High-density Plasma Research Articles

Inductively Coupled Plasma Reactive Ion Etching of (−201) β‐Ga2O3 Nano‐Fins

This study reports the effect of inductively coupled plasma reactive ion etching using a BCl3/Ar gas mixture on the sidewall taper angle (STA), etch rate, and surface morphology of (−201) β‐Ga2O3 substrates, as well as (−201)‐oriented β‐Ga2O3 films on sapphire, for nano‐fins of ≈200 nm width. Various etch parameters are systematically investigated for the β‐Ga2O3 films on sapphire, revealing that the STA is lower at high plasma density when chemical component increases, and the ion scattering decreases; achieving a STA of 1.5° ± 0.7°, accompanied by an etch rate of 114 nm min−1 and a reduction in surface roughness compared to before etching. Additionally, a strong positive correlation between the STA and the crystal orientation in the β‐Ga2O3 substrates is observed at lower plasma density, while a weak correlation is found between the STA and the crystal orientation at high plasma density. This study provides valuable insights for the fabrication of β‐Ga2O3 devices.

Laser-based approach to measure small nuclear cross sections in plasma

We demonstrate an approach successfully measuring very small nuclear isomeric excitation cross sections (on the order of 10 to 100 picobarns) via laser-cluster interaction. The interaction between an intense laser pulse and Kr atomic clusters generates a high-temperature and high-density plasma ball in which nuclear excitations are facilitated by inelastic electron scattering. The electron temperature reaches several hundred keV (corresponding to 10[Formula: see text] K), similar to a stellar environment. The very small nuclear excitation cross sections are extremely challenging to measure otherwise, for example using traditional accelerator-based approaches. Our method opens a broad avenue of measuring small nuclear cross sections and holds promise for advancing understanding of nuclear transition mechanisms and exploring pathways in the evolutionary dynamics of elements within stellar environments.

Numerical simulation study on discharge characteristics of the Dual hollow cathode plasma source using magnetic mirror field

The Dual hollow cathode plasma source uses hollow cathode cylindrical sputtering and ionizing metal elements, which has a high material utilization rate and has potential application in the field of material modification. Its performance index requires the cooperation of electrode discharge parameters and the rational utilization of the inner surface of the hollow cathode. In this paper, a two-dimensional fluid model of a Dual hollow cathode structure is developed to simulate the basic distribution of the Dual hollow cathode discharge, which fills the central region of the Dual hollow cathode structure and the electrons oscillate on the axis. The effects of electrode parameters and hollow cathode radius on the discharge were investigated. The results show that the radial electron density of the hollow cathode increases with increasing heated cathode voltage, but as the hot cathode voltage continues to expand beyond 1200 V this trend becomes less obvious. The enhancement of the wall ion current by the heated cathode voltage is very pronounced, and the hollow cathode discharge is the result of the mutual enhancement of the heated cathode voltage and the hollow cathode voltage. Hollow cathode sputtering needs to reach a voltage threshold of 300 V, and the existence of an optimal radius (r = 4 mm−6 mm) to achieve a high plasma density and at the same time favor hollow cathode sputtering.

The impact of toroidal mode coupling on high-density discharges in J-TEXT

Density limit has long been a widely studied issue influencing the operating range of tokamaks. The rapid growth of the m/n = 2/1 (where m and n are poloidal and toroidal mode numbers, respectively) tearing mode is generally regarded as a primary precursor to the density limit disruption. In this experiment, the coupling of the m/n = 1/1 mode and the m/n = 2/1 mode in high-density plasma was observed. During a sawtooth cycle, the frequencies of the two modes gradually converge until they become equal. After that, toroidal coupling occurs between the 1/1 and 2/1 modes, resulting in a mutually fixed phase relationship. With the occurrence of toroidal coupling, the 2/1 mode is stabilized. Prior to the disruption, the cessation of the 1/1 and 2/1 mode coupling, along with the rapid growth in the amplitude of the 2/1 mode, can be observed. Additionally, under the same parameters, comparing discharges with or without the 1/1 mode, it is found that the presence of the 1/1 mode leads to higher plasma density and temperature parameters.

