Magnetic-field study in a compact helical system

Implementation of the designed magnetic field in the compact helical system torsatron device has been verified both experimentally and numerically. A misalignment of the outer two sets of poloidal field coils has been identified both quantitatively and qualitatively. A magnetic-field-mapping experiment has indicated that closed magnetic surfaces have been successfully achieved up to 95% of the outermost surface and that islands exist on the m=2 and 3 major rational surfaces. The experimental results and the computational analysis consistently explain that these islands are mainly caused by the ambient field resulting from magnetization of the building structure. The m=2 island width is estimated to be several millimeters during regular plasma heating experiments when the toroidal field is more than 1 T.