High-temperature Plasma Research Articles

Effect of H Plasma Treatment of the Nucleation Layer on AlN Epitaxy in Microwave Plasma Chemical Vapor Deposition

The crystallinity of nucleation layers has significant influence on the crystallinity of aluminum nitride (AlN) epilayers. In this work, the etching effect of hydrogen (H) plasma was used to improve the nucleation-layer crystallinity. AlN nucleation layers were first cooled in a H-plasma atmosphere; then, AlN films were deposited on the processed nucleation layers, and the whole epitaxial process was carried out in high-temperature microwave plasma chemical vapor deposition (MPCVD) without the participation of ammonia. X-ray rocking curves (XRCs) showed that the cooling process obviously improved the crystal quality of AlN films, which was attributed to the preferential etching effect of H plasma on low-crystallinity nucleation islands. When a H2 flow rate of 135 sccm was used to process the nucleation layer, the AlN film with the narrowest (002) XRC full width at half-maximum (FWHM) of 86 arcsec was obtained. Possible growth mechanisms were suggested for the observations.

Modelling post-plasma quenching nozzles for improving the performance of CO2 microwave plasmas

Given the ecological problems associated to the CO2 emissions of fossil fuels, plasma technology has gained interest for conversion of CO2 into value-added products. Microwave plasmas operating at atmospheric pressure have proven to be especially interesting, due to the high gas temperatures inside the reactor (i.e. up to 6000 K) allowing for efficient thermal dissociation of CO2 into CO and O2. However, the performance of these high temperature plasmas is limited by recombination of CO back into CO2 once the gas cools down in the afterglow. In this work, we computationally investigated several quenching nozzles, developed and experimentally tested by Hecimovic et al., [1] for their ability to quickly cool the gas after the plasma, thereby quenching the CO recombination reactions. Using a 3D computational fluid dynamics model and a quasi-1D chemical kinetics model, we reveal that a reactor without nozzle lacks gas mixing between hot gas in the center and cold gas near the reactor walls. Especially at low flow rates, where there is an inherent lack of convective cooling due to the low gas flow velocity, the temperature in the afterglow remains high (between 2000 and 3000 K) for a relatively long time (in the 0.1 s range). As shown by our quasi-1D chemical kinetics model, this results in a important loss of CO due to recombination reactions. Attaching a nozzle in the effluent of the reactor induces fast gas quenching right after the plasma. Indeed, it introduces (i) more convective cooling by forcing cool gas near the walls to mix with hot gas in the center of the reactor, as well as (ii) more conductive cooling through the water-cooled walls of the nozzle. Our model shows that gas quenching and the suppression of recombination reactions have more impact at low flow rates, where recombination is the most limiting factor in the conversion process.

Preparation of diamond on GaN using microwave plasma chemical vapor deposition with double-substrate structure

Combining diamond with GaN can significantly improve the heat dissipation performance of GaN-based devices. However, how to avoid the destructive damage to the GaN epi-layer caused by high-temperature hydrogen plasma during the diamond growth is still a problem. This study employed a Si transition layer and double-substrate structure microwave plasma chemical vapor deposition (MPCVD) to prepare diamond film on GaN epi-layer. The effects of double-substrate structure on the diamond growth were studied. The microwave plasma parameters of both single-substrate structure and double-substrate structure MPCVD diagnosed by emission spectra were comparatively investigated. It has been found that the microwave plasma energy of double-substrate structure MPCVD is relatively more concentrated and has higher radicals activity, which is beneficial to the diamond growth. The impacts of the Si transition layer on the diamond growth were also investigated. It demonstrates that the Si transition layer can effectively protect the GaN epi-layer from being etched by hydrogen plasma and improve the diamond growth. The relationship between the thickness of the Si transition layer and the diamond growth and the relationship between diamond film thickness and adhesion has been studied in detail.

