Flux Surface Mapping in LHD

Magnetic flux surface measurements have been carried out in the Large Helical Device (LHD) in the standard magnetic field configuration with toroidal magnetic field strength up to 2.75 T. An electron beam launched with a small electron gun moving across the flux surfaces was detected with a fluorescent screen or a probe array. Nested surfaces could clearly be visualized with both methods. Even the stochastic region was detected. In the experiment, unfavorable m/n = 1/1 and 2/1 magnetic islands were discovered near the last closed flux surface, where m and n are poloidal and toroidal mode numbers, respectively. There is a possibility that the source of the error field, in the low magnetic field of 0.0875 T, is terrestrial magnetism. On the other hand, in the standard magnetic field of 2.75 T, the main source of the error field is thought to be ferromagnetic materials near the torus of LHD. Fortunately, it was demonstrated that such magnetic islands can be reduced or eliminated by applying a correction field with some perturbation coils.