Effects of central conducting pole and choking current on confinement of spheromak with flux hole

Stability against internal modes is investigated by use of the Mercier criterion. The flux hole was known to enhance stability of the spheromak by previous investigations. Introduction of a central conducting pole to the spheromak with the flux hole does not show always a stabilizing effect, i.e., it depends on the current profile in the plasma. Effect of a choking magnetic field is also studied and the flux hole is shown to disappear if a strong choking field is applied. The upper limits of the beta ratio on the magnetic axis are determined as functions of the choking current by use of a model pressure distribution and the optimized pressure distribution. For the latter pressure distribution, the Mercier criterion is marginally satisfied on all magnetic surfaces. The maximum value of them is shown to be 6%. (author)