Finite Beta Electro-Magnetic Effects at the Edge and the Role of the Scrape-off Layer in the L-H Transition

摘要

Magnetic induction becomes important when the skin-depth equals the wavelength of the electrostatic drift turbulence: c/ (4pisigmaomega)~(1/2)=k_perpendicular~-1. It is predicted that this suppresses the long-wave perturbations responsible for the bulk of the turbulent transport, providing an explanation for the L-H transition. Since k_perpendicular rho_s 1 at the edge (rho-s - ion Larmor radius), this may occur at rather low beta, typical of the separatrix parameters. The above equality is used to derive thresholds for the L-H transition: beta approx approx (k_perperdicularrho_2)~2(m_e/m_i) in the collisionless skin-depth limit of very low density plasmas and T_e~3 approx B~2L_perpendicularZ_eff/(m_i)~(1/2) (L_perpendicular - perpendicular decay length) in the collisional skin-depth limit of medium to high density plasmas. The latter is a broad agreement with experiment, pointing to a possible role of low temperature collisional plasma (which is in the SOL) in the physics of the L-H transition.