A new local field quantity is presented which gives the high gradient performance limit of accelerating structures due to vacuum rf breakdown. The new field quantity, a modified Poynting vector $S_c$, is derived from a model of the breakdown trigger in which field emission currents from potential breakdown sites cause local pulsed heating. The field quantity $S_c$ takes into account both active and reactive power flow on the structure surface. This new quantity has been evaluated for many X-band and 30 GHz rf tests, both traveling wave and standing wave, and the value of $S_c$ achieved in the experiments agrees well with analytical estimates.

• Received 28 January 2009

DOI:https://doi.org/10.1103/PhysRevSTAB.12.102001

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