Positive self-bias in a magnetized CCP discharge

Radio frequency (RF) plasmas are commonly used for surface treatments and plasma heating processes. Controlling the heat flux from the plasma to the RF electrode is a crucial issue for optimizing these processes and is, therefore, the subject of considerable research in the low- and high-temperature plasma physics communities. In an asymmetric capacitively coupled plasma discharge, the ions accelerated by the direct current (DC) self-bias are the prime factor of the wall heating process. In this work, investigations have been performed with the aim to act on the DC self-bias in a linear magnetized RF environment. The lateral side and one face of the electrode have been covered by ceramic in order to limit the electron flux toward these surfaces. The variations in DC self-bias voltage as a function of the gas pressure, coupled RF power, and tilt angle between the RF electrode and the axial magnetic field have been studied. A new regime was discovered at low pressures, higher magnetic fields, and grazing angles for which the self-bias is positive. An analytical model was developed, which is in agreement with the experimental results.

Electromagnetic Phenomena Accompanying the Passage of an Iron Meteorite Through the Earth’s Atmosphere

Based on observational facts, the author proposes a model for the formation of an electric charge in iron meteorites in the lower layers of the atmosphere through photoelectron emission when exposed to high-temperature plasma radiation on its surface. It is shown that at high Mach numbers, the explosive destruction of an iron meteorite can be caused not only by inertial, but also by electrostatic forces. The sub sequent charging of the fragments greatly increases their total charge, and the movement with acceleration causes electromagnetic radiation in the low frequency range. The electrostatic and electromagnetic fields accompanying the movement of meteoroids in the atmosphere can be the cause of the observed effects on the Earth’s surface, i.e., radio interference, electric shock to a person, and St. Elmo’s fire, among others.

Spray coagulation as an alternative to argon plasma coagulation for bleeding portal hypertensive gastropathy

Spray coagulation as an alternative to argon plasma coagulation for bleeding portal hypertensive gastropathy

Production of a large volume non-equilibrium region in an atmospheric argon arc plasma with a counter injection of cold gas from an annular anode

In this letter, an annular anode is designed for producing arc plasmas with a large non-equilibrium region by using a counterflow cold gas through the annular anode. The coupled mass-momentum-energy exchange processes in an argon arc plasma are studied numerically and experimentally. The counter-injection of the cold argon gas from the center of the anode leads to a steep gradient of the heavy-particle temperature due to the formation of a thin stagnation layer resulting from the interaction of the high temperature plasma with the cold gas; and in particular, a large volume non-equilibrium ‘dark’ plasma region is obtained above the anode surface. The results show that, with the enhancement of the convective heat transfer process in the plasma core region, the fraction of the non-equilibrium region to the whole arc plasma region reaches 92.2% where the heavy-particle temperature can be reduced significantly, e.g. ∼2300 K, while simultaneously, the electron temperature and number density are remained at high levels greater than 8000 K and 2.4 × 1020 m−3, respectively, under the operating condition studied in this letter. This research not only deepens the understanding to the non-equilibrium synergistic transport mechanisms of arc plasmas, but also provides a method for producing a large volume non-equilibrium plasma region so as to promote various existing applications, or even creating new applications in the future.

First measurements of p11B fusion in a magnetically confined plasma

Proton-boron (p11B) fusion is an attractive potential energy source but technically challenging to implement. Developing techniques to realize its potential requires first developing the experimental capability to produce p11B fusion in the magnetically-confined, thermonuclear plasma environment. Here we report clear experimental measurements supported by simulation of p11B fusion with high-energy neutral beams and boron powder injection in a high-temperature fusion plasma (the Large Helical Device) that have resulted in diagnostically significant levels of alpha particle emission. The injection of boron powder into the plasma edge results in boron accumulation in the core. Three 2 MW, 160 kV hydrogen neutral beam injectors create a large population of well-confined, high -energy protons to react with the boron plasma. The fusion products, MeV alpha particles, are measured with a custom designed particle detector which gives a fusion rate in very good relative agreement with calculations of the global rate. This is the first such realization of p11B fusion in a magnetically confined plasma.

