High Electron Temperature Research Articles

Vlasov simulation of the emissive plasma sheath with energy-dependent secondary emission coefficient and improved modeling for dielectric charging effects

A one-dimensional Vlasov–Poisson simulation code is employed to investigate the plasma sheath considering electron-induced secondary electron emission (SEE) and backscattering. The SEE coefficient is commonly treated as constant in a range of plasma simulations; here, an improved SEE model of a charged dielectric wall is constructed, which includes the wall charging effect on the SEE coefficient and the energy dependency of the SEE coefficient. Pertinent algorithms to implement the previously mentioned SEE model in plasma simulation are studied in detail. It is found that the SEE coefficient increases with the amount of negative wall charges, which in turn reduces the emissive sheath potential. With an energy-dependent SEE coefficient, the sheath potential is a nonlinear function of the plasma electron temperature, as opposed to the linear relation predicted by the classic emissive sheath theory. Simulation combining both wall-charging effect and SEE coefficient’ energy dependency suggests that the space-charged limited sheath is formed at high plasma electron temperature levels, where both sheath potential and surface charging saturate. Additionally, different algorithms to implement the backscattering in the kinetic simulation are tested and compared. Converting backscattered electrons to secondary electrons via an effective SEE coefficient barely affects the sheath properties. The simulation results are shown to be commensurate with the upgraded sheath theory predictions.

Nitrogen discharge characteristics and species kinetics in helicon plasma source

Nitrogen (N2) helicon plasma is produced with radio frequency (RF) right-helical antenna at low pressures. Several wave modes and their transitions of N2 helicon discharge are observed experimentally. Blue Core phenomenon is achieved at high magnetic fields and high RF powers, with strong local blue light emissions of N II lines and high electron temperature inside the core area. Based on actinometric ratio and pressure balance model, species kinetics of N2 helicon plasma are analyzed. It is shown that about 79% of N2 molecules are dissociated and about 49% of the neutrals are ionized inside the Blue core in high magnetic field of 850 G and RF power of 2200 W. Nearly 99% of N2 molecules inside the core area are depleted considering the neutral density before and after discharge, from 7.3 × 1013 cm−3 to 6.5 × 1011 cm−3. Serious neutral depletion of N2 and N neutrals and high electron temperature are suggested to be the dominant causes for significant enhancement of central N II emissions (blue lights). Meanwhile, evolution of reaction processes indicates that N ionization and N+ excitation become dominant in BC mode. Besides, external magnetic field is an important factor to control the discharge mode transitions as well as the radial distributions of plasmas. From the calculated results of dispersion relation, the cavity mode resonance, rather than antenna coupling resonance, of helicon waves plays a dominant role on the wave mode formation and RF energy coupling between RF antenna and plasma. The mode transition results from excitation of helicon wave of higher axial eigenmode. N2 helicon plasma shows different characteristics from argon in mode transition, spectral emission and Blue Core formation. It is due to the high dissociation energy of N2 molecules (9.8 eV) and extensive dissociation and ionization processes. This results in a higher RF power as well as magnetic field for helicon wave coupled mode in N2 helicon plasma than that in Ar plasma.

First insights into the ISM at z > 8 with JWST: possible physical implications of a high [O iii] λ4363/[O iii] λ5007

ABSTRACT We present a detailed analysis of the rest-frame optical emission line ratios for three spectroscopically confirmed galaxies at z > 7.5. The galaxies were identified in the James Webb Space Telescope (JWST) Early Release Observations field SMACS J0723.3 − 7327. By quantitatively comparing Balmer and oxygen line ratios of these galaxies with various low-redshift ‘analogue’ populations (e.g. Green Peas, Blueberries, etc.), we show that no single analogue population captures the diversity of line ratios of all three galaxies observed at z > 7.5. We find that S06355 at z = 7.67 and S10612 at z = 7.66 are similar to local Green Peas and Blueberries. In contrast, S04590 at z = 8.50 appears to be significantly different from the other two galaxies, most resembling extremely low-metallicity systems in the local Universe. Perhaps the most striking spectral feature in S04590 is the curiously high [O iii] λ4363/[O iii] λ5007 ratio (RO3) of 0.048 (or 0.055 when dust-corrected), implying either extremely high electron temperatures, >3 × 104 K, or gas densities >104 cm−3. Observed line ratios indicate that this galaxy is unlikely to host an AGN. Using photoionization modelling, we show that the inclusion of high-mass X-ray binaries or a high cosmic ray background in addition to a young, low-metallicity stellar population can provide the additional heating necessary to explain the observed high RO3 while remaining consistent with other observed line ratios. Our models represent a first step at accurately characterizing the dominant sources of photoionization and heating at very high redshifts, demonstrating that non-thermal processes may become important as we probe deeper into the Epoch of Reionization.

