Divertorlets concept for low-recycling fusion reactor divertor: experimental, analytical and numerical verification

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Divertor systems of fusion devices exhaust intense heat loads from the plasma, which degrades solid plasma-facing components (PFCs). Fast liquid metal (LM) flow divertors may be more advantageous for this purpose. However, LMs have risk of piling due to intense magnetohydrodynamic (MHD) drag. Despite this, severe deceleration of the flow could be countered with the injection of currents that are transverse to external magnetic fields, allowing to thrust the flow with jxB forces. The injection of currents as an approach to propel LM-divertor flows has remained experimentally understudied. This article focuses on the evaluation of jxB-thrust and finding its drawbacks. This paper evidences that the simple operation of a LM-flow divertor with jxB-thrust, without any of the instabilities caused from reactor plasmas or parasitic currents, already presents intrinsic challenges.jxB-thrust was experimentally tested with free-surface-LM flows, a vertical magnetic field and and externally applied current. Experiments were reviewed with a theoretical model, showing agreement in the trends of theory and experiments. Full 3D-MHD-free-surface flow simulations were also performed with FreeMHD and confirmed the sensitivity to unstable flow behavior in LM systems when applying external currents. Furthermore, excessive power requirements are expected for the implementation of jxB-thrust at the reactor scale, making these systems inefficient for commercial devices.

Section: J × B-thrustmentioning

confidence: 93%

Section: Simulation Setupmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Experimental, numerical and analytical evaluation of j × B -thrust for fast-liquid-metal-flow divertor systems of nuclear fusion devices

Castro¹,

Fisher²,

Al-Salami³

et al. 2023

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“…There are concerns regarding liquid Li injection and draining at high velocities (>10 m s −1 ), particularly due to issues such as MHD drag in flowing LM within magnetic fields and droplet formation [40][41][42][43]. It should be noted that the SOLPS and MHD/heat transfer computations cannot guarantee the establishment of such high-velocity flows in a real fusion device.…”

Section: Fnsf Demands On Upstream N E C Li and Divertor Heat Fluxmentioning

confidence: 99%

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 ^*

Islam,

Lore,

Smolentsev

et al. 2024

The SOLPS-ITER code is utilized to analyze the boundary plasma associated with a fast-flow lithium (Li) divertor configuration in the Fusion Nuclear Science Facility (FNSF) tokamak and identify operational regimes with acceptable divertor and core conditions. Plasma transport from the SOLPS-ITER code has been coupled with a liquid metal (LM) MHD/heat transfer code to model a Li open-surface divertor design and assess its impact on the scrape-off-layer (SOL) and core plasma performance. Simulations with only Neon (Ne) impurity seeding have been conducted to evaluate its impact on meeting FNSF design demands for the divertor and upstream plasma parameters. Simulation results indicate that Ne seeding significantly mitigates divertor heat flux but potentially reduces both upstream electron and main ion density due to fuel dilution. The combined application of Ne seeding and deuterium (D2) puffing is required to satisfy the FNSF design requirements on upstream density (1e20) and divertor energy flux ( < 10 MW/m2). D2 puffing plays a role in counteracting upstream density drops and augmenting energy and momentum losses through atomic and molecular processes.The inlet Li flow velocity is systematically varied across a wide range to identify acceptable flows and corresponding LM surface temperatures. This comprehensive analysis identifies the acceptable Li flow parameters, LM surface temperature, and emitted Li fluxes necessary to meet the major design constraints. The emitted Li fluxes exhibit minimal impact on the main plasma at surface temperatures up to approximately 525^C, corresponding emitted Li fluxes of up to 2e23 atoms/s. Uncertainties in the Li emission processes from the surface are also investigated, primarily influencing Li loss in the lower surface temperature range (525C). Conversely, evaporation predominantly drives the Li loss processes at higher surface temperature ranges (525C), contaminating both the divertor and upstream plasma.

“…External J × B force was used to offset the negative effects from MHD drag, as well as oppose any J × B forces created by plasma currents. Finally, as an alternative to having a single flow path, the divertorlets concept introduces many flow paths by juxtaposing vertically oriented inlets and outlets [39,40].…”

Section: Introductionmentioning

confidence: 99%

Magnetohydrodynamics in free surface liquid metal flow relevant to plasma-facing components

Sun

Al-Salami

Khodak³

et al. 2023

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While flowing liquid metal plasma-facing components (PFCs) represent a potentially transformative technology to enable long-pulse operation with high-power exhaust for fusion reactors, magnetohydrodynamic (MHD) drag in the conducting liquid metal will reduce the flow speed. Experiments have been completed in the linear open-channel LMX-U device [Hvasta et al., Nucl. Fusion. 58 (2018) 01602] for validation of MHD drag calculations with either insulating or conducting walls, with codes similar to those used to design flowing liquid metal PFCs for a Fusion Nuclear Science Facility [Kessel et al., Fusion Science Technol. 75 (2019) 886]. We observe that the average channel flow speed decreased with the use of conducting walls and the strength of the applied transverse magnetic field. The MHD drag from the retarding Lorentz force resulted in an increase of the liquid metal depth in the channel that ‘piled up’ near the inlet, but not the outlet. As reproduced by OpenFOAM and ANSYS CFX calculations, the magnitude and characteristics of the pileup in the flow direction increased with the applied traverse magnetic field by up to 120%, as compared to the case without an applied magnetic field, corresponding to an average velocity reduction of ~ 45%. Particle tracking measurements confirmed a predicted shear in the flow speed, with the surface velocity increasing by 300%, despite the 45% drop in the average bulk speed. The MHD effect makes the bulk flow laminarized but keeps surface waves aligned along the magnetic field lines due to the anisotropy of MHD drag. The 3D fringe field and high surface velocity generate ripples around the outlet region. It was also confirmed that the MHD drag strongly depends on the conductivity of the channel walls, magnetic field, and volumetric flow rate, in agreement with the simulations and a developed analytical model. These validated models are now available to begin to determine the conditions under which the ideal liquid metal channel design of a constant flow speed and fluid depth could be attained.