MAGNETIC ERROR FIELD ANALYSIS BY MEASUREMENT OF LOCAL ROTATIONAL TRANSFORM IN A LOW-ASPECT-RATIO TORSATRON

In magnetic fusion toroidal confinement devices, the confining magneti c fields may have errors which produce magnetic islands. The standard method of investigating magnetic field errors involves measuring the m aximum radial spatial extent of a given magnetic island at a fixed tor oidal location and calling this measurement the island width. However, because of magnetic flux conservation inside an island, the spatial w idth of the island is,not,toroidally invariant. Magnetic error fields and their associated islands have been investigated in the Compact Aub urn Torsatron through the measurement of the local rotational transfor m about the fixed points of the island. This method of magnetic field error investigation is valid at any poloidal or toroidal angle because the local rotational transform is spatially invariant. The technique of measuring the local rotational transform is described and the resul ts are compared to those obtained using standard methods of measuring island widths.