LNF: LIMPLASH, Liquid Metal Plasma Shields as new generation power exhaust solutions for magnetic fusion devices (2025-2028)

LNF - Nationally funded project

Title: LIMPLASH, Liquid Metal Plasma Shields as new generation power exhaust solutions for magnetic fusion devices

Reference: PID2024-161233OA-I00

Programme and date: Proyectos de Generación de Conocimiento, convocatoria 2024

Programme type (Modalidad de proyecto): Tipo A

Area/subarea (Área temática / subárea): Ciencias Físicas/Física de partículas y nuclear

Principal Investigator(s): Alfonso de Castro

Project type: Proyecto individual

Start-end dates: 01/09/2025 - 31/08/2028

Financing granted (direct costs): 40625 €

Description of the project

The development of new baseline, large scale, 24/7 availability, and carbon-free energy sources is paramount to reduce the global dependence on fossil fuels and its effects on biosphere modification, questions that are accelerating the climate change and nature degradation trends. Under such scenario, nuclear fusion is seen as hopeful option with intrinsic 24/7 availability and tremendously high energy density potential. It plans to use raw materials inexhaustible in the human time scale and not geographically concentrated, being the power generation theoretically free of long-lived radioactive wastes and inherently safe (no runaway reaction or explosions unlike fission energy). Within the magnetic fusion research, one of the key remaining issues for its development is the performance and resilience of the elements in unavoidable contact with the fusion plasma, the so-called Plasma Facing Components (PFCs) that will need to handle extreme heat fluxes (steady state 10 MW/m2 and transients of GW/m2 range in ms timescales). As the physics/technical feasibility of conventional PFC solutions (based on solid tungsten elements) and the related power exhaust scenario does not appear guaranteed or straightforward [1], active research on novel, advanced and alternative Liquid Metal (LM) PFCs has emerged mainly using Tin (Sn), lithium (Li) and their alloys (SnLi) as candidates [2]. Amid a fusion-relevant heat load irradiation scenario, LM surfaces can act as a sacrifice interface by favoring the creation of LM vapor/plasma clouds in front of it. The process is generally denominated vapor shielding [3] and offers an alternative and novel approach to the power exhaust problem. In this way, the power load fraction that utterly reaches the PFC substrate surface/structure can be decreased due to the volumetric dissipation that takes place in the LM cloud, thus allowing to enhance the total power exhaust capabilities beyond single conductive transfer that characterizes tungsten PFCs (additional vaporization, radiation and convection channels in the LM layer and vapor/plasma cloud). The experimental study and data analysis work on the fundamental characterization of LM enriched plasmoids and their associated thermal shielding regimes are the main objective of this proposal. Such plasmoids will be experimentally generated by the irradiation of prototypic LM targets with fusion relevant heat fluxes by means of both particle beam and high power laser irradiation [4]. This research on LM PFCs and their associated plasmoids attempts to explore an alternative solution that can help to handle the extreme power exhaust scenario expected in future magnetic fusion devices. The activities aim to answer scientific fundamental questions and generate basic knowledge to significantly advance in the comprehension and understanding of LM plasmoids capable of contributing to the major task of power exhaust in future fusion devices. This proposal pursues to continue a recently open research line with Sn prototypes (in which a novel LM embedded Langmuir Probe configuration has been developed [5] for LM plasmoid diagnosis), fully extend it to SnLi alloy targets and develop technological upgrades to eventually apply this research to pure lithium PFCs. The proposed works will be conducted in collaboration with world class, leading institutions in the fields of nuclear fusion and LM PFCs both in Europe (DiFFER, Netherlands) and USA (University of Illinois at Urbana-Champaign).

Project Documentation

• Memoria Cientifico Tecnica - Convocatoria 2024 "Proyectos de Generación de Conocimientos"

Main results

• In A. de Castro et al. Nucl. Fusion 2025 65 056034, the onset of a vapor-shielding regime was identified under the OLMAT particle beam irradiaton as a consequence of the formation of a local Sn plasma in front of the Sn CPS target. The plasma was characterized, and the species densities were absolutely quantified using a novel Langmuir probe (LP) configuration (directly embedded in the liquid-metal target) together with spectroscopy. A simple 1D heat-transfer model for the target, benchmarked against slab heat-conduction calculations (including evaporation and blackbody-radiation losses), enabled the identification of the vapor-shielding onset at a target temperature of approximately 1630 ± 85 K, where roughly 10% of the incoming OLMAT beam heat flux was dissipated through processes occurring in the Sn cloud. At this temperature, the plasma consisted mainly of Sn⁺ and Sn²⁺ ions (together accounting for 99% of the ionic content), with densities of approximately 1×10²⁰ m⁻³ and 2.5×10¹⁹ m⁻³, respectively.