A highly active and cost-effective tungsten modified Ni-based catalyst for the production of hydrogen via methane dry reforming

Catalyst community requires a low-cost, high-performance catalyst immediately for cost-effective H2 production through dry reforming of methane. Herein, cost-effective 1–9 wt% tungsten oxide-alumina supported Ni catalysts were prepared, examined and characterized by x-ray diffraction, 27Al solid-state nuclear magnetic resonance, X-ray photoelectron spectroscopy, high resolution-transmission electron microscopy and temperature programmed experiments. On 5.0 wt% W incorporation; minimum crystallinity, additional CO2-surface interacting species, maximum oxygen replenishment capacity by CO2, thermally retainable coordinated Al sites and minimum % weight loss pivot optimum hydrogen production 77% H2 yield, minimum deactivation factor −0.9, and maximum rate of hydrogen formation of 1.1301 molH2gcat−1h−1 at 440-min time-on-stream.

Electron-Phonon Coupling and Nonthermal Effects in Gold Nano-Objects at High Electronic Temperatures.

Laser irradiation of metals is widely used in research and applications. In this work, we study how the material geometry affects electron–phonon coupling in nano-sized gold samples: an ultrathin layer, nano-rod, and two types of gold nanoparticles (cubic and octahedral). We use the combined tight-binding molecular dynamics Boltzmann collision integral method implemented within XTANT-3 code to evaluate the coupling parameter in irradiation targets at high electronic temperatures (up to Te~20,000 K). Our results show that the electron–phonon coupling in all objects with the same fcc atomic structure (bulk, layer, rod, cubic and octahedral nanoparticles) is nearly identical at electronic temperatures above Te~7000 K, independently of geometry and dimensionality. At low electronic temperatures, reducing dimensionality reduces the coupling parameter. Additionally, nano-objects under ultrafast energy deposition experience nonthermal damage due to expansion caused by electronic pressure, in contrast to bulk metal. Nano-object ultrafast expansion leads to the ablation/emission of atoms and disorders the inside of the remaining parts. These nonthermal atomic expansion and melting are significantly faster than electron–phonon coupling, forming a dominant effect in nano-sized gold.

Electron velocity distribution on the abrupt change in source–drain current of GaN devices

An analytical model of the channel electron energy distribution in an on-state GaN transistor has been proposed based on the assumption that drift velocities of channel electrons obey the two-dimensional Maxwell–Boltzmann distribution. The validity of such an assumption was confirmed by Monte Carlo simulation. It was found that there could be a larger number of high-energy channel electrons whose energy is higher than the intervalley energy between [Formula: see text] and [Formula: see text] valleys in a GaN transistor with a high electron temperature. The fraction of hot electrons with its energy higher than the intervalley energy between [Formula: see text] and [Formula: see text] valleys to the total channel electrons can easily reach 50% when the electron temperature is higher than 3000 K. Such an electron temperature in a GaN transistor had been determined in experiments. Thus, hot electrons in the [Formula: see text] valley can transit into [Formula: see text] valleys. It suggests that intervalley transitions could be one possible physical origin of the abrupt change in the source−drain current in GaN devices. The proposed model can well explain how an abrupt change in the source–drain current in GaN transistor experiments depends on the voltage-dependent gate, the trap, etc.

Nonthermal effects in H-doped tungsten at high electronic temperatures

In a fusion reactor, the plasma with very high temperature intensely and continuously emits high-energy photons, which irradiate the tungsten material of the first wall. The irradiation with high-energy photons causes nonthermal events in the tungsten material. In this paper, the structural features of H-doped tungsten metal at highly electronic excitation states are studied using density functional theory. It has been found that the system can not only undergo nonthermal expansion with increasing electronic excitation energy, but also a phase transition from body-centered cubic (BCC) phase to face-centered cubic (FCC) or hexagonal close-packed cubic (HCP) phase occurs spontaneously with a threshold value of excitation energy. The relationship between the excitation energy threshold associated with the phase transition and the concentration of doped impurity in tungsten is disclosed, which can be expressed using an empirical formula. In addition, the synergistic effect of electronic excitation and impurity atoms on the phase transition is revealed. Our results give a deep insight into the irradiation effect of the high-energy photons on the first-wall tungsten in a fusion reactor.

