A Study of Flat Ring Emitter Electron Guns (Part B)

Abstract

<p> The design and performance of a flat emitting ring on-axis electron gun suitable for use in a small reflected beam accelerator was investigated. The design constraints include a low emittance (approximately 5π cm mrad), and a small beam size with a focus approximately 13 cm from the emitting surface.</p> <p> A suitable geometry was determined theoretically and was tested with a dispenser cathode. A beam with a focus at 12.7 ± 1 cm and an emittance of approximately 7π cm mrad was obtained. However, the dispenser cathode response time to heater current changes is too large for the required gun current control.</p> <p> Experiments were done to study the mechanical and thermal properties of flat emitting foil rings, since a directly heated foil has a fast response time. Two foils were tested: 1.27 x 10^-3 cm thick tungsten and 4.57 x 10^-4 cm thick tantalum. The present simple design requires impractically thin foils (≤ 0.25 microns thick) to reach emission temperatures at feasible heater currents.</p>