Advanced high power gyrotrons

Summary form given only, as follows. A 1 MW, 140 GHz gyrotron with diamond window for continuous wave operation and with a single-stage depressed collector has been designed and constructed in collaboration with CRPP Lausanne and TTE Velizy. It operates in the TE28,8 cavity mode and provides a linearly polarized TEM0,0 beam. The gyrotron consists of a magnetron injection gun, an improved cavity, an optimized non-linear up-taper and an improved launcher. RF power measurements at short pulse lengths gave 0.65 MW output power. In long pulse operation (150 ms) an output power of 0.6 MW was measured. Despite careful optimization it was not possible, to improve the output power. Measurements of the RF beam profile showed a strong shift of the beam by 30 mm downwards (design error). This deviation leads to power reflections of more than 20% back into the gyrotron, which causes a reduction of the generated power. The depressed collector could be operated up to 33 kV, which gives an improvement factor of 1.7 for the efficiency. The collector has been tested successfully in long pulse operation (30 s) without RF generation at I MW electron-beam power level. The measurements are currently continued, new results with a redesigned mirror system will be given. With the 165 GHz coaxial cavity gyrotron, an output power of 2.2 MW with an efficiency of 28% has been measured at a beam current of 84 A in short pulse operation. The maximum efficiency has been measured at 1.5 MW output power. With a voltage depression of about 35 kV an efficiency of 48% has been achieved (30% without SDC). Up to that depression voltage no reduction in output power has been observed. These results are in very good agreement with numerical calculations, if one includes internal losses of 10% and a velocity spread of 5% for the electron beam.