The Dependence of Cold and Hot Patches on Local Plasma Transport and Particle Precipitation in Northern Hemisphere Winter

AbstractBy using a database of 4,634 cold patches (high density and low electron temperature) and 4,700 hot patches (high density and high electron temperature) from Defense Meteorological Satellite Program F16 in 2005–2018 winter months (October–March), we present a statistical survey of the distributions of polar cap patches for different interplanetary magnetic field (IMF) orientations and ionospheric convection geometries. We investigate the dependence of cold and hot patches on local plasma transport and soft‐electron precipitation. Our results indicate that: in winter, (a) more cold and hot patches occur in the stronger anti‐sunward flow organized by different IMF orientations. (b) cold patches are frequent near the central polar cap, while hot patches are closer to the auroral oval. (c) enhanced anti‐sunward flow (E × B drift) mainly contributes to cold patch occurrence under Bz < 0, and soft‐electron precipitation contributes to hot patch occurrence both under southward and northward IMF.

Multi-scale turbulence simulation suggesting improvement of electron heated plasma confinement

Turbulent transport is a key physics process for confining magnetic fusion plasma. Recent theoretical and experimental studies of existing fusion experimental devices revealed the existence of cross-scale interactions between small (electron)-scale and large (ion)-scale turbulence. Since conventional turbulent transport modelling lacks cross-scale interactions, it should be clarified whether cross-scale interactions are needed to be considered in future experiments on burning plasma, whose high electron temperature is sustained with fusion-born alpha particle heating. Here, we present supercomputer simulations showing that electron-scale turbulence in high electron temperature plasma can affect the turbulent transport of not only electrons but also fuels and ash. Electron-scale turbulence disturbs the trajectories of resonant electrons responsible for ion-scale micro-instability and suppresses large-scale turbulent fluctuations. Simultaneously, ion-scale turbulent eddies also suppress electron-scale turbulence. These results indicate a mutually exclusive nature of turbulence with disparate scales. We demonstrate the possibility of reduced heat flux via cross-scale interactions.

55‐3: Development of Low‐Temperature Metal Dry‐Etching Equipment via ECR Plasma Source

This paper presents the dry etching performance and expectation of highly conductive thin metal films that are hardly etched at low temperature with conventional plasma dry etching equipment. Dry etching is performed using a combination of H2 and HCl gases in a reactive ion etching system with low temperature susceptor and electron cyclotron resonance (ECR) plasma source. We could achieve high electron temperature enough to dissociate and ionize H and Cl radicals from H2 and HCl molecules.

Velocity‐resolved Laser‐induced Desorption for Kinetics on Surface Adsorbates

AbstractInvited for this month's cover are the groups of Alec M. Wodtke at the Georg‐August University in Göttigen (Germany) and at the University of Crete (Greece). The cover picture shows ammonia (NH3) molecules adsorbed on an atomically flat platinum surface. An ultra‐short laser pulse hits the surface and heats up the metal electron rapidly to several thousand Kelvin within a few 100 fs. This high electronic temperature causes a fraction of the ammonia molecules to desorb with hyperthermal velocity from the surface. Depended on the laser intensity, the number of desorbing molecules is proportional to the surface coverage. Read the full text of their Research Article at 10.1002/cmtd.202200017.

Cover Picture: Velocity‐resolved Laser‐induced Desorption for Kinetics on Surface Adsorbates (Chem. Methods 5/2022)

The Front Cover shows ammonia molecules adsorbed on an atomically flat platinum surface. An ultra-short laser pulse hits the surface and heats up the metal electron rapidly to several thousand Kelvin within a few 100fs. This high electronic temperature causes a fraction of the ammonia molecules to desorb with hyperthermal velocity from the surface. More information can be found in the Research Article by Kim Pappendorf et al..

