Exhausting of the impurities accumulated inside the internal transport barrier (ITB), which is the heat-insulating layer formed inside the plasma, is demonstrated in JT-60.
A high temperature is obtained in the central region of the plasma with ITB due to a large temperature gradient formed by reduced heat transport. Therefore, plasma with ITB is considered to be in an advanced steady-state operation mode in the International Thermonuclear Experimental Reactor (ITER). However, impurity accumulation inside the ITB leading to the reduction of the fusion reaction rate is one of the largest operational concerns. The impurity density is determined by the balance between a diffusive flow from the high-density central region to the low-density edge region and an inward convective flow for accumulation in the central region. The diffusive flow is proportional to the impurity density gradient, and its proportional coefficient is called diffusivity. On the other hand, the impurity inward convection velocity is theoretically predicted to be proportional to the plasma density gradient, which is consistent with experimental results. Since particle transport is also reduced with the heat transport in the ITB region, the plasma density gradient becomes large. Therefore, the impurity inward convection velocity becomes large. However, since the impurity transport is reduced, the diffusivity becomes small. As a result, the diffusive flow from the central region to the edge region is decreased and the inward convective flow becomes large; thus, impurities tend to be accumulated inside the ITB. A flat density profile with a large temperature gradient is considered for ITER to avoid impurity accumulation.
In JT-60, density flattening by injecting HFW, which has been observed in tokamak and helical devices, has received attention. In an experiment where the HFW was injected inside the ITB in plasma where argon (Ar) had accumulated beforehand inside the ITB, a significant decrease in the X-ray intensity emitted from the Ar was observed in the central region, as shown in Fig. 2-12. In this case, the ion temperature of the ITB remains as shown in Fig. 2-13, and the high temperature is retained in the central region. On the other hand, the plasma density profile becomes flat, and the plasma density of the ITB is almost insignificant. At the same time, the central Ar density is significantly decreased. The decrease in the Ar density can be explained by the decrease in the inward impurity convection velocity due to the decrease in the plasma density gradient. Based on these results, it is expected a method will be established to avoid impurity accumulation in a future steady-state fusion reactor.
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