Filamentary structures in tokamaks and their importance to anomalous electron energy loss

This theoretical paper speculates on the possible existence of fine-scale filamentary structures in magnetically confined plasmas. These structures would be the result of non-linear development of the electrothermal instability and are predicted to occur with a wavelength of a few ion Larmor radii. The linear growth rate is essentially of the order of the heating or cooling rates (e.g. Ohmic heating equipartition, radiation loss rates) and is small compared to MHD instability growth rates. However, the structures could be generated early on in the time history of the discharge, and be steadily maintained by the current as quasi-stationary filaments. A non-linear time-dependent model shows the creation from random noise of narrow current filaments in which the electron temperature is increased locally by a factor of 10 or more with a corresponding drop in density. Saturation of the thermal instability occurs following the triggering of ion-acoustic turbulence in the filament because the local electron drift velocity exceeds the sound speed locally. There is enhanced electron resistivity in the filament, but the high-energy ion tail (produced by direct heating of the ions by the ion-acoustic turbulence and necessary to achieve its saturation) is distributed over a larger volume because of the large Larmor excursions of these hot ions. This increases the amount of direct heating of the ions so as to maintain a fraction (msub(e)/msub(i))sup(1/4) of the ions in the filamentary region at approximately the local electron temperature. The resulting energy balance indicates that a significant fraction of the Ohmic heating goes directly into the ion species, the hot ions mainly losing their energy classically to the colder bulk ions. This might remove the need to postulate an anomalous electron thermal conduction. Ion neoclassical thermal conduction (and radiation loss) would then remove the energy from the plasma. (author)