Liquid Metal Target Research Articles

Development of liquid metal targets for high-repetition intense laser and pulsed-power discharge applications

In this study, we investigate the response of linear liquid metal flow to a magnetic field when used as a load for high-repetition pulsed-power discharges and as a sheet for a laser target. When a magnetic field was applied perpendicular to the liquid metal flow, the flow deformed while maintaining its thickness. This approach may reduce current and enhance light sources in Z-pinch and alternative X-pinch discharges. However, low-frequency fluctuations were more prominent when the magnetic field was applied parallel to the liquid metal flow in sheet form. These findings suggest that combining liquid metal flow with a magnetic field could effectively facilitate high-repetition pulsed-power discharges and laser operations.

CFD Investigations on Heavy Liquid Metal Alternative Target Design for the SPS Beam Dump Facility

This study introduces numerical advancements in an alternative design for the Super Proton Synchrotron (SPS) Beam Dump Facility (BDF) at the European Laboratory for Particle Physics (CERN). The design envisions a high-power operation target made of flowing liquid lead. The proposed BDF is a versatile facility for both beam-dump-like and fixed-target experiments. The target behavior is studied, assuming a proton beam with a momentum of 400 GeV/c, a pulse frequency of 1/7.2 Hz, and an average beam power of 355 kW. Using various Computational Fluid Dynamics (CFD) codes, we evaluate the behavior of liquid lead and predict the thermal stress on the target vessel induced by the pulsed heat source generated by the charged particle beam. The comparison increases the reliability of the results, investigating the dependencies on the CFD modeling approach. The beam is a volumetric heat source with data from the beam-lead interaction simulations provided by the European Laboratory for Particle Physics and obtained with a Monte Carlo code. Velocity field and stress profiles can enhance the design of the lead loop and verify its viability and safety when operated with a liquid metal target.

Future ESS upgrade to Medium Pulse Length – What are the technical challenges for the accelerator and target?

A compression of the ESS proton pulse from the present 2.86 milliseconds to a medium pulse length of a few tens of microseconds which is better matched to the moderator time-constant of thermal neutrons would considerably boost the performance for many instruments at ESS. Generating such a proton pulse with preserved instantaneous beam power requires a storage ring to be added to the ESS accelerator. Such a ring has been studied within the ESSnuSB neutrino super-beam study. The proton pulse length extracted in single turn extraction from this ring would be approximately 1 microsecond long which could be destructive for the present ESS target and is very short compared to the moderator time constant. The more desirable medium length pulse could possibly be generated by multi-turn extraction and a new concept using a cyclotron like extraction scheme in a synchrotron or a FFA. Another way to generate the longer pulses is to extract a bunch train using fast strip line kickers but this would require a larger storage ring. Using a “bunch train” has been successfully applied at the CERN ISOLDE facility to avoid destruction of sensitive liquid metal targets used for Nuclear Physics experiments. Other challenges are linked to the injection into the storage rings and the understanding of the target, moderator and neutron extraction systems with short and medium pulse length.

Towards Integrated Target–SOL–Core Plasma Simulations for Fusion Devices with Liquid Metal Targets

Self-healing liquid metal divertors (LMDs) based on the Capillary Porous Structure (CPS) concept are currently being considered among the possible solutions to the power exhaust problem in future fusion reactors. Indeed, the passive replenishment of the plasma-facing surface by capillary forces and the self-shielding of the target via vapor emission can potentially improve the divertor lifetime and its resilience to transient loads. On the other hand, the LMD target erosion can be significant due to evaporation and thermal sputtering, on top of physical sputtering, possibly leading to unacceptable core plasma dilution/power losses (for a low-Z/high-Z metal such as Li and Sn, respectively). For this reason, it is necessary to assess whether an LMD is compatible with an European DEMO (EU-DEMO) plasma scenario. This requires a self-consistent model of the impurity emission from the target, the plasma in both the scrape-off layer (SOL) and the core regions and the transport of impurities therein. In this paper, an an integrated modelling approach is proposed, which is based on SOLPS-ITER and includes its coupling with a target erosion model written in FreeFem++ and a core plasma model (ASTRA/STRAHL). An application of the coupled SOL-target model to simulate experiments performed in the Magnum-PSI linear plasma device with a CPS target filled with Li is also included to provide a first demonstration of the capabilities of the approach. Results are promising, being in good agreement (within a few degrees) with the measured target temperature distribution. In perspective, the modelling framework presented here will be applied to the EU-DEMO with an Sn divertor.

