Empirical comparison of high gradient achievement for different metals in DC and pulsed mode

TL;DR

This paper compares the high gradient performance of various metals in DC and pulsed modes for use in advanced electron guns, focusing on breakdown thresholds, stability, and efficiency.

Contribution

It provides an empirical assessment of different metals and preparation methods for high gradient applications in electron gun development.

Findings

Certain metals achieved breakdown voltages above 100 kV in DC tests

Stable operation with minimal dark current was demonstrated over several days

Electrodes in the electron gun showed effective high gradient performance and quantum efficiency

Abstract

For the SwissFEL project, an advanced high gradient low emittance gun is under development. Reliable operation with an electric field, preferably above 125 MV/m at a 4 mm gap, in the presence of an UV laser beam, has to be achieved in a diode configuration in order to minimize the emittance dilution due to space charge effects. In the first phase, a DC breakdown test stand was used to test different metals with different preparation methods at voltages up to 100 kV. In addition high gradient stability tests were also carried out over several days in order to prove reliable spark-free operation with a minimum dark current. In the second phase, electrodes with selected materials were installed in the 250 ns FWHM, 500 kV electron gun and tested for high gradient breakdown and for quantum efficiency using an ultra-violet laser.