Hydrogen Plasma Research Articles

Mechanism of droplet ejection from liquid gallium interacting with inductively coupled hydrogen plasma

In this study, we observed droplet ejection from liquid gallium interacting with an inductively coupled hydrogen plasma. We observed the formation of a bubble in the liquid gallium, and the droplet ejection was synchronized with the burst of the bubble. We examined droplet ejection frequency by varying the ion flux –7.1) × 1014 cms, the ion energy (15–215 eV), and the liquid gallium temperature (370 K–500 K). As a result, we found that the droplet ejection frequency increased with the ion flux and the liquid gallium temperature, whereas it decreased with the ion energy. We observed that the inside of the bubble in the liquid gallium was occupied by hydrogen. Furthermore, the amount of hydrogen dissolved in the liquid gallium was dependent on the discharge conditions. On the basis of the experimental results, we have proposed a mechanism for bubble formation and droplet ejection. The key in the mechanism is the supersaturated dissolution of hydrogen due to the ion irradiation.

Observation of impurity accumulation and its compatibility with high plasma performance in W7-X

At the W7-X stellarator, the bolometer system has measured an intensive radiation zone in the inner plasma region (at a normalized radius ρ ∼ 0.3–0.4) in the hydrogen plasma generated by electron cyclotron resonance heating; it differs from the normal plasma radiation distribution with an edge-localized emission zone. Spectroscopic diagnostics have recorded high-Z elements such as iron. This phenomenon happens in the plasma phases after gas supply turn-off, which results in all impurity relevant diagnostic signals increasing for several seconds. Despite the enhancement of the core radiation, the plasma energy confinement is improved. A transport analysis shows that this impurity radiation behavior is associated with a low diffusion coefficient (D∼ 0.02 m2 s−1) and a reversal of the convection around the radial position of the emission peak, which, under normal conditions, separates the zones of outward convection in the central (|V| ∼ 0.1 m s−1) and inward convection in the outer region (|V| ∼ 0.3 m s−1). An impurity accumulation around this radial position has been identified. The transport coefficients obtained are comparable with the theoretical predictions of collisional impurity transport. In the plasma phases studied, both impurity and energy confinement are enhanced. The mechanism responsible for the improvement is believed to be a reduction of micro-instabilities associated with the observed steepening of the density profile, initiated by a low edge plasma density (<1.0 × 1019 m−3) after switching off the gas fueling. The normalized temperature and density gradients fulfil the condition for the suppression of ITG turbulence.

(Digital Presentation) Application of Plasma Cleaning in the Preservation and Restoration of Cultural Heritage

The chemical reactivity of metals at inappropriate climate conditions can be a threat to metallic cultural heritage. It is known that oxygen plays an important role in the corrosion and decay of metallic objects. Oxide formation is among the most prevalent destructive phenomena in this field. The metallic archaeological objects lose their original shape due to the oxide layer that develops on their surface. Among physical-mechanical and chemical techniques used in the past to remove the oxide layer from these antiques, Cold plasma technology has been proposed as a novel safe method to successfully achieve this goal. In a plasma medium, which can be formed by an electrical discharge, chemically reactive species exhibit a unique behavior in response to the injected gas type, electrical and chemical parameters, etc. This special behavior aids in achieving the user's goal through physicochemical phenomena. In this study, Hydrogen gas, with its reductive character, in the plasma phase provides free radicals of Hydrogen reacting to the copper oxide layer on ancient coins. As a result of this plasma reduction reaction, separation of the oxide layer from the copper surface is achievable. This phenomenon which is called Plasma cleaning, is a type of etching. Plasma cleaning does not destroy surface structure and information contained within metallic heritage artifacts. The feasibility of this non-destructive cleaning method has been investigated for the preservation and restoration of ancient copper coins dating back roughly 300 years ago. These ancient copper coins were exposed to a cold hydrogen plasma for 120 minutes. Results show that Tenorite and Spertiniite formed on the surface of coins have been successfully removed. The black color of Tenorite, a type of copper oxide, gives a dark appearance to copper coins' surface, and Spertitnite, as a copper hydroxide, exhibits a blue-green color on copper coins. According to the results, after 2 hours of hydrogen plasma treatment, not only these green spots have been removed, but also the dark appearance of the entire surface of the coin has been reduced. This amount of clearance is a good result considering the untreated area. Moreover, Figure 1 illustrates the coin's surface using SEM (A) before and (B) after Hydrogen plasma treatment. These images were obtained at a 200x magnification from the surface. In contrast to Figure 1(A), which had heavily polluted surfaces, case B's dark pollution has been removed, reducing the amount of darkness on the coin's surface. This study shows the efficacy of Plasma cleaning for the preservation and restoration of ancient copper coins. Figure 1

