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J. F. Parisi

Publications and source records attributed to J. F. Parisi.

At least 19 recordsLinked to original sources
arXiv2026

Scalable production of nuclear battery alpha emitters using fusion neutrons

Nuclear batteries powered by alpha decay have been deployed successfully for over 60 years, on a worldwide $^{238}$Pu supply of kilograms per year. We show that the 14 MeV neutrons of a single deuterium-tritium fusion plant can produce alpha emitter battery fuels up to tons per year, in three classes: fuels with completely new production pathways ($^{236}$Pu, $^{227}$Ac, $^{210}$Pb), fuels previously proposed whose scarce feedstock the same pathways now breed at scale ($^{232}$U, $^{228}$Th), and the established $^{238}$Pu. OpenMC simulations of actinide channels in a tokamak blanket give, per GW yr of fusion: 11 to 57 kg of $^{236}$Pu, whose chain releases 18 GJ per gram over a century, ending at stable $^{208}$Pb, plus up to 5.2 t of co-product $^{238}$Pu; up to 1.4 t of $^{231}$Pa from thorium, and, from channel with $^{231}$Pa feedstock, up to $\sim$15 t of $^{232}$U or $\sim$122 kg of $^{210}$Pb, with $^{227}$Ac produced at 21 g/yr per tonne of $^{231}$Pa. Neutron capture also upgrades $^{241}$Am to a $^{242}$Cm/$^{242m}$Am/$^{241}$Am/$^{238}$Pu blend with up to 10 times higher power density. The same $^{236}$Pu and $^{232}$U also serve as proliferation safeguards: the $^{237}$Np, $^{232}$Th, and $^{231}$Pa channel products are self-protecting, the plutonium by $^{236}$Pu and $^{238}$Pu decay heat and the 2.6 MeV gammas from $^{208}$Tl content, and similarly the uranium from its $^{232}$U. Many of these fuels ($^{236}$Pu, $^{232}$U, $^{228}$Th, $^{227}$Ac) have an order of magnitude higher power and energy density than current alpha emitters, and at human spaceflight-relevant doses the $^{227}$Ac and $^{210}$Pb chains need less shield mass than $^{238}$Pu or $^{241}$Am above a few hundred watts. Fusion neutrons could therefore enable nuclear batteries at the kilowatt to megawatt scale and unlock new possibilities for power sources requiring exceptionally high energy density.

physics.plasm-ph↗
arXiv2026

Enhanced alpha channeling with spin-polarized fuel

The nuclear spin state of deuterium-tritium (D-T) fuel sets both the D-T fusion cross section and the emission direction of the fusion-born alphas and neutrons. We show two ways that spin-polarized fuel (SPF) could enhance alpha channeling, the wave-mediated damping of alpha power onto fuel ions rather than electrons, which is predicted to increase fusion power significantly. First, the enhanced SPF cross section produces more alphas, and second, the perpendicular (to the magnetic field) bias of the alphas' kinetic energy couples more efficiently to the perpendicular-resonant channeling waves. The birth anisotropy survives slowing-down and appears as a population inversion of the bulk alpha distribution over a broad region of velocity space, so resonant alphas can drive a suitably tuned channeling wave rather than damp it. Without channeling, SPF roughly doubles the fusion power density through the cross-section boost and its temperature feedback on the reactivity, a well-known result. Our velocity-space calculations find the channeling efficiency about 1.5 times higher for vector-aligned fuel than for unpolarized fuel, and channeling raises the fusion power enhancement to three or four times as the channeling efficiency improves, provided the waves do not depolarize the fuel. A transport model of an ARC-class equilibrium with stiff critical-gradient transport gives an enhancement of 2.2, rising to 3.4 for less stiff transport and to 4.7 in the zero-dimensional model when the critical gradients rise with the hotter ions. Channeling also transports helium quickly to the divertor: at fixed pumping the core helium fraction nearly halves, and a divertor pump several times less selective for helium supports the same core helium dilution. Spin-polarized fuel thus enhances fusion power through the anisotropic alpha distribution, beyond its increase of the reactivity.

