  A novel quantity for predicting the high-gradient performance of radio
frequency accelerating structures is presented. The quantity is motivated,
derived and compared with earlier high-gradient limits and experiments. This
new method models a nascent RF breakdown as a current-carrying antenna and
calculates the coupling of the antenna to an RF power source. With the help of
an electron emission model to describe a nascent breakdown, the antenna model
describes how a breakdown modifies the local surface electric field before it
fully develops in any given structure geometry. For the structure geometries
that this method was applied to, it was found that the calculated
breakdown-loaded electric field was well-correlated with observed spatial
breakdown distributions, and gave consistent values for the maximum
breakdown-limited accelerating gradient between different geometries.
