Current Discharge Plasma Research Articles

Formation of Graded TiO2 Layer on Ti Wire by Direct Alternating Current Discharge Plasma at Atmospheric Pressure

Although metallic materials have been used as load-bearing materials in dental and biomedical fields since they have good mechanical properties such as good ductility and strength, their aesthetic properties are inferior to those of ceramic or resin. To obtain aesthetically improved Ti dental devices, the formation of white titanium oxide on pure Ti dental devices was studied. Direct atmospheric pressure plasma (APP) treatment using alternating current was carried out on pure a Ti plate and wire. It was found that a titanium oxide layer with enough whiteness can be obtained on pure Ti wire using direct APP treatment. Although delamination of the titanium oxide layer was found after a bending test, the concept of functionally graded materials (FGMs) can overcome the shortcoming.

A scintillating-fiber detector for making high-time-resolution secondary D-T neutron measurements in KSTAR.

A scintillating fiber (Sci-Fi) detector for the middle neutron flux range was installed in KSTAR as part of a collaboration between the National Institute for Fusion Science and the Korea Institute of Fusion Energy. The detector could make relatively high-time-resolution measurements of secondary deuterium (D)-tritium (T) neutron fluxes to investigate the degradation of D-D-born triton confinement, which is crucial for demonstrating alpha particle confinement, particularly above 0.9 MA in KSTAR. The pulse-height spectrum of the Sci-Fi detector exhibited two peaks, the higher of which corresponded to D-T neutrons. A discrimination technique was applied to extract the D-T neutron signal, revealing the time evolution of the D-T neutron flux during relatively high plasma current discharges with a 50ms temporal resolution. Future research will involve investigating the causes of the degradation of the triton burnup ratio above 0.9 MA in KSTAR.

Effects of duty ratio on liquid- and polymer-surface treatment by a unipolar microsecond-pulsed helium atmospheric-pressure plasma jet

Effects of duty ratio of a unipolar microsecond-pulsed helium atmospheric-pressure plasma jet (APPJ) on liquid- and polymer-surface treatments were investigated. In addition, changes in the plasma plume length, gas temperature, excitation temperature, discharge current, absorption power, and optical emission spectra were examined by varying the other operating parameters, such as applied voltage and additive flow of oxygen or water vapor. As an example of liquid sample, de-ionized water (DW) was exposed to an APPJ, and the concentrations of the reactive species generated in the DW were measured as functions of the operating parameters. Polycarbonate, polypropylene, and polymethylmethacrylate were employed as exemplary substrate materials to investigate the effect of plasma treatment on polymeric surfaces. The APPJ treatment increased the surface energy and changed the wetting characteristics of the surface from hydrophobic to hydrophilic. X-ray photoelectron spectroscopy results showed that a short-time plasma treatment with He and/or He/O2, He/H2O affects the surface wettability owing to the introduction of polar groups.

Ion temperature profile importance in collisional sheath modelling

A plasma fluid model is being developed for the simulation of a direct current plasma discharge simulation including the sheath regions. The code uses a second order centered finite difference scheme and time integration is done by strong stability preserving third order Runge-Kutta method. The separation of scalar and vectorial quantities in two different grids gives stable results. After validation by comparison with theoretical ion sheath profiles, a one dimensional direct current argon discharge was simulated and compared to 1D3v particle-in-cell simulation results. It is shown that the inclusion of a non constant ion temperature profile is mandatory in fluid models in order to recover correct increase of ion velocity in sheaths and thus to simulate direct current (DC) discharges where collisions are not negligible in the sheaths.

A multi-chord, two-color interferometer using Hilbert transform phase detection for measuring electron density in spheromak plasmas.

A new, four-chord, CO2/He-Ne heterodyne interferometer has been designed and built for measuring line-averaged plasma density in the HIT-SI3 and subsequent HIT-SIU sustained spheromak devices. The two-color system successfully eliminates vibration-induced errors caused by mirrors that are secured to the vacuum chamber and is able to resolve electron densities ne in the full operating range of 1018-1020m-3 in both experiments with an integrated error of 4.7 × 1017m-2. Data are presented from high toroidal current plasma discharges, showing the time evolution of electron densities ne and jϕ/ne along multiple chords.