physics.plasm-ph↗
arXiv2026

Production of Nuclear Battery $\beta^{-}$ Emitters Driven by Fusion Neutrons

Nuclear batteries require radioisotopes with specific combinations of half-life, decay mode, and radiation properties, yet most candidate fuels lack scalable production routes. We show how the future availability of deuterium-tritium (D-T) fusion neutrons could enable manufacturing nuclear battery radioisotopes at many orders of magnitude higher rate than at present. We assess the capability of 14 MeV D-T fusion neutrons to produce nuclear battery radioisotopes by simulating feedstock material irradiation with neutrons. Promising radioisotope candidates include ${}^{147}$Pm, ${}^{63}$Ni, ${}^{39}$Ar, and ${}^{137}$Cs. Some feedstocks allow a radioisotope to be produced at scale while also closing the tritium fuel cycle, resulting in hundreds to over one thousand kilograms of high specific activity radioisotope per gigawatt thermal year of D-T fusion irradiation. We perform OpenMC simulations of an enriched ${}^{148}$Nd blanket for a tokamak, demonstrating that tritium self-sufficient designs can produce over one ton of ${}^{147}$Pm per gigawatt thermal year, equivalent to $\sim$one billion Curies per year of ${}^{147}$Pm. Operation of such a blanket would represent an unprecedented increase of nuclear battery radioisotope production capability.

physics.plasm-ph↗
arXiv2026

Self-subsidizing Mercury Remediation with Fusion Reactors

Fusion reactors can permanently remediate mercury by using it as a neutron multiplier: each (n,2n) reaction reduces the neutron number towards ${}^{197}$Hg which quickly decays into stable gold, irreversibly removing it from the environment while generating substantial economic value. Fusion energy is therefore not merely environmentally benign, but anti-polluting through the continuous consumption of an environmental pollutant. The history of nuclear fission demonstrates that environmental concerns can be decisive obstacles to low-carbon power deployment, suggesting that integrated pollution remediation fundamentally improves the policy calculus for fusion energy. We show that at high neutron flux (achievable in muon-catalyzed and inertial confinement fusion), nuclear reactions make all mercury isotopes eligible for gold transmutation, incentivizing mercury recovery and valuing the world mercury extractable stock at ${\sim}\$200$ trillion, exceeding all in-ground gold reserves. Co-producing gold alongside electricity can triple a fusion plant's revenue, aligning economic incentives with complete, permanent mercury remediation.

physics.plasm-ph↗
arXiv2026

Modeling Temperature Profiles in the Pedestal of NSTX with Reduced Models

This paper describes new modeling capabilities for predicting H-mode pedestal profiles in spherical tokamaks. Temperature profiles for NSTX discharges 132543 and 132588 are modeled by coupling the \textsc{astra} transport solver with neoclassical transport and gyrokinetic-based reduced models for electron temperature gradient (ETG) and kinetic ballooning mode (KBM) instabilities. A quasi-linear surrogate model for ion-scale transport is developed using linear \textsc{gene} simulations, requiring only a single free parameter calibrated to one discharge. Time-evolving the temperatures with fixed density yields good agreement with experiments for both discharges. Systematic analysis of the transport mechanisms reveals that neoclassical transport is huge across the entire pedestal region for the ion channel. ETG turbulence is large in the plasma edge and low density gradient region, contributing substantially to the electron channel. However, KBM/MHD-like modes also drive significant transport in both the ion and electron thermal channels, making them essential for accurate pedestal modeling. Further refinements, including explicit $E \times B$ shear suppression and scaled ETG transport, produce quantitative but not qualitative improvements. This work lays the foundation for predictive modeling of future devices.

physics.plasm-ph↗
arXiv2026

The Value and Cost of Fusion Neutrons

Deuterium-tritium fusion reactions produce high-energy neutrons that can transmute materials into valuable isotopes. Over the next ten years, the cost of fusion neutrons is projected to decrease by roughly seven orders of magnitude. Most ($\sim$5 orders of magnitude) is technological overhang driven by the low availability of current experiments; the remaining $\sim$2 orders of magnitude require higher plasma gain and lower capital intensity. We introduce the levelized cost of a neutron (LCON), an economic metric analogous to the levelized cost of energy that gives the minimum neutron value for economic breakeven of a fusion system. LCON depends on plasma gain, capital intensity, availability, and neutron flux, and is offset by revenue from co-produced electricity, precious metals, and radioisotopes. The revenue per neutron spans at least ten orders of magnitude, from electricity and gold ($\sim$\$$10^{-20}$/neutron) to actinium-225 ($\sim$\$$10^{-10}$/neutron), defining a `neutron ladder': a staged, revenue-positive development pathway from current fusion devices to terawatt-scale power plants.

