Present status of the new multifrequency ECRH system for ASDEX upgrade

A new multifrequency electron cyclotron resonance heating system is under construction for the Axially Symmetric Divertor EXperiment (ASDEX) Upgrade tokamak experiment. For the first time in a fusion device, this system employs multifrequency gyrotrons that are step-tunable in the range 105-140 GHz. In its final stage the system will consist of four gyrotrons with a total power of 4 MW and a pulselength of 10 s. The first two gyrotrons, working at 105 and 140 GHz, were installed and tested. Transmission line elements such as corrugated waveguides, polarizer mirrors and vacuum windows are designed to cope with this frequency band. The system includes fast steerable launchers at the front end that will allow for localized feedback-controlled power deposition in the plasma. Index Terms--Electron cyclotron resonance heating (ECRH), fast steerable launcher, step-tunable gyrotron.