Current generation in fusion plasmas by injection of radiofrequency waves: Finite-element models on IBM 3090/VF

Abstract

The generation of plasma current by means of electromagnetic waves in the lower-hybrid (LH) range of frequency in the presence of an opposing d.c. electric field is examined within the context of a two-dimensional quasi-linear numerical model. This model, implemented in the finite-element code ADLER, solves for the simultaneous evolution of the electron distribution function and the wave spectral distribution in two dimensions in velocity and wave number space respectively. As a result, detailed information on the plasma evolution can be obtained without any simplifying assumption neither on the shape of the electron distribution function, nor on that of the self-consistently generated turbulent wave spectrum. One of the main goals of the present investigation is to estimate the threshold value of the opposing electric field beyond which the LH power is no longer able to maintain the initial plasma current against the collisional losses and the decelerating effect of the electric field. Other issues related to the physics of lower-hybrid current in the presence of an opposing electric field are also examined. In particular, special attention is paid to the details of the shape of the electron distribution function which, due to the combined action of the opposing electric field and the Coulomb collisional deflection, develops a genuinely two-dimensional structure. Typical features associated with this two-dimensional structure, like a bi-Maxwellian behaviour alongw forv∼0 (v andw denoting the electron velocity parallel and perpendicular to the ambient magnetic field, respectively) and inverted profiles forv<0, have been clearly detected. Finally, local (in configuration space) power balances based on the self-consistent quasi-linear wave damping rates are formulated. From these power balances one can evaluate α, the fraction of the total LH power input which needs to be absorbed by the resonant electrons in order to sustain or increase the plasma current against the collisional losses and the counter-driving electric field. The present study indicates that α is very sensitive to the location of spectral bounds in velocity space. In particular, for the case of an injection of 300 kW of LH power into a 1 keV plasma with a density of 2·1012 cm−3, in order to obtain values of α close to 50%, an upshift of the lower boundary of the spectrum down to less that three electron thermal velocities is required.