Alfvén-drift turbulence suppression as triggering mechanism for LH transition

Abstract

The Alfvén drift turbulence suppression at the plasma edge is suggested as a triggering mechanism for the L to H transition. The stability theory of Alfvén drift-waves shows that with increasing plasma pressure the Alfvén waves get coupled to electron drift waves and as a consequence the unstable long wavelength perturbations (most important for transport) are suppressed. The instability can be characterised by two significant parameters, i.e. the normalised plasma beta, β _n , and the normalised collision frequency, v _n . The resulting turbulent transport coefficient is suppressed when the normalised beta is greater than a critical value, i.e. β _n >1+v ^2/3 _n , which depends on the normalised collision frequency v _n . The transport coefficients change their dependence on plasma parameters at this threshold. Therefore, the possible scenario for the development of the H-mode could be associated with the stabilisation of the electron fluctuation at the plasma edge. The Alfvén drift-wave model predicts the experimental trend of a roughly linear dependence of threshold temperature on magnetic field, with a weak dependence on density at high densities and a strong dependence on density at lower densities.