High-density Plasma Research Articles

Complex Whistler‐Mode Wave Features Created by a High Density Plasma Duct in the Magnetosphere

AbstractA Van Allen Probes observation of a high‐density duct alongside whistler‐mode wave activity shows several distinctive characteristics: (a)—within the duct, the wave normal angles (WNA) are close to zero and the waves have relatively large amplitudes, this is expected from the classic conceptualization of ducts. (b)—at L‐shells higher than the duct's location a large “shadow” is present over an extended region that is larger than the duct itself, and (c)—the WNA on the earthward edge of the duct is considerably higher than expected. Using ray‐tracing simulations it is shown that rays fall into three categories: (a) ducted (trapped and amplified), (b) reflected (scattered to resonance cone and damped), and (c) free (non‐ducted). The combined macroscopic effect of all these ray trajectories reproduce the aforementioned features in the spacecraft observation.

Transition of Cu film to Cu2O film through oxygen plasma treatment

Metal–oxide-semiconductor-based thin-film transistors have attracted increasing research attention owing to their wide bandgap and minimal leakage current. However, the prevalent n-type behavior of several oxide semiconductors prompted the exploration of p-type alternatives. Among these, Cu films are promising candidates for different fields. In this study, the effectiveness of oxygen plasma treatment for the transition of Cu films to their p-type oxide semiconductor phase, i.e., Cu2O, was demonstrated with a focus on low-temperature fabrication for future integration in flexible devices. We reported the adjustability of the oxidation depth of a Cu thin film and the effect of oxidation dynamics by controlling the bottom radio frequency power of a high-density plasma system. This novel approach enabled metal-to-semiconductor transitions and modulation of the physical properties of Cu2O thin films by inducing changes in their composition and microstrain. The oxidation dynamics, oxidation depth, crystallinity, and film surface were analyzed. Moreover, the changes in the optical bandgap were determined, with the transition of the Cu (111) phase to the Cu2O (111) phase confirmed as a function of the process time. As oxidation progressed, particles formed on the surface of the Cu2O thin film and the particle size increased. Further, the oxidized Cu thin film was determined to be Cu2O rather than CuO. Therefore, oxygen plasma treatment is expected to be a new approach to low-temperature oxidation.

A Vlasov-Fokker-Planck-Landau code for the simulation of colliding supersonic dense plasma flows

A Vlasov-Fokker-Planck-Landau (VFPL) code is developed for the study of colliding supersonic dense plasma flows, in which the VFPL equations for both electrons and ions are solved by the time splitting strategy in the two-dimensional Cartesian coordinate space and the two-dimensional velocity space (2D2V). To accurately handle two colliding supersonic plasma flows with their velocities even much higher than the thermal velocities of ions, the full Fokker-Planck-Landau operator is employed for the collision terms. Using advanced numerical methods such as the fast spectral method and the asymptotic-preserving scheme, the ion-ion and electron-electron collision terms can be properly solved with a relatively large time step so that both high accuracy and efficiency of the developed code can be achieved. Further, the quantum degeneracy effect due to the high density and relatively low initial temperature of the electrons is included in the model. In addition, both the energy and momentum conservation are well satisfied. The developed code provides a unique numerical tool to study the interactions between high-velocity high-density plasma flows, which may be encountered in some advanced schemes of inertial confinement fusion and laser-driven laboratory astrophysics experiments.

The wave mode transition of argon helicon plasma

In this paper, multiple wave modes and transitions of argon helicon plasma excited by a half right-helical antenna in a system without any reflection endplate are investigated experimentally and theoretically at increasing radio frequency (RF) powers and external magnetic fields. Experiments show that above a critical magnetic field strength and pressure (about 250 G and 0.3 Pa in this work), two to four distinct wave-coupled modes and transitions were observed at increasing RF powers and/or magnetic fields. Theoretical analysis based on the dispersion relationship shows that in a high magnetic field, the helicon wave of the lowest order of axial eigenmode is always excited first, then the higher-order axial or radial mode; hence, the plasma density increases after mode jumping. There are two mechanisms responsible for the wave mode transitions in the present system, i.e. axial and radial mode transitions due to the change of axial and radial wavenumbers from a lower eigenmode to a higher one. A higher plasma density and magnetic field are helpful for achieving more higher-order modes of helicon waves.

