Plasma Flux Research Articles

Power handling in a highly-radiative negative triangularity pilot plant

Abstract This work explores detailed power handling solutions for a class of high-field, highly-radiative negative triangularity (NT) reactors based around the MANTA concept [Rutherford et al. (2024)]. The divertor design is kept as simple as possible, opting for a standard divertor with standard leg length. FreeGS is used to create an equilibrium for the boundary region, prioritizing a short outer leg length of only ∼50 cm (∼40% of the minor radius). The UEDGE code package is used for the boundary plasma solution, to track plasma temperatures and fluxes to the divertor targets. It is found that for PSOL = 25 MW and nsep = 0.96 × 1020 m−3, conditions consistent with initial core transport modeling, little additional power mitigation is necessary. For a fixed impurity fraction of just 0.13\% Ne in the plasma, the peak heat flux density at the more heavily loaded outer targets falls to 7.8 MW/m2, while the electron temperature, Te, remains just under 5 eV. Scans around the parameter space reveal that even at densities lower than in the primary operating scenario, PSOL can be increased up to 50 MW, so long as a slightly higher fraction of extrinsic radiator is used. With less than 1% neon (Ne) impurity content, the divertor still experiences less than 10 MW/m2 at the outer target. Design of the plasma-facing components includes a close-fitting vacuum vessel with a tungsten inner surface as well as FLiBe-carrying cooling channels fashioned into the VV wall directly behind the divertor targets. For the seeded heat flux profile, Ansys Fluent heat transfer simulations estimate that the outer target temperature remains at just below 1550◦C. Initial scoping of advanced divertor designs shows that for an X-divertor, detachment of the outer target becomes much simpler, and plasma fluxes to the targets drop considerably with only 0.01% Ne content.

Post-mortem analysis of the deposit layers on the lower divertor after the 2023 high particle fluence campaign of WEST

We analysed deposits collected on the lower divertor of WEST after the first high fluence campaign performed in 2023. Deposits were collected on the high field side (thick deposition area) of 2 ITER-grade plasma facing units (PFUs), located on different toroidal positions. Using focused ion beam cross sectioning, the deposits were found to be very thick (12–55 µm). A significant difference was observed in the toroidal direction with thicker deposits for the PFU located at the maximal heat load in the inner side, showing a deposition pattern due to the toroidal magnetic field modulation. Deposits present a complex layer-by-layer structure with dense layers, some melted parts and porosities within the layers. The deposition is mainly composed of tungsten with oxygen, boron, carbon and traces of nitrogen whose composition vary along the radial direction. We identified W-rich deposits in the high plasma flux area near the inner strike point and deposits rich in O, B, C and N in the low plasma flux area further away from this strike point. W dense layers of about 5 up to 40 µm thick in the thick deposit area near the strike point were attributed to the high fluence campaign. Pure boron layers resulting from wall conditioning and processed by plasma wall interactions were also observed.

Spectral properties of solar wind plasma flux and magnetic field fluctuations across fast reverse interplanetary shocks

We have analyzed spectra of fluctuations in the solar wind plasma flux and the magnetic field magnitude near the front of a fast reverse shocks, using data from the BMSW device (Bright Monitor of Solar Wind) operating on the SPEKTR-R satellite. Its time resolution made it possible to study plasma flux fluctuations up to a frequency of 16 Hz. Magnetic field data was taken mainly from the WIND satellite, for which the frequency of the fluctuations considered was up to 5.5 Hz. The slope of the spectra of the solar wind flux fluctuations on MHD scales has been shown to be close to the slope of the spectrum of magnetic field fluctuations in the disturbed region. On kinetic scales, the difference can be significant. For the region ahead of the front, the difference in the slope of the spectrum can be quite large both in the MHD and in the kinetic region. The frequency of the break of the flux spectrum ranges from 0.6 to 1.3 Hz, which corresponds to the scale of the proton inertial length. In a number of events, however, the shape of the spectrum indicates the influence of the proton gyroradius frequency, which is usually 0.05–0.15 Hz. The break in the power spectrum of magnetic field fluctuations also more often ranges from 0.7 to 1.2 Hz. In this case, the slope of the MHD part of the spectrum changes little, but in the kinetic part it increases slightly when moving to the disturbed region.

