Recent results from the National Spherical Torus Experiment

Abstract

The National Spherical Torus Experiment (NSTX) is a low aspect-ratio fusion research facility whose research goal is to make a determination of the attractiveness of the spherical torus concept in the areas of high-β stability, confinement, current drive, and divertor physics. Remarkable progress was made in extending the operational regime of the device in FY 2002. In brief, β t of 34% and β N of 6.5 were achieved. H-mode became the main operational regime, and energy confinement exceeded conventional aspect-ratio tokamak scalings. Heating was demonstrated with the radiofrequency antenna, and signatures of current drive were observed. Current initiation with coaxial helicity injection produced discharges of 400 kA, and first measurements of divertor heat flux profiles in H-mode were made.

Observation of a high performance operating regime with small edge-localized modes in the National Spherical Torus Experiment

“…For reference the steady poloidal beta (b p ) was ~ 1.2 (bootstrap current fraction 40-50%) and the line-average density relative to Greenwald scaling 11 (n GW = I p /pa 2 ) was 0.85 just before the end of the discharge, which was terminated by I p ramp-down. We note that details of this high performance regime for similar discharges have been reported previously [12][13][14][15] . The new discovery presented in this paper pertains to 1) the positive identification of the D a oscillations as tiny ELMs, which are different from ELMs in the published literature, and 2) the recent observation of conventional Type III ELMs in NSTX which confirm the uniqueness of these type V ELMs.…”

Observation of a High Performance Operating Regime with Small Edge-Localized Modes in the National Spherical Torus Experiment

“…For reference the steady poloidal beta (b p ) was ~ 1.2 (bootstrap current fraction 40-50%) and the line-average density relative to Greenwald scaling 11 (n GW = I p /pa 2 ) was 0.85 just before the end of the discharge, which was terminated by I p ramp-down. We note that details of this high performance regime for similar discharges have been reported previously [12][13][14][15] . The new discovery presented in this paper pertains to 1) the positive identification of the D a oscillations as tiny ELMs, which are different from ELMs in the published literature, and…”

Accounting of the power balance for neutral-beam heated H-mode plasmas in NSTX

“…Note however that the peak heat flux was observed to increase non-linearly with NBI power, similar to that found in prior experiments. [4] This was found to result from the edge temperature, which increases with NBI power and tends to shrink the SOL width, so the heat flux increases rapidly compared to the total heat to the plates. In LSN discharges, the in/out power split shows that 75-80% of the heat incident on the outer strike plate, i.e., the ratio is 1:4.…”

“…Extrapolation of the temperature rise yields a tile temperature in excess of the 1200°C limit after 3 s. [4] In single-null diverted H-mode discharges on DIII-D, the power accountability is more than 85%. Core plasma radiation accounted for < 18% with the rest flowing through the scrapeoff layer to the inboard and outboard divertors.…”