Low- to high-confinement mode transitions in the National Spherical Torus Experiment

Edge data from plasmas in the National Spherical Torus Experiment (NSTX) [S. Kaye et al., Fusion Technol. 36, 16 (1999)] have been compared to theories of transport suppression that have been used to develop a physics framework for low-confinement (L) to high-confinement (H) mode transitions. The NSTX data were obtained in low aspect ratio (R/a≃1.3) discharges taken from a variety of discharge phases, including L-modes, L-H transitions and H-modes with and without edge localized modes. The comparisons show that the group of points taken just before the L-H mode transition are well mixed with the purely L-mode group to within the measurement uncertainties, indicating that changes in these parameters leading up to the transition are subtle. One of the theory parameters, αMHD=−Rq2 dβt/dr, does show a clear threshold (αMHD=1 to 2) between the H-mode grouping of points and those remaining in the L-mode or taken just prior to the transition. Additionally, there is no evidence for an edge temperature threshold necessary for transitioning into the H-mode. NSTX data indicate further a possible connection between L-H transitions and nonambipolar beam ion losses.