Plasma performance is enhanced by increasing plasma pressure at the edge. This is because an increase in edge pressure results in an increase in the total plasma pressure. The plasma edge pressure can be increased by triangular shaping (the degree of triangular shaping is referred to as triangularity). Since sustainment of high-performance plasma is an important issue, high-triangularity plasma has been extensively examined in JT-60. To increase triangularity, a large electric current must be passed through the poloidal field (PF) coils. However, due to power supply constraints, only a 'high-triangularity plasma for a short time' and a 'low-triangularity plasma for a long duration' were possible.
An improvement in the PF coils enables a 'high-triangularity plasma for a long duration'(Fig. 2-6). In addition, the discharge scenario has been optimized to suppress an instability at high plasma pressure. As a result, a high-pressure plasma (normalized beta, betaN, is 2.7: a value similar to that of ITER) has been sustained for 60 times the energy confinement time (7.4 s) (Fig. 2-7). Previously, the maximum duration of the auxiliary heating and high-triangularity plasma has been 10 s. The new result is important in that a high-normalized-beta plasma, with a value comparable to ITER, has been sustained at the machine limits of JT-60 (Fig. 2-8).
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