For the production of the CLIC RF structures, a material capable of sustaining high electric field, with a low breakdown rate and showing low damages after breakdowns is needed. A DC breakdown study is underway at CERN in order to test candidate materials and surface preparations, and also to have a better understanding of the breakdown mechanism.
The saturated breakdown fields of several metals and alloys have recently been measured: niobium, vanadium, stainless steel, tungsten carbide and Ti-Mo alloy for example. Stainless steel shows promising results with a very high saturated field. The conditioning speed of Mo can be significantly improved by removing oxides at the surface with a heat treatment, typically at 875°C for 2 hours.
DC breakdown rate measurements have been done with Cu and Mo electrodes, showing similar results as in RF experiments: the breakdown probability seems to exponentially increase with the applied field.