Experimental results and numerical simulations of a high power 140 GHz gyrotron

The design of a 0.5 MW 140 GHz gyrotron with axial RF output operating in the TE/sub 10,4/ mode is presented. Experimental results and numerical simulations are compared. In particular the effects of RF reflections at the output window, velocity spread of the helical electron beam and startup of oscillations by simultaneous increase of pitch factor /spl alpha/ and relativistic factor /spl gamma/ are discussed. In short pulse operation (/spl les/5 ms) an output power of 690 kW has been obtained with an electronic efficiency of 31% and a mode purity of about 99%. The experiments have shown that it is possible to use for step frequency tuning the azimuthal neighbours of the TE/sub 10,4/ mode, TE/sub 9.4/ at 132.6 GHz (420 kW) and TE/sub 11,4/ at 147.3 GHz (300 kW) as working modes by decreasing and increasing the magnetic field, respectively.>