X-band RADAR Reflected Signal Measurement of Gallium-based Liquid Metal

RADAR(Radio Detection and Ranging) is an important system for surveillance and reconnaissance by detecting a reflected signal which obtains the range from the radar to the target, and the velocity of the target. The magnitude of the reflected signal varies due to the radar cross section of the target, characteristic of the transmission and reception antenna, distance between the radar and the target, and power and wavelength of the transmitted signal. Thus, the RCS is the important characteristic of the target to determine if the target can be observed by the RADAR system. It is based on the material and shape of the target. We have measured the reflection signal of a simple square-shaped (20 × 20 cm) target made of a new material, a gallium-based liquid metal alloy and compared that of well-known metals including copper, aluminum. The magnitude of reflected signal of the aluminum target was the largest and it was 2.4 times larger than that of the liquid metal target. We also investigated the effect of the shape by measuring reflectance of the F-22 3D model(~1/95 ratio) target covered with/without copper, aluminium, and liquid metal. The largest magnitude of the reflected signal measured from side-view with the copper-covered F-22 model was 2.6 times greater than that of liquid metal. The reflectance study of the liquid metal would be helpful for liquid metal-based frequency selective surface or metamaterials.

Optical Measurements of the thickness of the Gallium Indium free surface jet for the SARAF beam dump and neutron source

The Soreq Applied Research Accelerator Facility (SARAF) is a proton/deuteron RF superconducting linear accelerator. Phase II of the project is underway and includes the development of the accelerator to its final specifications: energy of 40 MeV proton/deuteron, and a current of up to 5 mA. A beam dump and a neutron source will be required for the commissioning stage, daily operation, and basic physics experiments. A liquid metal target based on a high-velocity windowless gallium–indium jet has been designed to absorb a beam of 200 kW by generating a stable 6 mm-thick film flowing at a velocity of up to 5 m/s into a concave supporting wall (nozzle). The main goal of the present research was to demonstrate that the thickness and variations of the Ga–In flow satisfy the design requirement in the area corresponding to the beam footprint. This research also presents a novel method for optical non-contact measurements of the liquid Ga–In thickness by using a chromatic confocal sensor. The advantage of our system is that it uses an “off the shelf” non-expensive product, non-contact, very accurate, and with very high resolution.The results show that the average Ga–In thickness was 5.6 ± 0.3 mm at a flow speed of 5 m/s. These results are in agreement with the expected thickness of the nozzle.

Mechanical DesignVerification for the moderated neutron source at SARAF

The accelerator-driven thermal neutron source (TNS) at SARAF is under the advanced design stage, to provide an accelerator-based substitution to existing neutron imaging and diffraction utilities of the IRR1 aging research reactor. A challenging task of the system design is to provide a safe and maintainable coupling of 40 MeV deuteron beam and a 200 kW power liquid metal target with a compact light-water moderator and reflector assembly. The verification process of the mechanical design is performed through Monte-Carlo simulations over the detailed CAD models. We outline the method of verification of neutron production and safety requirements over the detailed system design and highlight the advantages of this approach.

Deposition of Ga2O3 thin films by liquid metal target sputtering

This paper reports on the deposition of amorphous and crystalline thin films of Ga2O3 by reactive pulsed direct current magnetron sputtering from a liquid gallium target onto fused (f-) quartz and c plane (c-) sapphire substrates, where the temperature of the substrate is varied from room temperature (RT) to 800 °C. The deposition rate (up to 37 nm/min at RT on f-quartz and 5 nm/min at 800 °C on c-sapphire) is two to five times higher than the data given in the literature for radio frequency sputtering. Deposited onto unheated substrates, the films are X-ray amorphous. Well-defined X-ray diffraction peaks of β-Ga2O3 start to appear at a substrate temperature of 500 °C. Films grown on c-sapphire at temperatures above 600 °C are epitaxial. However, the high rocking curve full width at half maximum values of ≈2.4–2.5° are indicative of the presence of defects. A dense and void-free microstructure is observed in electron microscopy images. Composition analysis show stoichiometry close to Ga2O3 and no traces of impurities. The optical properties of low absorptance (<1%) in the visible range and an optical band gap of approximately 5 eV are consistent with the data in the literature for Ga2O3 films produced by other deposition methods.