Анализ спектров флуктуаций величины потока плазмы и модуля магнитного поля на обратных ударных волнах

We have analyzed spectra of fluctuations in the solar wind plasma flux and the magnetic field magnitude near the front of a fast reverse shocks, using data from the BMSW device (Bright Monitor of Solar Wind) operating on the SPEKTR-R satellite. Its time resolution made it possible to study plasma flux fluctuations up to a frequency of 16 Hz. Magnetic field data was taken mainly from the WIND satellite, for which the frequency of the fluctuations considered was up to 5.5 Hz. The slope of the spectra of the solar wind flux fluctuations on MHD scales has been shown to be close to the slope of the spectrum of magnetic field fluctuations in the disturbed region. On kinetic scales, the difference can be significant. For the region ahead of the front, the difference in the slope of the spectrum can be quite large both in the MHD and in the kinetic region. The frequency of the break of the flux spectrum ranges from 0.6 to 1.3 Hz, which corresponds to the scale of the proton inertial length. In a number of events, however, the shape of the spectrum indicates the influence of the proton gyroradius frequency, which is usually 0.05–0.15 Hz. The break in the power spectrum of magnetic field fluctuations also more often ranges from 0.7 to 1.2 Hz. In this case, the slope of the MHD part of the spectrum changes little, but in the kinetic part it increases slightly when moving to the disturbed region.

Enhancing the performance of SiC-based varistors through the use of SPS processing and fluxes

SiC varistors, which are used as surge arrestors in high power/high energy applications, often contain high levels of porosity (∼25 %) which can degrade varistor performance. Starting with SiC-clay varistors (∼43 µm grain size SiC) we employed spark plasma sintering (SPS) and flux additions (to enable liquid phase sintering) to reduce the level of porosity and understand its impact on the microstructure and the electrical properties. Sintering by SPS at 1200 °C for 5 min increased the density to 94 % theoretical, increasing energy density capability by almost a factor of 10 (to ∼1100 J cm−3), but the non-linear coefficient (α) fell by 40 % (to 3.5) and breakdown field (Eb) fell by 75 % (to ∼200 V cm−1). Varistors prepared with nepheline syenite as a flux (replacing clay) which were pressureless sintered at 1200 °C had maximum densities of 80 %. The samples in which all the clay was replaced by the flux exhibited exceptional energy density performance (over 10,000 J cm−3) but suffered reductions in other parameters. Using small grain size SiC (∼3 µm) and 40 % syenite flux yielded consistently superior properties: low power range α value 12.2, high power range α value 32.5, breakdown field ∼5880 (V cm−1) and very low leakage currents ∼10−3 (mA/cm2).

Intermediate Descending Layers Emerged Simultaneously in Five Different Locations During the Solar Eclipse on 21 June 2020

AbstractSolar eclipse traveled across South China in the afternoon on 21 June 2020. Five ionosondes located from mid‐to low‐latitudes and on both north and south of the eclipse path were applied to investigate the ionospheric responses. Both the zonal and meridional ranges of the observation region have exceeded 1,000 km. All the five ionosondes had observed the Intermediate Descending Layers (IDLs) simultaneously just after the eclipse maximum and this is a very small probability event. During the solar eclipse, the multi‐hop echoes above the Es, the rising Es to 150 km altitude, the plasma flux from above F2‐layer were also observed and analyzed. The descending trend of the IDLs and the peak height of F2‐layer (hmF2) shows great consistency, indicating the close relationship between the eclipse induced plasma flux and the IDLs. The traces of gravity waves were also found in the IDLs and F‐layer. The plasma flux may carry the ions to valley region and the eclipse produced gravity waves were responsible for the formation of the IDLs.

Development of a diamagnetic loop in KAIMIR

We developed a diamagnetic loop for the estimation of plasma stored energy in the KAIST Magnetic Mirror magnetic mirror device [Oh et al., J. Plasma Phys. 90, 975900202 (2024)]. Diamagnetic loops are used to estimate the plasma stored energy from measurements of the diamagnetic flux in plasma with an applied external magnetic field. However, diamagnetic flux measurements are accompanied by the vacuum flux, which generally exceeds the diamagnetic flux by over 10 000 times. Therefore, it is critical to attain a high signal-to-noise ratio with minimized noise in diamagnetic flux measurements. In this study, we employed a novel method to reduce background noise and improve the signal-to-noise ratio. Using two identical loops with opposite polarities, we successfully removed parasitic capacitive noise from the external insulation while amplifying the inductive signal two times. To eliminate the vacuum flux, we utilized two coaxial loops with different radii positioned at the same axial location. Results obtained from six paired loops confirmed the successful removal of the vacuum flux. The plasma stored energy was also found to agree well with Langmuir probe measurements, which verifies the diamagnetic flux measurements using the developed loop.

