Mode locking in pre-disruptive plasmas on Tore Supra
On Tore Supra, as in other large tokamaks, disruptions are preceded by an m=2, n=1 growing mode, the amplitude and frequency of which are measured by arrays of pick-up coils and saddle loops. Particular experiments with gas puffing, pellet injections, and ergodic divertors allow changes of the pressure profile and the Delta' value. Ergodic divertors also introduce a new type of interaction of the mode with the plasma. Experimental data are analysed within a variational formulation of the self-consistency of the mode. Outside the resonant layer, this model includes MHD effects induced by the current profile. Inside the resonant layer, it takes into account the Rutherford current, the stabilising effect of the pressure gradient through the average curvature and the friction of the mode on ion and electron populations. The model is used to simulate the growth rate, the frequency and the poloidal components observed during the bifurcation from a rotating mode regime at the diamagnetic frequency to a quasi stationary mode regime. Such bifurcations are typically observed for q(psi) < 4. For limiter experiments, the bifurcation occurs at a relatively large critical island width, for which the friction of the mode with the wall balances the diamagnetic effect. For ergodic divertor experiments, it takes place at a much smaller mode amplitude which, in most cases, remains constant. This is consistent with theoretical results on the interaction of the mode with the static magnetic islands created by the divertor.
Bibliographic Reference: Paper presented: 18th European Conference of the EPS-Plasma Physics Division, Berlin (DE), June 3-7, 1991
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Record Number: 199110384 / Last updated on: 1994-12-02
Original language: en
Available languages: en