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Magnetic surface breaking due to finite pressure effect is analyzed for three-dimensional magnetohydrodynamic equilibria of l=2 heliotron/torsatron configuration. Characteristics of magnetic island formation are numerically studied in detail. Main results are as follows: (i) There exists a clear-cut boundary, which practically determines the position of the outermost closed surface. The boundary position shifts as beta increases, though not affected by the external axisymmetric fields. (ii) The clear-cut boundary is maintained for a higher beta plasma as the aspect ratio of the configuration increases. For a low aspect ratio configuration, however, high beta equilibria with clear surfaces can be obtained by controlling the external poloidal field, especially the vertical field. (iii) The external l=1 field is particularly viable in controlling the breaking of magnetic surfaces.
Hayashi et al. (1992) studied this question.