in polarized electron source development at Jefferson Lab was the discovery that, in addition to the core electron beam delivered to the experimental halls, the photoguns produced a small amount of unwanted, unintended photoemission from the edge of the photocathode that was not properly transported away from the photogun [11]
Summary
The photocathode lifetime of modern dc high voltage GaAs photoguns is limited primarily by ion bombardment [1], the mechanism where residual gas is ionized by the extracted electron beam and transported backward to the photocathode where the ions adversely affect photocathode yield commonly referred to as quantum efficiency (QE). Two styles of gun high voltage chamber were used (Fig. 4): one with a large-bore ceramic insulator [11], and the other with a compact ‘‘inverted’’ insulator [12] Both gun geometries utilize a ‘‘side-ceramic’’ approach with the insulator oriented perpendicular to the electron beam path and with photocathode samples inserted from behind into the hollow cathode electrode with 25 focusing angle and flat anode. 499 MHz pulse repetition rate, and 0.13 pC bunch charge emitted by approximately 50 ps long optical pulse), the measurements described here were performed with dc beam at milliampere average currents Both gun styles rely on vacuum pumping using NEG and ion pumps, with NEG pumps (SAES getters WP950 or WP1250) lining the bottom and sides of the high voltage chamber. Experience indicates this is adequate—the preparation chamber vacuum is not degraded following the puck transfer and photocathodes exhibit high QE (bulk GaAs QE $20% at 532 nm, spin polarized superlattice GaAs QE $1% at 780 nm)
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