Simulation studies of tungsten impurity behaviors in helium plasma in comparison with deuterium plasma via SOLPS-ITER

The sputtering and transport of tungsten (W) impurities in helium (He) and deuterium (D) plasma discharges are compared using the SOLPS-ITER code. To reduce the computational resources of modeling, W ions are treated using the bundled charge state model. The results show that the W erosion flux of He plasma is almost a factor of two higher than that of D plasma under the same upstream electron density and heating power due to the higher W sputtering yield in He plasma. Moreover, the W self-sputtering flux is significantly higher than the W flux sputtered by the main ions. The leakage and retention of W impurities in the divertor region is also analyzed. W ions mainly escape from the near scrape-off layer (SOL) region through the divertor entrance as the stagnation point of the average W impurity poloidal velocity is considerably closer to the target plates in the near SOL region. Furthermore, the leakage flux of W ions in He plasma is higher than that in D plasma, mainly because of the higher W sputtering level in He plasma, which results in a larger W density. W ions with low-lying charge states, mostly comprising the charge state of W10–12+, easily escape from the divertor through the near SOL flux tubes in both D and He plasmas. In addition, the effects of upstream electron density on W sputtering and retention in He and D plasma discharges are presented.

FreeGSNKE: A Python-based dynamic free-boundary toroidal plasma equilibrium solver

We present a Python-based numerical solver for the two-dimensional dynamic plasma equilibrium problem. We model the time evolution of toroidally symmetric free-boundary tokamak plasma equilibria in the presence of the non-linear magnetohydrodynamic coupling with both currents in the “active” poloidal field coils, with assigned applied voltages, and eddy currents in the tokamak passive structures. FreeGSNKE (FreeGS Newton–Krylov Evolutive) builds and expands on the framework provided by the Python package FreeGS (Free boundary Grad–Shafranov). FreeGS solves the static free-boundary Grad–Shafranov (GS) problem, discretized in space using finite differences, by means of Picard iterations. FreeGSNKE introduces: (i) a solver for the static free-boundary GS problem based on the Newton–Krylov (NK) method, with improved stability and convergence properties; (ii) a solver for the linearized dynamic plasma equilibrium problem; and (iii) a solver for the non-linear dynamic problem, based on the NK method. We propose a novel “staggered” solution strategy for the non-linear problem, in which we make use of a set of equivalent formulations of the non-linear dynamic problem we derive. The alternation of NK solution steps in the currents and in the plasma flux lends this strategy an increased resilience to co-linearity and stagnation problems, resulting in favorable convergence properties. FreeGSNKE can be used for any user-defined tokamak geometry and coil configuration. FreeGSNKE's flexibility and ease of use make it a suitably robust control-oriented simulator of plasma magnetic equilibria. FreeGSNKE is entirely written in Python and easily interfaced with Python libraries, which facilitates machine learning based approaches to plasma control.

Simulations of tokamak edge plasma turbulent fluctuations based on a minimal 3D model

A new simulation model for tokamak boundary plasma, SOLT3D, is implemented in the BOUT++ framework (Dudson et al 2009 Comput. Phys. Commun. 180 1467). The simulation model includes a set of dynamic equations describing collisional boundary plasma and neutral gas in the tokamak scrape-off layer and divertor region. The model is verified against standard linear plasma instabilities and available nonlinear results. For L-mode like conditions, SOLT3D reproduces characteristics of boundary plasma turbulent fluctuations that are similar to published experimental data, in terms of the amplitude and spatial dependence of the fluctuations. It also reproduces realistic plasma fluxes on material surfaces and expected Bohm-like effective radial transport. Plasma fluctuations inferred from the simulations imply inevitably a significant level of intrinsic ‘noise’ for axisymmetric tokamak plasma transport modeling, introducing errors on the order of unity. In particular, the toroidally averaged atomic rates below 5–10 eV are strongly modified by turbulent plasma fluctuations, which should significantly affect the standard axisymmetric modeling of the tokamak edge plasma and divertor.

