Design and experimental verification of a multi-pass ECRH plasma start-up scenario at a reduced magnetic field of 1.8T at W7-X

Abstract Wendelstein 7-X (W7-X) nominally operates at a magnetic field of 2.5T, but operation at a lower magnetic field strength is desired. The electron cyclotron resonance heating (ECRH) system of W7-X operates at 140GHz, corresponding to second harmonic (X2) heating at 2.5T, but also allows for heating at the third harmonic with a vacuum magnetic field around 1.7T to 1.8T. However, plasma start-up is not possible with X3-heating. Therefore one of the gyrotrons was tuned down to 101GHz for an X2 breakdown of the plasma at 1.8T on the magnetic axis. The available power in the plasma vessel for this detuned gyrotron is only 250kW with a pulse length of the order of 50ms. For this reason a multi-pass scenario was designed with six passes close to the magnetic axis to increase the effective power in the first few milliseconds during the breakdown. During the short 101GHz gyrotron pulse, this X2 multi-pass scenario creates a small plasma with temperatures up to several keV and line integrated densities around 8*10^18m^-2. It could also be shown that the multi-pass scenario is necessary to achieve plasma breakdown within 50ms. The plasma can be taken over by two sources of the neutral beam injection (NBI), starting at the same time as the detuned gyrotron to increase temperature and density. After about 1.5s at least 3MW of X3 ECRH are needed to take over from the NBI to maintain a stable plasma. This enables the low field operation of W7-X.