• Next step experiments with the Sn CPS target, combined exposure to 33 MW/m², 125 ms particle-beam pulses together with ELM-like loading (500 MW/m² absorbed heat flux, 2 ms) enabled the exploration of more advanced stages of the local Sn plasma and the associated vapor-shielding regime. A quasi–steady-state target-temperature plateau was reached at approximately 1800 ± 90 K. However, the laser pulse produced a pronounced spike in the LP ion-saturation current, causing saturation of the LP electronics (amplifiers and data-acquisition system) and preventing full plasma characterization at this stage. Subsequent laser-only tests indicated that this increase in ion-saturation current was likely associated with the ejection of Sn particles and their interaction with the pre-existing plasma. Upgrades to the LP system are planned to reproduce the experiment and enable full plasma characterization during this advanced regime in the next OLMAT campaign (June 2026), in which a dedicated scan of the effect of laser transients on plasma build-up and vapor shielding is also foreseen.

• Recent analyses have also investigated the physics of these local Sn plasmas in terms of sheath potential and Sn–Sn⁺ collisionality. Experimental values of the sheath potential were compared to theoretical calculation of the considering collisionless Bohm sheath (cold ion approximation), generalized to a plasma with Sn+, Sn2+ and H+ populations and cold ion approximation. The results shown no significant deviations between theory and measurements in the studied range restricted to Ttarget≤1300 K, Sn+ density≤5∙1018 m-3 and Sn plasma fraction ≤90 % (this latter condition corresponds to a proton one xH+>10%). This experimental range was limited due to problems related to the overlapping of the electron saturation current branch with the Langmuir probe diagnostic limits (circuit, biasing power source and amplifier) System upgrade and future experiments to study denser, almost pure Sn plasmas in further vapor shielding regimes deployed at Ttarget>1700 K are ongoing.

• On the other hand, Sn–Sn⁺ collisions have been identified as plausible mechanism contributing to the partial detachment of the incoming heat flux from the target surface. Sn-Sn+ rate coefficients, cross sections and mean free paths were estimated using a Langevin capture model.This mean free path was compared to the Sn+ plasma characteristic length (obtained from the measures with a 16 channel SnII -645.3 nm- spatial array emission) in order to get a collisionality metric Estimation of Sn+ plasma characteristic length defined as: ξ_(Sn-Sn+)=(2R_p)/λ_(Sn-Sn+) that also varies exponentially with radius (same Sn plasma characteristic length, 2Rp). This analysis has shown that the entire plasmoid volume would get moderately collisional (ξ_(Sn-Sn+)>1) and the surface boundary fully collisional (ξ_(Sn-Sn+)>5) at Ttarget>1650 K and nSn+ >1.35∙1029 m-3. These are the conditions previously identified as vapor shielding onset (A. de Castro et al. Nucl. Fusion 2025 65 056034). At Ttarget>1760 K the entire characteristic plasmoid is predicted (extrapolation from measurements limited to 1700 K due to commented LP issues) to become fully collisional which approximately coincides with the experimentally observed steady-state regime (≈1775 K).

To be completed at the end of the project (taken from the final report)

Dissemination of project results (peer-reviewed publications and conference presentations)

A. de Castro, M. Reji, D. Tafalla et al., “Dynamics of tin plasmoids and vapor shielding onset from a liquid metal CPS target using ITER intra-ELM energy-range H0/H+ beams”, Nucl. Fusion, 65 (2025) 056034

References

[1] J. Linke, J. Du, T. Loewenhoff et al., “Challenges for plasma-facing components in nuclear fusion”, Matter Radiat. Extrem. 4 (2019) 056201.

[2] R. E. Nygren and F. L. Tabares, “Liquid surfaces for fusion plasma facing components - A critical review. Part I: Physics and PSI”, Nucl. Mater. Energy 9 (2016) 6.

[3] G. G. van Eden T. W. Morgan et al., “Oscillatory vapour shielding of liquid metal walls in nuclear fusion devices”, Nat. Commun. 8 (2017) 192

[4] A. De Castro, et al. “Physics and technology research for liquid-metal divertor development, focused on a tin- Capillary Porous System solution, at the OLMAT high heat-flux facility”, J. Fus. Ener., 42 (2023) 45

[5] A. de Castro, M. Reji, D. Tafalla et al., “Dynamics of tin plasmoids and vapor shielding onset from a liquid metal CPS target using ITER intra-ELM energy-range H0/H+ beams”, Nucl. Fusion, 65 (2025) 056034

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