Impact of Bottom Electrode in HfO2-Based Rram Devices on Switching Characteristics

Resistive random-access memory (RRAM) devices with hydrogen plasma treated HfO2 have shown low power switching (1) and good conductance quantization with programing pulsed operation (2) that qualify them to be used for in-memory computing. Engineering the distribution of defects or oxygen vacancies near the top and bottom electrodes has a significant impact on reducing the switching power and improving the multi-level cell (MLC) characteristics of the device. A graded distribution with higher concentration of oxygen vacancies closer to the top electrode (TE) due to hydrogen plasma treatment and lower concentration near the bottom electrode (BE) reduces switching energy (power) (1). However, the impact of the bottom electrode is not well understood. In this work we investigated three RRAM stacks as shown in Figure 1 that have the same top electrode metal (50nm PVD TiN/5nm ALD TiN) and dielectric (HfO2 with hydrogen plasma treatment at the mid-point) but with different BE for each stack: TiN (device A (control)), TiN plus Mo with thermal anneal (device B), and TiN plus Mo with plasma nitridation (device C). Choosing a more inert metal as the BE provides a lower oxygen vacancy concentration at the interface between the BE and the dielectric (3) which in turn reduces the switching power as noted with device C (TiN plus Mo with plasma nitridation) which switches at a minimum compliance current (Icc) of 500pA. It is possible that the presence of nitrogen prevents the increase in oxygen vacancy concentration near the BE. On the other hand, a higher concentration of oxygen vacancies at the BE and the dielectric interface leads to a larger magnitude of band bending and increases the height of Schottky barrier. This possibly leads to an increase in the switching power in device A and device B as compared to device C (3-4). Note that device A (TiN) switched at Icc=30µA and device B (TiN plus Mo with thermal anneal) switched at Icc=8µA, which is superior compared to device A in switching power. This suggests that the alloy of TiN with Mo is reducing the oxygen vacancies near the BE. To study the MLC characteristics of the devices, we performed a train of 36 RESET pulses with fixed amplitude Vp= -1V and pulse width starting at 4µs and increasing by 20µs every 3 pulses, while we applied Vr=0.1V and tr=10µs after each pulse to measure the conductance. Figure 2 shows that device B (TiN plus Mo with thermal anneal) has good MLC with no overlap between the states while device A shows unstable MLC behavior due to time constant variation. Figure 3a shows the results of device C for the same RESET pulse train that was applied to devices A and B. Since device C switches at very low current and voltage, the conductance quantization was unstable after 15 pulses due to overstress of the conductive filament. Figure 3b shows the results after selecting the proper pulse set Vp= -0.5V and pulse width starting at tp=100ns and increasing by 100ns every 3 pulses, while we applied Vr=0.05V and tr=100ns after each pulse, resulting in a better conductance quantization compared to the previous setup. Appropriate engineering of the BE and managing of the oxygen vacancy distribution by hydrogen plasma treatment can significantly reduce the switching power.

Silicon Nanotubes on Copper Films: Additive-Free Anode Material for Lithium-Ion Batteries.

