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内容記述 |
The ion temperature gradient (ITG) mode is one of drift wave instabilities, which is considered to cause the anomalous transport of the ion thermal energy in high temperature plasmas. The purpose of this thesis is to clarify effects of magnetic configurations on the ITG mode based on the gyrokinetic model. The gyrokinetic equation for ions is used to consider kinetic effects such as finite gyroradii and wave-pariticle interactions. Also, the assumption of adiabatic electrons and the quasineutrality condition are used to obtain the dispersion relation. Phase mixing due to ∇B-curvature drift motion is investigated in detail in the local approximation. Effects of magnetic configuration on nonlocal mode structure are studied in straight and toroidal helical systems.<br />In the local approximation, initial value problem of the ITG mode is solved. Due to the toroidal magnetic drift, the Laplace-transformed density and potential perturbations have a branch cut as well as poles on the complex-frequency plane. The inverse Laplace transform shows that the temporal evolution of the density and potential perturbations consists of the normal modes and the continuum mode, which correspond to contributions from the poles and the branch cut, respectively. The normal modes have exponential time dependence with the eigenfrequencies determined by the dispersion relation while the continuum mode shows power-law decay oscillation. For the stable case, the long-time asymptotic behavior of the potential and density perturbations is dominated by the continuum mode which decays slower than the normal modes.<br />Next, poloidal localization of the mode structure is studied by means of the ballooning representation. In the first place, the straight helical system is considered in order to focus on the helical ripples' effects. The magnetic shear is assumed to be negative and the poloidal period number L is taken as L = 2. Then, the helical ripples with a larger toroidal period number M reduce the growth rate of the ITG mode. This stabilizing effect is understood based on the structure of the eigenfunction along the field line as follows. As M increases, the connection length between the good and bad curvature regions becomes shorter and the eigenfunction enters the good curvature region, which leads to the stabilization. For large M (M～10), unstable ITG modes are driven only by the very large temperature gradients.<br />Finally, toroidal helical systems are considered, in which toroidicity and helical ripples exist simultaneously. Equilibrium plasma parameters are chosen in reference to the LHD. experimental result (L = 2, M = 10). Because of the toroidal destabilization, the critical temperature gradient in which ITG mode becomes unstable is smaller than for the straight helical system. Numerical results suggest the existence of unstable ITG modes in LHD, The good curvature region is generated even in the outer torus region due to the helical ripples, which results in the reduction of growth rate compared to the tokamak cases without helical ripples. Also, dependences of the ITG mode properties on various plasma equilibrium parameters such as the helical ripple intensity, safety factor, magnetic shear, ballooning angle, poloidal wavenumber, temperature and density gradients are investigated. |
要旨・審査要旨 / Abstract, Screening Result (268.2 kB)
