ON TRANSPORT BARRIERS AND LOW-HIGH MODE TRANSITIONS

Transport barriers and transitions between modes of low and high confi nement in tokamak plasmas are often attributed to suppression of turbu lence by a shear flaw related to a plasma gradient, e.g., 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 f lux and gradient-it depends on the source distribution within the plas ma and on conditions at the plasma edge (e.g., imposed by the scrape-o ff layer). This edge gradient must lie within prescribed limits if a s tationary plasma profile (which may include an improved confinement zo ne) is to exist.