An estimate on the concept of convection cooling in the boundary-layer of toroidal fusion plasmas

A simple model is considered in which the heating power transmitted to a toroidally confined plasma is eventually being carried away by means of a convective plasma flow in the boundary layer. The boundary layer of thickness delta fills the interspace between the outermost - closed - magnetic surface and the first wall which encloses the plasma column toroidally. The model requires that delta > approx. 1015/n [cm], a quantity also of relevance for the cold-gas-blanket concept. For the convection cooling process proposed here, however, all magnetic field lines inside the boundary layer have to lead to the first wall after a finite number of toroidal pathlengths in order to permit a thermal plasma flow to the wall. Such a magnetic field configuration corresponds approximately to a helical magnetic limiter, as can be produced e.g. by resonant helical perturbation fields of short decay length. The model connects the total heating power with average values of the plasma parameters T and n in the boundary layer and of the angle epsilon which gives the direction of the plasma flow towards the wall. The values of epsilon and of delta may be controlled by the field configuration of the magnetic limiter. This results in an estimate on the plasma temperature T in front of the first wall: for epsilon n approx. 1012 [cm-3] one obtains T to be of the order of 10 [eV] or below, applied for the parameters of TEXTOR and INTOR. (orig.)