Damping and Dispersion of Non-Adiabatic Acoustic Waves in a High-Temperature Plasma: A Radiative-Loss Function

The behavior of acoustic waves in a rarefied high-temperature plasma is studied; as an example, the plasma of the solar corona is considered. Effects of thermal conductivity and a heating/radiative loss are taken into account; data on a temperature distribution of a radiation intensity obtained from the CHIANTI 10 code are used. The classical Spitzer expression for a full-ionized plasma is used for the thermal conductivity. Based on the found values of the radiation-loss function, the cubic spline method is used to construct an approximate analytical expression necessary for studying linear waves. A dispersion relation is obtained, and a frequency, a phase speed, and a damping coefficient are found. Dispersion and damping properties are considered for a temperature of about 106 K and a particle density of about 1015m−3, which are typical for the coronal plasma. In sum, superiority in the dispersion and damping of the thermal conduction is shown; the heating and radiation loss manifest themselves at large wavelengths. In accordance with general results by Field, a condition was found under which the acoustic oscillations become unstable. It is shown that at certain values of the temperature and density, the wave damping is dominated by the heating/radiative loss misbalance. Thus, the earlier results on mechanisms of damping of observed acoustic waves in the solar corona are refined here.

Detection and quantification of Pb and Cr in oysters using laser-induced breakdown spectroscopy

The quantitative determination of heavy metals in aquatic products is of great importance for food security issues. Laser-induced breakdown spectroscopy (LIBS) has been used in a variety of foodstuff analysis, but is still limited by its low sensitivity when targeting trace heavy metals. In this work, we compare three sample enrichment methods, namely drying, carbonization, and ashing, for increasing detection sensitivity by LIBS analysis for Pb and Cr in oyster samples. The results demonstrate that carbonization can remove a significant amount of the contributions of organic elements C, H, N and O; meanwhile, the signals of the metallic elements such as Cu, Pb, Sr, Ca, Cr and Mg are enhanced by 3–6 times after carbonization, and further enhanced by 5–9 times after ashing. Such enhancement is not only due to the more concentrated metallic elements in the sample compared to the dried ones, but also the unifying of the matter in carbonized and ashed samples from which higher plasma temperature and electron density are observed. This condition favors the detection of trace elements. According to the calibration curves with univariate and multivariate analysis, the ashing method is considered to be the best choice. The limits of detection of the ashing method are 0.52 mg kg−1 for Pb and 0.08 mg kg−1 for Cr, which can detect the presence of heavy metals in the oysters exceeding the maximum limits of Pb and Cr required by the Chinese national standard. This method provides a promising application for the heavy metal contamination monitoring in the aquatic product industry.

Detailed Analysis of Spectra from Ga-like Ions of Heavy Elements Observed in High-Temperature Plasmas

This study has systematically investigated the atomic number (Z) dependence of spectra from gallium-like (Ga-like) ions of heavy elements. We have mainly analyzed the experimental spectra recorded in high-temperature plasmas produced in the Large Helical Device (LHD) for various elements with atomic numbers from 57 onward. The measured wavelengths are compared with theoretical values calculated with a multi-configuration Dirac Fock code. As a result, we have successfully obtained Z-dependent wavelengths of several prominent transitions of Ga-like ions, including a magnetic dipole (M1) transition. Many of them have been experimentally identified for the first time in this study. The present results manifest the significant effects of configuration interaction and spin–orbit interaction for highly charged heavy ions.

Passivation of oxygen vacancy defects in conductive ZnO nanoparticles via low-temperature annealing in NF3

Oxygen vacancies (VO) often exist in wide-bandgap metal oxide semiconductors (MOSs) as deep-level defects and undermine the reliability of various optoelectronic devices based on MOSs. Conventional methods to passivate VO defects, such as high-temperature annealing or plasma treatment, can compromise device performance. This work demonstrates that passivation of VO defects in aluminum-doped ZnO (AZO) nanoparticles can be realized through low-temperature annealing (350 °C) in an NF3 atmosphere. After the NF3 annealing, the longitudinal optical phonon scattering mode in Raman spectroscopy, the visible light emission intensity in photoluminescence spectroscopy, and the oxygen deficiency-related peak in x-ray photoelectron spectroscopy decrease simultaneously, indicating the passivation of VO defects in AZO nanoparticles. As a result, AZO nanoparticles show higher visible light reflectance and better stability of electrical conductivity owing to the suppression of deep-level light absorption and gas molecule adsorption. This work also offers insights into the passivation mechanism of VO defects in MOSs.