Importance of the dusk-dawn interplanetary magnetic field component (IMF By) to magnetospheric convection in Earth’s magnetotail plasma sheet

The solar wind and its embedded magnetic field, the interplanetary magnetic field (IMF) together with magnetic reconnection power the large-scale plasma and magnetic flux circulation in the Earth’s magnetosphere-ionosphere system. This circulation is termed as convection and its strength is controlled by the north-south IMF component (IMF Bz). In recent years, an interest has arisen to investigate the lesser-known role of the dusk-dawn component (IMF By) in convection. It has been previously known though that prevailing nonzero IMF By can cause plasma flow asymmetries in the high-latitude ionosphere, but how the magnetospheric flows, for instance, in the magnetotail plasma sheet are affected, remains to be investigated. In this article, we introduce the recent progress and the latest achievements in the research of the influence of IMF By on tail plasma sheet convection. The research progress has been rapid and it has revealed that both fast and slow convection are affected in a manner that is in accordance with the asymmetries observed in the ionospheric convection. The results indicate the significance of the IMF By component on magnetospheric convection and they represent a major advance in the field of solar wind-magnetosphere coupling.

MODELING OF SPATIAL-TEMPORAL VARIATIONS OF PHYSICAL PROCESS PARAMETERS IN THE IONOSPHERIC PLASMA OVER UKRAINE DURING THE MAXIMUM PHASE OF THE 24-TH SOLAR ACTIVITY CYCLE (2012—2015)

The object of research is physical processes occurring in the ionospheric plasma. The subject of research: spatial-temporal variations of the main parameters of the ionospheric plasma, which were obtained using incoherent scatter radar. Research methods include the ground-based radiophysical method of incoherent scatter of radio waves, statistical analysis of observation results, and semi-empirical modeling of parameters of dynamic and thermal processes. Modeling and analysis of spatial-temporal variations of the dynamic and thermal processes parameters in the ionosphere during the maximum phase of the 24th solar activity cycle (2012—2015) have been performed using experimental data obtained by the Kharkiv incoherent scatter radar. For the periods of the equinoxes and solstices, diurnal dependences of process parameters in the ionospheric plasma at altitudes from 210 to 450 km were constructed. The values of plasma transfer velocity due to ambipolar diffusion, the density of the full plasma flux, and the flux of charged particles due to ambipolar diffusion were determined. The values of the energy supplied to the electron gas, the density of the heat flux transferred by electrons from the plasmasphere to the ionosphere, as well as the velocity of the equivalent neutral wind and the meridional component of the neutral wind velocity, were calculated. It was established that for the considered periods, the effects of space weather variations and geomagnetic activity were significantly manifested in variations of the plasma flux density due to ambipolar diffusion, the density of the full plasma flux, as well as the energy supplied to electrons per unit time. Quantitative and qualitative characteristics of these parameters were typical but changed significantly (by 2…3 times) in some cases, even with a slight increase in geomagnetic activity.

Limits on the Density of the Lunar Ionosphere: ARTEMIS Observations

The Moon of our Earth has a tenuous atmosphere, known as an exosphere. The ionization of this exosphere is speculated to possibly form a weak ionosphere. Some radio occultation (RO) experiments have suggested the presence of a dense ionosphere with an electron density on the order of hundreds of cm−3 near the surface. Using in situ measurements from the ARTEMIS mission during 2012–2021, we conduct statistical analyses and case studies to investigate the plasma density at near-surface altitudes. ARTEMIS measurements reveal no plasma densities at altitudes between 10 and 50 km that exceed 35 cm−3, and therefore they provide no evidence for a steady-state or global lunar ionosphere at the level suggested by some RO observations. Density profiles with local time and altitude show higher density in the sunlit sector than in the shadowed sector. These observations suggest that the natural variation of solar wind plasma flux with solar zenith angle plays a critical role in controlling the plasma population near the surface. This research provides a reference for a comparison with RO observations and a statistical view of the low-altitude plasma environment near the lunar surface.

