Stability analysis for the Big Dee upgrade of the Doublet III tokamak

Abstract

Ideal magnetohydrodynamic stability analysis has been carried out for configurations expected in the Big Dee tokamak, an upgrade of the Doublet III tokamak into a non-circular cross-section device which began operation early in 1986. The results of this analysis support theoretical predictions as follows: Since the maximum value of beta stable to ballooning and Mercier modes, which we denote |3 C> increases with inverse aspect ratio, elongation and triangularity, the Big Dee is particularly suited to obtain high values of j3 c and there exist high j3 c Big Dee equilibria for large variations in all relevant plasma parameters. The beta limits for the Big Dee are consistent with established theory as summarized in present scaling laws. High beta Big Dee equilibria are continuously accessible when approached through changes in all relevant input parameters and are structurally stable with respect to variations of input plasma parameters. Big Dee beta limits have a smooth dependence on plasma parameters such as |3 p and elongation. These calculations indicate that in the actual running of the device the Big Dee high beta equilibria should be smoothly accessible. Theory predicts that the limiting plasma parameters, such as beta, total plasma current and plasma pressure, which can be obtained within the operating limits of the Big Dee are reactor relevant. Thus the Big Dee should be able to use its favourable ideal MHD scaling and controlled plasma shaping to attain reactor relevant parameters in a moderate sized device.

Ideal MHD properties of an ellipsoidal shell tokamak

“…where V denotes the maximum plasma volume and AV denotes the maximum change in plasma volume when the current half-width 6j is varied through the operating range of the tokamak [12]. Here, 5j is defined as the fraction of the volume which contains half the current.…”

Neoclassical transport considerations for advanced tokamaks

“…A number of advanced tokamak configurations are being studied. Increased beta in the first stability regime is sought through elongation of the tokamak cross-section [1], through simultaneous elongation and triangularity [2], and through operation at lower aspect ratio [3]. In addition, operation in the second stability regime is being explored through indentation of the plasma cross-section with bean shaped [4] and crescent shaped [5] configurations.…”