Pulsed Fusion Research Articles

Pulsed fast neutron yield measurements based on 79mBr activation in LaBr3(Ce)

This work aims to test the concept of an activation detector for pulsed DD fusion neutron sources, based on the production of metastable Br79m within a LaBr3(Ce) scintillator crystal via (n,n′) inelastic scattering. The pulsed neutron source employed is the NX3 Plasma Focus (PF) device operated in deuterium gas, which yields about 109 neutrons per shot. A range of D2 gas pressures, from 1 to 13 mbar are used to vary the test conditions. For the sake of comparison, a beryllium fast-neutron activation detector is used simultaneously with the LaBr3(Ce), and for each NX3 PF shot we derive neutron yield values from both Be and LaBr3(Ce) detectors, denoted YnBe and YnLaBr. The two detectors are positioned in the equatorial plane (θ=90°) of the NX3 to expose them to bursts of neutrons with energies close to 2.5 MeV, to simulate a thermonuclear DD fusion source. Overall, the shot-to-shot values of YnBe and YnLaBr obtained compare reasonably well. At each D2 gas pressures the 10-shot averaged values ⟨YnBe⟩ and ⟨YnLaBr⟩ are mostly within 10% of one another; for the worst case (10 mbar) ⟨YnLaBr⟩ is 25% higher than ⟨YnBe⟩. Overall, it is concluded that LaBr3(Ce) scintillation detectors can function as a capable and readily obtainable fast-neutron activation detector for measuring neutron yields from pulsed DD fusion sources.

Modelling and experiment to stabilize disruptive tearing modes in the ITER baseline scenario in DIII-D

The achievement of high gain, stationary conditions in a tokamak scenario aimed at producing fusion energy in the ITER Project is crucial to the demonstration that this form of energy can be used in future reactors to provide cheap and clean energy globally. Disruptions are a challenge for the fusion energy field, in particular for the ‘ITER Baseline Scenario’ (IBS), as reproduced in the DIII-D tokamak. This work shows that a solution has been found for the m= 2/n = 1 tearing modes that have consistently caused disruptions in the IBS: stable operation down to zero input torque was achieved by modifying the current density profile at the beginning of the pressure flattop and the ELM character later in the discharges, guided by previous results showing that the most likely cause of these instabilities is the current density profile. The coupling between sawteeth, n>2 modes and the 2/1 TMs is shown to not be statistically significant, nor the leading origin for the evolution towards instability. Ideal and resistive MHD modeling provide positive verification that a steeper ‘well’ in the region of the q = 2 rational surface leads to worse ideal stability, higher tearing index Δ’ and lower threshold Δ’c for resistive instabilities, consistent with the experimental results. This provides confidence that the methods used in this work can be extrapolated to other devices and applied to avoid disruptions in ITER and pulsed fusion devices worldwide.

RELAP5/Mod3.3 thermal-hydraulics characterization of the steam generator mock-up during operational transients in STEAM facility in support of the design of the DEMO WCLL BoP

The Water Cooled Lithium Lead Breeding Blanket (WCLL BB) is a key candidate for the driver blanket of the European DEMO reactor, progressing toward its Conceptual phase by the end of 2027. To assess different water and lithium-lead technologies for the WCLL BB and Balance of Plant (BoP) systems, the Water-thermal-HYDRAulic (W-HYDRA) experimental platform is under development at the ENEA Brasimone Research Centre. Among the facilities constituting the new W-HYDRA multipurpose infrastructure, STEAM is going to experimentally investigate the DEMO WCLL BoP thermal-hydraulics, focusing on the Steam Generator (SG) of the Primary Heat Transfer Systems (PHTS), to qualify its performances and suitability under its unconventional operation. The paper aims at supporting the thermal-hydraulic characterization of the Steam Generator mock-up during the sudden power variations typical of a pulsed fusion reactor. The analyzed selected scenario is the operational transient dwell-pulse-dwell, which determines high thermal cycling and correspondent high thermo-mechanical stresses on the primary side components. Two control logics with their relative drawbacks have been analyzed with a RELAP5/Mod3.3 1-D model, the first regulating the primary side average temperature, the second monitoring the minimum one. The comparison of the two systems highlighted that neither approach leads to hazardous conditions for the facility. However, while the average temperature controller is characterized by reduced thermal stresses on the components, the minimum temperature controller is characterized by higher thermal gradients for the primary loop. Both methodologies will be tested in the dedicated experimental campaign, aiming at yielding insights and evaluations concerning control strategies applicable to the DEMO reactor.

