The enhanced pedestal H-mode in the National Spherical Torus experiment

Abstract

Typical H-mode pedestal electron and ion temperatures in NSTX range from 100–300 eV. A new operating regime termed the ‘enhanced pedestal’ (EP) H-mode has been observed in which the pedestal ion temperature increases to ∼600 eV in about 50 ms, or one energy confinement time, resulting in a global confinement improvement. Ion temperature gradients as high as 30 keV/m are observed. The regime is correlated with a localized braking of the edge toroidal rotation near the q = 3 surface, in which case the pressure gradient term in the radial force balance becomes dominant over the toroidal and poloidal rotation terms. Coupled with increased rotation just inside the barrier, the radial electric field shear is also increased. An MHD trigger event (large edge localized mode) is common to the formation of the EP H-mode phase, which can occur either during the I p ramp-up or flat-top phases of discharges. The observed characteristics of this scenario are presented.