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

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 entier 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) ==