Nuclear Analysis of High-Power LIEBE Molten Target at CERN for the Production of Radioisotopes

To enhance the production of short-lived isotopes, higher beam powers are sought, which require targets able to accommodate them. One such target prototype is a liquid metal target LIEBE, developed at CERN. In this paper, a simulation of the proton beam interaction with the LIEBE target is presented. Simulations were performed by a series of proton transport calculations using the MCNP Monte Carlo code. The latest LIEBE target MCNP input was created in high-fidelity geometry, and the FENDL-3.1 cross-section data library was used. Flux and dose rate maps in the LIEBE target obtained from the simulations are presented in the paper. The maximum obtained dose around the target is roughly 361 Sv/h for gamma rays and 214 Sv/h for neutrons. The 70 MeV–100 µA proton beam penetrates roughly 7 mm deep into the liquid eutectic lead–bismuth. Based on this, further required changes to the LIEBE target can be evaluated.

Conceptual design of reciprocating probes and material-testing manipulator for tokamak COMPASS Upgrade

Three new in-vessel manipulators are designed and built for the new COMPASS Upgrade tokamak with uniquely high vessel temperature (250–500 °C) and heat flux density (perpendicular to divertor surface q ⊥ ∼ 80 MW/m2 and q ∥ ∼ GW/m2 at separatrix), which challenges the edge plasma diagnostics. Here we show their detailed engineering designs supported by heat conduction and mechanical models. Deep reciprocation of electrostatic probes near the separatrix should be possible by optimizing older concepts in (a) the head and probe geometry, (b) strongly increasing the deceleration up to 100× gravity by springs and strengthening the manipulator mechanical structure. One reciprocates close to the region of edge plasma influx (the outer midplane), the other at the plasma sink (between the outer divertor strike point and X-point), for studying the plasma divertor (impurity-seeded) detachment and liquid metal vapor transport. Both probe heads are equipped with a set of ball-pen and Langmuir probes, measuring reliably and extremely fast (10−6 s) local (1 mm resolution) plasma potential, density, electron temperature and heat flux and even ion temperature with 10−5 s resolution. The divertor manipulator (without reciprocation) will place various material test targets at the outer divertor. Unique will be its capability to increase 15× the surface heat flux with respect to the surrounding tungsten tiles just by controllable surface inclination of the test targets. We plan to test liquid metal targets where such inclined surface was found critical to achieve the desired mode with lithium vapor shielding. Even in the conservative expected performance of COMPASS Upgrade, we predict to reach and survive the EU DEMO relevant heat fluxes.

Flow and heat transfer simulation in the ADS Dense Granular-flow Target using the Material Point Method

The Dense Granular-flow Target (DGT) proposed by Chinese Academy of Sciences has great potential compared with solid target and liquid metal target. In DGT, the number of metal grains is up to ten million. To study the flow and heat transfer behavior of such a large number of particles, experiments and Discrete Element Method (DEM) simulations are extremely expensive. Therefore, this paper focus on develop a continuum approach suitable for simulating the flow and heat transfer behavior in DGT. This approach is based on the Material Point Method (MPM) and chose the μ(I) rheology as the constitutive model for granular material. It can be concluded from this work that the μ(I) rheology implemented in MPM is able to reproduce quantitatively the granular behavior in hopper flow. The velocity distributions calculated by MPM are in good agreement with those calculated by DEM, although the fluctuation information in particle scale is filtered. The temperature results of MPM are basically consistent with DEM in mesoscopic scale, which is a great improvement compared with the results from uniform velocity estimation. The influences of the effective thermal conductivity, proton current and flow rate on the temperature field are investigated. According to the MPM calculation, the maximum proton current that the target can withstand and the minimum velocity that the granular needs to be met can be estimated quickly. The MPM procedure developed in this paper would be of considerable help in predicting the flow and heat transfer behavior for DGT.

Bubble formation in liquid Sn under different plasma loading conditions leading to droplet ejection

Liquid metals have been proposed as potential divertor materials for future fusion reactors, and surface stability is a vital requirement for such liquid metal divertors (LMDs). Capillary porous structures (CPSs) have been applied to the design of liquid metal targets as they can avoid MHD instability by surface tension and provide a stable liquid surface. However, our previous work has found that liquid Sn surfaces can be very unstable in hydrogen plasma even in cases without magnetic fields. To increase our understanding of the interaction of liquid Sn surfaces with plasmas, in this work we systematically investigated the surface behaviors of liquid Sn in different plasma exposures in linear plasma devices, either in Nano-PSI at low flux and without magnetic field, or in Magnum-PSI with strong magnetic field strength. Surface instability leading to droplet ejection has been observed and recorded in the experiments. The ejection of droplets is not dependent on magnetic fields and plasma currents, and is found to be dependent on the plasma species and plasma flux and surface temperature. The CPS meshes applied in the experiments cannot completely avoid droplet ejection but can decrease droplet size and lower droplet production rate. In H plasma, droplets were observed once Sn melted even at low fluxes. For the case of N plasma, the appearance of droplets started at a temperature marginally higher than tin–nitride decomposition temperature. Only at high fluxes (∼1023–24 m−2 s−1) and high temperatures (900–1000 °C) were a few droplets observed in Ar or He plasma. For all cases, the ejection velocities of most droplets were around 1–5 m s−1. Bubble formation, growth and bursting in the plasma-species-supersaturated liquid Sn is proposed as the primary mechanism for the ejection of droplets. Plasma-enhanced solubility is responsible for the achievement of H/N-supersaturated liquid Sn, while high plasma flux implantation is responsible for Ar/He-supersaturated liquid Sn. Once the concentration of plasma species in liquid Sn reaches a certain supersaturation level, nucleation and growth of bubbles occur due to the desorption of dissolved plasma species from the liquid Sn. The formation and bursting of bubbles have been directly observed in the experiment. The sizes of most bubbles were estimated in the range of 40–400 μm or even smaller. A bubble growth model based on Sievert’s and Henry’s laws is invoked to describe bubble growth in liquid Sn.

