Transport barrier in ion temperature gradient driven turbulence

Abstract

Fluid simulations of long η_i modes are presented that demonstrate that the density profile plays a much more profound role in controlling global energy confinement than would be expected from simple marginal stability considerations. Specifically, local steepening of the density gradient produces an effective transport barrier that allows ion global confinement to improve dramatically. The threshold value η_i =d ln(T_i)/d ln(n) required for instability η_ic plays a fundamental role in the formation of the transport barrier—no transport barrier forms for η_ic<1. The implication of these results for understanding ion energy confinement in tokamaks is discussed.