Low-pressure Discharge Research Articles

The Collaborative Optimization of the Discharge Pressure and Heat Recovery Rate in a Transcritical CO2 Heat Pump Used in Extremely Low Temperature Environment

Considering the excellent environmental properties and heating capability under wide running conditions of the natural fluid CO2, the transcritical CO2 heat pump system has widely been used in the application of water heaters, commercial heating and cooling, electric vehicle thermal management, etc. Since the performance was highly affected by the discharge pressure and heat recovery rate in a transcritical CO2 system, the collaborative optimization of these two parameters was analyzed in detail in this study. The results showed that the optimal value of the system heating COP, which was the ration of heating capacity to power consumption, was better under a higher heat recovery rate and relatively lower discharge pressure, which is why these kinds of operating conditions are highly recommended from the perspective of collaborative optimization. Additionally, the heat recovery rate had a positive effect on the system performance when the discharge pressure was lower than its optimal value, while the heat recovery rate would present a passive effect on the system performance when the discharge pressure was higher than its optimal value. The relevant conclusions of this study provide a good theoretical basis for the efficient and stable operation of the transcritical CO2 heat pump technology under the conditions of a wide ambient temperature range.

Gas Desorption and Secondary Electron Emission from Graphene Coated Copper Due to E-Beam Stimulation

The gas desorption and secondary electron multiplication induced by electron bombardment tend to induce severe low-pressure discharge effects in space microwave device cavities. Nevertheless, few studies have focused on both secondary electron emission and electron-stimulated gas desorption (ESD). Although the suppression of secondary electrons by graphene was found to be better in our previous study, it is still unclear whether the surface modification of graphene, which brings about different interfacial states, can also be manifested in terms of ESD. The deep mechanism of gas desorption and secondary electron emission from this extremely thin two-dimensional material under electron bombardment still needs further investigation. Therefore, this paper investigates the mechanism of graphene modification on Cu metal surface on the gas release and secondary electron emission properties under electron bombardment. The surface states of graphene-modified Cu were characterized, and the ESD yield and secondary electron yield of Cu/GoCu were investigated using a self-researched platform and analyzed using molecular dynamics simulations and electron Monte Carlo simulations. The results of the study showed that the most released component on the Cu surface under the bombardment of electrons was H2O molecules, while the most released component on the GoCu surface was H2 molecules. The graphene-modified samples showed a significant suppression effect on the secondary electron yield and ESD only in the low-energy region below 400 eV. This study can provide a valuable reference for suppressing low-pressure discharge and multipactor phenomena in space microwave components.

Comparison between 1D radial and 0D global models for low-pressure oxygen DC glow discharges

This work focuses on the comparison between a zero-dimensional (0D) global model (LoKI) and a one-dimensional (1D) radial fluid model for the positive column of oxygen DC glow discharges in a tube of 1 cm inner radius at pressures between 0.5 Torr and 10 Torr. The data used in the two models are the same, so that the difference between the models is reduced to dimensionality. A good agreement is found between the two models on the main discharge parameters (gas temperature, electron density, reduced electric field and dissociation fraction), with relative differences below 5%. The agreement on other species average number densities, charged and neutral, is slightly worse, with relative differences increasing with pressure from 11% at 0.5 Torr to 57% at 10 Torr. The success of the 0D global model in describing these plasmas through volume averaged quantities decreases with pressure, due to pressure-driven narrowing of radial profiles. Hence, in the studied conditions, we recommend the use of volume-averaged models only in the pressure range up to 10 Torr.

Optical diagnosis of an inductively coupled plasma source for atmosphere-breathing electric propulsion system

Atmosphere-breathing electric propulsion (ABEP) systems capture atmospheric particles for use as propellant. In the best-case scenario, such systems can undertake long-life space missions without carrying propellant from the ground. The present research mainly focuses on plasma discharge processes based on inductively coupled plasma generation from atmospheric particles in very low Earth orbit (120–250 km). The optical diagnosis is done when the N2 and O2 mixture is injected into the low-pressure discharge chamber. Numerous active groups can be identified from the emission spectra, including excited molecules, molecular ions, atoms, and excited atoms. The generation mechanism of active groups is also clarified to understand the ionization process. The variations of plasma parameters are analyzed for different ratios of N2 to O2, which can also be expanded to describe the potential behavior of ABEP systems in low Earth orbit. Note that this research is only a preliminary study and is not fully representative of the potential of ABEP systems. However, to develop ABEP systems, it is essential to understand the plasma behavior of discharge systems.

One-Step Crank-Nicolson Direct-Splitting Algorithm with Enhanced Absorption to Evaluate Low-Pressure Discharge for Satellite Sensors in Outer Space.

