Studies of compact toroid injection into spherical torus and its propagation process

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1. In the HIST device, we have extended discharge duration time of helicity-driven spherical torus plasma by improvements of the device. We have examined comprehensively the role of the n=1 instability and relaxation on current generation processes for ST and spheromak plasmas. For the spheromak, n=1 mode activity plays an important role in current drive mechanism. For the ST configuration, n=1 mode tends to be stabilized by the external toroidal field. So we have proposed that repetitive plasmoid injection and merging process are essential for current drive.2. We have identified the characteristics of the Compact Torus (CT) plasma propagating in an external field using the FACT device. The shift motion of CT has been observed in the external field as well as in the JFT-2M device. We have concluded that this phenomenon is responsible for the Lorentz force J_gxB_<ext>. It has been found that the external magnetic field apparently decelerates the CT plasma, but the Hall effect may be able to reduce the drag force in the drift region.