Current drive by using lower hybrid fast wave in VEST

An efficient central or off-axis current drive is necessary for the steady-state operation of tokamak fusion reactors. The fast wave branch in the frequency range above two times the lower hybrid resonance frequency at high density, the so-called lower hybrid fast wave (LHFW), could be such an efficient current drive scheme in high density and high temperature of reactor-grade tokamak plasmas. This is because it has a higher parallel wave electric field for efficient Landau damping, compared to the fast wave branches in other frequency ranges, and it can more deeply penetrate high density plasmas than the slow wave in the same frequency range. An experimental study has been carried out to confirm the feasibility, in collaboration with Korea Atomic Energy Research Institute, Seoul National University, KwangWoon University, and Korea Accelerator and Plasma Research Association, in VEST. The results show that plasma current can be driven by the fast electrons generated by the LHFW. The details are reported including the theoretical background and RF system as well as the experiment results.

A hybrid plasma model for Cr thin film deposition by deep oscillation magnetron sputtering

A time-dependent hybrid plasma model composed of a zero-dimensional global model and a two-dimensional fluid model is proposed for simulation of plasma chemistry and transportation of plasma during Cr thin film deposition by deep oscillation magnetron sputtering (DOMS). The global model deals with plasma reactions in the ionization region near the target with discharge voltage and current waveforms as inputs. The temporal plasma characteristics calculated by the global model are utilized as a boundary condition for the two-dimensional fluid model to simulate high-density plasma transportation in the diffusion region through the entire macropulse period. The full momentum equation taking inertia force into consideration is applied for ion momentum conservation in the fluid model instead of using the drift-diffusion approximation, which ensures validity of the simulation for low-pressure plasmas. The deposition flux as well as the kinetic and potential energy fluxes transferred to the growing films are calculated by the hybrid model. Microstructure evolution of the DOMS deposited Cr thin films from zone I to zone T is attributed to the growing kinetic and potential energies as the charging voltage increases according to the structure zone diagram. The deposition rate loss in DOMS is explained by the back attraction effect, sputtering yield effect, and densification of the films.

Influence of a laser intensity on EUV brightness and ion speed from a laser-assisted discharge-produced plasma

Laser-assisted discharge-produced plasma (LDP) is one of the ways to generate extreme ultraviolet (EUV) light used in the semiconductor manufacturing processes. This light source uses a pulsed laser and a high-current pulsed electrical discharge to make a high-temperature and high-density tin plasma. One of two rotating disk electrodes, of which surfaces are coated by liquid tin (Sn), is irradiated by the laser to produce tin plasma. The plasma propagates from one electrode (cathode) to the other (anode) and ignites an electrical breakdown between the electrodes. The low-inductance circuit connected to the electrodes provides a current of approximately 15 kA and 150 ns to the tin plasma. The plasma implodes due to its own magnetic pressure, and EUV radiation is emitted from the resultant hot and dense plasma. High-speed ions are also emitted from the plasma and limit the lifetime of the mirror used to collect the EUV light. We need to maximize the light emission and minimize the ion emission. The role of the laser is essential in the LDP EUV source not only to ignite the discharge but also to condition the initial plasma. It influences EUV energy, EUV brightness, and emitted ion speed distribution of the plasma. The experiment suggested that laser intensity higher than 25 GW/cm2 produced high EUV brightness and low emitted ion speed.

