Effect of the limiter configuration on the electron temperature gradient instability in the tokamak scrape-off layer

A strong instability has been discovered caused by the interplay of two factors: (i) the gradient of the electron temperature across the magnetic field lines and (ii) the formation of Debye sheaths at the conducting end walls terminating the plasma along the magnetic field. The instability can be of some importance for the physics of tokamak scrap-off layers as it can determine the layer thickness. The analysis to the case of non-orthogonal intersections of the magnetic field with the wall surface, i.e. to the situation which is in fact most typical for a tokamak with a limiter is generalized. The new effects which appear in this more realistic model are caused by the change of the flux-tube length in the course of its displacement in the radial direction. Depending on the sign of the scalar product n·grad Te, where n is a unit vector normal to the wall, there can occur either suppression of the initial instability or its considerable enhancement. The possible influence of these effects on the transport coefficients in the scrape-off layer is evaluated, and conclusion is drawn that, under realistic assumptions on plasma parameters, this influence can be quite significant. (author)