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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