Slit extraction and emittance results of a permanent magnet minimum-B quadrupole electron cyclotron resonance ion source

We present an experimental and simulation study of high charge state ion beams produced with a permanent magnet electron cyclotron resonance ion source (ECRIS) with minimum-B quadrupole magnetic field topology and slit extraction system. The unconventional topology generates fan-shaped plasma flux, favouring a rectangular aperture for beam extraction. We demonstrate successful low-energy transport of the slit beam, achieving up to 25 times higher beam currents compared to a round extraction aperture. Maximum beam intensities are 1.5 and 15 μA for argon 11＋ and 9＋, respectively. The results indicate microwave power-limited beam production. Emittance values are comparable to traditional ECRIS. Unlike in conventional ECRIS, the emittance increases monotonically with increasing charge state due to slit extraction, magnetic topology, and beam transport characteristics. The experimental transport efficiency for helium ranges from 32% to 50%, depending on the total beam current. The losses primarily occur due to the dipole magnet’s insufficient vertical aperture.

A fully kinetic study on the plasma detachment processes in the collisionless propulsive magnetic nozzle

A fully kinetic axisymmetric particle-in-cell model is employed to simulate and study the detachment processes of electrons in the propulsive magnetic nozzle. The detachment ratio is adopted to evaluate the extent to which the electrons detach from the magnetic field. The theoretical expression for the electron detachment ratio is derived and indicates that the electron detachment is driven by two mechanisms: inertia effect and gyro-viscous effect. The simulation results show that the detachment direction of electrons are outward in the upstream and inward in the downstream. In addition, the dominating detachment mechanisms in the upstream is inertia effect, while the gyro-viscous effect is equally, if not more, important as the inertia effect in the downstream, especially around the periphery of the magnetic nozzle under lower magnetic field strength. Moreover, the electron detachment is found to contribute to no more than 20% of the axial flux of plasma in the downstream, while the majority of it is caused by the electrons flowing along the magnetic field.

Studies of the formation and distribution of cracks and various defects on the heated tungsten plate surface during pulsed plasma flux impact

In this work, cracking of the surface of a tungsten plate was studied using a PW-7 pulsed plasma accelerator. The surface of the tungsten plate was exposed at an energy density and plasma flux duration of 1.1 MJ/m2 and 200 μs, respectively. Before each shot, the base of the tungsten plate was heated to a temperature of 823 K. Heating of the base of the tungsten plate above the ductile–brittle transition temperature made it possible to control the temperature gradient on its surface under pulsed plasma exposure. In other words, the temperature gradient of the tungsten plate before and after exposure to the plasma flux decreased. Compared to the data at room temperature, the results of heating of the tungsten plate to the tungsten ductile–brittle transition temperature of 823 K showed a reduction in crack width to several hundred nanometers under the influence of plasma. It should be noted that heating of the plate did not completely eliminate cracking of the surface of the tungsten plate. However, this significantly minimizes the emission of dust from the surface of the tungsten plate under the influence of the plasma flux (barely no dust was observed).

Correspondence of Pi2 Pulsations, Aurora Luminosity, and Plasma Flux Fluctuation Near a Substorm Brightening Aurora: Arase Observations

AbstractAlthough many substorm‐related observations have been made, we still have limited insight into propagation of the plasma and field perturbations in Pi2 frequencies (∼7–25 mHz) in association with substorm aurora, particularly from the auroral source region in the inner magnetosphere to the ground. In this study, we present conjugate observations of a substorm brightening aurora using an all‐sky camera and an inner‐magnetospheric satellite Arase at L ∼ 5. A camera at Gakona (62.39°N, 214.78°E), Alaska, observed a substorm auroral brightening on 28 December 2018, and the footprint of the satellite was located just equatorward of the aurora. Around the timing of the auroral brightening, the satellite observed a series of quasi‐periodic variations in the electric and magnetic fields and in the energy flux of electrons and ions. We demonstrate that the diamagnetic variations of thermal pressure and medium‐energy ion energy flux in the inner magnetosphere show approximately one‐to‐one correspondence with the oscillations in luminosity of the substorm brightening aurora and high‐latitudinal Pi2 pulsations on the ground. We also found their anti‐correlation with low‐energy electrons. Cavity‐type Pi2 pulsations were observed at mid‐ and low‐latitudinal stations. Based on these observations, we suggest that a wave phenomenon in the substorm auroral source region, like ballooning type instability, play an important role in the development of substorm and related auroral brightening and high‐latitude Pi2, and that the variation of the auroral luminosity was directly driven by keV electrons which were modulated by Alfven waves in the inner magnetosphere.

