H-Mode Pedestal Width and Neutral Penetration

ITER confinement depends sensitively upon the \textit{width} of the H-mode pedestal, the physics of which remains poorly understood. Here we report on analytical studies of a simple model of the L$\to $H bifurcation in heat and particle transport, with central heating and edge fueling. A simple change-of-variables enables the reduction of this coupled system to simpler one field systems, \textit{without} any ad-hoc assumptions concerning the transport coefficients in L and H mode. Results indicate that a transition occurs locally if the product of heating and fueling lies within an interval set by the transport parameters. Thus, the pedestal width is inexorably linked to the neutral penetration depth, and an exponentially increasing amount of power is required to push beyond it. The major unknown is the particle diffusivity in H-mode. Implications for ITER, where neutral opacity is thought to be high, will be discussed.