Magnetic surface breaking in three-dimensional l=2 heliotron/torsatron equilibria

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Abstract
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.