Assessing global beryllium erosion via tomographic reconstruction of 3D beryllium emission profiles in ITER

A new method for estimating the global erosion of beryllium (Be) in ITER is proposed. The method uses ray tracing-aided tomography to reconstruct the three-dimensional (3D) profile of beryllium visible-light emissivity in boundary plasma from images captured with filtered cameras of VIS/IR wide angle viewing system, H-alpha (and Visible) Spectroscopy diagnostics and signals collected with divertor impurity monitor. The light reflected into the detectors from metallic plasma-facing components (PFCs) is filtered out in the process. The reconstructed Be emissivity is then used to assess the Be influx density distribution along all Be PFCs by integrating the product of the emissivity and the S/XB coefficient along the normal to the PFC surface. The accuracy of this method is evaluated by a comparison with synthetic emissivity data produced by recent simulation of global Be erosion and migration in ITER using the ERO2.0 code. The impact of the uncertainty of PFC light reflection properties on the error in reconstructing the 3D Be emissivity profile and Be influx density is analyzed. The method allows to recover with good accuracy the Be influx density in plasma-wetted areas under the conditions of H-mode fusion power operation with high plasma density in far scrape-off layer (SOL). Under the conditions of lower far-SOL plasma density and L-mode operation, only the total Be influx integrated over the area of the first wall panels with relatively high Be erosion can be reconstructed with a high accuracy. It is shown that neglecting the effects of light reflection may lead to a twofold overestimation of the total Be influx.

Study of the spatial growth of stimulated Brillouin scattering in a gas-filled hohlraum via detecting the driven ion acoustic wave

In an experiment performed on the Shenguang-III prototype laser facility, collective Thomson scattering (TS) is used to study the spatial growth of stimulated Brillouin scattering (SBS) in a gas-filled hohlraum by detecting the SBS-driven ion acoustic wave. High-quality time-resolved SBS and TS spectra are obtained simultaneously in the experiment, and these are analyzed by a steady-state code based on the ray-tracing model. The analysis indicates that ion–ion collisions may play an important role in suppressing SBS growth in the Au plasma; as a result, the SBS excited in the filled gas region is dominant. In the early phase of the laser pulse, SBS originates primarily from the high-density plasma at the edges of the interaction beam channel, which is piled up by the heating of the interaction beam. Throughout the duration of the laser pulse, the presence of the TS probe beam might mitigate SBS by perturbing the density distribution around the region overlapping with the interaction beam.

Formation of fast-ignition hotspots and propagartion of burning waves in pre-compressed isochoric plasmas

The formation and evolution of hotspots is important for achieving ignition and high energy gain in inertial fusion process. However, most of relevant studies are carried out on pre-compressed plasmas with an isobaric configuration, the evolution of the hotspot in a plasma with an isochoric configuration is rarely studied. In this paper, a semi-analytical model is developed to describe the evolution of the hotspot boundary and propagation of fusion burning waves for a high-density pre-compressed plasma with an isochoric configuration in the double-cone ignition scheme. For the shock wave, the strong shock wave approximation and the quasi-isobaric approximation are reasonable. The quasi-isobaric approximation shows that as the plasma density behind the shock wave increases, the plasma temperature decreases. Because of these, the range of <inline-formula><tex-math id="M4">\begin{document}$ {\mathrm{\alpha }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231474_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231474_M4.png"/></alternatives></inline-formula>-particles decreases rapidly behind the shock wave, forming an <inline-formula><tex-math id="M5">\begin{document}$ {\mathrm{\alpha }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231474_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231474_M5.png"/></alternatives></inline-formula>-particle absorption peak. Therefore, considering that the hotspot is the main region where <inline-formula><tex-math id="M6">\begin{document}$ {\mathrm{\alpha }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231474_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231474_M6.png"/></alternatives></inline-formula>-particles are produced and deposited, the position of the shock peak can be used to identify the boundary of the hotspot in a high-density plasma with an isochoric configuration. It also shows that a “self-regulating burning process” exists in the burning process of the isochoric hotspot, most of <inline-formula><tex-math id="M7">\begin{document}$ {\mathrm{\alpha }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231474_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231474_M7.png"/></alternatives></inline-formula>-particles are deposited in the stable region and behind the shock, and finally, transport through the shock peak and heat the cold fuel, resulting in the temperature rising. In the high-density hotspots of plasma with an isochoric configuration, the deposition of α-particles behaves as an obvious non-uniform distribution effect. By analyzing the non-uniform deposition of α-particles, the deposition rate of α-particles at the edge of spherical uniform hotspot is calculated, then the temperature and density evolution of the isochoric hotspot can be well described. The model can be used to estimate the Lawson parameter of the hotspots at the end of the early stage of ignition. It is found that a lower fast electron energy is more beneficial to ignition and high gain operation of fusion plasma. It is also shown that the high density of the hotspots in the isochoric plasma will lead to a higher fusion burning rate, which can offset the negative influence of the shock wave and even achieve higher energy gain. The semi-analytical model is verified by the hydrodynamic simulations of O-SUKI-N.

