30E04P	 
Transport Analysis of Oscillatory Phenomena for Plasma Dynamics in Helical Plasmas
ヘリカルプラズマでのプラズマダイナミクスにおける振動現象に関する輸送解析
S. Toda, K. Itoh and A. Wakasa
登田慎一郎，伊藤公孝，若狭有光
National Institute for Fusion Science, Oroshi 322-6, Toki, Gifu, 509-5292
核融合科学研究所，〒509-5292 岐阜県土岐市下石町 322-6
The formation mechanism of transport barriers is important issue to realize improved confinement modes
in toroidal plasmas. One possible mechanism to explain the transport barriers is the spatial transition in
the profile of the radial electric field Er and the suppression of turbulence by its shear. In thenonaxisymmetric system, the radial electric field is determined by the ambipolar condition. In tokamaks,
the model of L/H transition and the edge localized modes (ELMs) which is based on the bifurcation of theradial electric field was presented. In helical plasmas, two kinds of the oscillation for the plasma
quantities are experimentally observed. Firstly, the limit cycle phenomena in the temporal evolution of
the electrostatic potential, namely the electric pulsation, have been observed in the core region. Relatedwith the electric pulsation, the electron internal transport barrier is observed in the electron temperatureprofile. Therefore, the physical mechanism, which realizes the oscillatory plasma state, is critical for thestudy of improved confinement modes. In our recent work, temporally oscillating solutions of the radial
electric field are obtained. Secondly, the density limit oscillation in the W7-AS experiments [1] wasreported. The achievable limit of the density due to the radiation collapse has been studied, because thestrong degradation of the confinement occurs if the radiation collapse happens. Dynamics of the radialstructure for the plasma quantities are important for the study of the density limit. The zero-dimensional
dynamical model was shown for the oscillation of the density near the density limit. The oscillatory state
for the plasma dynamics has not been obtained yet by use of a one-dimensional transport analysis, which
includes the radiative loss.
To theoretically predict the electric pulsation in the core region of helical plasmas, the transport model for
LHD-like plasmas is constituted by the one-dimensional diffusion equations of the density, the electronand ion temperatures, and the radial electric field. A neoclassical transport database DCOM/NNW for
LHD (DGN/LHD) has been constructed [2]. In order to estimate the neoclassical transport and theambipolar radial electric field for LHD in details, the DGN/LHD for the nonaxisymmetric part of theradial flux is adopted in the diffusive equations. The DGN/LHD is used for the simulation to reproduce
the electric pulsation in the core region and to predict the parameter region for the electric pulsation in theLHD experimental results. The dependence of the transition point for the radial electric field on theparticle source is studied. The variation of the solution type (the stationary or oscillatory state) is also
examined.
To examine the density limit oscillation in helical plasmas, we add the term of the radiative loss due tothe impurity ions to the temporal equation for the electron temperature. The temporal evolution of thedensity profile is newly included in the transport model equations. The result for two states, which aredominated by the transport or the radiation, is shown. Dynamics of the plasma quantity is studied and thetemporal transition from the transport-dominated state to the radiation-dominated state is examined.
[1] L. Giannone, et al., Plasma Phys. Control. Fusion 42 (2000) 603[2] A. Wakasa, et al., J. J. Appl. Phys. 46 (2007) 1157