Stability Analysis of Spheromak by ERATO-J Code against Internal Modes*

Abstract

The stability of the spheromak against low n ( n =1, 2, 3, 4, 5) and localized modes is investigated by use of ERATO-J code and by means of the Mercier criterion. Here n is the toroidal mode number. The shape of the plasma is assumed to simulate the experimental plasma in the flux conserver of CTCC-II at Osaka University. The beta limits for these modes are evaluated as a function of q axis to elucidate which mode gives a lower limit. Here q axis is the value of the safety factor on the magnetic axis. As a result, the beta limit of the spheromak is shown to be determined by the n =1, the n =2 and the localized modes. However, the Mercier criterion is shown to give an excellent measure for the stability against internal modes, if q axis ≤0.9.