Finite beta electro-magnetic effects at the edge and the role of the scrape-off layer in the L-H transition

Magnetic induction becomes important when the skin-depth equals the wavelength of the electrostatic drift turbulence: c/root 4 pi sigma omega = k(perpendicular to)(-1). It is predicted that this suppresses the longwave perturbations responsible for the bulk of the turbulent transport, providing an explanation for the L-H transition. Since k(perpendicular to)rho(s) much less than 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 approximate to (k(perpendicular to)rho(s))(2)(m(e)/m(i)) in the collisionless skin-depth limit of very low density plasmas and T-e(3) similar to (BL)-L-2 perpendicular to Z(eff)/root m(i) (Lperpendicular to-perpendicular decay length in the collisional skin-depth limit of medium to high density plasmas. The latter is in 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.