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

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, λq,intmid∼3--7 mm 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 λq,intmid scales approximately as Ip−1. 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. (paper)