Non-thermal argon plasma jets of various lengths for selective reactive oxygen and nitrogen species production

Plasma jets simultaneously create a wide range of reactive oxygen species (ROS) and reactive nitrogen species (RNS), making selective production challenging. In this study, an attempt was made to address this problem by adjusting the downstream length of the plasma jet to regulate reactive species production in an AC-driven argon plasma jet. Jets with short downstream lengths are dominated by RNS, whereas longer ones are dominated by ROS. The energy impact collision between energetic electrons and diffused ambient air is primarily responsible for the generation of RNS in argon plasma. Short plasma jets have a higher electron temperature in the plasma effluent, allowing for better excitation and ionization of N2 molecules. In contrast, in longer jets, electrons transfer most of their energy to neutrals and moisture, even before the diffusion of ambient air. Thus, the production of OH radicals and H2O2 is dominant in longer plasma jets. Further, the ratio of NO2- to H2O2 in plasma-activated water decreased from 1.8 to 0.07, as the downstream length was increased from 1 cm to 10 cm. RNS-enriched short plasma jets are appropriate for immune cell activation, jets of intermediate length are suitable for gold nanoparticle formation because of the synergistic action of ROS and electron energy, and ROS-dominated longer plasma jets are excellent for methylene blue degradation. These findings are applicable to the design of plasma devices in which the generation of ROS and RNS in practical quantities is required to realize plasma technology for a wide range of applications.

Line identification of extreme ultraviolet (EUV) spectra from iron, copper and molybdenum ions in EAST tokamak

Extreme ultraviolet (EUV) spectra emitted from iron and copper ions in a range of 8–500 Å and molybdenum ions in a range of 8–400 Å were measured in Experimental Advanced Superconducting Tokamak (EAST) discharges accompanied with spontaneous sputtering events, by which metallic impurity influxes are suddenly increased in the discharge. Several spectral lines from L-, M-, and N-shell partially ionized ions have been successfully observed with two EUV spectrometers named EUV_Short and EUV_Long working in wavelength ranges of 8–130 Å and 20–500 Å, respectively. The wavelength position is accurately calibrated in-situ based on several well-known spectra of low-Z impurity ions. The line identification is carefully performed based on the National Institute of Standards Technology (NIST) database and previously published experimental data. As a result, it is found that B-, Be-, and Li-like iron and copper spectra Al-, Mg-, and Na-like molybdenum spectra appear in discharges with high central electron temperature (>1.8 keV). The highest ionization stages identified here are Li-like iron and copper and Na-like molybdenum, e.g. Fe XXIV at 192.028 Å (1s22p 2P3/2 → 1s22s 2S1/2), Cu XXVII at 153.513 Å (1s22p 2P3/2 → 1s22s 2S1/2) and Mo XXXII at 127.868 Å (2p63p 2P3/2 → 2p63s 2S1/2). Two unresolved transition arrays of molybdenum ions (Mo-UTAs) appear at wavelength ranges of 15–30 Å and 65–95 Å, and are preliminarily identified as Mo XX-Mo XXXII and Mo XVII-Mo XXXII, respectively. In addition, six spectral lines are newly found by comparing a time evolution of the line intensity from impurity ions in different ionization stages, i.e. Fe XVIII at 17.60 ± 0.02 Å, Cu XXI at 13.15 ± 0.02 Å, Cu XVIII at 260.32 ± 0.07 Å, Cu XVIII at 261.00 ± 0.06 Å, Mo XXVIII at 85.30 ± 0.03 Å and Mo XXIX at 89.59 ± 0.03 Å. The wavelengths of EUV spectra identified in this work are summarized in tables with transitions for each impurity species.

