High voltage performance of a dc photoemission electron gun with centrifugal barrel-polished electrodes

Abstract

The design and fabrication of electrodes for direct current (dc) high voltage photoemission electron guns can significantly influence their performance, most notably in terms of maximum achievable bias voltage. Proper electrostatic design of the triple-point junction shield electrode minimizes the risk of electrical breakdown (arcing) along the insulator-cable plug interface, while the electrode shape is designed to maintain <10 MV/m at the desired operating voltage aiming at little or no field emission once conditioned. Typical electrode surface preparation involves diamond-paste polishing by skilled personnel, requiring several weeks of effort per electrode. In this work, we describe a centrifugal barrel-polishing technique commonly used for polishing the interior surface of superconducting radio frequency cavities but implemented here for the first time to polish electrodes for dc high voltage photoguns. The technique reduced polishing time from weeks to hours while providing surface roughness comparable to that obtained with diamond-paste polishing and with unprecedented consistency between different electrode samples. We present electrode design considerations and high voltage conditioning results to 360 kV (∼11 MV/m), comparing barrel-polished electrode performance to that of diamond-paste polished electrodes. Tests were performed using a dc high voltage photogun with an inverted-geometry ceramic insulator design.

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

“…This contribution discusses the photogun electrostatic design which incorporates a triple-point junction shield used to linearize the potential across the length of the insulator [22], the performance of barrel-polished electrodes [23], the alkali-antimonide photocathode deposition chamber and load-lock features that permit rapid photocathode replacement, the diagnostic beam line and emittance measurement techniques used to measure geometric emittance of the beam produced across the full active area of the photocathode and to evaluate the intrinsic thermal emittance of Cs x K y Sb photocathodes fabricated on GaAs substrates [24,25], and an rf-pulsed green-light drive laser that employs an extremely versatile and reliable gainswitched diode laser master oscillator and fiber amplifiers. Importantly, this contribution describes a highly successful method to prolong photocathode QE lifetime using a biased anode to minimize ion bombardment of the photocathode by repelling ions produced in the beam line.…”

“…The cathode electrode was made of two 15.2 cm diameter hydroformed hemispherical shells (316L stainless steel) welded together. The assembly composed of spherical body and screening electrode was barrel polished to achieve a mirrorlike surface finish using two types of abrasives, with total polishing time of just a few hours [23]. Another key feature of the photogun design is the manner in which the drive laser beam is delivered to the photocathode.…”

Stable operation of a high-voltage high-current dc photoemission gun for the bunched beam electron cooler in RHIC

“…A photoemission electron gun injector, followed by a superconducting rf cavity for acceleration [23][24][25][26][27][28], was adapted as an electron source for LEReC. This choice took advantage of the high-voltage electron gun experience and the excellent beam quality achieved at Cornell University [29][30][31][32][33][34][35][36][37], in Japan [38][39][40][41][42][43][44] and at Jefferson National Lab [45][46][47][48]. It has been demonstrated that a high-voltage dc photoemission gun can have good beam quality, such as low emittance [30,31] and high average current [29].…”

Thermal emittance and lifetime of alkali-antimonide photocathodes grown on GaAs and molybdenum substrates evaluated in a −300 kV dc photogun

“…The focusing electrode and the side-insulator design introduce field nonuniformity at the photocathode surface and within the cathode/anode gap that can impact measured values of emittance. However, the emittance was measured and found to be uniform across a significant portion of the photocathode [24]. For comparative evaluation of thermal emittance for different photocathode samples, beam always originated from the center of the active area fabricated off axis by 1 to 2 mm relative to the photocathode substrate center.…”