Sputtered magnesium as a photocathode material for rf injectors

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/mn², 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.