Low-pressure discharge causes air ionization resulting in performance degeneration or failure for the satellite sensors in outer space. Here, a one-step Crank-Nicolson Direct-Splitting (CNDS) algorithm is proposed to evaluate the electrical behavior of satellite sensors under the low-pressure discharge circumstance. To be more specific, the CNDS algorithm is proposed in the Lorentz medium, which can accurately analyze the ionized air and generated plasma. Higher order perfectly matched layer (PML) is modified in the Lorentz medium to efficiently terminate the unbounded lattice. It can be concluded that the proposed algorithm shows entire considerable performance in the low-pressure discharge evaluation. The proposed PML formulation behaviors enhanced absorbing performance compared with the existing algorithm. Through the experiments, it can be observed that the low-pressure discharge phenomenon causes performance variation, which shows a significant influence on the satellite sensors. Meanwhile, results show considerable agreement between the simulation and experiment results which indicates the effectiveness of the algorithm.

Assessment of a thrust induced by a water-fueled magnetron sputtering source

Water-fueled low-pressure discharge is sustained by a magnetron sputtering source consisting of a copper cathode and a ring anode, and a force exerted to a target plate located downstream of the source is measured, where the water gas is continuously introduced by using a mass flow controller operational for a differential pressure lower than the water vaper pressure at a room temperature. The source and the dc power supply for the discharge are electrically floating from the grounded vacuum chamber to ensure zero net current between the source and the surroundings; the mass ejection of the sputtered and electrically neutral copper atoms or clusters are responsible for the thrust generation, while the ionized particles do not contribute to the thrust generation due to the plasma confinement near the cathode. The measurement shows the thrust generation of about 3–4 mN/kW at a discharge power level of about 80 W. The results would potentially lead a propulsion device with no neutralizer and no high-pressure gas storage tank.

Structure of the surface layer of high-chrome steel nitridated in the elion regime in the plasma of a suspended arc discharge with a heated cathode

The purpose of this work is to reveal the patterns of formation of the structure of the surface layer of high-chromium steel subjected to nitriding in a low-pressure gas discharge plasma using a plasma generator with a hot cathode "PINK". Heating of the samples to the nitriding temperature was carried out by the ionic component of the plasma, as well as by the electron and ionic components of the plasma (elion mode). The object of the study was heat-resistant corrosion-resistant steel of the austenitic class grade 20X23H18 (foreign analogue of AISI310S). The relevance of research is due to the relatively low level of hardness and wear resistance of steels of this class. Nitriding of steel was carried out on the TRIO installation, retrofitted with a switching unit for implementing the aelion (electronic and ionic) processing mode. It has been established that the thickness of the hardened layer is (55-60) pm and weakly depends on the method of nitriding, temperature (in the range of 793-873 K), and duration (3-5 hours) of the process. A nitriding regime has been revealed that makes it possible to form a surface layer with a microhardness of 13.7 GPa (ionic heating mode) and 10.8 GPa (elion heating mode). It has been established that the high strength and tribological properties of nitrided steel are due to the formation of a nanocrystalline structure in the surface layer, the main phases of which are iron nitrides Fe4N and chromium nitrides CrN. It is shown that heating the samples to the nitriding temperature in the aelion mode, which uses the electron and ion components of the plasma, leads to a significantly lower level of material roughness compared to the samples heated during nitriding by the ion component of the plasma.

Low-pressure CO2 discharges: 1D modeling

A 1D model of glow low-pressure CO2 discharges is developed. In the framework of this model, simulation of stationary and repetitively pulsed discharges at pressure ranging from 0.5 to 5 Torr and current from 10 to 50 mA is performed. The obtained plasma characteristics are compared with the available experimental results and with the data evaluated based on the approximate 0D approach. The results of 0D and 1D calculations agree for most of plasma parameters, except for the molar fraction of CO molecules produced at CO2 dissociation by electron impact. Agreement between the measured and calculated, in the framework of the 1D model, values of the CO molar fraction is provided by modifying the expression of the dissociation rate constant vs the reduced electric field.

Воздействие низкотемпературной плазмы на пленки поликетона: изменение химической структуры и морфологии поверхности

Using X-ray photoelectron spectroscopy, the change in the chemical composition of the polyketon films surface as a result of treatment by the low-pressure direct current discharge was studied. The films were placed at the anode and cathode, and filtered atmospheric air was used as the working gas. The formation of a significant amount of oxygen-containing groups on the polymer surface was shown. Using atomic force microscopy, it was found that plasma treatment leads to a significant increase in the surface roughness of the films. Such changes improve the contact and adhesion properties of polyketon films.