physics.plasm-ph↗
arXiv2025

Isotope Production in Muon-Catalyzed-Fusion Systems

Producing valuable isotopes with high-flux high-energy neutrons generated by muon-catalyzed fusion ($μ$CF) reactions could substantially improve the economic prospects for muon-catalyzed fusion. Because no external heating is required for $μ$CF, heat flux constraints are significantly relaxed compared with fusion systems requiring external heating. This could allow $μ$CF to attain much higher neutron flux without breaching material heat flux limits. If muon production rates can be increased, $μ$CF systems employing transmutation could be viable well before energy breakeven is possible. For $μ$CF systems transmuting valuable isotopes, the required number of catalyzed fusion events per muon and muon energy generation cost can be relaxed by several orders of magnitude relative to electricity-generating systems, making $μ$CF an attractive high-flux neutron source. We show an example $μ$CF system with a 10 gram ${}^{226}\mathrm{Ra}$ feedstock and a steady-state muon rate of $10^{12}$ muons / second - roughly half a kilowatt of fusion power - could produce 20 mg of ${}^{225}\mathrm{Ac}$ per year - comparable to 400 times global supply in 2024. As higher muon rate sources become available, many other radioisotope transmutation pathways become viable. These findings motivate the accelerated development of $μ$CF systems for neutron-driven isotope production far before net energy generation is possible.

physics.plasm-ph↗
arXiv2025

Isotope Production in Fusion Systems

Fusion systems producing isotopes via neutron-driven transmutation can achieve economic viability well before reaching energy breakeven. Incorporating carefully selected feedstock materials in a blanket allows fusion systems to generate both electrical power and high-value isotopes, expanding the space of viable concepts, significantly enhancing the economic value of fusion energy, and supporting an accelerated path to adoption. We calculate the value of this co-generation and derive a new economic breakeven condition based on net present value. At lower plasma gain, $Q_{\mathrm{plas}}\lesssim 1$, high-value transmutation, such as medical radioisotopes, enables pure transmuter fusion systems operating at only watts to megawatts of fusion power: for example, a 3 megawatt system transmuting ${}^{102}\mathrm{Ru}\rightarrow{}^{99}\mathrm{Mo}$ could fulfill global ${}^{99}\mathrm{Mo}$ demand with $Q_{\mathrm{plas}} \ll 1$. At higher gain $Q_{\mathrm{plas}}\gtrsim 3$, it becomes viable to generate electricity in addition to isotopes. For example, co-production of electricity and gold, transmuted from mercury in a fusion blanket, can reduce the required plasma gain for economic viability from $Q_{\mathrm{plas}}\sim 10$-$100$ to $Q_{\mathrm{plas}}\sim 3$-$5$. We further highlight techniques to enhance transmutation with asymmetric neutron wall loading. Fusion neutron-driven transmutation therefore offers a revenue-positive pathway for deploying fusion energy at terawatt-scale, starting from smaller watt-to-megawatt-scale machines for radioisotope production and then scaling up to co-producing electricity and gold in larger fusion power plants.

physics.plasm-ph↗
arXiv2025

Production of High-Specific-Activity Radioisotopes Using High-Energy Fusion Neutrons

We show that transmutation driven by high-energy neutrons from deuterium-tritium (D-T) fusion reactions can produce many important medical radioisotopes - including $^{32}$P, $^{60}$Co, $^{64}$Cu, $^{89}$Sr, $^{90}$Y, $^{89}$Zr, $^{99}$Mo/$^{99\mathrm{m}}$Tc, $^{103}$Pd, $^{111}$In, $^{117}$In/$^{117\mathrm{m}1}$Sn, $^{123}$I, $^{125}$I, $^{131}$I, $^{133}$Xe, $^{153}$Sm, $^{166}$Ho, $^{177}$Lu, $^{188}$Re, and $^{192}$Ir-and emerging isotopes such as $^{47}$Sc, $^{67}$Cu, $^{103}$Ru/$^{103\mathrm{m}}$Rh, $^{103}$Pd/$^{103\mathrm{m}}$Rh, $^{119}$Sb, $^{124}$I, $^{155}$Tb, $^{161}$Tb, $^{195\mathrm{m}1}$Ir/$^{195\mathrm{m}}$Pt, and $^{225}$Ac with high specific activity and in large quantities. These reactions involve stable, abundant feedstocks and non-fission transmutation channels that change the proton number, enabling chemical separation of the product. Fusion-based transmutation could provide a flexible and proliferation-resistant platform for supply of high-purity isotopes. A D-T neutron source operating at a few megawatts of fusion power could meet or exceed global demand for most major radioisotopes. Further research is required to develop tailored approaches for feedstock processing and product extraction.

