Frequency and Temperature Dependence of Electrical Breakdown at 21, 30, and 39 GHz

A TeV-range $e^{+}{e}^{-}$ linear collider has emerged as one of the most promising candidates to extend the high energy frontier of experimental elementary particle physics. A high accelerating gradient for such a collider is desirable to limit its overall length. Accelerating gradient is mainly limited by electrical breakdown, and it has been generally assumed that this limit increases with increasing frequency for normal-conducting accelerating structures. Since the choice of frequency has a profound influence on the design of a linear collider, the frequency dependence of breakdown has been measured using six exactly scaled single-cell cavities at 21, 30, and 39 GHz. The influence of temperature on breakdown behavior was also investigated. The maximum obtainable surface fields were found to be in the range of 300 to $400\mathrm{M}\mathrm{V}/\mathrm{m}$ for copper, with no significant dependence on either frequency or temperature.

Figure 1

Geometry of a 30 GHz cavity. Distances in mm.

Figure 2

Photograph of the six single cell cavities tested.

Figure 3

Envelope of typical rf pulses with and without pulse shortening from a 30 GHz cavity.

Figure 4

Frequency dependence of the maximum surface field.

Figure 5

Comparison of the measured surface resistivity with literature data of copper.

Figure 6

Temperature dependence of maximum surface field.