Abstract

Motivated by the ongoing feasibility study of a future multi-TeV e+e- Compact LInear Collider (CLIC), a comprehensive investigation of the DC breakdown behavior of various metallic electrode materials has been performed. The breakdown conditioning results for each material is presented and discussed.
In addition to the intrinsic properties of materials, other factors such as the type of pre-treatments are important. Changes in breakdown characteristics due to variations in surface finishing techniques have been studied for three different electrode materials: Molybdenum, CuZr and GlidCop. In all cases, electro discharge machining (EDM) leads to a significantly slower conditioning speed compared to milling. Also heat treatments are shown to influence strongly the breakdown characteristics. ‘Ex-situ’ vacuum annealing of molybdenum reduces, or even removes, the conditioning process towards breakdown saturation.
Significant pressure increase within the CTF3 accelerator structure at the moment of RF breakdown events has been observed. Mass spectroscopy measurements show that the gas released from the molybdenum electrodes at the moment of DC breakdown consist mainly of H₂ and CO. By careful mass spectroscopy measurements using properly calibrated vacuum gauges, quantitative measurements of gas releases have been performed. These results will later be used as parameters to calculate the true pressure and composition inside the accelerator structures.