DIMENSIONLESS FORM FOR THE L-H POWER THRESHOLD

The near separatrix region in tokamaks provides a new length scale par ameter x(0) = (-dln p(0)/d(x))(-1) of the order of several Larmor radi i. Modified drift and interchange instabilities occur in this region d ue to longitudinal losses. The related turbulence gives rise to perpen dicular transport and regulates the width x(0). The H-mode is set up w hen the shear flow stabilizes these instabilities leading to the condi tion ($) 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 lo ss mechanism. Classical thermal conduction provides a threshold scalin g similar to the experimentally observed scaling, but the free-streami ng loss yields a cubic dependence on B-0.