High Gradient Testing of off-Axis Coupled C-band Cu and CuAg Accelerating Structures

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.