Fast computation approach of electron-impact ionization and excitation cross-sections for atoms and ions with medium- and high-<i>Z</i> elements

The atomic data of medium- and high-<i>Z</i> elements, such as electron-impact ionization and excitation cross-sections, possess extensive applications in fields such as fusion science and X-ray interactions with matter. There are atoms and ions in high energy density plasma, with different charge states and energy states ranging from ground states to highly excited states, and the cross-sections of each charge state and energy state need to be calculated. The bottlenecks limiting computational performance are the inevitable relativistic effects of medium- and high-<i>Z</i> elements and the extremely complex electronic configurations. Taking tantalum (Ta) for example, by using the relativistic Dirac-Fock theory and distorted wave model, we compute the electron-impact ionization and excitation cross-sections of Ta from the ground state atom up to Ta<sup>72+</sup> with the incident electron energy range of 1–150 keV. The detailed configuration accounting (DCA) reaction channel cross-sections are derived by summing and weighting the original detailed level accounting (DLA) cross-sections. After examining the data, two regularities are found. In terms of DLA, the pre-averaging DCA cross-sections have varying initial DLA energy levels but are typically close to each other. There is not a straightforward function that can explain the discrepancies between them. In terms of DCA, inner subshells typically contribute very little to the total cross-section as their ionization and excitation cross-sections are orders of magnitude smaller than those of outer subshells. We provide two techniques to reduce the computational costs based on the regularities. To minimize the total number of DLA reaction channels used in the computation, the initial DLA energy levels can be randomly sampled. Through a Monte Carlo numerical experiment, we determine the appropriate number of sampling points that can reduce the total number of DLA channels by an order of magnitude while maintaining a 5% error margin. In terms of impact ionization, since small cross-section DCA channels are insignificant, only a tiny portion of the DCA channels are required to preserve a 95% accuracy of the entire cross-section. It is possible to use the analytical Binary Encounter Bethe (BEB) formula to determine which DCA channels should be neglected before the computation to reduce computational costs. In terms of electron-impact excitation, just the cross-sections of the same excited subshells as the preserved ionized subshells, which are determined in the previous electron-impact ionization (EII) calculations, are needed. Finally, we compare our EII results with theoretical and experimental results. In the low incident electron energy range of below 2 keV, our results accord with the theoretical result of the 6s EII cross-section of the Ta atom and the experimental result of the total EII cross-section of the Ta<sup>1+</sup> ion. In the high energy range of below 150 keV, our results are also consistent with the theoretical result of the 1s EII cross-section of the Ta atom and the experimental result of the 1s EII cross-section of the Cu atom. Our results reasonably match the previous experimental and theoretical results in low-energy range and high-energy range, inner subshell and outer subshell, indicating the accuracy of our calculation. The proposed optimizing strategy can be applied to various medium- to high-<i>Z</i> elements and is compatible to most computation codes.