nucl-ex↗
arXiv2025

Persistence of Deuterium and Tritium Nuclear Spin-Polarization in Presence of High-Frequency Plasma Waves

We present first-principles numerical calculations of the depolarization rate of spin-polarized deuterium and tritium nuclei in realistic tokamak plasmas, driven by resonant interactions with plasma waves. Backed up by first-of-a-kind linear and nonlinear simulations, we find that alpha particle-driven Alfvénic modes cause only negligible depolarization, which is contrary to expectations in prior literature. Other Alfvénic instabilities can in principle degrade polarization, but only under conditions unlikely to be realized on transport timescales. By combining full-orbit particle tracing with a dedicated depolarization solver, we demonstrate that wave-driven depolarization is surprisingly weak in SPARC and ITER-scale devices. These results provide strong evidence that spin-polarized fuel can maintain its polarization long enough to boost fusion reactivity, opening a viable path toward substantially enhanced performance in magnetic confinement fusion power plants.

physics.plasm-ph↗
arXiv2025

Validation of the GFS model for Gyrokinetic Stability of NSTX Pedestal Data

This study presents a large database validation of the Gyro Fluid System (GFS) model for linear gyrokinetic stability for high-mode (H-mode) edge transport barrier conditions in the National Spherical Torus Experiment (NSTX) tokamak. The database of linear stability calculations with the CGYRO gyrokinetic code was produced using plasma profile measurements from NSTX discharges to identify kinetic ballooning modes (KBM), trapped electron modes (TEM), and micro-tearing modes (MTM) that limit the pressure profile gradient in the H-mode barrier. A novel Bayesian optimization approach determines optimal resolution parameters for GFS specifically for spherical tokamak pedestal conditions. Our results demonstrate that GFS, with optimized resolution, can achieve accurate linear stability analysis in NSTX pedestal conditions for reduced resolution compared to CGYRO. GFS can accurately find the KBM, TEM, and MTM instability branches. Parametric analysis reveals that GFS accuracy in this extreme pedestal parameter range is degraded for low magnetic shear and near the separatrix conditions. These findings establish GFS as a fast linear eigenmode solver for spherical tokamak pedestal gyrokinetic stability and demonstrate a systematic methodology for determining the optimum resolution settings.

physics.plasm-ph↗
arXiv2025

The impact of kinetic and global effects on ballooning 2nd stable pedestals of conventional and low aspect ratio tokamaks

The EPED model [P.B. Snyder et al 2011 Nucl. Fusion 51 103016] had success describing the pedestals of the Type-I ELM and QH-mode operations in conventional tokamaks, by combining kinetic ballooning mode (KBM) and peeling-ballooning (PB) constraints. Within EPED, the KBM constraint is usually approximated by the ideal ballooning mode (IBM) stability threshold. It has been noted that quantitative differences between local ideal MHD and gyro-kinetic (GK) ballooning stability can be larger at low aspect ratio. KBM critical pedestals are consistent with observations in initial studies on conventional and spherical tokamaks. In this work, the application of a reduced model for the calculation of the kinetic ballooning stability boundary is presented based on a novel and newly developed Gyro-Fluid System (GFS) code [G.M. Staebler et al 2023 Phys. Plasmas 30 102501]. GFS is observed to capture KBMs in DIII-D as well as the NSTX(-U) pedestals, opening a route integrating this model into EPED. Finally, high-n global ballooning modes are observed to limit the access to the local 2nd stability and thus provide a transport mechanism that constrains the width evolution with beta_p,ped. The high-n global ballooning stability is approximated by its ideal MHD analogue using ELITE. It is shown that nearly local high-n with k_y*rho_s~1/2 modes can provide a proxy for the critical beta_p,ped when a 2nd stable access exists on DIII-D plasmas. The use of GFS and ELITE scaling in EPED provided an improved agreement in comparison to EPED1 with DIII-D pedestal data.

physics.plasm-ph↗
arXiv2025

Turbulent Transport-Limited Pedestals in Tokamaks

H-mode operation of tokamak fusion plasmas free of dangerous Type 1 edge-localized-modes (ELMs) requires a non-ELM mechanism for saturating the edge pedestal growth. One possible mechanism is turbulent transport. We introduce a transport threshold model to find pedestal width-height scalings for turbulent transport-limited pedestals. The model is applied to electron heat transport resulting from electron-temperature-gradient (ETG) turbulence. The width-height scalings are highly sensitive to the relative contribution of density and temperature to the pedestal pressure. Pressure that builds up mainly through temperature is more likely to be transport-limited, and hence ELM-free. A relative radial inward shift of the temperature to density pedestal location is also more likely to transport-limit the pedestal. A second constraint such as flow shear is required to saturate pedestal growth. We also calculate width-height transport scalings resulting from particle and heat transport arising from ETG and kinetic-ballooning-mode turbulence. Comparisons are performed for ELMy and ELM-free experiments in MAST-U, NSTX, and DIII-D. This is a first step towards a pedestal width-height scaling for transport-limited ELM-free pedestals.

