-
Author(s):
K. Itoh, S.-I. Itoh, A. Fukuyama, M. Yagi and M. Azumi
-
Title:
Self-sustained Turbulence and L-Mode Confinement in Toroidal Plasmas
Date of publication:
Apr. 1993
-
Key words:
Anomalous transport, L-mode, Thermal conductivity, Current diffusivity, Ion viscosity, Nonlinear theory, Turbulence, Ballooning instability, Interchange instability, Magnetic shear, Magnetic well, Tokamak, Helical systems, Stellarators, Collisionless skin depth
-
Abstract:
Theory of the L-mode confinement in toroidal plasmas is developed. The quantitative effect of the anomalous transport, which is caused by microscopic fluctuations, on the pressure -gradient- driven modes is analyzed. The ExB nonlinearity is renormalized in a form of the transport coefficient such as the thermal diffusivity, the ion viscosity and the current diffusivity. The destabilization by the current-diffusivity and the stabilization by the thermal transport and ion viscosity are analyzed. By use of the mean-field approximations, the nonlinear dispersion relation is solved. Growth rate and stability condition are expressed in terms of the renormalized transport coefficients. The transport coefficients in the steady state are obtained by the marginal stability condition for the least stable mode. This method is applied to the microscopic ballooning mode for the toroidal plasma with the magnetic well (such as tokamak). The formula of the anomalous transport is obtained. The role of pressure gradient in enhancing the anomalous transport is identified. Effects of the geometrical parameters such as the rotational transform and magnetic shear are also quantified. The comparison with experimental observations are made. A good agreement is found in a various aspects of the L-mode plasmas; such as the power degradation of the confinement time, large transport coefficient at edge, ion mass effect and the favourable effect of the plasma current. The typical wavenumber and level of the fluctuations for the self-sustained turbulence is also obtained. Important role of the collisionless skin depth is found. The analysis is also made for the plasma with magnetic hill and shear (such as torsatron/Heliotron devices). This method is applied to the interchange modes. Formula of the anomalous transport is obtained. Also investigated is the case of the magnetic well and low magnetic shear (conventional stellarator). The roles of the pressure gradient and the collisionless skin depth in determining the anomalous transport are found to be generic in toroidal plasmas. The difference in the magnetic configuration affects the transport coefficient. These formula explain major experimental observations on the L-mode confinement in helical plasmas including the differences from tokamak experiments.
-
|
