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

Abstract

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.

Overview of coordinated spherical tokamak research in Japan

“…The effect of the helical field application on the plasma horizontal position was observed with magnetic field measurements [29], [30] and with a high-speed camera [31], [32] in TOKASTAR-2. Suppression of vertical instability was also studied in elongated plasmas but no clear effects of the helical field were observed [33]. It was found that the effective radial field B eff r generated by helical coils was not appropriate for stabilizing the plasma vertical movement.…”

“…New local helical coils were designed and manufactured so that a large stabilizing B eff r could be obtained [33]. The new coils, named ULT coils, are shown in Fig.…”

“…The details of the poloidal field coils and the waveforms of the coil currents are described in Ref. [33]. In the case without the ULT field, the elongation started to increase gradually at t 2.65 ms, and finally the upward vertical displacement event (VDE) and the plasma current quench occurred.…”

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

“…Stabilization effects of the helical field was observed on the plasma horizontal position [1][2][3]. Those on the plasma vertical position was, however, not clear [4]. According to field line calculation, it was possible to generate closed flux surfaces (LCFSs) in vacuum with local helical coils.…”

“…The problem was that effective radial field made by upper/lower local helical coils, AHF coils, was weak because they are located 230 mm away from the equatorial plane; the distance is six times as large as the plasma minor radius, 40 mm. To reinforce effective radial magnetic field, new local helical coils (ULT coils) have been designed and fabricated [4]. Four ULT coils, two on upper side and two on lower side were installed inside of the toroidal field (TF) coils, as shown in Fig.…”