RESULTS FROM AN X-BAND COAXIAL EXTENDED LENGTH CAVITY

Experiments and simulations demonstrate high-power microwave generatio n at 9 GHz in a coaxial geometry. The 9 cm diameter annular electron b eam is propagated between inner and outer drift tube conductors, a con figuration which increases the beam current and reduces the structure fields from existing high-power sources. Since the TEM mode of the coa xial guide reduces the quality factor of small-gap cavities, especiall y at high frequency, the interaction is provided by an extended length cavity loaded with dielectric. A single 16 cm cavity generates 200 MW of power from the 400 keV, 7 kA electron beam. Although the cavity ca n oscillate at a number of resonances, a single mode is selected with 10-30 kW of input power from a magnetron. A coupler samples 25 MW of t he power from the interaction region, precisely measured using a singl e-shot calorimeter. Simulations indicate that the efficiency of the de vice is limited to 7% by saturation effects, and can be improved by re ducing the length of the cavity.