(EX/1-4) Evolution of Bootstrap-Sustained Discharge in JT-60U

Y. Takase1), S. Ide2), Y. Kamada2), H. Kubo2), O. Mitarai3), H. Nuga1), Y. Sakamoto2), T. Suzuki2), H. Takenaga2), JT-60 Team
 
1) University of Tokyo, Kashiwa, Chiba, Japan
2) Japan Atomic Energy Agency, Naka, Ibaraki-ken, Japan
3) Kyushu Tokai University, Kumamoto, Japan

Abstract.  A self-sustained state driven by the bootstrap current was achieved in JT-60U. Only perpendicular and counter tangential neutral beam injections were used so that the neutral beam driven current was negative. The current in the Ohmic heating coil was kept constant so it does not cause induction. The dynamics of such a completely self-driven system with no external current drive is studied. At a toroidal field of 3.7 T, beta collapses were often observed. The internal transport barrier (ITB) shrinks radially at such collapses, and the stored energy and the plasma current are reduced. Subsequent self-recovery of the ITB radius, as well as the stored energy and the plasma current, is observed, although the recovery is not complete. It is also demonstrated that beta collapses can be avoided by raising the toroidal field to 4 T. A plasma current of greater than 0.55 MA was maintained for 2.5 s. After an initial transient in which the plasma current increases due to stored energy increase in spite of the reduced heating power, the loop voltage becomes zero and the plasma current is maintained at a nearly constant level for 1.3 s. This plasma has a normalized beta of 1.2, poloidal beta of 3.0, a large ITB radius, and a current density profile peaked near the edge. The neutral beam driven current is calculated to be approximately - 50 kA and the calculated bootstrap current is approximately equal to the total plasma current. The inductively driven current is nearly zero, but still slightly positive in the outer half radius.

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