Low-temperature tensile properties of meta-stable β titanium Ti-5Al-5V-5Mo-3Cr alloy fabricated by pulsed laser powder bed fusion

Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy is increasingly used in structural airplane components, such as landing gear and fuselage, thanks to its high strength, good ductility, and low density. These components are consistently subjected to sub-zero temperatures during the flight. In this study, pulsed laser powder bed fusion (L-PBF) parameters were firstly optimized to fabricate highly dense (>99%) parts, and their effect on microstructure and microhardness was analyzed. Finally, tensile properties were evaluated under sub-zero temperature conditions and compared to the standard room ones to validate the feasibility of using Ti5553 under low-temperature environments.

Dawn of clean energy: Enhanced heat transfer, radiative cooling, and firecracker-style controlled nuclear fusion power generation system

Global climate change has become a major environmental threat and development challenge facing humanity. Controllable nuclear fusion is a globally recognized ideal solution for clean energy, but its required high-energy triggering conditions and intense energy release prevent existing technologies from achieving safe, stable, and long-term continuous operation. Here, inspired by the traditional Chinese firecrackers, we propose a pulsed fusion reaction flywheel energy storage multi-reactor relay operation to drive the steam turbine to continuously and stably generate electricity for a long period of time; meanwhile, to install cleaning rotors in the cooling medium pipeline to enhance heat exchange, and to apply radiative cooling technology on the surface of the cooling tower to improve cooling efficiency and to reduce energy consumption, thereby improving system safety and overall energy efficiency. Proposing the combination of original technologies at both the hot end and the cold end of the system, we strive to open up a new way for controllable nuclear fusion power generation.

Multimetal Research in Powder Bed Fusion: A Review.

This article discusses the different forms of powder bed fusion (PBF) techniques, namely laser powder bed fusion (LPBF), electron beam powder bed fusion (EB-PBF) and large-area pulsed laser powder bed fusion (L-APBF). The challenges faced in multimetal additive manufacturing, including material compatibility, porosity, cracks, loss of alloying elements and oxide inclusions, have been extensively discussed. Solutions proposed to overcome these challenges include the optimization of printing parameters, the use of support structures, and post-processing techniques. Future research on metal composites, functionally graded materials, multi-alloy structures and materials with tailored properties are needed to address these challenges and improve the quality and reliability of the final product. The advancement of multimetal additive manufacturing can offer significant benefits for various industries.

Exploring the Feasibility of a Power-Generating Pulsed Nuclear Magnetic Nozzle

Crewed missions to Mars and robotic missions to the gas giant planets are challenging because of the current lengthy trip times (2 years to Mars, ~20+ years to the gas giants) with current propulsion technology. These trips endanger astronauts due to the harmful effects of radiation and microgravity and represent a significant fraction of a PI’s (Principal Investigator's) lifespan for uncrewed gas giant missions. To make these trips safer and more reliable, trip times need to be reduced dramatically. Pulsed nuclear fusion propulsion systems promise to reduce these trip times down to 1-3 months for the Mars mission and 1-4 years for gas giant missions. However, widespread use of these systems is hampered by many technical factors, including efficient conversion of directed jet power for thrust and generation of input power for fusion reactor operation. To address both challenges, the present authors propose using the novel power-generating magnetic nozzle; this nozzle uses high-strength magnetic fields for thrust generation and low-strength fields for power generation. Most approaches in the literature consider the effect of either the high-strength fields or the low-strength fields but, for this work, the authors would like to show their combined effect. To address this, we use two computational tools in tandem from prior work: the Smoothed Particle Fluid with Maxwell equation solver (SPFMax) and a plasma flux compression generator code. The former will determine the effect of the high-strength fields and the latter will determine the effect of the low-strength fields. Combined, they show the effect on thrust, efficiency, and power generation. The present authors find that the inclusion of a power-generation system reduces nozzle efficiency by 7% and thrust by the same amount, however, this is a relatively small reduction. The authors also confirm prior work regarding non-dimensional scaling parameters of the power generation system. These results reduce the technical risk associated with these nozzles, hopefully allowing for their application in current concepts/programs, make interplanetary trips safer and more reliable, and allowing humanity to venture out and explore the solar system. Keywords: Mars, Plasma, Magnet, Nuclear, Power

