  We report the results of high gradient testing of two single cell off axis
coupled standing wave accelerating structures. Two brazed standing wave side
coupled structures with the same geometry were tested one made of pure copper
Cu and one made of a copper silver CuAg alloy with silver concentration of 0.08
percent. A peak surface electric field of 450 MV per m was achieved in the CuAg
structure for a klystron input power of 14.5 MW and a 1 mirco s pulse length
which was 25 percent higher than the peak surface electric field achieved in
the Cu structure. The superb high gradient performance was achieved because of
the two major optimizations in the cavity geometry 1 the shunt impedance of the
cavity was maximized for a peak surface electric field to accelerating gradient
ratio of 2 for a fully relativistic particle 2 the peak magnetic field
enhancement due to the input coupler was minimized to limit pulse heating.
These tests allow us to conclude that C band accelerating structures can
operate at peak fields similar to those at higher frequencies while providing a
larger beam iris for improved beam transport.
