Waveguide Transmission Line for 106-GHz Electron Cyclotron Heating in Heliotron-E

A 106-GHz electron cyclotron heating system is installed and operated for plasma production and heating of the Heliotron-E helical device. The Gaussian beam radiated from the gyrotron is coupled to the HE11 waveguide mode by the matching optics unit (MOU), then transmitted through 15-m corrugated waveguides and four miter bends. The system is closed for safety to prevent spurious modes from radiating into the free space and is operated at atmospheric pressure. The transmitted wave is launched from the outside of the torus, and the launched beam is focused on the magnetic axis so that the power deposition is expected to be localized at the desired resonance region. The measured transmission efficiency from the MOU output to the launcher output is 86%, which is in good agreement with the theoretical estimate. The power losses arise mainly at the waveguide mouth where the Gaussian beam is coupled to the HE11 mode, at the miter bends and in the launching system.