Beam intensity of N2 plasma controlled surface engineering on WO3 for enhanced hydrogen evolution and oxygen evolution

Tungsten oxide is a promising electrocatalyst for water splitting reactions, due to its flexible chemical state, strong structural malleability and good corrosion resistance. Herein, the surface engineering on WO3 was conducted by a controllable N2 plasma for enhancing the performance of water splitting reactions. Under the specific N2 plasma treatment, the surface crystalline phase and the surface morphology of the tungsten oxide can be changed, as well as the creation of oxygen vacancies. These changes lead to a greatly increased electrochemical surface area, a narrowed bandgap, and a distorted crystal lattice. For oxygen evolution reaction (OER) in 1.0 M KOH, the activity of the modified tungsten oxide electrodes presents a positive correlation with the input plasma, and an optimal overpotential of 269 mV @ 10 mA cm−2 is achieved with a Tafel slope of 122 mV dec-1. With a high N2 plasma flux, the W-N bond can be formed on the tungsten oxide surface, which contributes to the promotion of the hydrogen evolution reaction (HER) activity, and an overpotential of 93 mV @ 10 mA cm−2 in 1.0 M KOH is exhibited, and the Tafel slope largely reduced to 76 mV dec-1. Such a facile strategy provides an engineering route of preparing efficient electrodes for water splitting reactions.

Gas-Dynamic Multiple-Mirror Trap GDMT

This work is devoted to the project of a new-generation open trap, gas-dynamic multiple-mirror trap (GDMT), proposed at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. The aim of the project is to substantiate the possibility of using open traps as thermonuclear systems: a source of neutrons and, in the future, a thermonuclear reactor. The main objectives of the project are to develop technologies for long-term plasma maintenance in an open trap, optimize neutron source parameters based on the gas-dynamic trap, and demonstrate methods for improving plasma confinement. The magnetic vacuum system of the facility consists of a central trap, multiple-mirror sections that improve the longitudinal plasma confinement, and expanders designed to accommodate plasma flux absorbers. The facility is to be built in several stages. The starting configuration is broadly similar to the GDT facility and includes a central trap with strong magnetic mirrors and expanders. It solves two main problems: optimization of the parameters of the neutron source based on the gas-dynamic trap and study of the physics of the transition to the configuration of a diamagnetic trap with a high relative pressure β ≈ 1, which significantly increases the efficiency of the system. This work describes the technical design of the starting configuration of the facility and outlines the physical principles on which the GDMT project is based.

Callisto's Atmosphere: The Oxygen Enigma

AbstractObservations of Callisto's atmosphere have indicated an O2 component should exist, but the evolution from its initial source to its inferred steady‐state abundance is not well understood. Herein we constrain the production of O2 via radiolysis within Callisto's exposed ice patches and determine the corresponding O2 column density. To do so, for the first time we simulate the thermal and energetic components of the Jovian magnetospheric plasma irradiating Callisto's atmosphere and estimate energy deposited therein by the impinging charged particles along their trajectories to the surface. We then calculate O2 source fluxes corresponding to the energy of the impacting plasma fluxes, which is coupled with estimated atmospheric lifetimes to determine the steady‐state abundance of O2. Our results suggest that production of O2 via radiolysis within the exposed ice on Callisto's surface does not produce a sufficiently dense atmosphere relative to the column densities inferred from observations by about 2–3 orders of magnitude. To resolve this discrepancy between estimated and observed abundances, we provide the first estimates for other potential sources of atmospheric O2. We also make similar estimates for the production of H2 in Callisto's atmosphere relative to constraints provided in the literature, and the conclusion is the same: a sufficiently dense atmosphere is not produced. Thus, we have shown that a better understanding of the production and fate of radiolytic products in Callisto's regolith is required in order to place firmer constraints on the generation mechanisms of its atmosphere in preparation for future observations.