Thedevelopment of lightweight, long-lasting Li-ion batteries is of great technological importance for critical applications, including but not limited to aerospace and power grid applications. Increasing the specific capacity of the electrodes is the most direct strategy to increase the energy density of the battery. Currently, batteries with silicon anode materials have been commercially available only on a small scale or at a low composition of silicon. Great impact will be made when 100% silicon anode material can be fabricated at large scale. Specifically, the electric vehicle industry will experience disruption when a silicon solution enters the market which can offer durability as it delivers the expected high capacity.Silicon-based anode materials have a theoretical capacity an order of magnitude beyond the currently implemented graphitic materials- 3579 mAh/g when fully lithiated at room temperature to Li4Si15, as compared with 372 mAh/g. Silicon incorporates lithium into its microstructure as an alloying process as opposed to direct intercalation, like graphite; as such, a volume change of 300% causes mechanical degradation upon cycling if conventional processing is used. Nanoscale 0-D and 1-D materials have been shown to withstand volume expansion and enable Li-intercalation and de-intercalation reactions to occur; recent effective strategies include nanowires, nanoparticles embedded in polymeric matrices, and yolk-shell morphologies with carbon. [1-4]The abundance of silicon on earth is second only to oxygen; it makes up 28% by mass of the earth’s crust. The natural abundance, non-toxicity, extremely high specific capacity, low cost, chemical stability, and low average delithiation potential of silicon make it an ideal energy material. [5] Research interest has only increased over the last 15 years, as many simultaneous attempts are made at solving the challenges of nanoscale silicon- the successful execution of which will make large scale manufacturing of silicon anode materials possible. [6]The combination of nanosizing and engineering porosity within the silicon anode are the chief strategies to bring the cycle life necessary for commercialization. We report fabrication of anode material which consists of an additive free layer of silicon nanotubes bonded to copper foil, which has exhibited durability over 100 cycles. This promising novel silicon anode material has areal capacity of >2mAh·cm-2.[1] Keller, C., et al., Effect of Size and Shape on Electrochemical Performance of Nano-Silicon-Based Lithium Battery. Nanomaterials (Basel), 2021. 11(2). [2] Yang, K., et al., 3D growth of silicon nanowires under pure hydrogen plasma at low temperature (250 degrees C). Nanotechnology, 2021. 32(6): p. 065602. [3] Shi, J., et al., A surface-engineering-assisted method to synthesize recycled silicon-based anodes with a uniform carbon shell-protective layer for lithium-ion batteries. J Colloid Interface Sci, 2021. 588: p. 737-748. [4] Xiao Hua Liu, L.Z., Shan Huang, Scott X. Mao, Ting Zhu, and Jian Yu Huang, Size-Dependent Fracture of Silicon Nanoparticles During Lithiation. ACS Nano, 2012. 6(2): p. 1522-1531. [5] Naoki Nitta, F.W., Jung Tae Lee, and Gleb Yushin, Li-ion battery materials: present and future. Materials Today, 2015. 18(5). [6] Jaramillo-Cabanzo, D.F., et al., One-dimensional nanomaterials in lithium-ion batteries. Journal of Physics D: Applied Physics, 2021. 54(8). Fig. 1. Micro and nanostructure. Areal capacity. Figure 1

SiO2/SiC Nanowire Surfaces as a Candidate Biomaterial for Bone Regeneration

Tissue engineering (TE) and nanomedicine require devices with hydrophilic surfaces to better interact with the biological environment. This work presents a study on the wettability of cubic silicon-carbide-based (SiC) surfaces. We developed four cubic silicon-carbide-based epitaxial layers and three nanowire (NW) substrates. Sample morphologies were analyzed, and their wettabilities were quantified before and after a hydrogen plasma treatment to remove impurities due to growth residues and enhance hydrophilicity. Moreover, sample biocompatibility has been assessed with regard to L929 cells. Our results showed that core–shell nanowires (SiO2/SiC NWs), with and without hydrogen plasma treatment, are the most suitable candidate material for biological applications due to their high wettability that is not influenced by specific treatments. Biological tests underlined the non-toxicity of the developed biomaterials with regard to murine fibroblasts, and the proliferation assay highlighted the efficacy of all the surfaces with regard to murine osteoblasts. In conclusion, SiO2/SiC NWs offer a suitable substrate to develop platforms and membranes useful for biomedical applications in tissue engineering due to their peculiar characteristics.