Comparison of the evolution of Rayleigh–Taylor instability during the coasting phase of the central ignition and the double-cone ignition schemes

The Rayleigh–Taylor (RT) instability has been a great challenge for robust fusion ignition. In this paper, the evolution of the RT instability at the fuel inner interface during the coasting phase is investigated for the central ignition scheme [Hurricane et al., Rev Mod Phys. 95, 025005 (2023)] and the double-cone ignition (DCI) scheme [Zhang et al., Philos. Trans. R. Soc. A. 378, 20200015 (2020)]. It is found that the spherical convergent effect can be helpful for smoothing the disturbance by merging the spikes in the azimuthal direction. For the DCI scheme, the pressure gradient in the same direction with the density gradient at the fuel inner interface can further prevent the disturbance from growing. For the example case with an initial disturbance amplitude as large as 20 μm, the DCI scheme can still reach a high-density isochoric plasma with an areal density of 2.18 g/cm2 at the stagnation moment, providing favorable conditions for fast ignition by the relativistic electron beam.

Development and Validation of a Risk Prediction Model for NAFLD: A Study Based on a Physical Examination Population.

To construct and validate a precise and personalized predictive model for non-alcoholic fatty liver disease (NAFLD) to enhance NAFLD screening and healthcare administration. A total of 730 participants' clinical information and outcome measurements were gathered and randomly divided into training and validation sets in a ratio of 3:7. Using the least absolute shrinkage and selection operator (LASSO) regression and multiple logistic regression, a nomogram was established to select risk predictor variables. The NAFLD prediction model was validated through the receiver operating characteristic (ROC) curve, calibration plot, and decision curve analysis (DCA). After random grouping, the cohort comprised 517 in the training set and 213 in the validation set. The prediction model employed nine of the 20 selected variables, namely gender, hypertension, waist circumference, body mass index, blood platelet, triglycerides, high-density lipoprotein cholesterol, plasma glucose, and alanine aminotransferase. ROC curve analysis yielded an area under the curve values of 0.877 (95% Confidence Interval [CI]: 0.848-0.907) for the training set and 0.871 (95% CI: 0.825-0.917) for the validation set. Optimal critical values were determined as 0.472 (0.786, 0.825) in the training set and 0.457 (0.743, 0.839) in the validation set. Calibration curves for both sets showed proximity to the ideal diagonal, with P-values of 0.972 and 0.370 for the training and validation sets, respectively (P > 0.05). DCA indicated favorable clinical applicability of the model. We constructed a nomogram model that could complement traditional NAFLD detection methods, aiding in individualized risk assessment for NAFLD.

Experimental investigation into efficiency loss in rotating magnetic field thrusters

An experimental investigation into the low thrust efficiency of a rotating magnetic field (RMF) thruster is presented. This technology is a low maturity but potentially enabling candidate for high-power in-space propulsion for use with alternative propellants. Direct thrust stand measurements of a 5 kW class RMF thruster were performed and show the thrust efficiency was 0.41 ± 0.04% with a specific impulse of 292 ± 11 s—typical values for RMF thruster operation. A suite of far-field probes were used to inform a phenomenological efficiency model for RMF thruster performance that accounted for divergence, power coupling, mass utilization, and plasma/acceleration efficiency. It was found that the plasma efficiency was the critically low term at 6.4 ± 1.0%. This indicates that the majority of the energy coupled to the plasma from the RMF antennas was lost before being converted to directed kinetic energy in the thruster beam. To determine the source of these losses, time-resolved measurements of the internal plasma properties were performed using a triple Langmuir probe. It was found that collisional excitation radiation and wall losses were the two dominant loss processes. This trend can be explained by the unusually high plasma density ( >1019 m−3) exhibited by this device compared to other electric propulsion architectures. Limitations in the probing techniques and strategies for improving RMF thruster performance are discussed given the results from the efficiency analysis.

