Calculation of renormalized viscosity and resistivity in magnetohydrodynamic turbulence

A self-consistent renormalization (RG) scheme has been applied to nonhelical magnetohydrodynamic turbulence with normalized cross helicity $\sigma_c =0$ and $\sigma_c \to 1$. Kolmogorov's 5/3 powerlaw is assumed in order to compute the renormalized parameters. It has been shown that the RG fixed point is stable for $d \ge d_c \approx 2.2$. The renormalized viscosity $\nu^*$ and resistivity $\eta^*$ have been calculated, and they are found to be positive for all parameter regimes. For $\sigma_c=0$ and large Alfv\'{e}n ratio (ratio of kinetic and magnetic energies) $r_A$, $\nu^*=0.36$ and $\eta^*=0.85$. As $r_A$ is decreased, $\nu^*$ increases and $\eta^*$ decreases, untill $r_A \approx 0.25$ where both $\nu^*$ and $\eta^*$ are approximately zero. For large $d$, both $\nu^*$ and $\eta^*$ vary as $d^{-1/2}$. The renormalized parameters for the case $\sigma_c \to 1$ are also reported.Comment: 19 pages REVTEX, 3 ps files (Phys. Plasmas, v8, 3945, 2001

Influence of plasma boundary shape on helical core/long-lived mode in tokamak plasmas

Helical distortion of the core part of tokamak plasma, which is called a helical core or a long-lived mode, is investigated by means of three-dimensional magnetohydrodynamic equilibrium calculations. It is found that the magnitude of the helical distortion strongly depends on the shape of the plasma boundary for weakly reversed shear plasmas. The triangularity of the boundary enhances the amplitude of helical distortion. In addition, reversed D-shape plasmas also exhibit a helical core. It is also found that the triangularity lowers the critical β for the onset of a helical core; furthermore, the critical β vanishes when the triangularity exceeds a certain value. On the other hand, the influence of the ellipticity on the amplitude of helical distortion strongly depends on β. The ellipticity enhances the amplitude at high β, while it reduces the amplitude at low β

Global gyrokinetic nonlinear simulations of kinetic infernal modes in reversed shear tokamaks

The nonlinear evolution of electromagnetic instabilities in reversed shear plasmas is investigated by means of global gyrokinetic simulations. It is found that the kinetic infernal mode (KIM), which is a pressure-driven instability with low to intermediate toroidal mode number excited in a region of low magnetic shear, is unstable at high β, while the ion temperature gradient mode is unstable at low β, where β is the ratio of the plasma kinetic pressure to the magnetic pressure. The β threshold of the KIM is much lower than that of the kinetic ballooning mode (KBM) appearing in a normal shear plasma, while both the KIM and KBM are strong at the unfavorable curvature region, and the KIM has the same parity as the KBM. Nonlinear simulations show that the KIM gets saturated by exciting strong zonal flows and fluctuations of low toroidal mode number. The amplitude of the KIM turbulence is similar to that of the KBM turbulence in spite of the fact that the linear growth rate of the KIM is much higher than that of the KBM. This is because the excitation of zonal flows and fluctuations at low toroidal mode number is stronger in the reversed shear plasma than that of the normal shear plasma. On the other hand, the energy flux and particle flux due to the KIM turbulence are about two or three times larger than those by the KBM turbulence

Multi-machine analysis of turbulent transport in helical systems via gyrokinetic simulation

We have investigated drift-wave instability and nonlinear turbulent transport in two configurations with different magnetic field structures by means of electromagnetic gyrokinetic simulations. Here, one is the neoclassically optimized Large Helical Device (LHD) plasma and the other is the Heliotron J (HJ) plasma. First, we show that the validation against the turbulent transport in the LHD plasma is successful, and that the neoclassically optimized configuration has smaller turbulent transport. Second, the neoclassical optimization through an enhanced toroidal mirror ratio, which is a capability of non-axisymmetric plasma, is found to improve the turbulent transport in the HJ plasma, which is qualitatively consistent with the observation in the HJ. Hence, the neoclassical optimization reduces the turbulent transport in both the LHD and HJ plasmas. Third, as a trial in evaluating the performance of a helical system designed with different concepts for stability, we compared turbulent transport in these plasmas and found that both the mixing-length-estimated diffusion and nonlinear turbulent transport of the HJ plasma are smaller than those of the LHD plasma in gyro-Bohm units. The significant difference is stronger zonal flows in the HJ plasma than in the LHD plasma

Fermi Surfaces of Diborides: MgB2 and ZrB2

We provide a comparison of accurate full potential band calculations of the Fermi surfaces areas and masses of MgB2 and ZrB2 with the de Haas-van Alphen date of Yelland et al. and Tanaka et al., respectively. The discrepancies in areas in MgB2 can be removed by a shift of sigma-bands downward with respect to pi-bands by 0.24 eV. Comparison of effective masses lead to orbit averaged electron-phonon coupling constants lambda(sigma)=1.3 (both orbits), lambda(pi)=0.5. The required band shifts, which we interpret as an exchange attraction for sigma states beyond local density band theory, reduces the number of holes from 0.15 to 0.11 holes per cell. This makes the occurrence of superconductivity in MgB2 a somewhat closer call than previously recognized, and increases the likelihood that additional holes can lead to an increased Tc.Comment: 7 pages including 4 figure