A method is presented for inferring the deuterium fuel ion temperature from neutron counts measured with fast liquid scintillators in conditions where the ion velocity distribution is Maxwellian. Local neutron count rates at each scintillator position are combined to estimate total neutron yield from the plasma, where absolute detection efficiency is determined via Monte Carlo N-Particle (MCNP) neutron scattering simulation based on a three-dimensional model of the experiment structure. This method is particularly advantageous for magnetized target fusion applications as it yields a time-resolved diagnostic and does not require a direct line of sight to the plasma or collimation of the neutrons. The instrumentation configuration, pulse-shape discrimination and pile-up correction algorithms, detector calibration, and ion temperature calculation method with uncertainty characterization are discussed. An application of the method to General Fusion’s Plasma Injector 3 spherical tokamak device is demonstrated, and the results are compared to an ion Doppler spectroscopy ion temperature diagnostic.

Compact nuclear fusion reactors will require advanced plasma-facing components capable of withstanding the higher heat and neutron fluxes characteristic of these machines. Liquid metal walls, based for instance on lithium, have emerged as a promising alternative to solid walls. This study investigates lithium redeposition on plasma-facing walls in a deuterium plasma environment using a self-consistent 1D3V Particle-in-Cell code. The model accounts for the interplay between lithium evaporation and the electric potential with a simplified but relevant physics. Four key parameters were analyzed for their impact on lithium redeposition: the ratio γs of the Li+ outflux over the D+ influx, the magnetic field inclination θ, the ionization mean free path Λiz, and the magnetic field intensity B. Results indicate that γs and θ significantly influence redeposition, leading to redeposition rates higher than 95%, while variations in Λiz and B had secondary effects. In general, an increase in one of these parameters drives to a lower redeposition, except for θ, where potential “bumps” formed under specific conditions, alters ion trajectories and results in a non-monotonic redeposition dependency. The role of lithium in flattening the potential profile and its evaporation rate highlight the need to take Li impurity into account in the calculation of the potential profile.

Silicon clathrates are crystalline, cage-like silicon allotropes with potential for unique optoelectronic applications. Here, we report a novel discovery in solid-state hydrogen storage using low-sodium type II silicon clathrate films that retain molecular hydrogen under ambient temperature and pressure. Hydrogen was introduced via deuterium plasma at moderate temperatures, forming D2 molecules within clathrate cages. The structure remains essentially intact, with minimal conversion to diamond-cubic silicon after incorporation and thermal release. Supporting evidence shows that only a small fraction of the incorporated deuterium forms SiD or NaD bonds, while the majority remains as molecular D2. Thermal desorption measurements indicate that most deuterium is released below 200 °C. This work introduces a fundamentally new storage mechanism based on molecular encapsulation rather than surface binding or chemisorption. Our findings establish silicon clathrates as the first known solid-state silicon-based material to stably store molecular hydrogen at ambient conditions and point the way toward capacity enhancement.

Abstract The required heating power, P LH , to access the high confinement regime (H-mode) in nitrogen seeded deuterium plasmas is investigated at ASDEX Upgrade. It is shown that a stepwise increase of the nitrogen seeding level increases P LH monotonically by up to a factor of two with respect to the unseeded reference. At the highest seeding rates a stable plasma discharge from a detached low confinement mode (L-mode) to a detached H-mode without interrupting the divertor detachment during the L-H transition is achieved. The results suggest that the impurity seeding level required for continuous divertor detachment, as it is mandatory for a reactorscale fusion plasma, inevitably leads to a substantial increase in P LH . Therefore, the impact of required impurity seeding for detachment purposes on P LH must be taken into account for future reactor-scale plasma scenarios.

This study uses a one-dimensional two-temperature magnetohydrodynamic model to simulate the Z-pinch process in deuterium plasma. The onset time of viscous effects is determined using the Reynolds number. The energy evolution and temperature profile during the implosion process, with and without viscosity, are systematically compared. Results reveal that viscosity significantly influences system implosion performance during the later stages of Z-pinch, particularly after 38.4 ns in the reflected-shock and expansion stages. Specifically, viscosity reduces the implosion speed and shock propagation velocity. It smooths the density distribution and delays the maximum compression time by approximately 3.3 ns. Additionally, viscosity enhances the conversion of kinetic energy to thermal energy, raising the peak temperature of electrons while lowering temperature of ions.

