Characterization of H-mode Discharges in NSTX

We report the observation of the first H-mode transitions in the National Spherical Torus Experiment (NSTX). Energy confinement time transiently increased by more than a factor of 2, to order 100 ms. The heating power requirement for accessing the H-mode regime (power threshold) was significantly higher than predicted by a multi-machine database compiled from conventional aspect ratio tokamaks. Our power threshold results are similar to other STs, such as the START and MAST machines. These H-mode observations in STs challenge and extend the existing international power threshold and energy confinement time scalings based on conventional aspect ratio tokamaks. The remainder of this paper discusses characteristics of the H-modes in NSTX..