N. Mizuguchi

T. Hayashi

Y. Kagei

Y. Suzuki

National Institute for Fusion Science (NIFS-Repository)

Referencesof the magnetic polarity 3). The time evolutions ofmagnetic fields are shown in Fig. I. After the toroidalmagnetic field on the center post is reversed at (I TA = 100,where TA is the Alfven transit time, a large distortion offields is seen at (I TA = 117 (b). A sudden change in thepath of open field lines means that the magneticreconnection between open and closed field lines takesplace. As the reconnection events collapse initial fluxsurfaces, not only the direction of toroidal magnetic fieldbut also that of poloidal magnetic field (that is, toroidalcurrent) is reversed at (ITA =222 (c). The reconnectionprocess proceeds and an axisymmetric steady flippedconfiguration has been established at (I TA = 820 (d). Thisbehavior well reproduces the observations.a t/TA=O badioroT0.4IaO.2di °<:1o.aroTFig. 1. Time evolutions of magnetic flux, current andmagnetic field lines during the self-reversal ofmagnetic polarity. Initial normal ST (a) relaxestoward the flipped state (d) through thedeformation (b) and the collapse of initial fluxsurfaces (c) due to the magnetic reconnection event.1) Jarboe, T. R. et al.: 19th IAEA Fusion Energy Conf.,Lyon, (2002) IAEA-CN-94/IC/P-IO.2) Nagata, M. et al.: Phys. Rev. Lett. 90 (2003) 225001.3) Kagei, Y. et al.: Plasma Phys. Control. Fusion 45 (2003)L17.§ 16. Simulation Study of Magnetic HelicityInjection into Fusion PlasmasSuzuki, Y., Kagei, Y. (JAERI-Naka)Hayashi, T., Mizuguchi, N.Magnetic helicity injection using coaxial electrodes(Coaxial Helicity Injection: CHI) has successfullydemonstrated the non-inductive current generation onHIT-II and NSTX spherical torus (ST) devices I), however,it is not still evident if closed flux surfaces are produced.Since closed flux is surrounded by open flux linkingcoaxial electrodes, large fluctuations have been observedin the core, which makes it difficult to conclude thepresence of closed flux surfaces. On the other hand,another novel ST configuration in CHI-driven systems,called "flipped ST", has first been formed and sustainedin the HIST device 2). It is characterized by oppositepolarity to normal ST in closed flux surfaces. Since in theflipped state the closed flux is isolated from the open flux,closed flux is expected to be form without large globalfluctuations. In order to investigate such a favorableconfiguration, numerical experiments using 3-D MHDsimulations are executed. In this work, we pay attentionto the formation process ofthe flipped configuration.The simulation region consists of two cylinders withthe center post. One is a gun region, where the biasmagnetic flux penetrates inner and outer boundaries in theradial direction corresponding to a pair of electrodes. Theother is a confinement region, whose boundaries areassumed to be a perfectly conducting wall. The initialmagnetic state is a normal ST configuration given bysolving the Grad-Shafranov equations numerically undersuch boundary conditions. The simulation starts runningwhen the toroidal magnetic field on the center post startsOur simulation demonstrates the formation of a flippedST configuration accompanied by a spontaneous reversalto reverse.337

Simulation Study of Magnetic Helicity Injection into Fusion Plasmas

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