Optimization of Magnetic Field Based on Electron Orbit Measurement in TOKASTAR-2 Helical Plasmas

By using newly installed local helical coils (ULT coils), it is expected that helical magnetic field will be reinforced and then the rotational transform and the cross-sectional area of the last closed flux surface (LCFS) will get larger in TOKASTAR-2. Electron beam mapping and plasma measurement with an electrostatic probe were made to confirm improvement in the helical field. As the result, large closed flux surfaces were measured by using the ULT coils. In plasma measurement, it was observed that the plasma pressure changed according to the movement of the calculated LCFS, though change in the plasma pressure at the position of the calculated LCFS was not clear. Furthermore, it was confirmed that helical magnetic field confined plasma from plasma decay after turning off the plasma heating power.