Toward a 170 GHz, 2 MW, CW Coaxial Cavity Gyrotron

- experimental results and design considerations -

B. Piosczyk

Forschungszentrum Karlsruhe, Association EURATOM-FZK,

Institut für Hochleistungsimpuls- und Mikrowellentechnik (IHM), D-76021 Karlsruhe

e-mail: bernhard.piosczyk@ihm.fzk.de

Abstract

The development work on coaxial gyrotrons has been supported within the ITER task of ECRF Advanced Source Development. As a result of this development the feasibility of fabrication of coaxial cavity gyrotrons with an output power up to 2 MW, CW has been demonstrated and information necessary for a technical design has been obtained. Experiments with a gyrotron equipped with newly designed components - electron gun, cavity, RF output system have been performed. In short pulses a maximum RF output power of 2.2 MW has been reached in stable operation. At the nominal output power of 1.5 MW an efficiency of 30 % has been achieved. This has been enhanced to 48 % in operation with a single-stage depressed collector.

The stability of the coaxial insert has been measured to be within ± 0.03 mm under operating conditions. The losses at the coaxial insert have been found to be about 0.1% of the RF output power. Investigations of the microwave stray radiation captured inside the tube have been performed with the following results: (1) The captured stray radiation due to diffraction losses is approximately uniformly distributed inside the mirror box. (2) About 8% of the captured microwave power is radiated through a relief window with 100 mm diameter in the used setup. (3) The total amount of stray radiation has been found to be about 11 % of the RF output power. Parasitic low frequency oscillations have been successfully suppressed and stable operation has been achieved over a wide parameter range. Fast (~ 0.1 ms) frequency tuning has been demonstrated by applying a rapid variable bias voltage at the coaxial insert. In particular, step frequency tuning by ±  2.2 GHz due to switching from the nominal mode at 165 GHz to its azimuthal neighbors has been done and continuous tuning by up to 70 MHz within the bandwidth of the TE31,17 mode has been performed.

Within the talk first the experimental set up will be described and the special problems of coaxial gyrotrons will be discussed. Then recent experimental results will be given and a first integral design of a 2 MW coaxial gyrotron will be presented.