Fig. 2-7 Cross-section of the simulation model of JT-60 poloidal field coils and vacuum vessel system and computational domain The vacuum vessel was simulated by 100 separate toroidal ring elements as shown by open rectangles. The computational domain was divided by 80 x 80 meshes (each of 5 cm in size). A cross-section of the equilibrium plasma shape is also shown by elongated circular-shaped contours in solid lines.

Fig. 2-8 Vertical plasma displacement during disruptions Experimental results on JT-60 are shown for two cases; a stable case where the vertical location of the plasma is optimized according to the prediction of the computer simulation, and a not-optimized unstable case. In the stable case the plasma was produced about 15 cm vertically above the geometrical center of the JT-60 vacuum vessel. The growth rate of the instability (unstable case) is also obtained from the simulation. A dotted line shows the decaying plasma current during the disruption.

To establish stable operation of a tokamak fusion reactor, an intensive research is quite important to understand the physical mechanism of a plasma disruption, a tokamak-specific phenomenon, and to develop an effective method to control it. The plasma disruption involves a sudden loss of plasma confinement and a rapid decay of the plasma current, leading to the termination of the tokamak discharge. During the decay of the current, a fast vertical movement (displacement) of the whole plasma is frequently observed, causing very large electromagnetic forces on the tokamak structural components such as the divertor plates and other plasma-facing components. A detailed computer simulation study of the dynamical behavior of plasma disruptions has been carried out at JAERI in close cooperation with the experimental study of the phenomena on JT-60. Figure 2-7 shows a cross-section of the simulation model of the poloidal field coils (rectangles with an X) and the vacuum vessel (open rectangles) system of JT-60. The simulation computation has shown that a fast vertical displacement observed at disruption in JT-60 can be explained by an asymmetric distribution of eddy currents induced on the vacuum vessel that has a geometrical asymmetry in shape and structure, and also that this fast displacement can be avoided by optimizing the vertical location of the plasma just prior to the disruption. Figure 2-8 is a typical result of experiments done based on this finding obtained by computer simulation. While the plasma moves downward rapidly when its vertical location is not optimized, the displacement is suppressed or at least moderated if the location is optimized. The speed of the displacement (the growth rate of the instability) can be explained quantitatively by taking account of a deterioration due to the effect of induced eddy currents of the initial magnetic field configuration required for the plasma equilibrium. The computer simulation described here can be used to systematize experimental knowledge about plasma disruptions and to develop a clear physical picture of the contributing phenomena to establish the design guidelines of a tokamak reactor.

Reference Y. Nakamura et al., Mechanism of Vertical Displacement Events in JT-60 Disruptive Discharges, Nucl. Fusion, 36, 643 (1996).

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