First tests of the new multi-frequency ecrh system at ASDEX upgrade

Summary form only given. The power deposition in ECRH (electron cyclotron resonance heating) of fusion plasmas is primarily determined by the magnetic field. For a single frequency ECRH system this has the consequence that for central heating the magnetic field is no longer a free parameter. However, for tokamak plasmas with different plasma currents or different equilibria, the magnetic field should be a free parameter in order to operate at a reasonable edge safety factor q(a). Furthermore, in a plasma with given parameters, some experimental features, like suppression of neoclassical tearing modes (NTM), require to drive current on the high field side without changing the magnetic field. These requests can be satisfied if the gyrotron frequency is variable. A new broadband ECRH system is currently under construction at the ASDEX Upgrade tokamak at IPP Garching. This system will employ multi-frequency gyrotrons which are step-tunable in the frequency range 105-140 GHz. In its final stage the system will consist of 4 gyrotrons with a total power of 4 MW and a pulse length of 10 s. It employs a fast steerable launcher in the plasma vessel for feedback controlled power deposition that allows for poloidal steering of 10 deg, within 100 ms. Transmission line elements, such as corrugated waveguides, polarizer mirrors and vacuum windows, are designed to cope for this frequency band. The first two-frequency gyrotron, operating at 105 GHz and 140 GHz, is currently being put into operation at ASDEX Upgrade