Minutes of the CLIC Meeting - 27 January 2006

Abstract:

The need for high accelerating gradients for the future 30 GHz multi-TeV e+e- Compact Linear Collider (CLIC) at  CERN has triggered a comprehensive study of DC breakdown fields of metals in ultra-high vacuum. The custom-made experimental setup is based on a capacitor discharge across a gap junction. The simple design and fully automated computer control enable breakdown fields and dark current of numerous materials to be measured in a reliable way.  
The study shows that molybdenum, tungsten and titanium reach high breakdown hold-off fields, and are thus good candidates for the iris material of CLIC structures. For untreated molybdenum the breakdown field is higher than copper but the conditioning speed is slower. Titanium, on the other hand, shows acceptable conditioning speeds, but material erosion makes this solution problematic. A feasible solution to increase the spark conditioning speed for the case of molybdenum is presented. For molybdenum and tungsten a significant reduction in the saturated breakdown field was observed upon gas exposure during intensive spark conditioning. As an example, a 50% decrease of the breakdown field of molybdenum is recorded when spark conditioning is carried out in an environment of 1∙10-5 mbar dry and laboratory air.

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