JET-P(89)56
The Role of Current Profile Broadening in L and H-Mode Plasmas
Auxiliary heating by neutral beams (NBI) or ion cyclotron resonance heating (ICRH) can broaden or peak the plasma current density profile. A measure of changes to the current densitprofile Jf is established by a combination of current moments which yields a profile broadening parameter x. JET experiments are demonstrated to increase x (broaden Jf) when NBI is applied to discharges which exhibit either L or H mode confinement; pellet injection and ICRH are demonstrated to decrease x (peak Jf). The time evolution of the parameter x is shown to correlate with the changes to the Da emission seen in a H-mode discharges with and without ELMs. The broadening of Jf by NBI, especially for H-mode pulses, starts in the edge region. It is caused by the non-inductive bootstrap current component as a consequence of steep density profiles. The ohmic current component undergoes only a nominal change. Associated with current profile broadening is an increase in edge pressure and total stored plasma energy W. An analysis of global confinement (variations of W with power P) is made by splitting W into three parts: Wf, the fast ion energy; W1, the edge energy which is empirically shown to be proportional to the current profile broadening parameter x; Wp, the bulk plasma thermal energy. It is shown that the scaling for Wp in the L and H-mode confinement regimes is similar.