ICRF hydrogen minority heating in the boronized ASDEX tokamak

Since the divertor of ASDEX has been modified (1986-87) the hydrogen concentration in deuterium plasmas could not be reduced below 10%, although the machine was operated for long periods of time with deuterium injection. This is probably due to desorption in the divertor as indicated by the increasing H-concentration during a deuterium injection pulse. As a consequence for H-minority heating in deuterium, the maximum power into ohmic plasmas without causing a disruption was limited to few hundred kW. A partial solution was ICRH in combination with deuterium injection which allowed us to apply up to 1.5 MW ICRH to the plasma. The beneficial role of the injection is attributed to an improved ICRH absorption and to the higher energy flux and temperature in the divertor. During the last ICRH campaign we operated mainly in helium plasmas for a lower hydrogen concentration and the vessel was boronised. The H-concentration is measured routinely by a mass spectrometer in the divertor chamber. This measurement does not give a fast response to eventual changes and also no absolute concentrations in the main plasma, but it gives a reliable estimate of the time evolution during one discharge or from shot to shot. The data from the mass spectrometer were often cross-checked with charge exchange measurements from the main plasma. In helium discharges the hydrogen concentration is around 2% in the ohmic phase but it increases up to 8% as ICRH is applied. Under these conditions the maximum available power (2.7 MW) could be applied to the plasma without causing a disruption. This is partly due to the low H-concentration in helium at the beginning of the ICRH pulse but also to the boronisation, as discussed in a later. (author) 4 refs., 6 figs