Parametric decay in the edge plasma of ASDEX during fast wave heating in the ion cyclotron frequency range

Two types of parametric decay instabilities were observed in the scrape-off layer of ASDEX during hydrogen second harmonic heating, in a single as well as in a two ion species plasma. The first type was identified as decay into an ion Bernstein wave and an ion cyclotron quasi-mode, and the second as decay into an ion Bernstein wave and a low frequency electron quasi-mode. The parametric decay processes are due to the high electric fields near the fast wave antennas. Theoretical growth rate calculations predict a threshold for the electric fields near the antennas. In the relevant experiments the electric fields, estimated from a full-wave code, indeed exceeded the thresholds. These parametric decay processes, as well as the harmonics of the generator frequency also seen in the scrape-off layer, provide a mechanism that might contribute to the observed direct energy deposition in the edge plasma and to ICRF induced impurity production.