Vertical compact torus injection into the STOR-M Tokamak

Abstract

Central fuelling is a fundamental issue in the neat generation tokamak – ITER (International Thermonuclear Experimental Reactor). It is essential for optimization of the bootstrap current which is proportional to the pressure gradient of trapped particles. The conventional tokamak fuelling techniques, such as gas puffing and cryogenic pellet injection, are considered to be inadequate to fulfill this goal due to premature ionization caused by high plasma temperature and density. Fuelling by injecting a compact torus (CT) may be the only viable method suitable for a reactor-grade tokamak. CTs can be injected at different angles with respect to the tokamak toroidal magnetic field, either horizontally or vertically. In vertical injection, deeper CT penetration is expected due to the absence of the gradient of tokamak toroidal magnetic field in that direction. This thesis contributes to experimental investigation of vertical compact torus injection into the STOR-M tokamak. To perform vertical injection, the original injector- USCTI (University of Saskatchewan Compact Torus Injector) was modified by attaching a segment of 90° curved drift tube to bend the CT trajectory from horizontal to vertical. Bench tests have shown that a CT injected horizontally can be deflected effectively to the vertical direction. The velocity of 130 kmsˉ¹ has been achieved while the CT passes through the 90° curved drift tube. It was found that the CT magnetic field structure kept intact as a typical structure of compact torus plasma. By further optimization of the USCTI configuration, the velocity has been increased to 270 kmsˉ¹ Based on the encouraging bench test results, actual vertical CT injection experiments have been performed in the STOR-M tokamak. Experimental results demonstrated, for the first time, vertical CT injection into a tokamak. Prompt increases both in line averaged electron density and in soft X-ray emission (central cord) are observed following vertical injection. Some H-mode phenomena, characterized by suppression of the m =2 Mirnov oscillation level and drop in Hα radiation level, have also been observed following the vertical injection. Fuelling effects caused by vertical injection and by tangential injection are discussed. The experimental results suggest that vertical CT injection is a feasible tokamak fuelling technique.