A. "Sanpei

N." Mizuguchi

National Institute for Fusion Science (NIFS-Repository)

Comparative analyses between the nonlinear three-dimensional MHD simulation results1) and the experi-mental observation at RELAX device2) have been carriedout for the low-aspect-ratio reversed-field-pinch(RFP)plasma to reveal the physical mechanism of the forma-tion processes of helical structures. The simulation re-sults show a clear formation of n=4 structure as a resultof dominant growth of resistive modes, where n repre-sents the toroidal mode number. The resultant relaxedhelical state consists of a unique bean-shaped and hol-low pressure profile in the poloidal cross section for bothcases of resonant and non-resonant triggering instabilitymodes.To avoid the degradation of confinement due tothe chaotizing of the field lines in the core region ofRFP, a unique control method making use of the self-concentrating nature of the plasma perturbations intoa small number of modes has been proposed both ex-perimentally and theoretically. Several types of suchstates have been observed, such as the quasi-single he-licity (QSH) and the single helical axis (SHAx) states.However, the physical mechanisms for the formation anddeformation of the structures have not been clarified well.We solve a standard set of the nonlinear, resistive,and compressive MHD equations by the MIPS code ina full-toroidal three-dimensional geometry to investigatethe dynamical behavior of RFP plasma on the structuralchanges within the MHD time scale on the order of sub-millisecond. The initial conditions for the simulation aregiven by a numerical equilibrium that roughly follows theexperimental conditions of RELAX. The equilibria arecalculated by the Grad-Shafranov solver with a fittingreconstruction, the RELAXFit code. Two typical cases,where the q = 1/4 rational surface does and does notexist, are examined.The simulation results show the dominant growthof the n=4 mode for both cases with and without theq = 1/4 surface. For the resonant case, the m/n = 1/4mode grows with a large single magnetic island, showinga typical behavior of the tearing modes. The originalmagnetic axis shrinks gradually, whereas the created O-point of the island forms a new magnetic axis. In thenonlinear stage, the system experiences a relaxed statewith helically twisted shape. The poloidal cross sectiondeforms into a bean shape with a highly hollow pressureprofile. Such a hollow profile are found to be formedthrough a magnetic reconnection process which is in-duced near the core region due to the plasma flows ofthe instability modes.Comparing with the experimental observations, wemay acknowledge the validity of the simulation. Thehelical deformation of SXR emissivity predicted by thesimulation described in upper panel of Fig. 1 can ex-plain an experimental result of successive SXR imagingdiagnostic from vertical port shown in middle panel.Left lower panel shows poincare map obtained inrelaxed state of the simulation. As mentioned above, wecan observe a large magnetic island due to m=1 inter-nally resonant tearing mode. Right lower panel shows ex-perimental SXR emissivity distribution on poloidal crosssection calculated from vertical SXR images with com-puted tomography technique. Both of these m=1 defor-mations are corresponding to m/n = 1/4 helical defor-mation.This comparative analyses would deepen our un-derstanding of the self-organizing phenomena in a low-aspect and high-beta fusion plasma. To find out thebasic constraint of the relaxation and its application tothe experimental improvement is the next step of ourresearch.Fig. 1: Simulation and experimental results of the heli-cal structures in RFP plasma. A hellically twisted overallstructure (upper panels) and a bean-shaped deformationor a magnetic island structure (lower panels) are shown.1) N. Mizuguchi, A. Sanpei, et al. : 24th IAEA-FEC,San Diego (2012), TH/P3-26.2) A. Sanpei, et al. : IEEE Transaction Plasma Science,39, 2410 (2011).394§18. MHD Dynamics of High-beta RFP Plasma on Structure FormationsSanpei, A. (Kyoto Inst. Tech.), Mizuguchi, N.

MHD Dynamics of High-beta RFP Plasma on Structure Formations

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