Drift waves can be excited in a uniform pressure very high-β plasma because of the existing density and temperature gradients. The growth rate is calculated and an estimate for the resulting diffusion is given.
Equations have been derived which describe the nonlinear evolution of standard drift waves (driven by inverse Landau damping) and of dissipative trapped electron modes. Assuming that all but the n=0 modes are shear stabilized, it is shown that the nonlinear equations admit eigenfunctions of the same form as the linear equations with, however, the shear length Ls=q/(ε dlnq/dr) replaced by another length ls<Ls. ls decreases with increasing turbulence thereby stabilizing the mode. The turbulence level and the diffusion coefficient are self-consistently calculated. In the case of the standard drift wave, good agreement is found with experimental results in linear devices. In the case of the trapped electron mode, estimates obtained with parameters derived from published profiles of ST tokamak discharges are quite reasonable. Stabilization arises through nonlinear diamagnetic drifts.
The convection mode and its relation with the lower-hybrid waves are investigated. It is demonstrated that the convection mode can be unstable in the presence of a drift or beam, and that it can also be driven parametrically.
A general procedure is presented to evaluate the tokamak particle and energy losses in the presence of a poloidal divertor or a toroidal limiter, assuming cold collisional electrons and hot collisionless ions in the scrape-off layer. The case of the limiter is treated in detail and approximate expressions for the losses, to be incorporated in the tokamak transport equations, are obtained. The importance of the barely trapped particles in determining the radial profiles is emphasized.
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