Experimental and numerical modelling of picosecond laser ablation of thin aluminium Polyethylene Terephthalate (PET) films

An experimental and theoretical study on 10 ps single pulse ablation of 10 – 50 nm aluminium films on Polyethylene Terephthalate (PET) substrate has been carried out. Both front and rear side ablation with different film thicknesses were measured experimentally and modelled numerically. It was found that the ablation threshold varied linearly with film thickness between 10 nm and 50 nm and showed a reasonable experimental and theoretical agreement. The rear side ablation threshold is always lower than the front side, with tearing at the edges. The front and rear side ablation thresholds for a 10 nm thick film were measured to be Fth(f) = 0.021 ± 0.008 Jcm−2 and Fth(r) = 0.016 ± 0.002 Jcm−2 respectively, with a ratio of Fth (f)/Fth(r) ∼ 1.3. This ratio drops to ∼ 1.1 with 50 nm films. The maximum electron temperature of the aluminium is also related to the film thickness. The highest electron temperature appears with the thinnest film, optical density = 0.5, t ∼ 10 nm, reaching Te ∼ 7,600 K under fluence F = 0.20 Jcm−2 at a delay time t = 13 ps while the thermal equilibrium is established at ∼ 30 ps. There is also a temperature gradient at the Al/PET interface due to the electron insulation and the sharp transition of the Al and PET thermal diffusivity. With a PET damage threshold, Fth PET = 3.46 Jcm−2, a clear processing window avoiding PET substrate damage was observed in single pulse exposure, and multi-beam parallel processing with the aid of a spatial light modulator was demonstrated, accelerating film patterning significantly.

Plasma assisted CO2 dissociation in pure and gas mixture streams with a ferroelectric packed-bed reactor in ambient conditions

Carbon dioxide decomposition is a challenging target to combat climate change. Nonthermal plasmas are advantageous for this purpose because they operate at ambient conditions and can be easily scaled-up. In this study, we attempt the CO2 splitting into CO and O2 in a parallel plate packed-bed plasma reactor moderated with Lead Zirconate Titanate (PZT) as ferroelectric component, achieving conversion rates and energy efficiencies higher than those obtained with BaTiO3 in our experimental device. The analysis of the reaction mechanisms with optical emission spectroscopy under various operating conditions has shown a direct correlation between energy efficiency and intensity of CO* emission bands. These results and those obtained with a LiNbO3 plate placed onto the active electrode suggest that high temperature electrons contribute to the splitting of CO2 through an enhancement in the formation of CO2+ intermediate species. Results obtained for CO2 + O2 mixtures confirm this view and suggest that back recombination processes involving CO and O2 may reduce the overall splitting efficiency. The study of mixtures of CO2 and dry air has proved the capacity of ferroelectric packed-bed reactors to efficiently decompose CO2 with no formation of harmful NXOY subproducts in conditions close to those in real facilities. The found enhancement in energy efficiency with respect to that found for the pure gas decomposition supports that new reaction pathways involving nitrogen molecules are contributing to the dissociation reaction. We conclude that PZT moderated packed-bed plasma reactors is an optimum alternative for the decompositon of CO2 in real gas flows and ambient conditions.

Development of Langmuir probe array for the new lower tungsten divertor in EAST

The divertor Langmuir probes (Div-LPs) system is a key diagnostic, which is widely used in advanced tokamaks including ITER, for divertor plasma measurement. With the improvement of EAST performance by more available heating power, the lower graphite divertor has been upgraded to a new tungsten structure recently. Therefore, the Div-LPs for the EAST lower divertor have undergone an overall upgrade, including the structure of the probe tip, the layout of the probe arrays and the optimized power supply. In the EAST 2021 experimental campaign, the new Div-LPs, which can measure the electron temperature, plasma density, heat and particle fluxes at the divertor plates, have been tested by long-term discharges with long-pulse plasmas. In the strike point sweeping experiments, the particle flux distribution and evolution measured by the Div-LPs match the inversed strike point by EFIT code excellently. In addition, the Div-LPs have been successfully employed in long-pulse plasmas over 100 s with high core electron temperature operation. In the long-pulse discharges, the data measured by Div-LPs are reliable by cross-validation with key plasma parameters. Moreover, it is observed that the electron temperature, particle and heat fluxes measured by Div-LPs significantly drop after impurity seeding, which is consistent with the surface temperature measured by the infrared camera. These experimental results demonstrate the success on the upgrade of the new divertor Langmuir probe system, which will provide essential diagnostic data for divertor physics study and plasma control, especially for long-pulse high-performance operation.

