Evolution and Termination of H-modes in NSTX
The dynamic evolution of the first National Spherical Torus Experiment (NSTX) H-modes [high-confinement 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 neutral-beam injection (NBI) power level affects both whether or not the discharge exhibits ELMs [edge localized modes] and the duration of the ELM-free phase. The ELM-free discharges had energy confinement as high as 120 ms, whereas the few discharges with ELMs had confinement times approximately 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 [low to high] 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 (GPI) of He-I light during the H-mode phase showed rapid formation of a narrow emission layer approximately 2 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(subscript th)] study showed the neutral-beam injection power [P(subscript b)] component of P(subscript th) to be less than or equal to 0.84 MW, higher than that predicted by the ITER [International Thermonuclear Exprimental Reactor] database scaling.
- Research Organization:
- Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ
- Sponsoring Organization:
- USDOE Office of Science
- DOE Contract Number:
- AC02-76CH03073
- OSTI ID:
- 793015
- Report Number(s):
- PPPL-3644
- Country of Publication:
- United States
- Language:
- English
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