Investigation of plasma parameters, distributions, and optical emission for the anti-microbial performance of non-woven fabric under direct current glow discharge

The distribution of electron temperature Te and density Ne for direct current glow plasma discharge was investigated, using a single Langmuir probe, inserted inside the plasma cell. The radial temperature distribution has the same values, except with a small increment variation at the cathode edge, and an axial decrement for the temperature Te distribution profiles from the cathode fall region, passing the abnormal glow region, up to the faraway axial region.The radial distribution of the electron density Ne has its highest value at the cathode, with very intense plasma at the cathode fall region, and more Ne decrement in the abnormal glow region, passing the abnormal glow region up to the faraway axial region. In the axial Ne distribution, an increase in Ne from the cathode fall region reaches maximum values in the abnormal glow region and decreases in the faraway axial region.The optimal plasma surface treatment of non-woven silk fabric (n-WSF) can be achieved by utilizing a high plasma density and low energy of electrons to inactivate viable cells attached to (n-WSF) at very short application times, leading to complete inactivation, where the bacterial inactivation rate increases in the abnormal glow region. Based on analyses of the experimental data of initial and final densities of viable cells using survival curves in the abnormal glow discharge region, a dramatic inhibitory effect of plasma discharge on the residual survival microbe ratio was observed.

Quantitative analysis method of laser-induced breakdown spectroscopy based on temperature iterative correction of self-absorption effect

Laser-induced breakdown spectroscopy (LIBS) is an ideal real-time on-line method of detecting minor elements in alloys. However, in the case of laser-produced high-density plasma, the self-absorption is usually an undesired effect because it not only reduces the true line intensity, leading the line intensity to become nonlinear with the increase of emitting species content, but also affects the characterization parameters of the plasma, and finally affects the accuracy of quantitative analysis. Since the plasma electron temperature <inline-formula><tex-math id="Z-20240228161235">\begin{document}$(T)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231541_Z-20240228161235.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231541_Z-20240228161235.png"/></alternatives></inline-formula>, radiation particle number density and absorption path length (<i>Nl</i> ) determine the degree of self-absorption and affect the corrected spectral line intensity, a new self-absorption correction method is proposed based on temperature iteration. The initial <i>T</i> is obtained by using this method through spectral line intensity, and the self-absorption coefficient SA is calculated based on the initial <i>Nl</i> parameter to correct the spectral line intensity. Then a new <i>T</i> is obtained from the new spectral line intensity and the new SA is calculated to further correct the spectral line intensity. Through continuous calculation and correction of these two parameters, self-absorption correction is finally achieved. The experimental results of alloy steel samples show that the linearity of Boltzmann plot is increased from 0.867 without self-absorption correction to 0.974 with self-absorption correction, and the linear correlation coefficient <i>R</i><sup>2</sup> of the single variable calibration curve for Mn element increases from 0.971 to 0.997. The relative error of elemental content measurement is improved from 4.32% without self-absorption correction to 1.23% with self-absorption correction. Compared with the commonly applied self-absorption correction methods, this method has obvious advantages of simpler programming, higher computation efficiency, and its independence of the availability or accuracy of Stark broadening coefficients. Moreover, this method can directly obtain the radiation particle number density and absorption path length, which is beneficial to the diagnosis and quantitative analysis of plasma.

Simulation of effect of metal particles on breakdown process of three-electrode gas spark switches

