Formation and Stability of Oblate Field-Reversed Configuration Plasmas

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An oblate field-reversed configuration (FRC) is formed by a control of a mirror ratio and a magnetic field index of the confinement field of a negative biased theta-pinch (FRTP). An elongation of FRC plasmas decreases less than about 2.5 by the increase of the mirror ratio (1.8) and the field index (0.3). The field index is possible to contribute to the adjustment of a radial implosion strength and an axial contraction.An n=1 mode motion of FRC plasmas is investigated by using a mirror and a cusp configuration of the bias field of the FRTP. The n=1 motion reaches 20-40% of the plasma radius in the former bias configuration, although it is a low level in the latter configuration. It is experimentally confirmed that the violent n=1 motion can be controlled by a multipole field. A critical strength needed to push back the plasma to the equilibrium position is theoretically derived by using a simple model including a conductor effect of the wall and is compared with the experimental results. Both values agree within the experimental reproducibility. The critical field strength of which is required to be about 15% of the confinement field. It is found that the n=2 rotational instability can also be stabilized by the same order.Several diagnostics for the n=1 motion, a plasma shape and an internal plasma structure are a newly developed. Optical diagnostics is developed for the n=1 motion. A plasma shape diagnostic by a diamagnetic measurement is improved. It is found that the plasma shape can be determined by solving a Grad-Shafranov equation iteratively. The boundary condition is the measured magnetic flux and magnetic field strength. The initial value is the plasma shape of the diamagnetic method. A plasma cross-sectional view can be observed by a CCD camera with fast-gated image intensifier