Status of Development of High Power Coaxial-Cavity Gyrotron at FZK

Abstract The development of high power gyrotrons at Forschungszentrum Karlsruhe (FZK), in cooperation with several European research institutions, is in progress. Gyrotrons are foreseen for heating, current drive and stabilization of plasmas in experimental fusion reactors. At FZK a high power 2 MW, 170 GHz pre-prototype gyrotron with coaxial cavity for ITER is being developed and experimentally tested at short pulse (a few ms) operation, a similar industrial prototype gyrotron has been tested at the Centre de Recherches en Physique des Plasmas (CRPP), Lausanne. Recent activities at FZK included the verification of its critical components, especially the electron gun, cavity and quasi-optical RF-output system. Low frequency oscillations at 265 MHz have been successfully suppressed due to a small modification of the geometry of the gyrotron components, increasing the measured power of the gyrotron up to 1.4 MW with an efficiency of 26% (non-depressed operation). However, additional parasitic oscillations around 152 - 160 GHz appear simultaneously with the desired gyrotron working mode. New beam tunnel prototypes which should avoid the excitation of parasitic oscillations in the GHz frequency range are fabricated and first tests have been performed. To increase the output power of the coaxial-cavity gyrotron up to 2 MW it is necessary to operate the tube at the nominal magnetic field, namely at 6.87 T. This has been realised with an additional normal conducting coil directly wounded on the gyrotron body in the cavity region. In first experiments at nominal operating parameters for the coaxial-cavity gyrotron a output power of 1.8 MW and a efficiency of 27% has been achieved. Further optimization of the quasi optical (q.o.) output system has been developed and according to the calculations an efficiency of the launcher with Gaussian mode content of about 96 % is expected. The new launcher antenna together with a suitable mirror system has been fabricated and will be verified.