physics.plasm-ph↗
arXiv2025

Prediction of ELM-free Operation in Spherical Tokamaks With High Plasma Squareness

We predict that high plasma squareness in spherical tokamaks (STs) could result in edge-localized-mode (ELM)-free H-mode. The effect of squareness on gyrokinetic and peeling-ballooning-mode width-height pedestal scalings is calculated for STs. Because STs can sustain H-mode in first ballooning stability, first-stable pedestals with lower gradients may be further from the peeling-ballooning-mode boundary and therefore naturally free of Type 1 ELMs. We show that while higher squareness destabilizes ballooning modes in first stability, the ELM stability boundary is essentially unchanged. Therefore, higher squareness could result in ELM-free discharges. Random Forest (RF) machine learning models for the gyrokinetic growth rate and distance from first stability are used to predict how squareness affects stability. A RF model with only three easily obtainable geometric inputs predicts proximity to the gyrokinetic width-height scaling on a test dataset with high accuracy, $R^2 = 0.965$.

physics.plasm-ph↗
arXiv2025

Revisiting Fusion in D-${}^{3}$He Plasmas With Spin-Polarized Fuel

Spin-polarized fuel (SPF) is recognized for enhancing fusion reactivity, but it could provide other advantages particularly relevant to advanced fusion fuels. In this work, we calculate how SPF in D-${}^3$He plasmas affects not only D-${}^3$He fusion reactions but D-D fusion and subsequent secondary reactions. By incorporating multiple effects, we show how, under optimistic assumptions, the fusion power relative to unpolarized D-${}^3$He fusion power could increase by more than a factor of three by polarizing the deuterium and helium-3. Such an increase may improve the feasibility of fusion concepts using D-${}^3$He fuel. We perform a case study with a hypothetical pulsed magneto-inertial fusion device using polarized D-${}^3$He fuel, showing how the net electric power could increase by almost an order of magnitude. We also consider the potential of SPF to provide a path to fully aneutronic fusion. This work is a new look at how SPF could improve the feasibility of fusion concepts using advanced fuels.

physics.plasm-ph↗
arXiv2025

HIPED: Machine Learning Framework for Spherical Tokamak Pedestal Prediction and Optimization

We introduce a Machine Learning framework, HIPED (HeIght and width Predictor for Edge Dynamics), for predicting and optimizing pedestal and core performance in spherical tokamak plasmas. Trained on pedestal and core datasets from the third MAST-U campaign, HIPED provides accurate estimates of pedestal height and width. The results reveal notable differences compared with conventional aspect-ratio studies; for instance, a simple power-law relation between pedestal width and height has very low accuracy. Instead, additional parameters such as normalized plasma pressure, elongation, and Greenwald fraction significantly improve accuracy. HIPED can also be trained only on `control room parameters' to inform experimentalists of which controllable parameters to adjust for improving core-integrated performance. The framework further includes a multi-objective optimization scheme that helps guide experimental planning and optimization. We find Pareto-optimal discharges with respect to various features, including distance from edge-localized modes and normalized plasma pressure, track their parameter trajectories over time, and identify the control room parameters required for these Pareto-optimal discharges. This provides a framework for systematically optimizing core and edge performance according to different experimental priorities.

physics.plasm-ph↗
arXiv2025

Electric Power Enhancement using Spin-Polarized Fuel in Fusion Power Plants

Using a range of fusion power plant (FPP) concepts, we demonstrate that spin-polarized fuel (SPF) can significantly enhance net electric power output, often by many multiples. Notably, the electric power gain from SPF generally exceeds the corresponding increase in thermal fusion power. Plants close to engineering breakeven stand to benefit most, where even modest boosts in fusion power produce disproportionately larger gains in net electricity. As a representative example, a 25% increase in fusion power via SPF could allow an ITER-like device (with an added turbine to recover thermal fusion power) to achieve engineering breakeven. These findings strongly motivate the development of spin-polarized fuel for FPPs.

physics.plasm-ph↗