Ultrafast deposition of ultrahard Cr films in tubes by bipolar-HiPIMS with small target-substrate distance

The PVD deposition of films with excellent surface properties has been a persistent area of research. In this paper, bipolar high power impulse magnetron sputtering with small target-substrate distance is proposed, and ultra-hard Cr films have been deposited with a higher rate in tubes. The microstructure and mechanical properties of the films were characterized using X-ray diffraction, scanning electron microscopy, nanoindentation and scratch test, and compared with films prepared under conventional conditions. With target-substrate distance of 7.5 mm, the ultrafine columnar structure is fabricated due to intensive glow discharge and direct particle-bombardment in high-density plasma. The small grain size of 7.4 ± 0.3 nm is observed although deposition rate as high as 10 μm/hr. The nanohardness of the films reaches 19.94 ± 1.14 GPa, much higher than that reported in the literatures. Meanwhile, the adhesion between film and substrate may be as high as 77 ± 5 N.

A model for fast electron-driven high-density plasma in the double-cone ignition scheme

Abstract A model for fast electron-driven high-density plasma is proposed to describe the effect of injected fast electrons on the temperature and inner pressure of the plasma in the fast heating process of the double-cone ignition (DCI) scheme. Due to the collision of the two low-density plasmas, the density and volume of the high-density plasma vary. Therefore, the ignition temperature and energy requirement of the high-density plasma vary at different moments, and the required energy for hot electrons to heat the plasma also changes. In practical experiments, the energy input of hot electrons needs to be considered. To reduce the energy input of hot electrons, the optimal moment and the shortest time for injecting hot electrons with minimum energy are analyzed. In this paper, it is proposed to inject hot electrons for a short time to heat the high-density plasma to a relatively high temperature. Then, the alpha particles with the high heating rate and PdV work heat the plasma to the ignition temperature, further reducing the energy required to inject hot electrons. The study of the injection time of fast electrons can reduce the energy requirement of fast electrons for the high-density plasma and increase the probability of successful ignition of the high-density plasma.

Process optimization of wet etching for split gate trench MOSFET

Abstract In this paper, the influence of etching technology on the performance of Split Gate Trench (SGT) MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistor) is studied, and the significance and contribution of these improvements in the fields of mechatronics, robotics and control systems are discussed. Etching can be categorized into wet etching and dry etching, with wet etching being widely used for etching the gates of SGT MOSFETs. Due to the isotropic nature of wet etching, issues such as undercutting, trenching, and insufficient link-up region length can arise, leading to short circuits between the gate and the source. The improvement of this problem directly promotes the enhancement of mechatronics system efficiency, the improvement of robot dynamic performance and accuracy, and the realization of efficient control strategies in control systems. By optimizing the process flow to enhance the adhesion of photoresist, increase the density of the HDP (High Density Plasma) deposited oxide film, and adjust the etching rate and frequency of wet etching, it is possible to mitigate the issues of isotropy and insufficient precision in wet etching. These improvements address failure phenomena and enhance product yield.

Plasma discharge experiments of a Faraday shieldless driver for high-power and long-pulse radio frequency ion source for NBI application.

Due to the high stability and less maintenance, the radio frequency (RF) driven ion source is preferred for the neutral beam injection (NBI) system. In a popular design of the RF ion source for NBI application, a Faraday shield (FS) is installed inside the RF plasma driver to protect the discharge tube. However, the FS also brings some drawbacks, such as lowering the RF power transfer and increasing the processing difficulty. A prototype of the RF plasma driver without FS and with mature manufacturing technology has been developed by using a water-cooled discharge tube. After basic testing, this prototype was further tested under the RF plasma discharge experiments in views of high power, long pulse, and long term. The reliability and its plasma characteristics were the focus of these experiments, also for the hidden issues. The results show that the prototype could generate stable and high-density plasma without any damage or sputtering mark. An RF plasma discharge of 50 kW and 20s has been achieved. The expected frequency tuning was equally effective on the prototype. Moreover, compared to the RF plasma driver with FS, the prototype could produce higher electron density in the extraction region under the same RF power. Of course, some of the shortcomings of the prototype have also been exposed in the experiments and will be improved in subsequent experiments.

Charged-particle transport in high energy density plasmas

This Special Topic Collection grew out of two gatherings of researchers active in the high energy density (HED) physics community: a mini-conference on charged-particle transport in HED plasma held during the 64th annual meeting of the American Physical Society's Division of Plasma Physics (Spokane, WA, November 2022) and a dedicated charged-particle transport coefficient code comparison workshop (Livermore, CA, July 2023). These gatherings provided opportunities for theoretical, computational, and experimental researchers to discuss the state of the field, including current capabilities and methods, needs of hydrodynamic simulations, and frontiers for future research. This special issue collects a total of 13 research and review articles on charged-particle transport in HED plasmas.