Properties of κ‐Ga2O3 Prepared by Epitaxial Lateral Overgrowth

AbstractThe structural and electrical properties of undoped and Sn doped κ‐Ga2O3 layers grown by epitaxial lateral overgrowth on TiO2/sapphire substrates using stripe and point masks show that the crystalline structure of the films can be greatly improved relative to conventional planar growth. The undoped films are semi‐insulating, with the Fermi level pinned near EC‐0.7 eV, and deep electron traps at EC‐0.5 eV and EC‐0.3 eV are detectable in thermally stimulated current and photoinduced current transient spectra measurements. Low concentration Sn doping results in net donor concentrations of ≈ 1013 cm−3, and deep trap spectra determined by electron traps at EC‐0.5 eV, and deep acceptors with an optical ionization threshold near 2 and 3.1 eV. Treatment of the samples in hydrogen plasma at 330 °C increases the donor density near the surface to ≈ 1019 cm−3. Such samples show strong persistent photocapacitance and photoconductivity, indicating the possible DX‐like character of the centers involved. For thin (5 µm) κ‐Ga2O3 films grown on GaN/sapphire templates, p‐type‐like behavior is unexpectedly observed in electrical properties and we discuss the possible formation of a 2D hole gas at the κ‐Ga2O3/GaN interface.

Laser light scattering (LLS) to observe plasma impact on the adhesion of micrometer-sized particles to a surface

Laser light scattering (LLS) method, combined with a long-distance microscope was utilized to detect micrometer-sized particles on a smooth substrate. LLS was capable to detect individual particle release, shrink, or fragmentation during exposure to a plasma or a gas jet. In-situ monitoring of hundreds of particles was carried out to investigate the effect of hydrogen plasma exposure on particle adhesion, morphology, and composition. LLS was calibrated with monodisperse melamine resin spheres with known sizes of 2.14 µm, 2.94 µm, and 5.26 µm in diameter. The lowest achievable noise level of approximately 3% was demonstrated for counting 5.26 µm spherical melamine particles. The accuracy for melamine particle size measurements ranged from 50% for 2.14 µm particles to 10% for 5.26 µm particles. This scatter was taken as the imprecision of the method. Size distribution for polydisperse particles with known refractive index was obtained by interpolating to an effective scattering cross-section of a sphere using Mie theory. While the Abbe diffraction limit was about 2 µm in our system, the detection limit for Si particles in LLS according to Mie approximation was assessed to about 3 µm, given the limitations of the laser flux, microscope resolution, camera noise, and particle composition. Additionally, the gradual changes in forward scattering cross-sections for Si particles during the exposure to the hydrogen plasma were consistent with Si etching reported in the literature.

Isotope dependence of transport in ST40 hot ion mode plasmas

The ST40 compact, high-field spherical tokamak, operating at 2.1 T and with 1.8 MW of neutral beam heating, achieved central carbon impurity ion temperatures in excess of 10 keV, surpassing their business milestone of 100 M ∘C (8.6 keV). The high temperature discharges were in the hot ion mode, with , and they were achieved in both hydrogen and deuterium plasmas with deuterium neutral beam injection. In order to achieve the high temperature scenarios, careful wall conditioning and scenario optimization was carried out in , , and finally in plasmas. The TRANSP transport code was employed to study the dependence of confinement and transport on isotopic mass, and the conditions that led to the high measured central ion temperatures in the and plasmas. The kinetic profiles input into TRANSP were inferred from line-of-sight and limited radial measurements as well as consistency with a number of other experimental constraints. The TRANSP results first showed that the main species central ion temperature was only 0.5–1 keV lower than the measured carbon impurity temperature, and thus in the high performance plasmas also surpassed the 100 M ∘C level. TRANSP also showed that while the electron thermal conduction loss was dominant, reductions in central ion transport, and an effective decoupling of the ions from the electrons, led to an increase in central ion temperatures with increasing plasma mass. In fact, the associated confinement times exhibited a strong dependence on the isotopic mass of the thermal plasma. These preliminary results are foundations for dedicated experiments with full kinetic profile measurements on ST40 in the next run campaign.