Structural and ultraviolet photo-detection properties of laser molecular beam epitaxy grown GaN layers using solid GaN and liquid Ga targets

GaN epitaxial layers have been grown on sapphire (0001) substrate by laser molecular beam epitaxy technique using laser ablation of polycrystalline GaN solid and liquid Ga metal targets in the ambient of nitrogen plasma. In-situ reflection high energy electron diffraction and exsitu atomic force microscopy observations revealed that GaN growth using solid target yields rough surface under three-dimensional growth mode while a flat surface GaN is obtained using liquid Ga target. From X-ray rocking curve measurement, it is also observed that the GaN layer grown using solid GaN target has a relatively better structural quality. X-ray photoelectron spectroscopy confirmed Ga–N bond formation and near-stoichiometric composition of the GaN epilayers. The influence of threading dislocation density on the ultraviolet (UV) photoresponse properties of GaN layers have been studied using metal-semiconductor-metal (MSM) based device structure. It is found that the GaN MSM structure obtained using solid GaN target with lesser screw and dislocation densities exhibits a higher responsivity with fast response and recovery time.

The EU strategy for solving the DEMO exhaust problem

Exhaust of power and particles is crucial for the DEMO device and the EU has developed a strategy to address the challenges. This strategy consists of a conventional approach based on extrapolation of the ITER solution (detached lower single null divertor) as well as the development of alternatives as risk mitigation. These comprise alternative magnetic divertor geometry, liquid metal targets and intrinsically ELM-free operational scenarios. On the experimental side, the EUROfusion programme has initiated both upgrades to existing linear and toroidal devices as well as plans to engage in new devices presently under construction in the EU. In parallel, the theory and modelling efforts are ramped up in a targeted effort to obtain the necessary understanding for safe extrapolation to DEMO. This is especially important for the alternatives, which cannot be tested in ITER.

Modeling of COMPASS tokamak divertor liquid metal experiments

Two small liquid metal targets based on the capillary porous structure were exposed to the divertor plasma of the tokamak COMPASS. The first target was wetted by pure lithium and the second one by a lithium-tin alloy, both releasing mainly lithium atoms (sputtering and evaporation) when exposed to plasma. Due to poorly conductive target material and steep surface inclination (implying the surface-perpendicular plasma heat flux 12–17 MW/m 2 ) for 0.1–0.2 s, the LiSn target has reached 900 °C under ELMy H-mode. A model of heat conduction is developed and serves to evaluate the lithium sputtering and evaporation and, thus, the surface cooling by the released lithium and consequent radiative shielding. In these conditions, cooling of the surface by the latent heat of vapor did not exceed 1 MW/m 2 . About 10 19 lithium atoms were evaporated (comparable to the COMPASS 1 m 3 plasma deuterium content), local Li pressure exceeded the deuterium plasma pressure. Since the radiating Li vapor cloud spreads over a sphere much larger than the hot spot, its cooling effect is negligible (0.2 MW/m 2 ). We also predict zero lithium prompt redeposition, consistent with our observation.