High-quality GaN thin film deposition at low temperature by ECR plasma-assisted sputter deposition method and its dependence of sapphire substrate misorientation angle

Gallium nitride (GaN) thin films were deposited by electron cyclotron resonance (ECR) plasma-assisted sputtering, which combines GaN-magnetron sputtering with argon and nitrogen plasma assistance using an ECR high-density plasma. GaN films on the misorientation-angle-0.0° (just) sapphire substrate showed very good crystallinity with a GaN(0002) rocking curve (XRC) full width at half maximum (FWHM) of 0.042° and epitaxial growth confirmed by φ-scan measurements at a low heating temperature of 350 °C. However, the GaN thin film had a rough surface with circular grains about 100 nm in diameter and a surface root-mean-square height (Sq) of 1.21 nm. Therefore, the misorientation angle of the sapphire substrate was varied from 0.2° to 10.0°. As a result, the grains observed on the just substrate disappeared at 0.5°. The film had Sq: 0.33 nm, and the FWHM of the XRC of GaN(0002) was 0.066°, indicating improved surface flatness while maintaining crystallinity. This is considered to be due to the step flow, which promotes ECR plasma-assisted diffusion on the terrace even at the low temperature of 350 °C. The polarity of the GaN thin film was analyzed by time-of-flight atomic scattering surface analysis and found to be N-polar on all substrates.

Spatiotemporal dynamics of fast electron heating in solid-density matter via XFEL

High-intensity, short-pulse lasers are crucial for generating energetic electrons that produce high-energy-density (HED) states in matter, offering potential applications in igniting dense fusion fuels for fast ignition laser fusion. High-density targets heated by these electrons exhibit spatially non-uniform and highly transient conditions, which have been challenging to characterize due to limitations in diagnostics that provide simultaneous high spatial and temporal resolution. Here, we employ an X-ray Free Electron Laser (XFEL) to achieve spatiotemporally resolved measurements at sub-micron and femtosecond scales on a solid-density copper foil heated by laser-driven fast electrons. Our X-ray transmission imaging reveals the formation of a solid-density hot plasma localized to the laser spot size, surrounded by Fermi degenerate, warm dense matter within a picosecond, and the energy relaxation occurring within the hot plasma over tens of picoseconds. These results validate 2D particle-in-cell simulations incorporating atomic processes and provide insights into the energy transfer mechanisms beyond current simulation capabilities. This work significantly advances our understanding of rapid fast electron heating and energy relaxation in solid-density matter, serving as a key stepping stone towards efficient high-density plasma heating and furthering the fields of HED science and inertial fusion energy research using intense, short-pulse lasers.

Effect of gas injection pattern on magnetically expanding rf plasma source

Abstract Argon gas is injected from a back plate having either a radial center hole or shower-patterned eight holes into a 13.3-cm-diameter and 25-cm-long radio frequency (rf) plasma source attached to a 43.7-cm-diameter and 65cm-long diffusion chamber under an expanding magnetic field, which resembles the magnetic nozzle rf plasma thruster. The source has a double-turn loop antenna powered by a 13.56 MHz rf generator at a maximum power level of ~2.8 kW in low-pressure argon, providing a plasma density of about 1018 m−3 in the source. A high plasma density and a slightly low electron temperature are obtained for the shower-pattered case in both the source tube and the diffusion chamber, compared with the center hole case, suggesting that the neutral density profile significantly affects the plasma density profile. This result will provide an improvement in the thruster performance by the gas injection pattern.

Elevating electron energy gain and betatron x-ray emission in proton-driven wakefield acceleration

The long proton beams present at CERN have the potential to evolve into a train of microbunches through the self-modulation instability process. The resonant wakefield generated by a periodic train of proton microbunches can establish a high acceleration field within the plasma, facilitating electron acceleration. This paper investigates the impact of plasma density on resonant wakefield excitation, thus influencing the acceleration of a witness electron bunch and its corresponding betatron radiation within the wakefield. Various scenarios involving different plasma densities are explored through particle-in-cell simulations. The peak wakefield in each scenario is calculated by considering a long pre-modulated proton driver with a fixed peak current. Subsequently, the study delves into the witness beam acceleration in the peak wakefield and its radiation emission. Elevated plasma density increases both the number of microbunches and the accelerating gradient of each microbunch, consequently resulting in heightened resonant wakefield. Nevertheless, the scaling is disrupted by the saturation of the resonant wakefield due to the nonlinearities. The simulation results reveal that at high plasma densities, an intense and broadband radiation spectrum extending into the domain of the hard x-rays and gamma rays is generated. Furthermore, in such instances, the energy gain of the witness beam is significantly enhanced. The impact of wakefield on the witness energy gain and the corresponding radiation spectrum is clearly evident at elevated densities.

