Plasma rotation caused by radial electric field in tokamaks: a calculation using the guiding field line model

摘要

The equilibrium currents and flow velocities of species are calculated with the guiding-field line (GFL) model for the plasma in tokamaks with a radial electric field, including the case of large gradients of the field. In the model, all quantities such as the particle position and distribution function are defined on the basis of the GFL-a field line that goes through the centre of the particle's drift orbit and moves at the bounce-averaged drift rate. The distribution function is formed by all orbits that have their GFLs at a given magnetic surface. When the orbits are being squeezed by large-gradient electric field, their position is not changed so that the GFL distribution function remains unchanged; the modification of the current is explicitly given by the wobble-magnetization term that is related to the orbits' shape in the model. The gradient of electric field is shown to affect the parallel component of the current or flow velocity of the ions, rather than the perpendicular one. The effect of the electric field gradient on the electrons is negligible. The results obtained with the model match well with the fluid equations and close to the results of neoclassical theory, though different in detail. It is shown that for the typical conditions of the Tokamak Fusion Test Reactor, the presence of radial electric field enhances the toroidal flow velocity of plasma species up to 20-100 km s(-1) ,opposite to the Ohmic current when the electric field is directed inward. The poloidal flow velocity of ions is of the order of 1-2 km s(-1), and is irrelevant to the electric field unless the electric field has a large gradient. [References: 28]