Study on Stabilization of Vertical Position of Tokamak Plasma with Local Helical Coils in TOKASTAR-2

We investigated effects of the local helical magnetic field on the plasma vertical position to suppress Vertical Displacement Event (VDE) in TOKASTAR-2. Conditions for VDE occurrence were investigated on the vertical plasma position and the current of coils for elongating the plasma, with and without the helical field, and no clear effects of the helical field were observed. Even though the helical coil currents were increased and the plasma vertical position were adjusted, the existing local helical coils were not effective to stabilize the plasma vertical position, resulting in the plasma current quench. We evaluated the effective radial field, which is expected to stabilize the plasma vertical position, using magnetic field line trace calculation. We found that the distribution and magnitude of the effective radial field generated by the existing helical coils were not appropriate for stabilization of the plasma vertical position. We designed new local helical coils consisting of triangular coils located on the upper and lower sides of the plasma. The new coils can generate the effective radial magnetic field, which is expected to stabilize the plasma vertical position.