Plasma facing components with capillary porous system and liquid metal coolant flow

Abstract

Liquid metal can create a renewable protective surface on plasma facing components (PFC), with an additional advantage of deuterium pumping and the prospect of tritium extraction if liquid lithium (LL) is used and maintained below 450 °C, the temperature above which LL vapor pressure begins to contaminate the plasma. LM can also be utilized as an efficient coolant, driven by the Lorentz force created with the help of the magnetic field in fusion devices. Capillary porous systems can serve as a conduit of LM and simultaneously provide stabilization of the LM flow, protecting against spills into the plasma. Recently, a combination of a fast-flowing LM cooling system with a porous plasma facing wall (CPSF) was investigated [A. Khodak and R. Maingi, Nucl. Mater. Energy 26, 100935 (2021)]. The system takes an advantage of a magnetohydrodynamics velocity profile as well as attractive LM properties to promote efficient heat transfer from the plasma to the LL at low pumping energy cost, relative to the incident heat flux on the PFC. In the case of a disruption leading to excessive heat flux from the plasma to the LM PFCs, LL evaporation can stabilize the PFC surface temperature, due to high evaporation heat and apparent vapor shielding. The proposed CPSF was optimized analytically for the conditions of a fusion nuclear science facility [Kessel et al., Fusion Sci. Technol. 75, 886 (2019)]: 10 T toroidal field and 10 MW/m2 peak incident heat flux. Computational fluid dynamics analysis confirmed that a CPSF system with 2.5 mm square channels can pump enough LL so that no additional coolant is needed.

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

“…The lower outer divertor target (orange walls in figure 3) is given evaporation consistent with a 500 • C lithium surface, which amounts to 1.514×10 22 Li/s for the given surface area. This temperature was found to be reasonable for an advanced capillary porous systems using liquid lithium [8] when the heat flux was limited to 11 MW m −2 . In the present analysis the effect of thermal and physical sputtering are ignored, as the model for target evaporation here is very simple.…”

“…In the present analysis, any increase in lithium emission due to sputtering effectively corresponds to a smaller assumed target temperature. A reduced temperature should, in principle, be possible given changes to the design criterion of the advanced capillary porous system [8]. Accurate calculation of target evaporation is a point of future work where more advanced modeling shall be performed to give self-consistent target temperatures that include the effect of sputtering with the lithium emission, which has a non-negligible effect as seen in other work modeling lithium capillary porous systems [24,25].…”

“…The target is assumed to be evaporating at 500 • C which should retain very little deuterium [8,9,28]. However if the temperature was not well controlled and reduced below that value, lower recycling may occur.…”

“…The lithium concentration in the detached scenario was (n Li /n e ) OMP,sep ∼ 0.05 [6]. While sub-10 MW m −2 has been shown to be acceptable for a tungsten target, a lithium wetted capillary porous system is expected to survive similar heat fluxes, with up to 16 MW m −2 being reported as acceptable for lithium filled tungsten-CPS targets in the literature [8,9]. The 10 MW m −2 limit can be seen as a conservative limit on the acceptable heat flux for a lithiumwetted CPS divertor target.…”

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 *

“…The use of liquid metal (LM) lithium (Li) offers significant benefits for PFCs owing to its low atomic number (Z = 3), low viscosity, high thermal conductivity, self-healing capabilities through liquid flow, and potential benefits to confinement under controlled Li concentration in the upstream via reducing recycling flux (D sticks on LM surface). 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].…”