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.
The equilibrium analysis including the eddy current effect was performed for TOKASTAR-2 tokamak plasma by using TOSCA code that treated the vacuum vessel as the 2D axisymmetric model. In measurement of the vacuum magnetic field without plasma, it was found that the measured magnetic field and the calculated one using the model configuration which was based on the TOKASTAR-2 design did not agree. To improve the model, some parameters in the model (e.g. coil positions) were adjusted to minimize the difference between the calculated field and the measured one. In the equilibrium analysis, the poloidal beta β P and the plasma internal inductance l i were determined minimizing the difference between the calculated field and the measured one for external probes. It was found that the solution calculated by the external measurement was roughly consistent with the internal field measurement, with difference in the plasma internal inductance of 0.10. The high-speed camera and the internal measurement results indicated that the shape and position of the plasma also have small errors. More improvement in the wall model would be needed to resolve these discrepancies.
The ion temperature of the smallest tokamak, the major radius of 0.1 m, is measured using Doppler broadening spectroscopy. Experiments are performed for helium discharge. Ion temperature Ti = 0.7eV is obtained from the Doppler broadened line spectrum of the helium ion. The electron temperature and density measured using line emission intensities of the helium atom are Te = 4.7eV and ne = 3.2 × 1018m−3. The major radius R0 = 0.11m and the minor radius a = 0.03m are obtained from magnetic measurements. Then, the energy flow from the electron to the ion is evaluated as well as ohmic heating and power losses due to atomic processes. The main loss channel for electron stored energy is conduction even though the tokamak is immersed in the residual neutral gas. Total energy confinement time τE = 2.3 µs is determined from the power balance, which is comparable with that deduced from the neo-Alcator scaling law.
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