Thermalization times of light ions in fusion plasmas are calculated using quantum and semiclassical dielectric models for the interaction of external particles with plasma electrons and ions. An accurate analytical approximation is obtained, which combines a classical Bohr-type description for the interaction with plasma ions with a quantum description for the interaction with plasma electrons. Scaling laws for thermalization times are found for different light ions in a characteristic range of intermediate energies of special interest for tokamak systems composed of pure deuterium and deuterium-tritium plasmas. The study covers a range of temperatures between 10^{7} and 10^{9}K and electron densities from 10^{13} to 10^{15}cm^{-3}. A useful fitting formula for the thermalization time is proposed and applied to the cases of plasma heating using energetic deuterium beams and the thermalization of alpha particles produced by D-T fusion reactions. Finally, a set of useful tables with reference values for these cases is provided.

Abstract At the National Institute of Fusion Science (NIFS), the deuterium plasma experiment was conducted using the large helical device (LHD) from 2017 to 2022 for the high performance of the plasma experiment. Through this experiment, a small amount of tritium was produced by D-D fusion reaction and released to the atmospheric environment through the stack. To understand the impact of tritium on the environment, environmental tritium monitoring was conducted before, during, and after the experiment for public acceptance and in accordance with local governments. From this monitoring, no impacts were observed on monthly precipitation and pine needle samples at the NIFS site. As the result of a comprehensive assessment combined with atmosphere and environmental water monitoring, it was concluded that the impact of discharged tritium from the stack of LHD to the surrounding environment would be none and/or negligibly small.

Abstract The neutron emission spectra associated with fusion reactions contain valuable information on both the bulk plasma and fast-ion behaviour before the reaction, including information, e.g., energy and momentum distribution, on another fusion product after the reaction, e.g., α-particle for T(d,n)α reaction. Anisotropic fuel-ion velocity distribution functions owing to anisotropic beam injection and/or recoil component (referred to as the knock-on tail) production via nuclear elastic scattering (NES) cause anisotropic deviation of the neutron emission spectrum from the typical Gaussian distribution function. In this investigation, the anisotropic slowing-down distribution functions of the deuterium beam (injected tangentially or perpendicularly to a toroidal magnetic field) and the recoil deuterons produced by NES when a tangential hydrogen beam was injected were evaluated using a 3D particle trajectory simulation, assuming JT-60SA relevant deuterium plasma. Using the obtained deuteron distribution functions, the double-differential neutron emission spectra from D(d,n)3He reactions were evaluated as the average value within the volume enclosed by magnetic flux surfaces as a function of the poloidal radius. The neutron currents that reached the neutron spectrometer per unit surface and time were estimated in several directions from the double-differential neutron emission spectra. Moreover, the neutron spectra for various directions of sight were obtained. It was demonstrated that the neutron spectrum from a Gaussian distribution, resulting from knock-on tail formation in the deuteron distribution function due to the injection of a 500 keV hydrogen beam, falls within the measurable range for the JT-60SA deuterium plasma, despite suitable plasma conditions being prepared.

Abstract To ensure a robust tokamak start-up, electron cyclotron heating (ECH) assisted plasma initiation has been proposed. In this study, a zero-dimensional (0D) model studying tokamak start-up assisted by ECH has been redeveloped. To validate the reliability of the model, an ohmic start-up discharge in deuterium plasma with carbon and oxygen impurities has been simulated. The results were benchmarked against a reference model and showed favorable agreement. Based on the established 0D model, it has been enhanced by the integration of the ECH module for studying ITER start-up discharge. Compared with pure ohmic heating, the simulation results demonstrate that ECH assisted start-up not only achieves plasma breakdown more efficiently, but also significantly enhances key parameters, increasing the electron density by 9% and plasma current by 50% under the same simulation time. An analysis has also been conducted regarding the influence of ECH on the ionization states of impurities. It was found that carbon and oxygen impurities could achieve higher ionization states at the same gas pressure with ECH-assisted discharge, consequently prompting a faster impurity burn-through. In addition, the ITER start-up has been conducted under different injected ECH power. The results demonstrate that overall energy balance remains favorable for the discharge despite the increased loss related to impurities under higher injected ECH power, and the ECH power absorption efficiency is also improved.

