Self-Orgamzation in Low-Aspect Ratio Tens Plasma and Healing of Magnetic Chaos

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In this research project, we have constructed an RFP machine RELAX, whose aspect ratio (A) is 2, the world's lowest value to date. During its 1st phase operation for about a year the following discharge parameters have been achieved: maximum plasma current of <80kA, minimum discharge resistance of 1 mΩ discharge duration longer than 2 ms with RFP configuration sustained for L5-2 ms. The MHD characteristics of low-ARFP plasmas can be summarizes as follows. In flat-topped discharges, we have observed two types of toroidal mode spectrum of the m=1 modes depending upon the discharge phases. In the later phase, where the magnetic fluctuation level tends to be the lowest, the edge magnetic fluctuation has a characteristic of multi-helicity (MH) state with dominant core resonant m=1/n=4,5 modes. In the early to mid phase, periodic growth and decay of a low-amplitude (but higher than in the later phase) single mode, either n=4 or n=5, has been observed. In round-topped RFP discharges, a m=1/n= … More 4 single dominant mode tends to grow quasi-periodically with suppression of the neighboring modes, and the toroidal mode spectrum has the quasi-single helicity (QSH) characteristic with dominant mode amplitude higher than in other RFP experiments. These observations are in agreement with theoretical predictions from low-ARFP equilibrium profiles. In RELAX, the self-reversal mode operation has also become possible, which discharge is characterized by a large-scale change in magnetic field profile. The measured profiles of asymmetric components are in good agreement with those of Helical Ohmic Equilibrium (HOE), in which RFP configuration is sustained without magnetic chaos. The measured oscillation may be an indication of toroidal rotation of the HOE configuration. A new equilibrium reconstruction code, RELAXFit, has been developed. We have extended the operation regime of the MSTFit code to the lower-A regime. This work has been performed as a part of US-Japan Fusion Collaboration Program. In the RELAXFit, we have taken into account the neoclassical effect in order to estimate the bootstrap current in RELAX. Throughout this research project, we have established the basis for further experimental study of low-A RFP configuration, a new regime of toroidal plasmas. Less