Control of NV, SiV and GeV centers formation in single crystal diamond needles

Single-crystal diamond needles in the form of micrometer-scale sharp pyramids were produced using a combination of chemical vapor deposition and thermal oxidation processes. These diamond needles with atomic-sized fluorescent defects (color centers) attract great interest for practical applications as a platform for creation of optical quantum sensing probes and optical quantum communication elements. Particular color centers localization providing suitable luminescence characteristics is required for these applications. Here we report about formation of nitrogen-vacancy (NV), silicon-vacancy (SiV) and germanium-vacancy (GeV) centers in the single-crystal diamond needles during direct current discharge plasma enhanced chemical vapor deposition (PE CVD). Luminescent characteristics of the needles were observed by confocal photoluminescence mapping. Analysis of experimental results allows us proposing possible mechanisms of impurities introduction into the crystal lattice during needles growth. These mechanisms include gas decomposition and etching of substrate material during CVD. The proposed mechanisms were approved by realization of CVD process protocols resulting in controllable formation of NV, SiV and GeV centers in single-crystal diamond needles including formation of thin NV- and SiV- enriched layers by tuning deposition process parameters. The proposed experimental approaches in combination with revealed mechanisms for introduction of favorable impurities make single-crystal diamond needles promising candidates for scanning quantum sensing, quantum communication and hyperpolarization experiments.

Electric signals measured during plasma thin-film etching and their connection to the electron concentration and the properties of the treated surface

The electric characteristics of a discharge are usually changed when a thin film is deposited on or etched from a discharge electrode or a substrate. The electric characteristics include the plasma potential, discharge voltage and discharge current, including higher harmonic frequencies of these quantities. This fact can be used for the monitoring of various plasma processes, but the mechanism by which the thin film influences the electric characteristics of the discharge has not been fully clarified. Our study of diamond-like carbon (DLC) film etching verified that variations of electric discharge parameters are caused by variations of electron concentration, which is caused by a difference in the electron emission yield between the DLC film and its substrate.

Recent advances towards aqueous hydrogen peroxide formation in a direct current plasma–liquid system

The aqueous phase hydrogen peroxide (H2O2aq) produced from the plasma–liquid interactions can directly or synergistically (with other substances) affect the liquid chemistry, and therefore it is important to unfold the H2O2aq formation mechanism. However, up to now, a consensus on the H2O2aq formation mechanism is not reached. This review aims to survey the recent advances on the understanding of the H2O2aq formation mechanism in the system of a direct current discharge plasma operated over a liquid electrode. Theoretical and experimental analyses indicate that the recombination of dissolved OH radicals (OHaq) is the dominant process for the H2O2aq formation, while the purported plasma-induced photolysis of water and the dissolution of gaseous H2O2 are ruled out.

Using a fiber optic sensor for registration of pulsed plasma discharge current

The problem of increasing the accuracy of recording the pulse shape of a rapidly changing current of a powerful plasma emitter under conditions of powerful electromagnetic interference is considered. To register high-current pulsed discharges, it is proposed to use a polarimetric fiber-optic current sensor based on a Spun-type fiber. Using the indicated current sensor, a technique for recording high-current pulsed discharges of a plasma emitter in the mode of time profiling of the current pulse of two high-voltage capacitive storage devices with a total energy of up to 1800 kJ is realized. The discharge chamber, power supply circuits, and the formation of a plasma discharge are described. The design and principle of operation of a prototype of a two-pass polarimetric fiberoptic sensor with a Spun-type fiber and a Faraday compensator of mutual optical anisotropy are presented. The general scheme of current measurements, the sensor calibration scheme is presented, and the experience of using a prototype sensor for measuring high-power pulse currents is described. The features of the preliminary calibration of the sensor are discussed. The results of measuring the current by optical and traditional recording methods based on the Rogowski coil and Hall sensors are compared with each other, and a noticeable increase in the accuracy of reproducing the current pulse shape by the optical recording method is noted. Possible options for upgrading a fiber-optic sensor to expand the range of current recording are considered.