Exposure of Reduced Activation Ferritic/Martensitic Steels to Deuterium Plasma: a Review of Data on Surface Modification and Deuterium Diffusion and Accumulation

Effect of ion energy and flux during deuterium plasma exposure of displacement-damaged tungsten

JT-60SA is currently the world's largest superconducting tokamak.It accelerates the realization of fusion energy by supporting ITER exploitation and complementing it in addressing key physics and engineering issues for DEMO reactors.The main targets of JT-60SA are breakeven-equivalent high-temperature deuterium plasmas and high-β steady-state plasmas.To achieve these goals, laser-aided diagnostics with high spatial resolution are essential for detailed physics studies.In Operation Phase 1, a tangential single-chord two-color CO2 laser interferometer was installed.The data acquisition and processing system featured fringe-jump detection and real-time processing, enabling density feedback control.In the upcoming operational phases, a CO2 laser polarimeter and Thomson scattering diagnostics will be implemented in Operation Phase 2, followed by the installation of phase-contrast imaging in Operation Phase 3.This paper summarizes the measurement results from the first operational phase and outlines the design and current status of the other three diagnostics.

In this study, the pre-ionization technique, achieved by applying an optimized shunt resistor, was employed on a Dense Plasma Focus (DPF) device to investigate the effects of characteristics such as the uniformity and velocity of the current sheath layer, as well as the repeatability and reproducibility of a high-quality deuterium plasma column on neutronic yield. Results obtained from a pair of identical magnetic probes, positioned in two different geometries relative to the current sheath layer configuration, demonstrated that this technique not only reduced system impurities and increased the current sheath velocity but also facilitated a smoother and slower transition of the current sheath layer from the breakdown phase to the axial phase by reducing non-homogeneities at both the 0^∘ and 180^∘ azimuthal positions of the layer. Further analysis of the frequency properties of the current sheath layer using Fast Fourier Transform (FFT) and wavelet time-frequency analyses showed that, despite the production of similar frequencies in the system, the pre-ionization technique contributed to a more homogeneous and uniform distribution of these frequencies throughout the entire time range — from the moment of capacitor bank discharge to the end of the axial phase. As a result, the high-quality plasma column improved repeatability and reproducibility, due to increased density and temperature, which contributed to a higher neutronic yield during the pinch phase.

Effect of pulsed deuterium plasma irradiation on dual-phase tungsten medium-entropy alloys

Abstract The fuel ion densities of tritium (T) and deuterium (D) are important parameters for reactor control in fusion experiments. The fuel ion densities typically need to be determined for a wide range of experimental conditions, anywhere between pure deuterium plasmas with trace amounts of T to pure tritium plasmas with trace amounts of D. Given the significantly larger cross section for D + T fusion reactions, compared to D + D or T + T, introducing small amounts of tritium to a deuterium plasma (or vice versa) quickly has a large impact on the neutron emission rates and the neutron energy spectrum. For experiments that use the neutron emission rate as a figure of merit, or for simulations, the time-resolved T and D densities often need to be considered. In this paper, we present a method for determining the densities using multiple neutron diagnostics together with simulations of supra-thermal fuel ions utilizing the plasma transport code TRANSP. The method is set up utilizing a Bayesian framework to estimate the most likely distribution of T and D densities given the available data and the modeled slowing-down ion velocity distribution, using the synthetic neutron diagnostics code DRESS to calculate the expected neutron emission for the given fuel ion distributions. The method is applied to experiments conducted at the JET tokamak, involving deuterium-dominated plasmas with tritium concentrations ranging from 0 to 10%, heated by D neutral beam injection and ion cyclotron resonance frequency heating.

Abstract Boron (B), a low-Z (atomic number) material, has been widely utilized in wall conditioning to improve plasma performance in fusion devices [1]. In 2023, boronization was successfully conducted on EAST featuring an ITER-like tungsten divertor and fully metallic first wall. The process employed predischarge coating with carborane (C2B10H12) as the working material, assisted by ion cyclotron wall conditioning (ICWC). After one time 12 g boronization, it was found the thickness of B film was approximately 120 nm. Post-boronization observations indicated that substantial hydrogen (H) release during initial plasma discharges compared with the consumed W/B wall, attributed to H co-deposition during the ICWC-boronization processing, which led to uncontrollable divertor neutral pressure and plasma density. The H/(H+D) ratio demonstrated a gradual reduction from ~85% to 30% over more than 1850 s of deuterium plasma, with a cumulative injected energy of 2325 MJ. The B coating significantly enhanced the stored energy in plasma and improved confinement performance. The stored energy in plasma showed an increase of about 20%, primarily due to a reduction in impurity radiation, including oxygen (O) and heavy impurities such as tungsten (W), iron (Fe), and copper (Cu). The effective ion charge (Zeff) decreased from 2.3 to 2.0. Following ICWC-boronization, the line-integrated radiation profile decreased by nearly 35% in the plasma core, plasma density and electron temperature exhibited an increase of ~7% and 12% due to enhanced wall fueling and reduced impurity radiation. The lifetime of boronization, as evaluated by the line emissions from boron and other impurity radiation, was about 1700 seconds of deuterium plasma, with a cumulative injected energy of 2125 MJ on EAST. These findings provide significant insights for evaluating ICWC-boronization applicability in ITER with full W wall structure.

