A 500 kW, 140 GHz gyrotron in an asymmetric volume mode for fusion plasma applications

A major goal of the KfK gyrotron development program is the design and construction of a high power, long pulse 140 GHz gyrotron for electron cyclotron wave applications in fusion research. As a first step a TE0,3 tube with a pulse length of 400 ms and an output power of up to 120 kW was constructed, tested, and successfully used for plasma heating and plasma diagnostics of neutral beam sustained, high density plasmas in the W7AS stellarator at IPP in Garching. These results have been reported elsewhere and will not be discussed further here. To reach higher power at increased pulse length a higher order cavity mode must be chosen. As a next step, KfK has designed, built, and is testing a gyrotron which operates in the TE10,4 mode with an output power of up to 690 kW, and frequency 140 GHz. The RF power is coupled out with an axial window. This phase of the program aims to demonstrate the capabilities of the asymmetric volume mode (AVM) approach in generating high power levels. The next version, using a new magnet will use a built in quasi-optical mode transducing antenna which converts the rotating TE10,4 mode to a free space Gaussian beam, and a radial window, in order to separate the mm-wave output and the electron beam. The influence of the quasi-optical convertor on the beam-field interaction in the resonator and the behavior of the convertor and mirror system under hot test conditions will be investigated. It may then be possible to extend the pulse length up to about 1/3 s (depending on the design of the window and collector). (orig.)