Understanding pulsed laser deposition process of copper halides via plasma diagnostics techniques

Due to the raised interest in copper halides for optoelectronic applications, a wide range of plasma diagnostic tools based on Langmuir probe (LP) and optical emission spectroscopy (OES) were implemented to investigate the pulsed laser deposition (PLD) process of selected copper halides. This represents the first report of complete diagnostics of copper halide plasma generated by laser ablation. High plasma potential and electron temperature were observed by a time-resolved LP analysis, which affected the kinetic energy of the ejected ions during expansion. The effect of argon gas addition on the copper halide plasma dynamics was investigated. Angle-resolved measurements revealed a narrow cone plasma expansion with a dual peak charge particle distribution, which was affected only above 2 Pa of Ar atmosphere. Plasma potential, charge density, and electron excitation temperature were increasing with Ar pressure, while electron temperature and expansion velocities were decreasing. Using a semi-empirical model, the estimation of an ion acceleration field, neutral temperature, and densities were computed. Space- and time-resolved OES confirmed the plume splitting indicated by LP measurements and provided insight into the composition of the two-plasma structure. All the plasma parameters were investigated in conditions similar to those used for thin film growth, and the results were discussed with respect to the PLD technology.

Modelling the influence of meteoric smoke particles on artificial heating in the D-region

Abstract. We investigate if the presence of meteoric smoke particles (MSPs) influences the electron temperature during artificial heating in the D-region. By transferring the energy of powerful high-frequency radio waves into thermal energy of electrons, artificial heating increases the electron temperature. Artificial heating depends on the height variation of electron density. The presence of MSPs can influence the electron density through charging of MSPs by electrons, which can reduce the number of free electrons and even result in height regions with strongly reduced electron density, so-called electron bite-outs. We simulate the influence of the artificial heating by calculating the intensity of the upward-propagating radio wave. The electron temperature at each height is derived from the balance of radio wave absorption and cooling through elastic and inelastic collisions with neutral species. The influence of MSPs is investigated by including results from a one-dimensional height-dependent ionospheric model that includes electrons, positively and negatively charged ions, neutral MSPs, singly positively and singly negatively charged MSPs, and photochemistry such as photoionization and photodetachment. We apply typical ionospheric conditions and find that MSPs can influence both the magnitude and the height profile of the heated electron temperature above 80 km; however, this depends on ionospheric conditions. During night, the presence of MSPs leads to more efficient heating and thus a higher electron temperature above altitudes of 80 km. We found differences of up to 1000 K in electron temperature for calculations with and without MSPs. When MSPs are present, the heated electron temperature decreases more slowly. The presence of MSPs does not much affect the heating below 80 km for night conditions. For day conditions, the difference between the heated electron temperature with MSPs and without MSPs is less than 25 K. We also investigate model runs using MSP number density profiles for autumn, summer and winter. The night-time electron temperature is expected to be 280 K hotter in autumn than during winter conditions, while the sunlit D-region is 8 K cooler for autumn MSP conditions than for the summer case, depending on altitude. Finally, an investigation of the electron attachment efficiency to MSPs shows a significant impact on the amount of chargeable dust and consequently on the electron temperature.

Compositional Variation of the Dayside Martian Ionosphere: Inference from Photochemical Equilibrium Computations

Abstract The Martian ionosphere plays a crucial role in driving the interactions of the planet with solar photons and solar wind particles. The variations of the dayside Martian ionosphere with several controlling factors, including the solar extreme ultraviolet radiation, the background atmosphere, and the underlying thermal structure, have been the topic of extensive research in terms of electron distribution. In contrast, how the ionospheric composition varies has not been systematically investigated, a topic that is attempted in this study based on photochemical equilibrium computations performed at 100–200 km altitude, including a large number of reactions. Our calculations reveal the following compositional variations as natural outcomes of the ionospheric chemistry on Mars. (1) With increasing solar irradiance, the proportions of the majority of nonterminal ions are enhanced at the expense of reduced proportions of terminal ions, including O 2 + , HCO+, NO+, and H3O+. (2) At high electron temperatures, the proportion of NO+ is modestly reduced, whereas the proportions of the other species are nearly unaffected. (3) The response of the ionospheric composition to the upper atmospheric composition is complicated, showing the strong negative response of many trace ions to ambient CO2, O, and CO, as well as the strong positive response of protonated ions to H2, nitrogen-bearing ions to N and N2, water-group ions to H2O, and HO 2 + to O2. As an application of the model results, the recent ion measurements made on board the Mars Atmosphere and Volatile Evolution are used to provide hints about the realistic composition of the Martian upper atmosphere.