Low-recycling, flat temperature profile discharges with higher performance, NBI-heating, and liquid lithium walls in the Lithium Tokamak Experiment-β
ORAL · Invited
Abstract
Recently, the Lithium Tokamak Experiment-β has achieved key prerequisites for a fusion device operating in the low-recycling regime. Previously, LTX and LTX-β demonstrated low-recycling flat Te profiles transiently in ohmic discharges with solid Li coatings. Now, LTX-β has sustained steady flat Te profiles and extended these operating conditions to neutral beam (NBI) heating and liquid Li walls with values of Ip, Te, Ti, pressure, and τE 50-200% higher than LTX.
Li retains hydrogen and suppresses edge neutral cooling, allowing increased Tedge ~ Tcore. Gas puffing can also cool the edge, so prior flat Te discharges had decaying ne after fueling ended. We show that careful fueling can exploit low ion-electron collisional coupling and sustain the unique high Te, low ne edge for multiple τE with steady ne ~1019 m-3.
The same low collisionality results in a long fast ion slowing down time τfi; thus heavily fueled, high ne discharges show the strongest NBI heating, though with a cold edge. Still, careful fueling during the Ip ramp enables NBI heating with Tedge > Tcore/2. Interpretive TRANSP analysis frequently shows τE ~ 2x H-mode scalings [DP Boyle et al NF 63 (2023) 056020], and generally, no degradation of τE with PNBI is observed. An imminent upgrade will re-aim the beam and enable pulses longer than τfi; NUBEAM predicts doubled NBI heating and fueling.
Improvements in Li deposition led to cleaner Li surfaces and good plasma performance with liquid coatings in LTX. After further improvements in LTX-β, the first observations of flat Te profiles with liquid Li have now been made, and plasma performance improvements similar to with solid coatings achieved. New experiments with visibly clean liquid Li coatings have reproduced and expanded on some of the solid Li results of the past two years, including quantification of the recycling coefficient.
Li retains hydrogen and suppresses edge neutral cooling, allowing increased Tedge ~ Tcore. Gas puffing can also cool the edge, so prior flat Te discharges had decaying ne after fueling ended. We show that careful fueling can exploit low ion-electron collisional coupling and sustain the unique high Te, low ne edge for multiple τE with steady ne ~1019 m-3.
The same low collisionality results in a long fast ion slowing down time τfi; thus heavily fueled, high ne discharges show the strongest NBI heating, though with a cold edge. Still, careful fueling during the Ip ramp enables NBI heating with Tedge > Tcore/2. Interpretive TRANSP analysis frequently shows τE ~ 2x H-mode scalings [DP Boyle et al NF 63 (2023) 056020], and generally, no degradation of τE with PNBI is observed. An imminent upgrade will re-aim the beam and enable pulses longer than τfi; NUBEAM predicts doubled NBI heating and fueling.
Improvements in Li deposition led to cleaner Li surfaces and good plasma performance with liquid coatings in LTX. After further improvements in LTX-β, the first observations of flat Te profiles with liquid Li have now been made, and plasma performance improvements similar to with solid coatings achieved. New experiments with visibly clean liquid Li coatings have reproduced and expanded on some of the solid Li results of the past two years, including quantification of the recycling coefficient.
*This work supported by US DOE contracts DE-AC02-09CH11466, DE-AC05-00OR22725, DE-AC52-07NA27344, DE-SC0019006, DE-SC0019239, DE-SC0023481, DE-SC0023274, and DE-SC0019308.
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Publication: DP Boyle et al Nuclear Fusion 63 (2023) 056020 https://doi.org/10.1088/1741-4326/acc4da
Physics of Plasmas paper to be submitted
Presenters
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Dennis P Boyle
- PPPL