The ultrahigh-vacuum electrical breakdown characteristics of copper, molybdenum, and tungsten have been explored in a setup based on a capacitor discharge. Upon repeated sparking, tungsten and molybdenum showed improvement of the maximum applicable field before breakdown (conditioning) in contrast to copper, which experienced alternate improvement and degrading. After conditioning, tungsten withstood the highest applied field followed by molybdenum and copper. This behavior was correlated with that of the field enhancement factor $\beta$ extracted from measurements of the field emission current. These results are compared with the tests performed on 30 GHz test accelerating structures for the future Compact Linear Collider.

• Received 11 March 2004

DOI:https://doi.org/10.1103/PhysRevSTAB.7.092003

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