NONLINEAR INCOMPRESSIBLE POLOIDAL VISCOSITY AND ITS IMPLICATIONS FOR H-MODE IN STELLARATOR HELIOTRON PLASMAS

Non-linear incompressible poloidal viscosity is an important ingredien t in understanding the L-H transition in both tokamaks and stellarator s. Usually two or more local maxima in poloidal viscosity correspondin g to this transition may appear in stellarator/heliotron devices. Depe nding on the relative magnitudes of the toroidal and helical component s of the magnetic spectrum, the local maxima can occur at a poloidal E x B Mach number M(p) somewhat larger than \m - nq\/m, where E (B) is the electric (magnetic) field strength, m(n) is the poloidal (toroidal ) mode number of the components of the \B\ spectrum and q is the safet y factor. Non-linear incompressible viscosities for the plateau Pfirsc h-Schluter regime are calculated for present and next generation stell arator/heliotron devices (Heliotron-E, CHS, LHD, W7-AS and W7-X) by us ing a magnetic spectrum near the edge region under the assumption of n egligibly small parallel flow. The possibility of the occurrence of th e L-H transition and the limitation due to the effect of the charge ex change momentum loss are discussed. When the ion-ion collision frequen cy and neutrals are reduced sufficiently, all these devices show a loc al maximum of poloidal viscosity at M(p) similar to 1. However, the re duction of the poloidal viscosity in the region beyond this maximum is not large compared with the tokamak case.