Cold test measurements on components of the 1 MW, 140 GHz, CW gyrotron forthe stellarator Wendelstein 7-X

For the development of a 1 MW, 140 GHz gyrotron for CW operation which will be installed at the stellarator facility Wendelstein 7-X at IPP Greifswald , a collaboration between different European research institutes and an ind ustrial company has been established. In order to prove the proper function ing of the millimeter wave components installed in the gyrotron - such as t he cavity, the waveguide taper and the quasioptical mode converter - these components should be cold tested, preferably before installation. However, due to lack of time as well as long delivery times, this was not possible. Therefore, two units of the quasioptical mode converter and the cavity were fabricated with identical geometry, one of those being used for measuremen ts on the low power test device. To perform these cold tests for tapers and mode converters, the gyrotron cavity output mode has to be simulated. This means that a high order rotating mode (TE28,8 mode) must be generated at l ow power. This can be achieved by means of a mode generator consisting of t wo mirrors and a coaxial cavity with a perforated outer wall. Before applyi ng the mode generator to the components, its proper behavior and the accura te alignment of the system must be verified either by radiation pattern mea surements or k-spectrometer measurements. As the coupling through the holes of the k-spectrometer is extremely low, a special vector network analyzer with a dynamic range of at least 100 dB had to be developed. This has been achieved by integration of a phase locked backward-wave oscillator with a l ine width of 100 Hz and an output power of 10 mW. A non-destructive measure ment of the resonance frequency and the quality factor of the cavity does n ot seem possible. The second cavity will be prepared for the cold measureme nt by drilling a small radial hole into its wall in the plane of the field maximum. This hole is then used for the input coupling. The accuracy requir ed for this hole is rather critical. The coupling coefficient must be high for sufficient excitation of the rf field, but on the other hand it must ne ither change the frequency nor the quality factor strongly. The transmissio n is measured by a probe at the output of the uptaper. (C) 2001 Elsevier Sc ience B.V. All rights reserved.