1 MW, 140 GHz, CW gyrotron for Wendelstein 7-X

Summary form only given. A continuous wave, 1 MW, 140 GHz gyrotron oscillator with a CVD diamond window has been designed and constructed as a joint collaboration between FZK Karlsruhe, CRPP-Lausanne, IPF Stuttgart, CEA Cadarache and TTE Velizy for the 10 MW ECRH system of the new stellarator plasma experiment Wendelstein 7-X at IPP Greifswald, Germany. The gyrotron will operate in the T/sub 28,8/ cavity mode and provides a linearly polarized, fundamental Gaussian output beam. It is composed of a diode type electron gun, an improved beam tunnel, a high mode purity low-Ohmic-loss cavity with rounded transitions. The design parameter for the cavity were chosen to minimize power reflection and mode purity. The electron gun and the magnetic field are designed for a velocity ratio of 1.3. The RF field is guided by an optimized non linear uptaper and converted to a Gaussian beam by a highly efficient quasi-optical mode converter consisting of a rippled wall waveguide launcher followed by a mirror of quasi-elliptical shape and two toroidally focussing mirrors. A large single stage depressed collector on ground potential is capable to withstand the electron beam power in continuous operation assuming a total efficiency of 45% which is achieved by a voltage of 30 kY to the cavity and -50 kV to the cathode. The RF beam is extracted from the gyrotron horizontally through an edge-cooled single-disk CVD-diamond window with an aperture of 88 mm and a disk thickness of 1.8 mm. With a loss tangent of 4.10/sup -5/ and a dielectric permittivity of 5.67, the absorbed power is only 700 W and thermal finite element computations a temperature rise of only 61/spl deg/C at the center of the window.