The role of current profile broadening in L-mode and H-mode plasmas

Abstract

Auxiliary heating by neutral beam injection (NBI) or ion cyclotron resonance heating (ICRH) can broaden or steepen the plasma current density profile. A measure of changes to the current density profile Jϕ is established by a combination of current moments which yields the profile broadening parameter ζ. JET experiments are demonstrated to increase ζ (broaden Jϕ) when NBI is applied to discharges which exhibit either L-mode or H-mode confinement; pellet injection and ICRH or current decay are demonstrated to decrease ζ (peak Jϕ). The time evolution of the parameter ζ is shown to correlate with the changes to the Dα emission seen in H-mode discharges with and without edge localized modes. The broadening of Jϕ by NBI, especially for H-mode pulses, smarts in the edge region and it is identified with the non-inductive bootstrap current component as a consequence of steep density profiles. Associated with current profile broadening is an increase in the edge pressure and the total stored plasma energy W. An analysis of global confinement (variations of W with power P) is made by splitting W into three parts: the fast ion energy, Wf, the edge energy, W1, which is empirically shown to be proportional to the current profile broadening parameter ζ, and the bulk plasma thermal energy, Wp. It is shown that the scaling for Wp in the L- and H-mode confinement regimes is similar.