The multi-beam transmission system for 140 GHz electron cyclotron heating, on the stellarator W7-X:: Concept, design and first tests

For the fusion plasma experiment Wendelstein 7-X which is built up at the Max-Planck-Institut fur 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 each generating 1 MW of CW power at 140 GHz. 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 beam waveguide design, the principle of multi-beam transmission is described and results for the shape of the mirrors producing minimum mode conversion in the MBWG transmission system are given. The design of the complete system is presented, and technical issues as well as the expected transmission efficiency are discussed. Measurements on a prototype system show a transmission efficiency of about 90 %.