Improvement of the Power Handling Capability of the Upgraded Multi-Frequency ECRH System at ASDEX Upgrade

8 gyrotrons are in routine operation in the Electron Cyclotron Resonance Heating (ECRH) system at the ASDEX Upgrade tokamak. All gyrotrons are step-tunable operating at 105 and 140 GHz with a maximum output power of about 1 MW and 10 s pulse length. The system includes 8 transmission lines, mainly consisting of oversized corrugated waveguides (I.D. = 87 mm) with overall lengths between 50 and 70 meters including quasi-optical sections at both ends. The transmission lines are operating in air and transmission at maximum power levels was limited by arcing in the past. Several measures led to a significant improvement of the power handling capability of the transmission lines.