Experimental investigation of quasi-steady current driving for spheromak plasmas by means of magnetic helicity injection.

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External drive of the plasma current of spheromak plasmas has been performed to demonstrate the feasibility of sustaining their magnetic configurations by the magnetic helicity injection method. The both of the conductive type and the inductive type of the helicity injection have been examined in the Tokyo University spheromak machine TS-3. Under appropriate current driving conditions. the spheromak configuration is maintained three to five times longer than non-driving conditions. A visible light computer tomography system has been developed to observe unstable plasma motions in the pasmas. In this measurement, the time evolutions of two dimensional emission profile of visible light on the mid-plane of plasmas have been observed by a newly developed three-direction ten-channel computer tomography system. This system can reconstruct the emission profile by the Cormack method based on the Fourier-Bessel-Neumann expansions. From reconstructed images, the relaxation is found to be caused by the inverse-cascade phenomena, in which instabilities with higher toroidal mode numbers are excited first, followed by the energy transmission to those with lower mode numbers. Magnetic measurements have also been performed and the flux conversion from poloidal to toroidal associated with the inverse-cascade phenomena is observed. Three dimensional magnetohydrodynamic computer simulations have also been carried out to examine the dynamic behaviors of the spheromak configuration in helicity injection periods. It is found that a magnetic relaxation cycle, consisting of the current peaking phase and the subsequent MHD relaxation phase, plays very important roles in the magnetic flux conversion from poloidal to toroidal. The nonlinear coupling of the current driven MHD instabilities with mode numbers of n=2 and n=3 takes place in the flux conversion. These findings are qualitatively consistent with the tomographic observations of the MHD activities.