Specific features of eddy current analysis with ANSYS® for fast plasma current decay event in W7-X

During the second operation phase (OP1.2) of the most advanced stellarator Wendelstein 7-X (W7-X), some events of fast plasma current decay with a time constant of about 1 ms were observed (for both toroidal and diamagnetic plasma currents). The events were triggered by Electron Cyclotron Current Drive (ECCD) and were much faster than the assumptions considered during the design phase of W7-X components: 100 kA and 2.7 MA plasma current discharge with the time constants of 140 and 50 ms for toroidal and diamagnetic currents respectively. During the observed 1 ms discharge of maximum expected plasma currents, significant eddy currents are expected to be induced in some components with resulting electro-magnetic (EM) forces. In order to avoid unnecessary changes on as-built components, evaluation of the eddy currents / EM forces with high accuracy is required, if the conservative simplified estimation shows criticality. As a result, a sophisticated EM model with plasma currents, superconducting (sc) coils, plasma vessel (PV) and multiple ports is developed in ANSYS® using the new element type SOLID236 / 237 (edge-based formulation), which is much more accurate than the legacy elements SOLID97 (vector potential formulation). The specific features of using SOLID236 elements for eddy current analysis are studied and presented in this paper. This includes the application of boundary conditions on sector EM model, modelling of excitation currents, field and eddy current accuracy, skin effect, shielding effect, voltage in conducting components, etc. Most of the issues are studied in comparison with SOLID97 elements. Finally, the pros and cons of SOLID236 and SOLID97 elements are summarized, and the experiences during the W7-X EM analyses are presented.

Calculation of Temperature Conditions of a Plasma Sputtering Cell for Decontamination of Nuclear Power Plant Constructions

We are developing a technology for the “dry” deactivation of nuclear power plant (NPP) constructions by ion sputtering of surfaces with microsized radioactive contaminants. Our technology is implemented by plasma discharge in an inert gas medium with mass transfer of sputtered material and its deposition in the diffusion mode on the anode substrate. In our technology, unlike traditional radiochemical methods, radionuclides do not transfer into liquid radioactive waste (LRW), but condense in a compact solid form, which makes it possible to use them. Since our technology can be applied both during decommissioning of NPPs (including deactivation of neutron-irradiated nuclear graphite) and during routine operation and scheduled repairs of nuclear reactors, it is possible to extract the necessary isotope concentrate in the required quantities using an intense neutron flux in the NPP. The design of a plasma sputtering cell to remove radionuclides from the irradiated graphite and the NPP construction surfaces involves ignition of a direct current plasma discharge in an inert gas (for example, argon) at a pressure P ~ 0.1–1 atm and control of the temperature conditions of the sputtering material deposition. In this paper, the temperatures of the anode (tantalum collector) and the cathode (sputtered graphite) have been calculated depending on the input power to the argon plasma discharge. Data on the temperatures of the cathode and anode (collector) surfaces make it possible to control the elemental composition of the sputtered atoms and to form nanoscale layers of radionuclide concentrate on collector for use in radiation medicine and new betavoltaic batteries. Thus, the technology is important not only for the deactivation of NPPs but also for the formation of nanoscale layers of beta-active materials.

Calculation of temperature conditions of a plasma sputtering cell for decontamination of nuclear power plant constructions

We are developing a technology for the “dry” deactivation of nuclear power plant constructions by ion sputtering of surfaces with micro-sized radioactive contaminants, our technology is implemented by plasma discharge in an inert gas medium with mass transfer of sputtered material and its deposition in diffusion mode on the anode substrate. Unlike traditional radiochemical methods, in our technology, radionuclides do not transfer into liquid radioactive waste, but condense in a compact solid form, which makes possible to use them. Весаuse our technology can be applied both during decommissioning nuclear power plants (including deactivation of neutron-irradiated nuclear graphite), and during routine operation and scheduled repairs of nuclear reactors, so it is possible to extract the necessary isotope concentrate in the required quantities, due to intense neutron flux in nuclear power plant. The design of plasma sputtering cell to remove radionuclides from the irradiated graphite and NPP construction surfaces involves ignition of a direct current plasma discharge in an inert gas (for example, argon) at the pressure of P ~ 0.1–1 atm and control of the temperature conditions of the sputtering material deposition. In this paper temperatures of the anode (tantalum collector) and the cathode (sputtered graphite) were calculated depending on the input power to the argon plasma discharge. Data on temperatures of cathode and anode (collector) surfaces make it possible to control the elemental composition of the sputtered atoms and to form nanoscale layers of radionuclide concentrate on collector for using in radiation medicine and new beta-voltaic batteries. Thus, the technology is important not only for the deactivation of nuclear power plants, but also for the formation of nanoscale layers of beta-active materials.

