Sputtered magnesium as a photocathode material for rf injectors

Abstract

In this article, we report the investigation of sputtered magnesium as a suitable photoelectron emitter in a rf injector. The measurements were conducted on a 20 μm thick Mg layer, ion sputtered on a copper substrate. UV laser induced damage threshold measurements on this sample indicate that laser energy densities of up to 100 μJ/mm2 at 266 nm, the irradiated surface is indistinguishable from the unirradiated surface. For laser energy densities in the range of 100–300 μJ/mm2, slight roughening of the surface was evident. At 600 μJ/mm2, damage is visible, with the depth estimated to be ∼1 μm. This damage threshold is six times larger than that of the bulk magnesium and approaches that of copper. Systematic laser cleaning has improved the quantum efficiency of the surface from 2×10−5 to 2×10−3. This combination of high quantum efficiency and high damage threshold of sputtered magnesium produces a record current density of 25 kA/mm2 for a metal cathode. The effect of laser cleaning on sputtered Mg, the sample preparation technique for reliable performance, and the parametric dependence of quantum efficiency are reported as well.