W. Kasparek, G. Dammertz, V. Erckmann, G. Gantenbein, H. Hailer, F. Hollmann, L. Jonitz, H. Laqua, W. Leonhardt, G. Michel, M. Schmid, P.G. Schüller, K. Schwörer, R. Wacker, M. Weissgerber

For the fusion plasma experiment Wendelstein 7-X which is built up at the Max-Planck-Institut für Plasmaphysik (IPP) at Greifswald, a powerful heating system using millimetre waves is under construction. The system is designed to heat the plasma at the second harmonic frequency of the electron cyclotron resonance and will include 10 gyrotrons at 140 GHz, generating 1 MW of CW power each. The millimetre wave power will be transmitted via two multi-beam waveguides (MBWG) over a distance of about 60 m.

This paper focuses on the transmission of the millimetre waves from the gyrotrons to the plasma. After a short discussion of multi-beam transmission with mirrors producing minimum mode conversion, the design of the complete system is presented. Additional issues, like cooling of the mirrors, alignment of the lines, power measurement with loads and directional couplers, and other beam diagnostics are discussed.

Measurements on various components as well as on a prototype system are presented. This includes investigations of the mirror surfaces under application of a heat load equivalent to the absorbed power from a 1 MW mm-wave beam, as well as successful long-pulse tests (e.g. 740 kW / 100s) during conditioning of a prototype gyrotron for W7-X. For the prototype system, calorimetric measurements show a transmission efficiency of about 90% in good agreement with theory, and low-power measurements of radiation patterns yield a high mode purity for the mllimetre wave beams.

This work has been performed in the frame of the project PMW, ECRH for W7-X hosted at FZK Karlsruhe (collaboration between FZK Karlsruhe, IPP Garching and Greifswald, and IPF Stuttgart).