Compared with two-electrode gas spark switch, three-electrode gas spark switch has the advantages of lower operating voltage, higher reliability and less discharge jitter, so it has been widely used in pulse power systems. However, due to the characteristics of pulse power technology, the gas spark switch is easy to cause ablation on the electrode surface during use, and the metal particles generated by ablation will significantly affect the stability and reliability of the switch. In this work the discharge process of the three-electrode gas spark switch under atmospheric pressure nitrogen environment is simulated first. In this model, the ionization coefficient near the trigger electrode is modified to compensate for the shortcomings of the local field approximation, and the relevant mathematical derivation process is given. The formation of the initial electrons is described by the field electron emission phenomenon, and the development process of electron collapse into the streamer is obtained. The physical mechanism of switch on is investigated, and the development process of each stage of switch discharge is described in detail. Then, the discharge process of the switch is studied when there are metal particles near the trigger. The study shows that the presence of metal particles enhances the electric field near the trigger and accelerates the formation of the initial electron cloud. In addition, in the presence of metal particles, the metal particles and the trigger will first break down, forming a high-density plasma channel after the breakdown, and becoming the source of the subsequent flow development. At the same time, because the metal particles on the channel have an obstructing effect on the streamer development, the streamer generates a discharge branch after contacting metal particles. In the end, the influences of metal particles of different shapes and sizes on the discharge process are discussed. The results show that metal particles with sharp shapes have stronger electric field distortion, when the electric field intensity is large enough, it may cause field emission on the surface of metal particle. And it is also made clear that the size of metal particle is small, the obstruction of the development path of streamer is small, and the streamers quickly converge behind the particles.

Experimental studies and COMSOL 1-D simulation in Ar capacitively coupled plasmas

This work systematically studies a capacitively coupled plasma (CCP) source using experiments and 1-D COMSOL simulations relevant to Ar plasmas. Two radio frequency compensated Langmuir probes (LPs) and optical emission spectroscopy (OES) were purposefully used to measure the plasma parameters, and the experimental results were compared with those of simulations. We studied the axial variation of plasma parameters using an axial LP between the power and ground electrodes of the CCP at various operating pressures ranging from 10 to 150 mTorr. The electron density showed a gradual increase in its value with rising pressures. In addition, we employed a radial LP at the axial location L = 4 cm from the surface of the power electrode to measure the plasma parameters and compare these data with those of the axial LP and simulations. The variations of plasma potential measured by the radial LP showed an opposite trend of variation to those of simulations and the axial LP at pressures 10–60 mTorr, which is attributed to the plasma diffusion at low pressures. LP and OES measurements and simulation data suggest stochastic heating that generates high electron temperatures at low pressures. In addition, data revealed that the high-density plasma generation at high pressures could be due to the effects of both collisional heating and stochastic heating. Analysis showed that electrons could gain energy from the strong field regime of the sheath closed to the electrodes, which has a similar variation to electron temperature. The results of simulations have shown excellent agreement with experiments, and this work has the basis for plasma applications like plasma-enhanced chemical vapor deposition.

Bright ultraviolet knots as possible sources of coherent microwave radiation

A distinctive feature of the September 6, 2012 event was that sources of narrow-band (2–4 GHz) sub-second pulses (SSP) were observed in small areas of flare loops with so-called bright ultraviolet knots with high plasma density up to 1011 10¹¹ cm⁻³. Time profiles of hard X-rays of the flare, although similar to microwave light curves, do not have structures corresponding to SSP. Analysis of microwave, X-ray, and ultraviolet data has shown that the observable pulses of microwave radiation with a narrow spectral band are coherent in nature and are generated by electrons with energies of several tens of kiloelectronvolt in bright knots at a double plasma frequency. The results of the observations suggest that the appearance of bright knots is associated with local processes of energy release due to interaction of flare loops.

Enhanced plasma nitriding assisted by combining hollow cathode discharge with rare earth oxide particles for iron-based metals

Plasma nitriding (PN) assisted by combining hollow cathode discharge (HCD) with rare earth (RE) oxide particles was conducted on Cr12MoV steel. The HCD generated a high-density plasma and induced the release of RE elements from the RE oxide particles, thereby promoting the formation of more ε-Fe2-3N phases and the diffusion of more nitrogen into the surface layer. The hardness of the nitrided layer with the assistance of the combination of HCD with RE oxide particles was superior to that achieved by HCD-assisted PN and conventional PN.