Recent Advances Towards a Lithium Vapor Box Divertor

Authors:  R.J. Goldston, A. Hakim, G.W. Hammett, M.A. Jaworski, J. Schwartz

Abstract:  Fusion power plants are likely to require near complete detachment of the divertor plasma from the divertor target plates, in order to have both acceptable heat flux at the target to avoid prompt damage and acceptable plasma temperature at the target surface, to minimize long-term erosion. However hydrogenic and impurity puffing experiments show that detached operation leads easily to X-point MARFEs, impure plasmas, degradation in confinement, and lower helium pressure at the exhaust. The concept of the Lithium Vapor Box Divertor is to use local evaporation and strong differential pumping through condensation to localize the gas-phase material that absorbs the plasma heat flux and so avoid these difficulties. We use ADAS calculations of εcool, the plasma energy lost per injected lithium atom, to estimate the lithium vapor pressure, and so temperature, required for detachment, taking into account power balance. We also develop a simple model of near-detachment to evaluate the required upstream density, based on further taking into account dynamic pressure balance. A remarkable general result is found, not just for lithium-induced detachment, that the upstream density divided by the Greenwald-limit density scales as (P5/8/B3/8) Tdet/(εcool + γTdet), with no explicit size scaling. Tdet is the temperature just before strong pressure loss, ~1/2 of the ionization potential of the dominant recycling species, and γ is the tradiational heat transmission factor.
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