Evolution and termination of H-modes in NSTX

The dynamic evolution of the first National Spherical Torus Experiment (NSTX) H-modes will be discussed. The H-modes were obtained in lower-single null divertor discharges with various forms of plasma heating. The exact timing of divertor formation and also the NBI power level affects both whether or not the discharge exhibits ELMs and the duration of the ELM-free phase. ELM-free discharges had energy confinement as high as 120 ms, whereas the few discharges with ELMs had confinement times similar to50-70 ms. Buildup of a steep edge density gradient and formation of 'ears' on the density profile were observed within a few ms of the L-H transition, yielding broader density and pressure profiles. The L-H transition was marked by a decrease in edge visible light and simultaneous increase in electron Bernstein wave emission, reflecting a steepening of the edge density gradient. Gas puff imaging of He-I light during the H-mode phase showed rapid formation of a narrow emission layer similar to2 cm wide in the H-mode phase, which returned within 20 microseconds at termination of the H-mode phase to a broader turbulent emission layer. All of the H-modes were terminated by an MHD reconnection event. The first power threshold (P-th) study showed the neutral beam injection power (P-b) component of P-th to be less than or equal to0.84 MW, higher than that predicted by the ITER database scaling.