We study the radial structure of high confinement modes in a simplified, one-dimensional model of the self-consistent interaction of fluctuations, shear flow, and pressure gradient. The model describes the plasma edge with an energy flux coming from the core, which is used as a boundary condition for the pressure transport equation. As the energy flux increases, there is an L–H transition bifurcation which is described near marginal instability using a reduced Ginzburg–Landau model for the shear flow coupled to a transport equation for the pressure. For higher values of the energy flux, a second transition takes place in which the H-mode exhibits a finite-k instability. Numerical results show that this instability leads in the nonlinear regime to the spontaneous formation of a pedestal in the pressure profile, where the effective diffusivity exhibits a sharp drop. A further increase in the energy flux leads to multiple pedestals across the simulation domain.

52.55.Dy General theory and basic studies of plasma lifetime, particle and heat loss, energy balance, field structure, etc.

52.40.Hf Plasma-material interactions; boundary layer effects

52.25.Gj Fluctuation and chaos phenomena

52.30.-q Plasma dynamics and flow

52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams

52.25.Fi Transport properties

Plasma physics

Issue 5A (May 2004)

Received 11 October 2003

Published 5 April 2004