Towards the 2 MW RF output power of the coaxial cavity gyrotron pre-prototype for iter

Abstract

To support the development of the first industrial prototype of a 2 MW, CW, 170 GHz coaxial cavity gyrotron, a short pulse (≪ few ms) coaxial gyrotron (“pre-prototype”) of modular type is used for experimental studies at FZK. As a consequence of the limitation on the magnetic field value (6.67 T) of the superconducting magnet (SC) at FZK, it became necessary to reduce the operating voltage in the preprototype tube to values below 80 kV, in order to be able to excite the TE 34,19 mode at 170 GHz. This resulted in decrease of the RF output power to 1.5 MW. During the experiments parasitic oscillations at high frequencies around 152 and 160 GHz have been observed at U c ≫74 kV simultaneously with the nominal gyrotron working mode at 170 GHz. In this region with additional parasitic oscillations operation a reduced of the gyrotron efficiency has been measured. There are some indications that the parasitic oscillations are generated inside the beam tunnel near the cavity. To study these parasitic beam tunnel oscillations different designs of the beam tunnel are under consideration. To make the experiments with the pre-prototype tube more relevant for the industrial prototype it is necessary to operate the tube at the nominal magnetic field, namely at 6.86 T. To enhance the magnetic field to the desired value an additional normal conducting coil has been wounded directly on the housing of the gyrotron around the position of the cavity. First experiments with the new beam tunnel at the nominal magnetic field have been already started. In addition, the improvement of the most critical component of the coaxial cavity gyrotron - the quasi-optical RF output system is in progress. Up to now the Gaussian content of the RF output beam is not satisfactory, namely less than 80 %. The main reason of the poor performance of the RF output system is the launcher antenna with its low efficiency of conversion. The new launcher has been designed using a novel optimization method and is currently fabricated together with a suitable mirror system consisting only of toroidal mirrors. The simulated conversion efficiency of the TE 34,19 mode to the desired fundamental Gaussian mode is near 97 %. The system will be experimentally verified first in low power measurements at the beginning of 2009. Achieved results will be presented and discussed.