Repetitive transport bursts in simulations of edge-localized modes in tokamaks

Abstract

The character of particle and energy transport in simulations of tokamak edge turbulence is determined by the magnitude of the density gradient. Edge turbulence becomes increasingly intermittent as the edge density gradient increases. Beyond a critical limit in the edge density gradient, the transport is dominated by short, repetitive bursts of particles and energy outward toward the wall. These bursts are extremely ballooning in character, strongly localized on the large major radius side of the torus. The duration of a burst is given by the ballooning mode growth time t0=(RLn)1∕2∕cs, where cs is the sound speed, R is the major radius of the torus, and Ln is the density gradient scale length. The bursts are coherent in structure with a poloidal scale size L0 that is proportional to the square root of the plasma resistivity η. With further increases in the edge density gradient, the fluxes of energy and particles in the bursts become much larger in magnitude. The particle and energy bursts seen in the simulations are similar to the bursts in Dα radiation seen during edge-localized modes in tokamaks.