A simple analytical model for a high recycling edge plasma, which relies on the presence of a small parameter – the ratio of the particle flows crossing the magnetic field to those impinging on the target – is proposed. The concept of a one-dimensional steady state (OSS) is introduced as a zero approximation in the small parameter. The mean number density of the particles – ions plus neutrals – in the magnetic flux tube is chosen as the most representative and convenient parameter of the problem. The OSS is shown to be ambiguous in a certain range for sufficiently high values of the energy flow entering the scrape-off layer from the bulk plasma. This ambiguity is confirmed by 1-D calculations. An equation describing quasi-steady variations in OSS is derived, and a mechanism of exciting self-sustained oscillations is developed. These oscillations are related to peculiarities of the neutral gas transport in the edge plasma and to the ambiguity of the OSS. The results of a 2-D simulation of the edge plasma oscillations are found to be in a good agreement with this mechanism, which might be responsible for the edge localized oscillations observed in H-mode divertor experiments.
Plasma oscillations (I)