Overview of L-H power threshold studies in NSTX

A summary of results from recent L-H power threshold (P-LH) experiments in the National Spherical Torus Experiment is presented. First P-LH (normalized linearly by plasma density) was found to be a minimum in double-null configuration, tending to increase as the plasma was shifted more strongly towards lower-or upper-single null configuration with either neutral beam or rf heating. The measured P-LH/n(e) was comparable with neutral beam or rf heating, suggesting that rotation was not playing a dominant role in setting the value of P-LH. The role of triangularity (delta(bot)) in setting P-LH/n(e) is less clear: while 50% less auxiliary heating power was required to access H-mode at low dbot than at high dbot, the high dbot discharges had lower ohmic heating and higher dW/dt, leading to comparable loss of power over a range of dbot. In addition, the dependences of P-LH on the density, species (helium versus deuterium), plasma current, applied non-axisymmetric error fields and lithium wall conditioning are summarized.