No abstract

The “divertorlets” concept is a potential non-evaporative liquid metal solution for heat removal at low recycling regime. A toroidal divertorlets [1] prototype was built and tested at LMX-U at PPPL to evaluate the performance of this configuration. In this paper, details of the design, experimental results, comparison with analytical theory and MHD numerical simulations of toroidal divertorlets are covered. Experiments, analytical model and simulations showed agreement and allowed the projection of properties at higher magnetic fields (reactor-like operation), proving the concept to be a compelling solution for divertor applications.

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.

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.