The effect of gas injection location on a lithium vapor box divertor in NSTX-U

Abstract

The lithium vapor box divertor is a proposed divertor design that will minimize contamination of the upstream plasma in a fusion device, while also ensuring protection of the target. In this design lithium is evaporated near the target by high temperature lithium surfaces, dissipating the plasma heat flux. The lithium vapor box has been predicted via the fluid-kinetic code SOLPS-ITER to achieve low (nLi/ne∽0.05) upstream concentrations of lithium and low target heat fluxes. Here we compare two choices of deuterium gas puff location using SOLPS-ITER, the Private Flux Region (PFR) and the Common Flux Region (CFR), and find significant differences in the contamination level required to reach an acceptable target heat flux (defined here as qmax tar≦10 MW/m2). Deuterium gas puffing from the PFR is seen to better reduce upstream lithium contamination. The difference in puffing location is seen to cause changes in the upstream flow of lithium ions. The PFR puff, having better access to the separatrix, can better reduce the upstream-directed flow of lithium near the separatrix which is the primary source of contamination due to a large thermal force in this region. Puffing from the CFR, partially due to inefficacy at reducing separatrix lithium flow, has higher lithium concentration within the plasma. Solutions that reduce the heat flux to below 10 MW/m2 have a range of lithium concentrations between nLi/ne∽0.01-0.12 depending on puff intensity, location, evaporator temperature and recycling at the various plasma facing components. The efficacy of the puffs is tested for sensitivity to deuterium recycling coefficient at the target, evaporators, and main chamber walls. A CFR located puff is found to be more dependent on the recycling coefficients used than a PFR located puff. regardless of the set of recycling coefficients chosen, PFR puffing achieves lower lithium contamination than CFR puffing for a given heat flux.

Optimization of lithium vapor box divertor evaporator location on NSTX-U using SOLPS-ITER

“…In alignment with previous analyses [5,6,8], a simulated NSTX-U magnetic equilibrium is employed as an example for a lithium vapor box device. The lower outer divertor PFCs are altered from NSTX-U to create a contained lithium evaporation region.…”

“…The locations used are shown schematically in figure 1, results of the variation will be discussed in section 3. The cross-field transport coefficients used here are the same as a previous analysis of the lithium vapor box using SOLPS-ITER [5]. The power boundary condition at the Core-Edge Interface (CEI) is 7 MW for all simulations presented in this article, indicating 10 MW of input power with 30% radiated power within the normalized poloidal flux function ψ n ∼ 0.85 surface, where the CEI is located.…”

“…The power dissipation should occur outside the Last Closed Flux Surface (LCFS), or risk reduction in energy confinement time of the main plasma [4]. Impurities, necessary to induce the power dissipation in detachment, must have limited concentration in the main plasma or risk reductions in fusion power production, either via upstream radiation or fuel dilution [5,7].…”

“…The 'box' structure of the lithium vapor box limits the ionization location to be near the target, while the flow of the main ions work to prevent lithium from reaching the main plasma via the friction force [8,9]. Past work using SOLPS-ITER [10] has shown that (n Li /n e ) LCFS ≲ 0.05 was possible in NSTX-U with moderate fuel puffing, indicating viability for a burning plasma scenario, should a reactor have similar results [5][6][7]. (n Li /n e ) LCFS ≲ 0.05 was possible across changes in power, density, recycling scenarios and transport coefficients.…”

Analysis and design of fast flow liquid Li divertor for fusion nuclear science facility (FNSF) using coupled plasma boundary and LM MHD/heat transfer codes *

“…Reduced recycling increases plasma temperature, resulting in improvements in core performance, as demonstrated in the [11][12][13][14][15][16][17]. Liquid Li can be used in various ways as a PFCs; it may be injected as a fast-flowing layer [4], employed as a gravity-driven system [18], utilized as a JXB driven [16] or Thermoelectric MHD flow system [19], or applied as an evaporative system known as a 'vapor box' [20,21]. The application of liquid lithium divertor has demonstrated effective heat flux reduction in NSTX [22].…”