Test of tokamak low‐mode–high‐mode transition theory in stellarators

Abstract

The tokamak L–H (low‐mode–high mode) transition theory can be tested in stellarators in a controlled manner by making use of the two or more local maxima in poloidal viscosity in these devices. Depending on the relative magnitudes of the toroidal and helical components of the magnetic‐field spectrum, the local maxima, and thus transition, can occur either at a critical poloidal E×B Mach number Mp of the order of unity, similar to that of a tokamak, or at an Mp of the order of ‖m−nq‖/m, similar to that of a helically symmetric torus. Here, E(B) is the electric (magnetic) field strength, m (n) is the poloidal (toroidal) mode number of the helical component of the ‖B‖ spectrum, and q is the safety factor. Possible limitations on the test due to the effects of the charge‐exchange momentum loss are discussed.