Study of plasma-assisted detonation initiation by quasi-direct current discharge

To study the effects of quasi-direct current discharge plasma on the initiation of a pulse detonation engine at multiple locations, we proposed a double-zones quasi-direct current discharge plasma ignition scheme. Based on the establishment of the plasma-assisted detonation initiation model, the process of detonation wave formation in the mixture of hydrogen and air by single and double ignition zone were studied by numerical method. The wave structure, component evolution history, and Zeldovich–von Neumann–Döring curve after forming a stable detonation wave were all discussed. The simulation results indicate that due to its higher total ignition energy and the synchronous propagation of multiple compression waves, double-zone plasma ignition has a 17.9% shorter deflagration to detonation transition time and 14.2% lower detonation distance compared to the single-zone scheme. The double-zone scheme does not modify the peak flow field temperature and pressure when the stable detonation wave is formed, resulting in smoother pressure and temperature increases.

A compact pulse-modulation cold air plasma jet for the inactivation of chronic wound bacteria: development and characterization

A compact low-temperature plasma jet device was developed to use ambient air as plasma gas. The device was driven by a 2.52-kV high-voltage direct-current pulse in a burst mode, with a repetition rate of 2 kHz. The maximum plasma discharge current was 3.5 A, with an approximately 10 ns full-width half-maximum. Nitric oxide, hydroxyl radical, atomic oxygen, ozone, and hydrogen peroxide—important reactive oxygen and nitrogen species (RONS)—were mainly produced. The amount of plasma-generated RONS can be controlled by varying the pulse-modulation factors. After optimization, the plasma plume length was approximately 5 mm and the treatment temperature was less than 40 °C. The preliminary bactericidal effects were tested on Staphylococcus aureus, Pseudomonas aeruginosa, and methicillin-resistant S. aureus (MRSA), and their biofilms. The results showed that the plasma can effectively inactivate S. aureus, P. aeruginosa, and MRSA in both time- and pulse-dependent manner. Thus, this produced plasma device proved to be an efficient tool for inactivating deteriorating bacteria. Further versatile utilization of this portable plasma generator is also promising.

Self-Assembled Magnetically Isolated Co Nanoparticles Embedded Inside Carbon Nanotubes

We investigate the influence of carbon nanotubes (CNTs) aligned array on the magnetic properties of an ensemble of densely packed Co nanoparticles (NPs) embedded inside CNTs. Samples are synthesized by chemical vapor deposition activated by current discharge plasma and hot filament. Each CNT contained only one NP of Co, which had preliminarily been formed on the surface of the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrate. Co NPs are elongated along the CNT axis. The reference Co NPs ensemble and Co NPs embedded inside CNTs behave differently in a magnetic field. In the former case, Co NPs are strongly coupled by the dipole-dipole interaction (DDI); the easy axis plane is oriented parallel to the substrate. For Co-CNT samples, Co NPs are magnetically isolated. The reason for suppressing the contribution of the DDI is the magnetic anisotropy. It increases significantly because of the peculiar morphology of Co embedded in CNT and stresses. We evaluate the values of shape, magnetocrystalline, and magnetoelastic anisotropy constants. The magnetoelastic anisotropy is estimated for both the crystalline structures of Co, fcc and hcp, observed in Co NPs. The maximum stresses are reached in the case of hcp Co, when the hexagonal axis is oriented along the radial CNT direction. The influence of stresses onto the magnetic structure of Co inclusions is investigated by the micromagnetic simulations.

Prediction of mechanical behavior of the KSTAR CS magnet based on PF coil currents

The KSTAR includes seven pairs of ring-shaped poloidal field (PF) superconducting coils with rectangular cross section. The central solenoid (CS) is a vertical stack of four pairs of PF coils compressed axially by preloading structures. The axial compression (remaining preload) on the CS coils is monitored by strain measurement, one of the important monitoring parameters for safe operation of KSTAR. The preload variation during plasma discharge can be predicted based on the PF coils and plasma currents without complicated structural analysis. The calculation of equivalent vertical EM force was proposed to estimate the bottom displacement of the CS magnet with the assistance of the joint diagram for bolted joints. The predicted vertical displacements according to the proposed methodology were reasonably well in agreement with the displacement measured during long pulse discharge #18437. The predicted displacement was used to quickly calculate the preloading reduction of the CS magnet structure. This approach will contribute to the development of operation scenarios for long pulse and large plasma current discharges. During plasma discharge, our algorithm can be applied to a real-time plasma control system.