Self-Controlled Flashing Nuclear Fusion in Stationary Magnetized Low-Temperature Plasma

The prerequisites and mechanism for the implementation of efficient pulsed (flashing) nuclear fusion in a low-temperature hydrogen plasma with a temperature of 10 to 20 eV in a constant magnetic field are considered. It is shown for the first time that the natural very frequent alternation of the processes of ionization of atoms and recombination of ions leads to the synchronous formation of coherent correlated states of hydrogen nuclei and its isotopes. The formation of such states leads to the generation of very large fluctuations of kinetic energy (up to 10 to 100 keV) at the initial stage of each ionization event, which exists for most of the lifetime of the ionized state before ion recombination. It is shown that the relatively long duration of the existence of these fluctuations and their very large amplitude are sufficient for efficient nuclear fusion in such a magnetized low-temperature plasma.

Continuum kinetic investigation of the impact of bias potentials in the current saturation regime on sheath formation

In this work, we examine sheath formation in the presence of bias potentials in the current saturation regime for pulsed power fusion experiments. It is important to understand how the particle and heat fluxes at the wall may impact the wall material and affect electrode degradation. Simulations are performed using the 1X-1V Boltzmann–Poisson system for a proton–electron plasma in the presence of bias potentials ranging from 0 to 10 kV. The results indicate that the sheath near the anode remains generally the same as that of a classical sheath without the presence of a bias potential. However, the sheath near the cathode becomes more prominent with a larger potential drop, a significant decrease in the electron density, and larger sheath lengths. The spatially constant current density increases to a saturation value with the increasing bias potential. For high bias potentials, the material choice needs to consider that the anode faces significantly larger particle and heat fluxes compared to the cathode. In general, the results trend with theory with differences attributed to the simplified assumptions in the theory and the kinetic effects considered in the simulations. Due to the significant computational cost of a well resolved 1X-2V simulation, only one such simulation is performed for the 5 kV case showing higher current.

A steady state vs pulsed fusion neutron science facility

Two major modifications to the existing steady state fusion neutron science facility (FNSF) concept (Kessel et al 2018 Fusion Eng. Des. 135 236–70) are investigated with the aim of determining whether or not its predicted performance can be improved. The modifications are high magnetic field and pulsed operation. We find that high field leads to major economic improvements in a steady state FNSF, although at the expense of lowering the engineering gain. Pulsed operation replaces the problems associated with low current drive efficiency, with hopefully more manageable engineering problems. Here, however, high toroidal field (TF) is not helpful, and a lower TF field is more desirable economically. Pulsed FNSFs also have a reduced engineering gain. Further modifications lead to FNSF designs satisfying the additional constraint of engineering gain equal to unity. For these designs there is a large cost penalty for the steady state FNSF but only a modest penalty for the pulsed FNSF. All of our modified designs show modest to large potential economic improvements over the existing design. Overall, our conclusion is that it may be desirable to carry out a more detailed analysis of one of our improved designs, the choice depending upon which issue in the existing design is most important.

Effect of Process Parameters on the Surface Microgeometry of a Ti6Al4V Alloy Manufactured by Laser Powder Bed Fusion: 3D vs. 2D Characterization

The influence of the main process parameters, laser power, point distance and time exposure, on the surface microgeometry of Ti6Al4V specimens produced by a pulsed powder bed fusion process was investigated. A 3D characterization was carried out and collected data were elaborated to reconstruct the surface and to determine both the 3D and the 2D material ratio curves along different directions. The 3D material ratio curve gives a slightly lower material ratio of peak zone Mr1 and higher material ratio of valley zone Mr2, reduced peak height Rpk and reduced valley height Rvk than the 2D curves. Roughness is greater in the 3D analysis than in the 2D one, skewness is the same and kurtosis increases from <3 in 2D to >3 in 3D. Roughness and skewness increase on increasing point distance and decreasing time exposure and laser power. Within the investigated ranges (27.3–71.2 J/mm3), an increase in energy density reduces the surface roughness while skewness and kurtosis are not significantly affected. The results indicate that a 3D approach allows better characterization of the surface microgeometry than a 2D one.