Design and Testing of Advanced Liquid Metal Targets for DEMO Divertor: The OLMAT Project

In a future fusion reactor like DEMOnstration reactor (DEMO) one of the main concerns is the handling of the power exhaust from the plasma, especially at the divertor. The expected power loads cannot easily be handled by traditional armor solutions based on solid materials like tungsten, especially when the effect of intense neutron bombardment is also considered. Interest in armor concepts based on liquid metals has been subsequently on the rise, as they prove to be more resilient against high, fast power loads and neutron bombardment. However, engineering solutions for those concepts are very complex, and need to be tested. For this purpose, Optimization of Liquid Metal Advanced Targets project (OLMAT) has been envisaged. The project will use the Neutral Beam Injection of the TJ-II stellarator to irradiate liquid metal targets with power densities (neutrals plus occasionally ions) relevant to DEMO steady state operation, in the range of 20 MW/m2. OLMAT design will allow a series of experiments that other divertor simulator devices cannot easily perform: in-situ measurements of hydrogen retention, redeposition, vapor shielding, material fatigue, dust and precipitates effects, etc. Moreover, a high-power fiber laser will be used to simulate Edge Localized Modes in a small area, or to simulate the strike point power deposition profile.

Shielding of liquid metal targets in plasma of linear devices

We apply a 0D model of shielding for simulations of liquid Li exposure experiments in He plasma of the Magnum-PSI linear device. The model accounts for all the most essential processes in the vapor cloud and plasma–surface interactions. The simulation results are in good agreement with the target surface temperature measurements. A factor of 10 increase in the erosion flux leads to a 100 °C decrease in the surface temperature, still giving a reasonable agreement with experiments. Therefore, the temperature measurements are inconclusive with respect to determining whether enhanced erosion takes place or not. We observe that only 10% of the eroded material is redeposited, while several other works suggest up to 99% redeposition. We show that low redeposition is a consequence of the low electron temperature, about 0.3 eV, in the vapor cloud and that the ratio of the ionization length to the transverse cloud size is the critical parameter to look at. Hence, the redeposition factor is not a universal quantity and can vary in quite a broad range depending on the plasma parameters and machine geometry.

Cavitation-induced erosion in liquid metal spallation targets – A comparison between a predictive method and measured erosion damage to stainless steel target modules at the Spallation Neutron Source

Neutrons are produced for neutron scattering instruments at the Spallation Neutron Source by bombarding liquid mercury with a pulsed high-energy proton beam. The liquid mercury target material flows through a replaceable stainless-steel target vessel and cavitates during operation due to the extremely high heating rate caused by each proton pulse. Cavitation-induced erosion damage to the mercury-facing surfaces of target vessels is a key concern for target lifetime and has been the subject of considerable research. While numerous experiments have studied and modeled the progression of cavitation erosion damage to liquid metal target containers, direct quantitative measurements of actual erosion of a spallation target vessel were previously not available. Recently, the erosion damage to multiple target vessels operated at the Spallation Neutron Source was measured using a high-resolution laser scanning system, which provided an opportunity to study the efficacy of a cavitation damage progression model. In this paper, a cavitation progression modeling procedure developed using empirical observations is compared to measurements of samples from target vessels after operation. The strengths and limitations of the technique are discussed and methods for improving future modeling and predictive accuracy are proposed.

High-density convergent plasma sputtering device for a liquid metal target using an unheated glass plate.

A high-density convergent plasma sputtering device has been developed for a liquid metal target, using an unheated glass plate. The convergent magnetic field lines, which are produced by an external solenoid coil and a permanent magnet positioned behind the liquid metal target, effectively transport high-density plasmas near the target. In this study, a liquid gallium target was sputtered with nitrogen plasmas, without additive gas required for depositing gallium nitride films on the unheated substrates. The deposition rate of the GaN film was estimated at ∼13 nm/min at a gas pressure of 0.2 Pa. A strong diffraction peak was observed along the GaN (002) axis, with the use of an unheated glass plate and a target-substrate distance of ∼45 mm.

Numerical flow simulation of the neutron source SINQ of PSI

The Swiss spallation source, SINQ, is in operation since 1996. Since its start-up a constant target development program has been pursued, which culminated in the operation of the liquid metal target MEGAPIE. However, the work-horse target consists of a bundle of Zircaloy-II rods filled with Lead, the so-called Cannelloni target. As the high intensity proton accelerator, HIPA, at the PSI constantly delivers higher beam currents to SINQ it is important to perform numerical simulations to understand the response of the target to the power deposition and to ensure the proper D2O cooling of the Cannelloni rod bundle. Steady-state 3D fluid simulations of the heavy water coolant flow are coupled with the heat source terms in the Cannellonis. The source terms are described as UDFs (User Defined Function) for all Cannellonis with its Zircaloy-II tube and Lead filling. The energy deposition functions for the heat source terms are calculated using MPCNX 2.7.0. Major results from the CFD simulations are the flow structure with the resulting pressure field and the temperature distribution in the target. The results show good agreement with respect to the measured values during operation of SINQ. Additionally the local convective heat transfer coefficients (HTC) are calculated.