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

Abstract

This contribution describes the latest milestones of a multiyear program to build and operate a compact −300 kV dc high voltage photogun with inverted insulator geometry and alkali-antimonide photocathodes. Photocathode thermal emittance measurements and quantum efficiency charge lifetime measurements at average current up to 4.5 mA are presented, as well as an innovative implementation of ion generation and tracking simulations to explain the benefits of a biased anode to repel beam line ions from the anodecathode gap, to dramatically improve the operating lifetime of the photogun and eliminate the occurrence of micro-arc discharges.

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

“…The −300 kV photogun used for these photocathode studies is described in detail in Ref. [23]. The photogun employs an "inverted insulator" geometry, with voltage applied using a high voltage cable that inserts into a conical-shaped alumina insulator.…”

“…The insulator extends into the vacuum chamber and supports the cathode electrode. The spherical cathode electrode includes a specially designed screening electrode that reduces the electric field strength at the triple-point junction where arcing is thought to originate [23]. The drive laser beam passes through entrance and exit holes in the anode electrode at 25°angles of incidence and reflection, thereby eliminating the need for in-vacuum laser mirrors which can restrict the effective aperture of the beam line.…”

“…The drive laser beam passes through entrance and exit holes in the anode electrode at 25°angles of incidence and reflection, thereby eliminating the need for in-vacuum laser mirrors which can restrict the effective aperture of the beam line. The anode is electrically isolated from ground potential using alumina washers to enable biasing as a means to repel downstream ions created by the beam [23]. The vacuum pressure inside the photogun while delivering electron beam is about 1 × 10 −11 Torr achieved using nonevaporable getter and ion pumps.…”

“…7 and were obtained using the wire scanner to determine x, y rms beam sizes. Finally, the emittance was measured across the photocathode vertical and horizontal axes showing uniform beam emittance across a significant portion of the photocathode [23].…”

“…As described in Ref. [23], uninterrupted beam delivery from the photogun was only possible when a positive bias voltage was applied to the photogun anode electrode. Table III summarizes the charge lifetime measurements for beam delivery under various conditions for the five fabricated photocathodes, each fabricated with an active area centered off axis by 1 to 2 mm relative to the photocathode substrate center.…”