Project Design of a Pulsed D-D Fusion Neutron Source Based on Field Reversed Configuration

The fusion neutron source is of great significance for conducting material testing for future fusion reactors, as it can genuinely reflect the change of material properties under fusion neutron irradiation. The International Fusion Materials Irradiation Facility (IFMIF) is an accelerator-driven neutron source. It still has some differences from the ideal fusion neutron source in terms of fusion neutron energy spectrum, which has led to the reconsideration of the fusion neutron source approach. In this paper, the magnetic field configuration, heating scheme design, and related calculations are carried out regarding the fusion neutron source. The plasma temperature, density evolution process, and the corresponding neutron yield of the field reversed configuration (FRC) plasma after two-stage cascade magnetic compression are analyzed, and the suppression of the magnetic fluid instabilities such as tilted and rotating modes of the FRC plasma after considering the two-fluid effect and the finite Larmor radius effect are studied. The fundamental physical parameters of the fusion neutron source are finally given. The calculation results show that the neutron source is expected to obtain fusion neutrons with an annual average power density higher than 2 MW/m², which can meet the requirements of material testing of the demonstration reactors (DEMO). The power estimation also shows that the scheme has the potential to become an energy source based on the pulsed deuterium-deuterium fusion.

Physics of large-area pulsed laser powder bed fusion

The build rate of powder bed additive manufacturing could be significantly accelerated if consolidation of metal powders evolved from a serial process to a parallel process. In this work, the physics of Large-Area pulsed laser Powder Bed Fusion (LAPBF) in 316L stainless steel was studied through high speed imaging and high-fidelity physics simulations. Laser pulses were found to rapidly melt the metal powder, with subsequent fast coalescence of the melted particles into larger droplets. Conduction of heat from the molten droplets melted the substrate surface, and the molten droplets then spread out over roughly 100 μs. For the laser and metal powder parameters used in this study, layer thicknesses of greater than 40 µm resulted in uneven distribution of added material onto the substrate surface and thus an increase in porosity in multilayer prints. Simulations showed that pit features could be created (that can result in pores) from overlying powder particles shadowing the underlying substrate and blocking sufficient laser energy to deposit into the substrate. Simulations suggested that for these laser and powder parameters using thinner powder layers would reduce shadowing and allow the laser pulse to effectively heat the substrate thereby mitigating the defect formation. Implementing this change ultimately demonstrated > 99.5% density in the simulation, and > 99.8% density experimentally in 316L stainless steel prints. During the LABPF process very little material ejection was observed, a known impediment to laser powder bed fusion scaling to larger volume part production. This absence of eject a suggests that LAPBF may be able to produce material with high quality, suitable for critical applications, and scalable to high volume production.

Ultra-short pulsed laser powder bed fusion of Al-Si alloys: Impact of pulse duration and energy in comparison to continuous wave excitation

Laser assisted powder bed fusion of Al-40 wt%Si using ultra-short laser pulses is presented. The effects of the pulse duration as well as the pulse energy on the melt pool size, solidification morphology and material properties have been investigated. The experiments have been carried out with a mode-locked fiber laser system delivering pulses from 500 fs up to 800 ps at a wavelength of 1030 nm. Comparative investigations have been performed using a continuous wave Yb-fiber laser operating at 1070 nm. The results show that the melt pool width is reduced at shorter pulse durations and higher repetition rates while maintaining the same average power. Additionally, keyhole melting is achieved in pulsed operation after exceeding the threshold fluence for single pulse ablation. In comparison to continuous wave radiation, powder bed fusion using ultra-short laser pulses leads to a more uniform melt pool shape with refined primary Si and eutectic structure that is accompanied with an improvement of the mechanical properties.