Residual hydrogen reduction and refining effect during two-step hydrogen-argon plasma arc melting of cerium

Herein, the two-step hydrogen-argon plasma arc melting technology (HPAM-APAM) namely hydrogen plasma arc melting (HPAM) followed by argon plasma arc melting (APAM), was put forward to successfully realize the application of HPAM on cerium refining. The residual hydrogen content was significantly decreased to 79 ppm with the increase of APAM time to 40 min. The contents of volatile impurities Li and Mg were greatly reduced to less than 0.1 ppm. Paradoxically, the contents of Ti and W with low vapor pressures could be reduced to 2 ppm, whereas the impurities Fe and Si with high vapor pressures were hardly removed. The analysis indicated that the reduction of W and Ti was ascribed to the formation of volatile WO3 and TiO, while the negligible reduction of Fe and Si was attributed to the strong interaction strength with cerium based on “alloy phase formation theory”. A new kinetic refining mechanism by the activated H during HPAM process was proposed. This work helps to extend HPAM to refine hydrogen absorbing metals and provides new insights into the removal mechanism of metallic impurities.

Effect of Hydrogen Plasma Treatment on the Sensitivity of ZnO Based Electrochemical Non-Enzymatic Biosensor.

Information on vitamin C-ascorbic acid (AA)-content is important as it facilitates the provision of dietary advice and strategies for the prevention and treatment of conditions associated with AA deficiency or excess. The methods of determining AA content include chromatographic techniques, spectrophotometry, and electrochemical methods of analysis. In the present work, an electrochemical enzyme-free ascorbic acid sensor for a neutral medium has been developed. The sensor is based on zinc oxide nanowire (ZnO NW) arrays synthesized via low-temperature chemical deposition (Chemical Bath Deposition) on the surface of an ITO substrate. The sensitivity of the electrochemical enzyme-free sensor was found to be dependent on the process treatments. The AA sensitivity values measured in a neutral PBS electrolyte were found to be 73, 44, and 92 µA mM-1 cm-2 for the ZnO NW-based sensors of the pristine, air-annealed (AT), and air-annealed followed by hydrogen plasma treatment (AT+PT), respectively. The simple H-plasma treatment of ZnO nanowire arrays synthesized via low-temperature chemical deposition has been shown to be an effective process step to produce an enzyme-free sensor for biological molecules in a neutral electrolyte for applications in health care and biomedical safety.

120 mA operation of J-PARC cesiated RF-driven H- ion source

The Japan Proton Accelerator Research Complex (J-PARC) cesiated RF-driven H- ion source (IS) has been stably operated for about eight years by using an internal-RF-antenna developed at the Spallation Neutron Source (SNS). From Sep. 2018, the J-PARC 400 MeV LINAC has been steadily operated with the design H- ion beam intensity (IH- ) of 50 mA, when an about 60 mA beam is injected from the IS. At each end of about 3 months J-PARC operation, the IS supplies a 52.5 keV 72 mA beam for the LINAC 60 mA acceleration. The IS superior emittances predicted in a test-stand were proven by the high RFQ acceleration efficiency of 94.3% (IH-RFQ = 67.9 mA). The high intensity beam with transverse emittances suitable for the RFQ is produced with several unique measures, such as, slight water molecules addition into hydrogen plasma, the low temperature (about 70°C) operation of 45°-tapered 16-mm thick plasma electrode with the precise cesium (Cs) density control, the impurity elimination in the hydrogen plasma along with the filter-field optimization, the continuous-wave igniter plasma driven with a 42-W 30-MHz RF, and so on. The 120 mA operation results of the IS in the test-stand are presented in this paper. A 69.9 keV 120 mA H- ion beam, about 107.8 mA of which was inside the transverse emittances used for a common RFQ design, was stably operated with a duty factor of 4% (1 ms × 40 Hz). The IS is the benchmark one for the next generation high energy H- LINACs aiming for 100 mA class intensity.

