Vacuum breakdown limit and quantum efficiency obtained for various technical metals using dc and pulsed voltage sources

摘要

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 ultraviolet 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. The authors show that gradient achieved for rough machined (Ra nm) metal electrodes followed by an argon glow plasma are similar to the one obtained using a mirrorlike electrode (Ra nm). 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 full width at half maximum, 500 kV electron gun and tested for high gradient breakdown and for quantum efficiency using an ultraviolet laser. Routine electron beam operation, breakdown-free, at 50 MV/m (6 mm gap, 10 Hz repetition rate) at various charges is now achieved using different metal electrodes.