Characterization of the image plate multi-scan response to mono-energetic x-rays.

Image plates (IPs), or phosphor storage screens, are a technology employed frequently in inertial confinement fusion (ICF) and high energy density plasma (HEDP) diagnostics because of their sensitivity to many types of radiation, including, x rays, protons, alphas, beta particles, and neutrons. Prior studies characterizing IPs are predicated on the signal level remaining below the scanner saturation threshold. Since the scanning process removes some signal from the IP via photostimulated luminescence, repeatedly scanning an IP can bring the signal level below the scanner saturation threshold. This process, in turn, raises concerns about the signal response of IPs after an arbitrary number of scans and whether such a process yields, for example, a constant ratio of signal between the nth and n + 1st scan. Here, the sensitivity of IPs is investigated when scanned multiple times. It is demonstrated that the ratio of signal decay is not a constant with the number of scans and that the signal decay depends on the x-ray energy. As such, repeatedly scanning an IP with a mixture of signal types (e.g., x ray, neutron, and protons) enables ICF and HEDP diagnostics employing IPs to better isolate a particular signal type.

A spectroscopic analysis code for spatially resolved x-ray absorption data from the COAX platform.

Sophisticated tools such as computer vision techniques in combination with 1D lineout type analyses have been used in automating the analysis of spectral data for high energy density (HED) plasmas. Standardized automation can solve the problems posed by the complexity of HED spectra and the quantity of data. We present a spectroscopic code written for automated and streamlined analysis of spatially resolved x-ray absorption data from the COAX platform on Omega-60. COAX uses radiographs and spectroscopic diagnostics to provide shock position and density information. We also obtain the more novel spectral-derived spatial profile of the supersonic radiation flow into a low-density foam. Considerable effort has been spent modernizing our previous spectroscopic analysis method, including the development of new tools characterized by a faster runtime and minimal user input to reduce bias and a testing suite for verifying the accuracy of the various functions within the code. The new code analyzes our spectroscopic images in 1-2min, with added uncertainty and confidence.

TECXY simulations of the power exhaust in the multi-impurity plasma of DTT reactor

Reduction of the heat load to plasma-facing components is a crucial problem for future fusion reactors like Divertor Test Tokamak (DTT). Mitigation of the power load via increased plasma radiation with the use of puffed ions of impurities would be one way to mitigate power in the scrape-off layer. This paper presents a numerical investigation of the impact of seeded impurities on the radiation pattern and the power load to the divertor plates of the high-field DTT reactor in the single null (SN) configuration. The simulations have been done with the use of the TECXY code, which solves multi-species plasma transport equations for multiple impurity species simultaneously and all associated ionization stages in a two-dimensional poloidal geometry. TECXY represents the model of plasma transport in the scrape-off layer region by a classical set of transport equations of multi-species plasma derived by Braginskii. The paper aims to compare the mitigation capabilities of neon and argon impurities seeded in the plasma of the DTT device and to obtain a significant energy flux reduction to the target plate at the smallest possible impurity concentration. Performed investigations showed the effects of neon and argon impurities seeding separately for constant electron density at the separatrix. It has been found that the decrease in electron temperature on the divertor plates up to 3 eV at the outer and the inner divertor plates and the peak power load below 15 MW m−2 at the outer divertor plate can be achieved with argon seeding and much lower impurity concentration than that in the case of neon impurity seeding. Studies have shown the complexity of the effect of neon and argon impurities on the boundary plasma. It was found that the reduction of temperature and the power on both divertor plates was the most effective for the high upstream plasma densities. The results show also that diffusive perpendicular transport strongly affects impurity radiation and thus plasma condition at divertor plates.

Enhanced energy deposition of laser-produced proton beams in high-density plasmas due to beam density self-steepening

The energy deposition of laser-accelerated proton beams in solid-density plasmas with different ion charge numbers is studied with detailed one-dimensional particle-in-cell simulations. In the plasma with a high ion charge number, in which the plasma collision frequency approaches electron oscillation frequency, the beam protons are strongly decelerated by the electric field induced by the plasma return current. The energy deposition is further enhanced as the beam travel distance increases due to beam density self-steeping, which is also confirmed by multi-dimensional particle-in-cell simulation. A simple analytical model is proposed to estimate the characteristic travel distance for significant density self-steeping, showing agreement with the simulation results. While in the plasma with a low ion charge number, in which the plasma collision frequency is much smaller than the electron oscillation frequency, the proton beam is modulated significantly by the excited two-stream instability.

