DIMENSIONLESS FORM FOR THE L-H POWER THRESHOLD

The near separatrix region in tokamaks provides a new length scale parameter x(0) = (-dln p(0)/d(x))(-1) of the order of several Larmor radii. Modified drift and interchange instabilities occur in this region due to longitudinal losses. The related turbulence gives rise to perpendicular transport and regulates the width x(0). The H-mode is set up when the shear flow stabilizes these instabilities leading to the condition ($) over cap rho = rho(i)/x(0) > c(1) where c(1) is of the order unity. The L-H power threshold depends strongly on the longitudinal loss mechanism. Classical thermal conduction provides a threshold scaling similar to the experimentally observed scaling, but the free-streaming loss yields a cubic dependence on B-0.