Self-sustained oscillations in the divertor plasma

Abstract

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 N̄ 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 N̄ 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.

2‐D Fluid Transport Simulations of Gaseous/Radiative Divertors

“…Bifurcations of the solutions are found for gas injection that persist as the mesh is refined. The existence of multiple solutions and bifurcations have been predicted from simpler models as arising from recycling [12,14] and impurities [15]. What we find here may well be related to these modes, but more direct comparison remains to be done.…”

“…Likewise, switching off recombination does not change the character of the solution although ni increases by about 25%. Low Te solutions have been found in models by Krasheninnikov, et al [12], where oscillatory modes are seen that may correspond to our bifurcation; this connection is being analyzed. Also, Petravic [13] finds a collapsed solution where recombination plays a dominant role.…”

Physical mechanisms in divertors and their impact on the core

“…At present, a few different type of bifurcation of the SOL plasmas were found theoretically [12]: thermal bifurcation caused by the plasma recycling and impurity radiation, and spontaneous breakdown of in-out symmetry of SOL plasmas. It was also shown by both theory and modeling that self-sustained oscillations of the SOL plasma parameters are possible.…”

Self-Sustained Divertor Plasma Oscillations in the JET Tokamak

“…Second, the effect of this oscillatory behavior on the main plasma and on the achievement of good confinement (H-mode) will be described. Finally, the experimental observations will be succinctly compared with theoretical predictions for the plasma edge [3,4]. From this comparison, it is postulated that the oscillatory behavior reported here constitutes a basic physical process at the edge plasma of fusion devices.…”