Thermionic emission of a tungsten surface in high heat flux plasma: PIC simulations

The surface temperature of a tungsten surface facing hot hydrogen plasma is evaluated, thanks to 1d/3v particle-in-cell simulations in floating wall conditions. At each iteration, the plasma heat flux to the cathode is equalized with the outgoing one, which is due to thermionic emission, surface radiation, and heat conduction through the wall. The thermal conductivity is chosen within the range 35–160 W m−1 K−1 in the different simulations in order to take into account the surface condition. A transition from a cold temperature surface to a hot one arises for a critical thermal conductivity, whose value depends on the plasma parameters. This transition is very abrupt and leads to a space charge limited regime where the thermionic current penetrating the plasma has reached its maximal value and is about three times the Bohm current. Changing the initial conditions in the code, more particularly, the timing of electron emission, can lead to a very different final surface temperature. This history effect and the associated hysteresis are evidenced by means of fluid calculations, which are in a good agreement with the simulation results as well as with previous experimental measurements.

Different elements, same results: time-resolved temperature determination by oxygen and nitrogen elements

The core purpose of this research is to use optical emission spectroscopy to determine the electron temperature (Te ) of a hydrogen plasma generated in a capillary discharge plasma, with a focus on its temporal variation. The plasma density (ne ) is first determined using the Stark broadening technique, which measures the broadening of spectral lines as a result of the electric field in the plasma. Subsequently, a passive spectroscopic technique is employed to estimate the electron plasma temperature by detecting the emitted light from the plasma. This spectral detection is performed using a visible range spectrometer. In this study, two elements, oxygen and nitrogen, are specifically selected based on the chemical composition of the capillary. The electron plasma temperature is estimated using the line ratio method, which involves comparing the intensities of two specific spectral lines emitted by the selected elements. By analyzing these line ratios, the electron plasma temperature can be inferred. The combination of the Stark broadening technique and line ratio method provides valuable insights into the plasma's physical characteristics, specifically its density and temperature.

Generation of Soft X-Ray and Vacuum Ultraviolet Radiation during the Interaction of a Hydrogen Plasma Flow with a Gas Jet

Generation of Soft X-Ray and Vacuum Ultraviolet Radiation during the Interaction of a Hydrogen Plasma Flow with a Gas Jet

Doping-induced Ti3+ state and oxygen vacancies in TiO2: A single-chip combinatorial investigation

TiO2 is widely recognized as a high performance photocatalyst for photocatalytic applications. Oxygen vacancies (VO) and titanium defects are known to make a significant contribution to the enhanced photocatalytic efficiency of TiO2, accordingly control of these two centers is pivotal to optimizing the surface photochemical activity. In this work, the formation of VO and Ti3+ ions induced by n-type doping of TiO2 with Nb is demonstrated using a combinatorial investigation approach. Chemical Beam Vapour Deposition (CBVD) is used to fabricate Nb concentration-graded (NbxTi1-x)O2 anatase films with 0.009 ≤ x ≤ 0.033 over a 5 mm length on a single chip. Cathodoluminescence (CL) microanalysis shows the luminescence intensity decreases with increasing Nb concentration; however, the presence of a self-trapped exciton (STE) emission, characteristic of anatase TiO2, indicates that the Nb incorporation does not affect the structural properties of TiO2. Notably, the introduction of Nb donors induces the formation of the signature luminescence bands of VO and Ti3+ at 2.05 and 2.60 eV at 80 K, respectively, and their intensities rise by up to 28% with increasing Nb content. The formation of VO and Ti3+ is attributed to the effect of excess electrons locally produced by Nb donor doping. The thermal activation energies for the STE and defect-related emissions are found to be identical at 32 ± 4 meV, suggesting that the recombination kinetics is mediated by Nb donors. Significantly, we demonstrate that the near-surface VO density can be further increased by two different post-growth treatment approaches: (i) remote hydrogen plasma and (ii) low-energy electron beam irradiation (LEEBI), without effecting the valence state of Ti. Our results open opportunities to optimize the photocatalytic properties of TiO2 by defect engineering.