Analysis of molten slag from high-temperature plasma treatment of oil-based drill cuttings

Traditional methods of disposing of oil-based drill cuttings do not provide the much-needed benign treatment. In this study, a skid-mounted thermal plasma processing device was developed with a designed maximum processing capacity of 0.8 kg/min. When the input power of the plasma torch is 10–50 kW, the temperature measured by the thermocouple is 385–1979 °C. After plasma treatment, the morphology of oil-based drill cuttings changes from jelly-like to vitreous state solid structure, and the content of organic matter decreases with the decrease in weight. Mass and volume reductions of 40.98–61.49% and 25.02–68.81%, respectively, were achieved in a treatment period of 0.5–3 min. The total organic compound reduction of 73.08–98.12% was equally achieved in the same treatment period. Due to the special structure of the glassy state residue, the leached concentration of toxic metals is far below the Chinese standard for landfill leachate discharge. This effectively solves the problem of secondary pollution of solid waste. Therefore, thermal plasma technology is an effective alternative method to treat oil-based drill cuttings.

Probing local electron temperature and density inside a sheared flow stabilized Z-pinch using portable optical Thomson scattering.

We report the first optical Thomson scattering measurements inside a high electron temperature (≳1 keV) and moderate electron density (mid 1016 cm-3) plasma. This diagnostic has been built to provide critical plasma parameters, such as electron temperature and density, for Advanced Research Projects Agency-Energy-supported fusion-energy concepts. It uses an 8J laser at 532nm in 1.5ns to measure the high frequency feature of the Thomson scattering profile at 17 locations along the probe axis. It is able to measure electron density from 5 × 1017 cm-3 to several 1019 cm-3 and electron temperatures from tens of eV to several keV. Here, we describe the design, deployment, and analysis on the sheared flow stabilized Z-pinch machine at Zap Energy named FuZE. The probe beam is aimed at an axial distance of 20cm from the central electrode and is timed within the temporal envelope of neutron emission. The high temperature and moderate density plasmas generated on FuZE lie in an unconventional regime for Thomson scattering as they are between tokamaks and laser-produced plasmas. We described the analysis considerations in this regime, show that the electron density was below 5 × 1016 cm-3 at all times during these measurements, and present a sample shot where the inferred electron temperature varied from 167 ± 16eV to 700 ± 85eV over 1.6cm.

On the Formation of a Plasma Cloud at the Ablation of a Pellet in a High-Temperature Magnetized Toroidal Plasma

The investigation of cold secondary plasma clouds near pellets ablating in the hot plasma of magnetic confinement devices (tokamaks and stellarators) provides valuable information on the physical characteristics of a pellet cloud. In this work, the characteristic sizes of emitting clouds around fusible polystyrene pellets and refractory carbon pellets have been analyzed. The calculation of the ionization length of C+ ions in both carbon and hydrocarbon clouds has shown that the contribution of only hot electrons is insufficient to ensure the experimentally observed decay lengths of the CII line intensity. Taking into account the strong shielding of the electron flux of the background plasma in the hydrocarbon pellet cloud, the ionization of C+ ions in this cloud is determined predominantly by electrons of the cold plasma of the cloud. Shielding near a refractory carbon pellet is weak because its ablation rate is lower. The contributions from hot electrons of the surrounding plasma and cold electrons of the pellet cloud to the ionization of C+ ions are comparable in the case of carbon pellets.

H-mode dithering phase studies on ST40.

The dithering H-mode phase, characterized by oscillations, is generally observed at input power values close to the L-H transition power threshold and low plasma collisionalities (low electron density and/or high plasma temperature). Measurements to characterize the dithering phase are presented for the low aspect ratio, high magnetic field tokamak, ST40. The dithering phase oscillation frequency is observed between 400 and 800 Hz and demonstrates an inverse relationship with core plasma density. Dithering phase H-modes are documented across a nonlinear, low-density power threshold operational space, with signature low- and high-density branches. The minimum power threshold for dithering H-mode access is measured at a core, line average electron density of 4.7(±0.5) × 1019 m-3, close to a predicted value of 4.1(±0.4) × 1019 m-3 from multi-machine studies. ASTRA calculated values of power coupled to the ion species, at the dithering H-mode transition, exhibit a similar nonlinear dependence on density. This analysis points to the important contribution of the ion thermal channel to the L-H phase transition. The low-frequency plasma density and D-alpha dithers appear to be accompanied by sudden bursts of magnetohydrodynamic (MHD) activity. A simple model is tested to demonstrate a possible scenario of self-regulation among turbulence, zonal flows, pressure (density) gradient and MHD activities. This article is part of a discussion meeting issue 'H-mode transition and pedestal studies in fusion plasmas'.

