Abstract
When realising future fusion reactors, their stationary burning must be maintained and the heat flux to the divertor must be reduced. This essentially requires a stationary internal transport barrier (ITB) plasma with a fast control system. However, the time scale for determining the position of the foot point of an ITB is not clearly understood even though its understanding is indispensable for fast profile control. In this study, the foot point of the electron ITB (eITB) was observed to be reformed at the vicinity of a magnetic island when the island started to form. In addition, the enhanced confinement region was observed to expand during the eITB formation according to the radial movement of the magnetic island toward the outer region. Compared to the time scales of the local heat transport, the faster time scales of the movement of the eITB foot point immediately after island formation (~0.5 ms) suggest the importance of the magnetic island for plasma profile control to maintain stationary burning.
Highlights
The key issues in realising future fusion reactors involve the maintenance of stationary burning and reduction of heat flux to the divertor with high-pressure core plasma[1,2,3]
Magnetic island on the profiles of electron ITB (eITB) plasmas was investigated in Heliotron J under constant electron density, which fulfils the condition of low ne required to form the eITB14
The time scale of the structure formation compared to that of the local heat transport suggests that the structure formation is affected by the radial electric field, which can be formed transiently
Summary
The key issues in realising future fusion reactors involve the maintenance of stationary burning and reduction of heat flux to the divertor with high-pressure core plasma[1,2,3]. The formation of the electron ITB (eITB), especially the determination of its foot point, has been reported to be affected by the neoclassical transport and existence of a rational surface and magnetic island. 7, the simulation showed that transport is reduced in the core plasma region adjacent to the magnetic island because of reduction in micro-turbulence, which is caused by the increase in flow shear around the magnetic island. Kishimoto et al.[8] theoretically studied the ITG mode simulation to explain the reduction in transport near the minimum q area on the rational surface This simulation predicts the suppression of turbulent transport around the minimum q surface due to the discontinuity of the phase relationship in the global wave structure across the minimum q surface. We discuss the expansion of the improved confinement region following the movement of the magnetic island
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