Measurement of Tokamak Plasma with the External Helical Field Using a High-Speed Camera in TOKASTAR-2

The high-speed camera was used to evaluate the position of the plasma in TOKASTAR-2. A plane mirror was installed in the vacuum vessel to expand the field of view and get the image in the whole region between inner and outer walls. The images were taken at 86400 FPS with 256 × 128 pixels (20 µm/pixel) per frame in the tokamak discharge with and without the external helical field. Under the condition of the weak vertical field, the plasma position control was improved by the external helical field. The outward plasma displacement was suppressed and the plasma was located stably in contact with the inner legs of the TF coils.