On transport barriers and L-H mode transitions

Transport barriers and L-H transitions in Tokamak plasmas are often attributed to suppression of turbulence by a shear flow related to a plasma gradient, eg. of density. However, such shear flow is also affected by the second derivative of density. When this is introduced there is no unique relation between flux and gradient - it depends on the source distribution within the plasma and on conditions at the plasma edge (eg. imposed by the scrape-off layer). This edge gradient must lie within prescribed limits if a stationary plasma profile (which may include an improved confinement zone) is to exist. (author)