Anisotropy of Assemblies of Densely Packed Co-Alloy Nanoparticles Embedded in Carbon Nanotubes

We report on the magnetic properties of an array of binary metal CoFe, CoNi, and CoPt nanoparticles (NPs) embedded inside vertically oriented carbon nanotubes (CNTs). Samples were synthesized by chemical vapor deposition activated by current discharge plasma and hot filaments. Assemblies of Co-based catalytic NPs were prepared on SiO2/Si substrates by sputtering ultrathin films followed by reduction in an H2/NH3 mixture. As a result of the CNT growth, each CNT contained only one ferromagnetic NP located at the top. For all samples, the easy axis of magnetization was along the CNT axis. The magnetic parameters, including effective anisotropy constant and the contributions of dipole interactions and shape, magnetocrystalline, and magnetoelastic anisotropies, were estimated based on the experimental data and a random-anisotropy model. The magnetoelastic contribution was decisive. From the magnetoelasticity, the stresses in the NPs embedded in the CNTs were determined. Finally, the magnetization distribution in CoFe, CoNi, and CoPt NPs was simulated considering the magnetoelastic contribution.

Impact of aligned carbon nanotubes array on the magnetostatic isolation of closely packed ferromagnetic nanoparticles

We investigate the influence of carbon nanotubes (CNT) aligned array on the magnetic properties of ensemble of densely packed Co nanoparticles (NPs) embedded inside CNT. Each CNT contains only one nanosized Co. Such a special structure was formed by catalyst chemical vapor deposition (CCVD) activated by current discharge plasma and hot filament. The Co NPs, previously deposited onto SiO2/Si substrate, acted as a catalyst. By varying the parameters of the CCVD process, we were able to also sputter the substrate instead of CNT growth. Co NPs were used as a mask and the structure of Si-based nanocones with Co NPs on the top of each cone was formed. Exhaustive investigation of the structural, morphology and crystalline properties of Co nanoparticles were performed. The magnetic properties of two kinds of samples, Co on the Si-based nanocone and Co inside CNT, were differ drastically. In the former case, the magnetic anisotropy of thin-film-type has been observed with large magnetic domains. Whereas for the Co-CNT samples ferromagnetic NPs were magnetically isolated. It was established that the magnetic anisotropy of nanosized Co plays more dominant role than the dipole interaction between Co NPs. The role of the CNT container in this is discussed.

Novel high-frequency energy-efficient pulsed-dc generator for capacitively coupled plasma discharge.

The circuit design, assembly, and operating tests of a high-frequency and high-voltage (HV) pulsed dc generator (PDG) for capacitively coupled plasma (CCP) discharge inside a vacuum chamber are reported. For capacitive loads, it is challenging to obtain sharp rectangular pulses with fast rising and falling edges, requiring intense current for quick charging and discharging. The requirement of intense current generally limits the pulse operation frequency. In this study, we present a new type of PDG consisting of a pair of half-resonant converters and a constant current-controller circuit connected with HV solid-state power switches that can deliver almost rectangular high voltage pulses with fast rising and falling edges for CCP discharge. A prototype of the PDG is assembled to modulate from a high-voltage direct current (HVdc) input into a pulsed HVdc output, while following an input pulse signal and a set current level. The pulse rise time and fall time are less than 500 ns and 800 ns, respectively, and the minimum pulse width is 1 µs. The maximum voltage for a negative pulse is 1000 V, and the maximum repetition frequency is 500 kHz. During the pulse on time, the plasma discharge current is controlled steadily at the set value. The half-resonant converters in the PDG perform recovery of the remaining energy from the capacitive load at every termination of pulse discharge. The PDG performed with a high energy efficiency of 85% from the HVdc input to the pulsed dc output at a repetition rate of 1 kHz and with stable plasma operation in various discharge conditions. The results suggest that the developed PDG can be considered to be more efficient for plasma processing by CCP.

Determination of q during sawtooth from inverse evolution of BAEs in Tore Supra

Measuring the value of the safety factor (q) in the core during sawtooth cycles is still an open issue. A new method to measure q in Tore Supra plasma core is presented here. It relies on the analysis of the time evolution of a set of MHD modes detected after the sawtooth crashes. These modes are in the frequency range of previously observed Beta-induced Alfvén Eigenmodes, but with a frequency declining in time. The mode frequency analysis shows that the q profile is reversed when we have ICRH, after the sawtooth crash. In high current discharges ( MA), the q-profile remains reversed for a longer time compared with lower plasma current discharges. Non-linear 3D MHD simulations of sawteeth performed with the XTOR-2F code (Lütjens and Luciani 2010 J. Comput. Phys. 229 8130–43) exhibit features that are similar to these observations.