No abstract

The magnetohydrodynamic (MHD) ballooning stability of the TJ-II heliac standard configuration is examined using a criterion derived for three dimensional (3-D) general configurations. From the ideal limit of this criterion, a critical ⟨β⟩ approximately=1.3% is obtained. This critical value is very close to that obtained from the Mercier criterion and is almost unaffected when those terms associated with compressibility are retained. The inclusion of resistivity is shown to have almost no effect on the attainable critical ⟨β⟩ even though unstable modes persist for lower ⟨β⟩ values due to their small growth rate. For parameters relevant to this configuration, the resistive pressure convection limit predicts unstable modes, scaling as γ~m2η for all ⟨β⟩ below the ideal limit, which are shown to disappear when compressibility terms are retained in the calculations. Unstable resistive ballooning modes with γτ A~10-3 are then found for ⟨β⟩ values down to 90% of the ideal critical ⟨β⟩. The growth rate rapidly falls if the pressure gradient is further decreased owing to the stabilizing effect of compressibility for γτA << 1

The configuration flexibility provided to the TJ-II Heliac by the helical hard core permits a significant change in the stability properties of the plasma. In this numerical analysis of full three-dimensional equilibria it is found that, by exploiting the flexibility of the machine, the stability beta limit given by the Mercier criterion can be varied almost continuously over values of average beta from zero to more than 6%.

A study of the stability limits for ideal ballooning modes in helical axis stellarators (heliac type stellarators) has been performed. Specific configurations similar to those of the TJ-II heliac have been taken, having a deep magnetic well and almost zero shear, which is characteristic for a heliac. The stability limits are those imposed by the destabilization of ideal ballooning and local interchange modes. The number of periods is varied in order to make a progressive approximation to helical symmetry as the aspect ratio increases. The very high stability limits for straight heliacs are recovered as the aspect ratio tends to infinity