MULTI-MODE GENERATOR FOR THE COLD TEST OF

BROADBAND QUASI-OPTICAL GYROTRON MODE CONVERTERS

D. Wagner

Max-Plank-Institut für Plasmaphysik, EURATOM-IPP, Boltzmannstr.2, D-85748 Garching, Germany

M. Thumm, G. Gantenbein, J. Flamm

Forschungszentrum Karlsruhe, EURATOM-FZK,

Institut für Hochleistungsimpuls- und Mikrowellentechnik, PO Box 3640, D-76021 Karlsruhe, Germany

J. Neilson

Calabazas Creek Research, 690 Port Drive, San Mateo, CA 94404-1010, USA

R. Vernon

University of Wisconsin, Madison, Wisconsin, 53706-1691, USA

Abstract

To test the performance of a broadband quasi-optical mode converter for step-tunable gyrotrons in cold-test experiments, several high order cavity modes at different frequencies have to be excited at low power levels. In order to avoid the huge effort of building a mode generator for each single mode a broadband quasi-optical method was chosen. Here a Gaussian beam is focused onto a mode specific caustic in a coaxial cavity through a translucent cavity wall. The mode generator can be tuned to different resonances by eventually exchanging the inner conductor of the cavity and by slightly adjusting the position of its quasi-optical components. The two main modes of interest are the TE_22,6 at 110 GHz and the TE_24,7 at 124.7 GHz. In first tests both modes could be excited with high mode purity and low counter-rotating content. The frequency mismatch was below 20 MHz compared to the calculation in both cases. Additional modes could be excited as well.

Possible collaboration:

Optimization and cold test of broadband quasi-optical antennas for multi-frequency gyrotrons. (US-EU)