The local 3-D ideal MHD stability properties for two different H-1 Heliac configurations with values of magnetic well of 1.3 and 3.2%, respectively, have been investigated. The pressure profile has been determined that approaches marginal stability to 3-D ballooning modes almost uniformly across the plasma radius for each case. In spite of the difference in well depths, the beta limits for both configurations are almost the same (at β ≃ 0.8%). The optimal pressure profile for the deep well configuration is slightly more peaked than that for the standard configuration. The ballooning imposed beta limits are more stringent than those of the Mercier criterion (β ≃ 1%) and the ballooning optimized pressure profiles are more peaked than the corresponding profiles which are marginal according to the Mercier criterion
Influences of ballooning modes with moderate wave number on MHD equilibrium in LHD