Rapid carbon-free iron ore reduction using an atmospheric pressure hydrogen microwave plasma

CO2 emissions from steel production account for about 8% of the global anthropogenic CO2 emissions and the majority (over 70%) of these emissions occur during the reduction of iron ore to iron. Hence, the steel industry is striving to reduce its dependence on carbon-based energy sources and reducing agents, like the coke used in a traditional blast furnace. Approaches such as hydrogen-based direct reduction are being considered since they can drastically reduce the overall CO2 emissions of the steel-making process. Here, we report an electrified process for reducing iron ore particles using atmospheric pressure hydrogen plasma powered by microwave energy. The process has the potential to be entirely carbon-free and overcome common challenges of other hydrogen reduction approaches, including other plasma-based approaches. Relative reduction rates achieved are as high as 15.5% /s, on par or faster than the highest rates reported in the literature operating at lower temperatures and hydrogen concentrations. When compared to thermal reduction under otherwise close to identical conditions, the microwave plasma reduction is three to four times faster, suggesting the importance of plasma generated reactive species like atomic hydrogen. A promising mass scaling is observed, with increasing the mass load 50 times requiring only 7 times longer reaction, which points to a good potential for further scale-up of the technology.

Removal of C, Si, Mn, S and P in Iron Melt under Hydrogen Plasma

Removal of C, Si, Mn, S and P in Iron Melt under Hydrogen Plasma

The development of a zero-dimensional collisional-radiative model for interpreting plasma emission in low temperature divertor plasmas in tokamaks

Collisional-radiative models are commonly used to analyse atomic and molecular processes in low temperature plasmas by determining the distribution functions of excited states as functions of various plasma parameters. This paper outlines the improvements to a zero-dimensional collisional-radiative model, developed at Keio University, for purposes relevant to the analysis of emission measurements in low temperature hydrogen plasmas. The extension of this 0D model centres on the inclusion of additional molecular species and reactions, a calculation of emission intensity in order to directly compare with experimental work, and the addition of a simple wall model to allow for a deeper understanding of recycling of atoms and molecules in low temperature plasmas close to plasma facing components in fusion machines. Results from the improved model are then compared with both an existing CR model for benchmarking and experimental emission spectroscopy data from an inductively-coupled plasma device at the University of Liverpool. These results show how the developments to the model have increased the relevance to experimental plasmas, such as those in the power exhaust regions of fusion machines, with the ratio of the Hβ/Hα line intensities outputted from the improved CR model mirroring more closely the emission measured experimentally in the ICP device.

The Etching Mechanisms of Diamond, Graphite, and Amorphous Carbon by Hydrogen Plasma: A Reactive Molecular Dynamics Study

AbstractUnderstanding how hydrogen plasma etches various potential products during the diamond growth process can contribute to improving the knowledge of diamond growth. However, due to the absence of an in situ characterization technique during the etching process, the complex chemical reactions involved in the process obscure the atomic‐scale etching mechanisms. In this paper, the etching mechanisms of diamond (001), graphite (0001), and amorphous carbon substrates irradiated by hydrogen plasmas are investigated and compared using molecular dynamics simulations based on ReaxFF. When the incident energy of H atoms is 1 eV, the rate of carbon loss from graphite and amorphous carbon are far higher than that from diamond. As the incident energy of H atoms increases, the etching rate of diamond shows a slow increase, while the etching rates of amorphous carbon and graphite exhibit more significant increases. It can be concluded that the etching rate of diamond is significantly lower than that of graphite and amorphous carbon under H plasma. In the Chemical Vapor Deposition (CVD) process of diamond growth, the generated graphite and amorphous carbon are rapidly etched, leaving only diamond. This offers a plausible explanation for the growth mechanism of diamond through CVD.