Study on toroidal current plasmas from RFP to tokamak

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Twisted magnetic-field-line structures appear commonly in many different space, astrophysical and laboratory plasmas, and they play an essential role in nonlinear dynamics of these systems. The creation of a magnetic flux rope is an important example of such space phenomena. Magnetic reconnection produce flux tubes with helicalmagnetic-field lines, which stay stable for a period of time. Structural change of the magnetic field is accompanied by energy transfer and transformation, and the relaxation of the magnetic energy results in heating of plasma particles.This research project was planed to explore, based on laboratory plasma experiments, the nonlinear dynamics of "twisted magnetic-field lines" in their most common structure. Using two toroidal experiments, REPUTE-1 (Univ.Tokyo) and EXTRAP-T2 (Alfven Lab.), we produced high-temperature plasmas in the ULQ and RFP regimes. A unique feature of our experiment, in comparison with other laboratory systems, is in that the twist of magneti … More c-field lines is rather tight. A high pitch magnetic field has a large free energy that can excite torsional magnetic fluctuations. In our experiments, we observe clear indications of strong nonlinear effects produced by magnetic fluctuations, which we call the MHD relaxation.The energy dissipation in the MHD relaxation should go much faster than that caused by the resistive dissipation of the mean current. The latter leads the plasma to a diffused state in a long time scale. A different quasi-steady state emerges within a shorter time scale. This bifurcation of a quasi-steady state in a typical "self-organization" of a structure through fluctuations. Enhanced energy dissipation, which is the most essential characteristic of a "dissipative structure", appears as anomalous heating of the plasma. The viscous dissipation heats ions, while the resistive dissipation heats electrons. We observe preferential ion heating in the direction parallel to the magnetic field lines. This observation is consistent to the theoretical prediction of ion viscous dissipation in the self-organization process. Hence, the microscopic mechanism of ion heating associated with the self-organization is now established. Less