Commissioning of the second two-frequency gyrotron in the new multi-frequency ECRH system for ASDEX upgrade

The power deposition in the plasma is primarily determined by the magnetic field B(r). 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 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 . In the experiments performed up to now in ASDEX Upgrade, the installed power was only 2 MW, of which 1.6 MW was coupled to the plasma. This imposes a limit for current drive, NTM stabilization or generation of internal transport barriers . The requirement for the new ECRH system is therefore an installed power of 4 MW. Since the current diffusion time in hot plasmas, like those with an internal transport barrier and Te > 10 keV, is several seconds, we need a pulse duration of 10 s compatible with the limit of ASDEX Upgrade flat top discharges. A further requirement is the capability for very localized power deposition such that its center can be feedback controlled, for instance to keep it on a resonant q-surface. For this purpose fast movable mirrors have been installed.