Study on Burn Control in a Tokamak Fusion Reactor

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We proposed previously the feedback control by compression-decompression to stabilize the thermal instability in a tokamak fusion reactor. In order to attain the stationary burning, the core plasma is compressed or decompressed in major radius direction through energizing the control coil according to the deviation of temperature or fusion power from the equilibrium values. In the previous studies, the shell effects on vertical field applied for the control were approximated by a timedelay. The present study deals with the magnetic circuits consisting a plasma, control coils and the shell and includes the effects of eddy currents in the conductive shell on the compression-decompression control. The eddy currents are decomposed into the fourier series and the first and second order modes are taken into consideration. The inductances between the current components such a plasma, control coils and the eddy currents are evaluated in the approximation of a large aspect ratio limit. The basc equations for the study consist of the magnetic circuits, temperature balance equation of a plasma and plasma equilibrium equation. We investigated the linear stability based on these equations for several scalings of an energy confinement time, and obtained the requirements of the feedback gain and the electrical resistivity of the shell. In case of S.S. shell, the thickness for the stabilization should be less than 30 cm for Keye-Goldstone scaling and 7 cm for Mirnov H-mode scaling. In conclusion, the compression-decompression control is shown to be an effective method to stabilize the thermal instability in a tokamak equipped with a moderately thick shell(several cm in s.s.).