Abstract
The dynamic evolution of the first National SphericalTorus 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 ∼50–70 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∼2 cmwide 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 (Pth) study showed the neutral beam injection power (Pb) component
of Pth to be0.84MW, higher than that predicted by the ITER database scaling.