Verlag des Forschungszentrums Jülich

JUEL-3864
Nicolai, Albert
TSC-Modelling of MAST-Discharges
78 S., 2001



The TSC -code had been applied to compute the time evolution of the main discharge parameters of MAST - shots during the first 120 msec.

This two dimensional, time dependent, free boundary code advances the MHD equations describing the evolution of an axisymmetric plasma on the transport timescale. The circuit equations for the poloidal field coils are coupled to the Maxwell - MHD equations for the plasma via boundary conditions.

The plasma description in TSC is completed by using e. g. the semiempirical Coppi - Tang model for the heat conductivities and neoclassical theory for the resistivity.

The attempt was made to align the theoretical data with the analogous experimental data:
(1) Either the plasma current or the current ofthe central solenoid are controlled by a 'lst or 2nd level' feedback allowing the computation of either the solenoid or the plasma current. The coil currents in the divertor coil P2, induction coil P3, and in the vertical field coils P4 and P5 were linked to the experimental currents by means of the '2nd level' feedback.
(2) To reproduce the maximum electron temperature, the coefficient a121 of the Coppi - Tang transport model was adjusted. Some characteristic shots e. g. a 1 MA - shot, an H - mode shot, a shot with a hollow temperature profile have been selected. Also a direct induction shot (with a plasma initiated by the central solenoid) was investigated. Furthermore it was attempted to understand the 'merging -compression' method by the 2d -TSC - simulation which averages over the 3d - effects. Some typical results are:

1. If the plasma current is controlled (by the 1st level feedback) the maximum current of the central solenoid can be reproduced with an accuracy of around 15%.
2. The range of the coefficient a₁₂₁ needed to reproduce the maximum electron temperature is about 0.08 < a₁₂₁ < 0.16. Good confinement ( a₁₂₁ = 0.08) is possibly a prerequisite for the H - mode as in the shot #2700. The coefficient a₁₂₁ = 0.16 may indicate the influence of impurities.
3. To generate the same plasma current by the direct induction method needs a larger current swing of the central solenoid than the merging -compression method. This increase is reproduced approximately.
4. The 'merging -compression' method ( shots #2274, #2700, #2482, #2875) leads to a plasma current of around 300 kA and to a total vessel current of around 400 kA.

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