Conceptual model of a solid energy storage for the pulsed demonstration fusion power plant

One of the fundamental issues of the pulse fusion power plants is the appropriate storage of thermal energy during the burn phase and idle phase to ensure the steady-state thermal and flow conditions of the coolant through the turbine. Accumulation of power into the solid heat accumulator is one of the feasible solutions, which, together with its controlled bypass, would allow to achieve steady-state coolant parameters during the burn and idle phases. This paper presents a prototypal model of the solid energy storage. The model is based on the CFD parametric calculations that determine the thermal and flow characteristics of the storage depending on its charge status.

Plasma processed tungsten for fusion reactor first-wall material

Tungsten is one of the prime candidates for a first-wall material near the divertor area due to its high temperature strength, high thermal conductivity, low erosion rate and low tritium retention. The erosion resistance of tungsten to the edge plasmas and transient events are carefully investigated in a simulated fusion environment. Here, we use the dense plasma focus (DPF) device operated in a D2 as a source for pulsed fusion plasma. The tungsten (α-W) substrates with a preferential growth direction along (110) plane were used. These pristine-W samples were nanostructurized using a (i) low-temperature continuous nitrogen RF plasma system and (ii) coated with 60 nm tungsten film, using high-temperature argon plasma in a dense plasma focus (DPF) device. The low- temperature plasma treatment created mesh-like porous nanostructure on the surface of pristine-W with change in crystalline orientation to (200), while the DPF-based deposition resulted in a nanocrystal (30–50 nm) decorated surface with enhanced (200) orientation. The crack propagation and bubble formation during DPF D2 plasma exposure were significantly controlled by the surface modification of tungsten. The mesh-like structure was modified to form loosely bound spherical nanoparticles, while the nanocrystals remained tightly bound and grew in size with D2 plasma exposure. The better adhesion of the nanocrystals and controlled growth along the (200) direction resulted in least change in hardness measurements for the nanocrystal decorated samples. Thus, nanocrystal decoration of tungsten with a preferential growth direction of (200) can help reduce the fusion-induced damage in first-wall materials.

Backwards extrapolation activation diagnostics and their dynamic range for pulsed neutron source measurements

Activation materials implanted within radiation detectors can be used to measure pulsed neutron fields. This work develops an instrument concept with the aim to maximise sensitivity to pulsed fusion neutron fields and, using a data-rejection algorithm combined with backwards extrapolation, enable neutron fluence estimates to be made over a large dynamic range. Through high-fidelity modelling of residual temporal emissions, and a parameterised approach, we study the sensitivity to neutrons of a plastic scintillator–Ag foil layer detector concept. For an optimal design we apply paralysable and non-paralysable deadtime models to the predicted response to D–D fusion neutron fields at various neutron field intensities. In high neutron fluence irradiation scenarios, where deadtime effects are strongly evident, we use our approach to make estimates of the fluence from instrument response data. We discuss the practical applications of such diagnostics used for plasma focus (PF) fusion experiments, such as at the PF-1000U facility in IPPLM, Poland, inertial confinement fusion and pulsed tokamak experiments, for example at MAST-U, where such diagnostics could complement fission counter-based neutron diagnostics in the future. Finally, we show that the calibration of such detection systems may be achieved using relatively low emission rate, steady state neutron sources, with calibration factors that are straightforward to apply to pulsed neutron field measurements.

Melt pool microstructure and morphology from single exposures in laser powder bed fusion of 316L stainless steel

Single depositions of 316L stainless steel in pulsed laser powder bed fusion were performed to understand the influence of energy density input strategy on melt pool morphology, microstructure and temperature fields. Depositions performed at equal energy inputs, but variable laser powers, produced keyhole mode depositions for laser powers ≥150 W and conduction mode depositions for laser powers below 150 W. The laser power combined to the laser exposure time was found to control the penetration depth, increasing from 50 μm to 300 μm as the laser exposure increases. An increase in laser exposure time was found to decrease the isotherm liquidus velocity, from 1.2 mm s−1 to 0.2 mm s−1. EBSD maps of deposition cross-sections highlighted the change in temperature gradients as the melt pool increased in depth during keyhole and conduction mode depositions. Strong sub-surface temperature gradients were found for keyhole mode with weak gradients above the surface. In conduction mode variation in the temperature gradient throughout the melt pool was less pronounced. The decrease in cooling rate as the exposure time is increased was highlighted in coarsening of the cell spacing produced by the solute redistribution during solidification, diverging between bottom and dome of the deposition melt pool.