Abstract We investigated isotope effects during the tokamak à configuration variable (TCV) ohmic discharge of a diverted positive triangular shape configuration of deuterium (D) and hydrogen (H) plasmas. The transition from the linear ohmic confinement (LOC) regime to the saturated ohmic confinement (SOC) regime was clearly identified from the shot-by-shot density scan experiments. The transport characteristics were almost identical in the H and D plasmas in the LOC regime, and clear improvements were observed in the heat and particle transports in the D plasma compared with the H plasma in the SOC regime. In the SOC regime, the global energy confinement was higher in the D plasma than in the H plasma. Improvements in the SOC regime were evident in the ion channel of the heat transport and the diffusion term of the particle transport. Intrinsic toroidal rotation was found. Its profiles were identical in the H and D plasma in the LOC regime. However, the steeper gradient of toroidal rotation was found in the D plasma than in the H plasma in the SOC regime. The gyrokinetic modeling of switching ion species and keeping identical input profiles showed no difference of the heat flux in the LOC regime and a clear reduction in the D plasma heat flux in the SOC regime. Additionally, collisionality is shown to play an important role in in the heat flux reduction in D plasmas relative to H plasmas. The gyrokinetic validation of the heat transport against the experimental profiles showed a qualitative agreement regarding the heat and particle fluxes. Quantitative agreement was better for the ion heat channel than for the other transport channels.

Abstract The typical pulse on the JET tokamak is ∼10 s during the main phase of the discharge, however long discharge operation (>30 s) is possible with sufficient preparation and care. During the last period of JET operation in 2023 long pulses in deuterium plasmas were developed to assess the sustainment of the plasma performance over several times the current resistive time scale and to address plasma-wall interaction physics in a full metallic environment with the ITER-like wall, with a W divertor and a Be first wall. To prepare for the long pulse operation an analysis of heatloads was required to ensure the pulse was safe for the machine, this defined a number of choices on toroidal field and plasma configuration. While the 30 s pulse was within the control and protection systems commissioned operating envelope the target 60 s pulse was beyond the normal operation of the control and protection systems. These systems were adapted and tested as far as possible to ensure they would work in the real pulse and a number of issues resolved over a series of tests. Significant modifications were required to carry out the experiment which had to be reversed before going back to standard operations. Even with these extensive preparations issues were found and resolved leading to the success of the 60 s pulse. The technical details of these preparations and their implementation will be discussed in detail.

Abstract A code has been developed for simulating the propagation and absorption of fast magnetosonic (FMS) waves excited in plasmas of controlled-fusion facilities. The longitudinal and azimuthal wavenumbers of an FMS wave propagating in a deuterium plasma with parameters characteristic of the ohmic-heating regime in the L-2M stellarator have been calculated. The wave frequency corresponds to the second harmonic of the ion cyclotron frequency of deuterium. The model of cold collisionless cylindrical plasma is used. The FMS wave absorption by ions and electrons has been calculated. It is shown that, for the second harmonic of the ion cyclotron frequency, 79% of the FMS wave power will be absorbed by ions. Therefore, in deuterium plasma, for efficient current drive using FMS waves, it is necessary either to use higher harmonics of the ion cyclotron frequency or to search for alternative mechanisms of FMS wave absorption by electrons, e.g., the mode conversion current drive method.

Tungsten erosion and re-deposition have been simulated by the Monte Carlo code ERO for the EAST upper outer divertor, with a comparative study specifically addressing the effects of carbon and lithium impurities on tungsten target erosion under helium versus deuterium plasma discharge conditions. The simulations indicate that tungsten erosion in helium discharges is significantly higher than in deuterium discharges due to a higher sputtering yield by helium ions. In deuterium discharges, the tungsten gross erosion rate initially rises with increasing carbon concentration in background plasma due to enhanced tungsten sputtering by carbon, and then decreases due to the protective effects of carbon deposition on the tungsten surface. In contrast, in helium discharges, an increase in background carbon concentration leads directly to a lower tungsten gross erosion rate, as helium sputtering dominates the tungsten erosion process. Lithium impurities play a notable protective role against tungsten erosion in both helium and deuterium discharges. Assuming suitable lithium and carbon concentrations, the modelling results align well with experimental data. The simulations offer key insights into tungsten erosion processes in deuterium and helium discharges.