A Cross-Sectional Study on the Prevalence of Subclinical Hypothyroidism in Metabolic Syndrome Patients at a Tertiary Care Hospital.

Metabolic syndrome (MetS) is defined as a "constellation" of cardiometabolic risk factors, characterized by hypertension, atherogenic dyslipidemia, hyperglycemia, prothrombotic and proinflammatory conditions which, jointly, increase the risk of suffering cardiovascular diseases (CVD) and type 2 diabetes mellitus.Thyroid dysfunction is also believed to affect parameters such as high-density lipoprotein (HDL) cholesterol, triglycerides, plasma glucose, and blood pressure, in turn increasing the risk of CVD. Subclinical hypothyroidism (SCH), which independently raises the risk of CVDs and their associated complications, is more frequently detected in patients with MetS compared to the general population. When both conditions coexist, the risk of CVD and its complications is significantly heightened. The objective of the study was to find out the prevalence of SCH in patients with MetS. A prospective cross-sectional study was conducted in the Department of General Medicine, New Civil Hospital, Surat, Gujarat, India. Eighty patients who fulfilled the criteria for MetS by the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III), were taken into the study. A detailed history, anthropometric measurements, blood pressure, fasting blood glucose, lipid profile, and thyroid profile (Free T3, Free T4, and serum thyroid-stimulating hormone (TSH)) were undertaken. The thyroid profile was done by chemiluminescence immunoassay (CLIA) method. Out of 80 patients with MetS, 48 were female and 32 were male, with the overall mean age of the study population being 46.5±9.5 years. Among them, 23.7% of the population was found to be having thyroid dysfunction. Among the thyroid dysfunction, SCH was highly prevalent (18.8%), 3.8% patients had overt hypothyroidism and 1.3% patients had subclinical hyperthyroidism. There were no overt hyperthyroid patients in our study.HDL (mg/dl) and TSH (mIU/L) were significantly higher in the SCH group as compared to other types of hypothyroidism group (p-value < 0.05). There is a statistically significant prevalence of SCH (18.8%) in MetS patients. It is clear from the study that one-fifth of MetS patients or every fifth patient with MetS had SCH. Thus, looking proactively for SCH in MetS and treating it would prevent conversion to overt hypothyroidism and complications of MetS.

Observation of tungsten emission spectra up to W46+ ions in the Large Helical Device and contribution to the study of high-Z impurity transport in fusion plasmas

Spectroscopic studies of emissions released from tungsten ions combined with a pellet injection technique have been conducted in Large Helical Device for contribution to the tungsten transport study in tungsten divertor fusion devices and for expansion of the experimental database of tungsten line emissions. The spectral intensities of W5+, W24+–W28+, W37+, W38+, W41+–W43+, W45+, and W46+ emission lines were measured simultaneously over a wide wavelength range from x-ray to visible. Time evolutions of the various tungsten line spectra indicate that the tungsten confinement time depends on the electron density of the plasma and is long in high density plasmas, on the order of seconds, and short in low density plasmas, on the order of sub-seconds. When the confinement time was long, the tungsten ions remained in the plasma until the end of the discharge, changing their dominant charge with the change in electron temperature. When the confinement time was short, the tungsten ions rapidly decreased in all charge states and disappeared. Space-resolved EUV and visible spectroscopy measurements have revealed that tungsten ions stayed in the core region of the plasma with changing their dominant charge state depending on the electron temperature in the discharges with the long confinement time. Detailed analysis of soft x-ray emission suggested that the confinement time increases with density and becomes saturated when the central electron density exceeds 2 × 1013 cm−3.

Etch characteristics of cobalt thin films using high density plasma of CH3COCH3/Ar gas mixture

Co thin films masked with SiO2/Si3N4 layers were etched using a high-density plasma of a CH3COCH3/Ar gas mixture. As the concentration of CH3COCH3 increased, the etch rate of the Co thin films and etch selectivity decreased. Optimal etch profiles of the Co films without redeposition were achieved owing to the formation of Co compounds and a passivation layer, which facilitated a high degree of anisotropy. Moreover, the etch characteristics of the Co films were examined using the ICP RF power, dc-bias voltage to the substrate, and process pressure. The active species in plasmas and Co compounds formed during etching were investigated using optical emission spectroscopy and X-ray photoelectron spectroscopy. Finally, the Co thin films patterned with 300 nm lines were etched using a CH3COCH3/Ar gas mixture under optimized etch conditions. The findings suggest that a CH3COCH3/Ar gas mixture can serve as an effective etch gas for fabricating dry-etched Co thin films.