Formation of Field Reversed Configuration Plasma with Toroidal Magnetic Field

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The application of an axial current was proposed to form a field reversed configuration (FRC) plasma with a toroidal magnetic field. Effects of the axial current and threshold values of a successful FRC formation and confinement were investigated by NUCTE-3. Various frequencies (<=4.7x10^5rad/s) and amplitudes(<=6OkA) of axial currents were examined. In the formation phase, two effects were clarified ;(1) heating of the embedded plasma in the bias field (2) amplification of bias flux. In the confinement phase, the effect of stabilization of rotational instability were clarified. Though the kink instability was excited by a large axial current in both phases, it was confirmed that the instability can be avoided by using alternating current. Threshold value of the formation phase seemed to have relation with the poloidal flux limitation of a FRC formation and disturbances of preheating plasma and bias field. The stabilizing current value is almost the same as the theoretical value. But the frequency was not so high to suppress the kink mode.A FRC plasma is usually considered as an elongated compact toroid that contains no toroidal field. But an azumuthal fluctuating magnetic field was observed by magnetic probes located on the surface of the discharge tube. The Fourier analysis of the signals showed that the n=1 mode was kept at a small level throughout the discharge but the n=2 mode grew with onset of the rotational in- stability. Current distributions should be a dipole axial current for n=1 mode and a quadrupole axial current for n=2 mode, both of which are symmetrical to the axial mideplane of FRC plasma. The mechanism of the quadruple current generation was explained from the both effects of charge separation by centrifugal force and end shortening of the charge. The dipole current seemed to generate due to a plasma motion in the confining field.