The effects of increasing lithium deposition on the power exhaust channel in NSTX

Abstract

Previous measurements on the National Spherical Torus Experiment (NSTX) demonstrated peak, perpendicular heat fluxes, qdep,pk⩽15 MW m−2 with an inter-edge localized mode integral heat flux width, during high performance, high power operation (plasma current, Ip = 1.2 MA and injected neutral beam power, PNBI = 6 MW) when magnetically mapped to the outer midplane. Analysis indicates that scales approximately as . The extrapolation of the divertor heat flux and λq for NSTX-U are predicted to be upwards of 24 MW m−2 and 3 mm, respectively assuming a high magnetic flux expansion, fexp ∼ 30, PNBI = 10 MW, balanced double null operation and boronized wall conditioning. While the divertor heat flux has been shown to be mitigated through increased magnetic flux expansion, impurity gas puffing, and innovative divertor configurations on NSTX, the application of evaporative lithium coatings in NSTX has shown reduced peak heat flux from 5 to 2 MW m−2 during similar operation with 150 and 300 mg of pre-discharge lithium evaporation respectively. Measurement of divertor surface temperatures in lithiated NSTX discharges is achieved with a unique dual-band IR thermography system to mitigate the variable surface emissivity introduced by evaporative lithium coatings. This results in a relative increase in divertor radiation as measured by divertor bolometry. While the measured divertor heat flux is reduced with strong lithium evaporation, λq contracts to 3–6 mm at low Ip but remains nearly constant as Ip is increased to 1.2 MA yielding λq's comparable to no lithium discharges at high Ip.

Theory based scaling of edge turbulence and implications for the scrape-off layer width

“…These rough estimates result in a midplane SOL heat flux width due to turbulence of  q ~ 0.2 cm in H mode and 0.8 cm in OH mode, similar in order of magnitude to previous estimates made for the subset database using a somewhat different method 11. Typical H mode inter-ELM midplane-mapped heat flux widths measured in NSTX are in the range of 0.2 to 1.5 cm (without Li injection) depending on I p , the plasma current 38. At the highest plasma currents the midplane width, correcting for diffusive broadening in the divertor legs, is  q ~ 0.2 cm comparable to the turbulence estimate deduced here.…”

“…The simulations therefore suggest that the effect of turbulence is neither dominant or negligible for this I p = 1 MA discharge. It is possible that other physics, such as neoclassical drift effects, 39 could explain the main  q ~ 1/I p scaling that is observed in NSTX 38 but that turbulence is still important, emerging as dominant only at larger I p (or in larger R devices). have not yet fully formed at this location, but is rather due to wavy density structures that convect plasma back and forth across the separatrix.…”

Recent progress on spherical torus research

“…Further progress in reducing the peak flux in NSTX was demonstrated through the PDD regime, resulting in a peak heat flux reduction of up to 60%. [259][260][261] Deuterium gas puffing into the divertor area reduces the electron temperature to about a few eV in front of the divertor plate, which is sufficient to facilitate radiative cooling. The radiative cooling effect can be seen in the SFD configuration in Fig.…”

Turbulent transport regimes and the scrape-off layer heat flux width

“…An international experimental effort to characterize k q in diverted high-confinement (H) mode discharges was carried out, [14][15][16][17][18][19][20][21] and the resulting database has been analyzed in detail. 22 This database yields a nearly linear inverse dependence on plasma current, i.e., k q / 1/I p with rather weak dependences on other parameters.…”