Plasma heating and improvement of lower hybrid current drive efficiency by electron cyclotron waves on EAST

The electron cyclotron (EC) system on EAST consists of four gyrotrons with a frequency of 140 GHz (second harmonic of the extraordinary mode), each of which is expected to deliver a maximum power of 1.0 MW and be operated at 100-1000 s pulse length. Significant progress in long-pulse operation has been achieved recently, including the pulse duration up to 1056 s with EC power injected into plasma of 0.55 MW and the pulse duration of 310 s with EC power of 1.6 MW (output by 3 gyrotrons). High electron temperature (Te >12 keV) plasma measured by Thomson scattering was produced with the combination of EC and lower hybrid (LH) waves. It is found that the plasma heating effect depends on the EC power location greatly. By adjusting the EC power location, the plasma current profile can be modified. As a consequence of the increment of electron temperature by electron cyclotron resonance heating (ECRH), the lower hybrid current drive (LHCD) efficiency is improved, benefiting for the long-pulse operation. In addition, a synergy effect between EC and LH current drive was observed in steady-state operation on EAST.

Исследование эволюции структурно-фазового состояния образцов сплава W – Сu в графитовой оболочке в процессе вакуумных отжигов и воздействия высокотемпературной плазмы

The change in porosity during isothermal vacuum annealing of the pseudo alloy W – 18 wt. % Cu obtained by powder metallurgy methods in the temperature range of 800 – 1200 °C and the resistance to oxidation under the action of high-temperature plasma of the outer surface of a hybrid laboratory structure, consisting of two composite elements made of dissimilar materials: graphite and alloy W – 18 wt. % Cu. The process of evolution of the microstructure controlled by diffusion along the internal interfaces (grain and subgrain boundaries) of copper atoms from copper particles located on these surfaces in the near-surface layers of the sample has been studied.

Atomic simulation of surface damage of fused silica under laser irradiation

Fused silica optical element is the core component of the inertial confinement nuclear fusion ignition device. Due to the requirement of ignition conditions of the device for high power laser, the damage to fused silica optical element under strong laser is the key to restricting the operation of the ignition device. Therefore, the study of the surface damage of fused silica irradiated by laser is crucial to the development of the ignition device for inertial confinement nuclear fusion. In this paper, large-scale non-equilibrium molecular dynamics simulation method and micro-structure analysis technology suitable for dynamic process are proposed to study the damage process of fused silica surface under laser loading. Based on the theoretical study of high-temperature plasma fireball model, the damage of high-temperature fused silica plasma ball to surface is simulated. By tracking the local structure, temperature distribution and surface morphology, the factors affecting the surface damage of fused silica are analyzed. Our research results show that the size, distance from the surface, and temperature of high-temperature fused silica balls have important effects on the surface damage. We find that there are two different damage modes under the combined effect of the above factors. One is related to a rapid damage process, generating U-shaped voids and no further obvious damages after the surface spraying, and the other is dependent of a slow damage process: continuously expanding and resulting in a larger damage area. The surface morphologies formed by these two damage modes are consistent with the two typical damage morphologies observed in the experiments. This research can provide a guidance for understanding the complex damage process in fused silica under laser irradiation.

Preparation and properties of Al2O3-PF composite functional protective coating on the surface of polymer matrix composites

In this paper, a metal-free ceramic-resin (Al2O3-PF) composite protective coating was prepared on 3240 glass fiber reinforced epoxy resin by high-energy supersonic plasma spraying technology and “double channel and large temperature difference” powder feeding technology. The composite coating not only has good morphology and structure, but also has excellent performance in various properties. It is found that the bond strength of the composite coating can reach 29.60 MPa, and the shear strength can reach 24.24 MPa, far exceeding the research results in related fields at home and abroad. In addition, under the same wear conditions and high temperature ablation, the composite coating can increase the wear resistance and high temperature ablation resistance of the resin matrix by about 2 times, and the insulation resistance of the coating can still reach 2.43 × 1013 Ω after 500 times of high temperature plasma jet ablation, showing excellent insulation performance. However, due to the structural particularity of the composite coating, its erosion resistance and thermal expansion performance need to be further improved.