H-MODE TURBULENCE, POWER THRESHOLD, ELM, AND PEDESTAL STUDIES IN NSTX

H-mode operation plays a crucial role in NSTX research, allowing higher beta limits due to reduced plasma pressure peaking, and long pulse operation due to high bootstrap current fraction. Here, new results are presented in the areas of edge localized modes (ELMs), H-mode pedestal physics, L-H turbulence, and power threshold studies, ELMS of several other types (as observed in conventional aspect ratio tokamaks) are often observed: 1) large, Type I ELMs, 2) "medium" Type II/III ELMs, and 3) giant ELMs which can reduce stored energy by up to 30% in certain conditions. In addition, many high performance discharges in NSTX have tiny ELMs (newly termed Type V), which have some differences as compared with ELM types in the published literature. The H-mode pedestal typically contains between 25-33% of the total stored energy, and the NSTX pedestal energy agrees reasonably well with a recent international multi-machine scaling. We find that the L-H transition occurrs on a ~ 100 μsec timescale as viewed by a gas puff imaging diagnostic, and that intermittent quiescent periods precede the final transition. A power threshold identity experiment between NSTX and MAST shows comparable loss power at the L-H transition in balanced double-null discharges. Both machines require more power for the L-H transition as the balance is shifted toward lower single null. High field side gas fueling enables more reliable H-mode access, but does not always lead to a lower power threshold e.g. with a reduction of the duration of early heating. Finally the edge plasma parameters just before the L-H transition were compared with theories of the transition. It was found that while some theories can separate well-developed L- and H-mode data, they have little predictive value. The National Spherical Torus Experiment (NSTX) is a medium-size, low aspect ratio spherical torus(1) with both neutral beam (NBI) and radio-frequency (RF) auxiliary heating, and main parameters: R=0.86m, a=0.67m, R/a ! 1.26, B t ≤ 0.6 T, I p ≤ 1.5 MA, P NBI < 7.4 MW, P RF < 6 MW. An assessment of the attractiveness of the spherical torus (ST) concept in the areas of high beta (β) stability, turbulence and transport, start-up and current drive, and boundary physics is the main research goal of NSTX. This paper describes H-mode physics research in NSTX with the primary focus on imaging of edge-localized modes (ELMs) with new diagnostics; an overview of recent NSTX results is given in (2). I. Edge localized modes Several ELM types from the higher aspect ratio tokamaks have been observed on NSTX. The large ELMs with stored energy drop between 5-15% were observed at high normalized beta (β N )<6 in lower-single null (LSN) divertor configuration and over a wider range of β N in double- null divertor configuration (DN). An ELM, which has characteristics of both Type II and Type III ELMs from the literature, was observed in discharge shapes close to and including balanced DN. A new small ELM (termed Type V(3) since the numeral IV was used by the DIII-D group in the mid-1990's) was observed over a broad range in β N and beta poloidal (β p ), almost exclusively in a lower-single null configuration while using the center stack midplane gas injector. Characteristics of these ELMs are presented below.