Evidence for edge gradients as control parameters of the spontaneous high-mode transition

Abstract

Measurements in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] show that edge gradients of ion and electron temperature and pressure, ∇Ti, ∇Te, ∇Pi, and ∇Pe, are good candidates for parameters which respond to the heating power to control the fundamental physics of the spontaneous Low-mode (L-mode) to High-mode (H-mode) transition. These gradients are measured in the region where the H-mode transport barrier forms and they are found to consistently increase in time during the L-phase of discharges which make a transition to H-mode. Moreover, for a fixed magnetic configuration, there is a well-defined boundary between the L-mode and H-mode states in the ∇Te-∇Pe operational space. The values of Ti, Te, Pi, and Pe, measured at this location, show smaller relative changes during the L-phase of these discharges, indicating that the scale lengths of these quantities are decreasing as the L-H transition is approached in time. These results are consistent with several theoretical models in which gradients of pressure or temperature cause the transition. © 2001 American Institute of Physics.