Investigation on the charge-state distribution diagnosis oflaser-produced Al plasmas

Laser-produced plasmas (LPP) have been known to possess great application potential under extreme ultraviolet (EUV) lithography, soft X-ray micrography, and heavy ion accelerator, highly charged ion optical frequency standards, pulsed laser deposition, astrophysics and nuclear fusion. The applications in these fields require a clearer understanding of the evolutionary behavior of the LPP. However, LPP is neither homogeneous nor static during their expanding and cooling process, which involves complex physical processes. For a deep understanding of the LPP, many diagnostic methods and techniques have been developed, such as spectroscopy, fast imaging, time-of-flight (TOF), laser interferometry and Thomson scattering. Among these methods, spectroscopy has been the most widely used diagnostic method, which can be used to obtain relatively more state information. However, the diagnosis results of charge-state distribution obtained by spatio-temporally resolved spectroscopy are local and transient, and cannot therefore obtain the overall distribution. In order to have a more comprehensive understanding of charge-state distribution and its evolutionary behavior of the LPP, other diagnostic methods are needed. The TOF method is used for the measurement of highly charged ions of the LPP, which can be used to obtain the overall charge-state distribution information. Therefore, in this work, spectrometry and TOF method were used for the diagnosis of charge-state distribution of the LPP in a vacuum. The EUV emission spectra of laser-produced Al plasmas were measured by using spatio-temporally resolved laser-produced plasmas spectroscopy technique. The charge-state distributions with different delay times at the distance of 2.0 mm from target surface were diagnosed by spectral simulation and their evolution behavior were also analyzed. The TOF spectra of laser-produced Al plasmas were measured by the TOF method, and the overall charge-state distribution of ions in laser-produced Al plasmas was obtained. The results revealed that spectroscopy can diagnose the local and transient charge-state distribution of ions in the laser-produced plasmas, and the TOF method can be used to diagnose its overall distribution, and the two diagnostic methods therefore complement each other. This work aims to clarify the complementarity of the two methods, and to provide a method support for the comprehensive and accurate understanding of the charge-state distribution and evolution evolutionary behavior during the expansion process of the LPP, and also provides a reference for further optimization and modification of the radiation hydrodynamics model of the LPP.

Pulsed fusion grade deuterium plasma induced structural phase transformation in stainless steel using medium energy plasma focus device

Samples of ferrite -austenite stainless steel (SS) have been exposed to fusion grade plasma and radiations using a medium energy plasma focus device, ‘MEPF-12’. Three different polished samples of SS (10 mm diameter ×2 mm thick) were irradiated using single and multiple ( five and ten) plasma focus discharges. Each plasma focus discharge comprised energetic and high intensity deuterium ions, energetic deuterium plasma streams and 2.45 MeV neutrons produced via D(d, 3He)n fusion reactions. At sample position, typical values of deuterium ion fluence and neutron fluence were measured to be ∼1018ions/m2 and ∼1010 neutrons/m2, respectively, in each plasma focus discharge. Post irradiations, the samples have been characterized using different techniques for study of their surface modification characteristics, specifically, crystallinity, surface morphologies and surface roughness. The XRD analysis of the SS sample exposed to single or more plasma focus discharges revealed structural phase transformation from initial mixed ferrite [α(BCC)] -austenite [γ(FCC)] phase to austenite phase. Effects of deuterium ion concentration and transient rise in surface temperature on structural phase transformation have been discussed in detail. Surface morphologies studied using SEM has revealed erosion, melting and recrystallization of surface layer along with various other defects like pores, blisters, open bubbles and so on. Measurements using surface profilometer confirmed increase in surface roughness of the samples on irradiation. Detail discussion on the variation in the surface damage features with increasing number of plasma focus discharges used for irradiating samples and its comparison with virgin sample have been included in this report.