Variable load modes and operation characteristics of closed Brayton cycle pumped thermal electricity storage system with liquid-phase storage

As a new type of large-scale energy storage system, pumped thermal electricity storage (PTES) requires frequent load variation to smooth out grid fluctuations. However, there is a lack of research on the variable load of PTES systems. Therefore, five variable load methods are innovatively proposed in this paper, and the dynamic characteristics of the closed Brayton cycle PTES system with liquid-phase storage are simulated for variable load, separately. The results show that: for the same total air discharge time, during charge process, the rotor speed fluctuation rates of the five modes are “air charge + low-pressure and high-pressure sequential air discharge (CLHS)” (+0.80%), “air charge + low-pressure air discharge (CL)” (+1.07%), “air charge + high-pressure air discharge (CH)” (+1.40%), “air charge + high-pressure and low-pressure sequential air discharge (CHLS)” (+1.51%), “air charge + high-pressure and low-pressure simultaneous air discharge (CHL)” (+2.51%), from small to large. During discharge process, the rotor speed fluctuation rates are “CL” (-3.31%), “CHLS” (-3.96%), “CLHS” (-4.39%), “CH” (-5.61%), “CHL” (-9.83%), from small to large. During both charge and discharge processes, the load regulation ranges are “CHL”, “CLHS”/“CHLS”, “CH”, “CL”, from large to small. It can be seen that the modes with smaller rotor speed fluctuation rates also have smaller load regulation ranges, while the modes with larger load regulation ranges have larger rotor speed fluctuation rates. Therefore, in operation, different variable load modes should be selected according to the actual needs of rotor stability and load regulation range. The results of this study can provide a reference for the actual operation of pumped thermal electricity storage system.

Rotational and vibrational temperatures of hydrogen nonequilibrium plasmas from Fulcher band emission spectra

A roto-vibrational resolved corona model is discussed for the simulation of the Fulcher spectrum in Hydrogen low pressure discharges. The model allows to derive H 2 rotational and vibrational temperatures from the analysis of emission spectra in the [600:640] nm range. The model is applied to the analysis of emission spectra collected at the SPIDER negative ion source. Results are presented for different values of the applied power to the plasma, pressure, and for different regions of the plasma, thus providing a characterization of the plasma with respect to the internal temperatures of the molecular component.

Mechanisms Involved in the Modification of Textiles by Non-Equilibrium Plasma Treatment.

Plasma methods are often employed for the desired wettability and soaking properties of polymeric textiles, but the exact mechanisms involved in plasma-textile interactions are yet to be discovered. This review presents the fundamentals of plasma penetration into textiles and illustrates mechanisms that lead to the appropriate surface finish of fibers inside the textile. The crucial relations are provided, and the different concepts of low-pressure and atmospheric-pressure discharges useful for the modification of textile's properties are explained. The atmospheric-pressure plasma sustained in the form of numerous stochastical streamers will penetrate textiles of reasonable porosity, so the reactive species useful for the functionalization of fibers deep inside the textile will be created inside the textile. Low-pressure plasmas sustained at reasonable discharge power will not penetrate into the textile, so the depth of the modified textile is limited by the diffusion of reactive species. Since the charged particles neutralize on the textile surface, the neutral species will functionalize the fibers deep inside the textile when low-pressure plasma is chosen for the treatment of textiles.

INFLUENCE OF THE MAGNETIC FIELD OF THE CURRENT PASSING THROUGH THE ANODE ON A GLOW DISCHARGE IN A COAXIAL SYSTEM

To study the influence of the magnetic field on the normal low-pressure glow discharge, an electrode coaxial system with a meshy tubular cathode and an internal anode was used. A current was passed through the anode to obtain the azimuthal magnetic field crossed with electric field. Oscillography of the signals from the electric probe and photodiode, together with video recording, made it possible to establish the movement of the discharge luminous region along the electrodes. When AC current with a frequency of 50 Hz was passed through the anode, the discharge movement was oscillatory – from one end of the system to another.

Radio-frequency plasma to clean ITER front-end diagnostic mirrors in geometry of Edge Thomson Scattering system

The ITER Edge Thomson scattering (ETS) system provides electron temperature and density profile measurements in the ITER tokamak. In collection optics, the front-end metallic first and second mirrors are expected to experience contamination with beryllium, tungsten and construction materials. Plasma cleaning based on a low-pressure radiofrequency discharge is expected to sputter contaminants. In the plasma cleaning system, a water-cooled first mirror is combined with a powered electrode. Water cooling was realized as a notch filter for the driving frequency with the electrode grounded for a DC-voltage. To understand plasma cleaning effects, a new test model reproducing the ETS First and the Second mirror geometries in a vacuum chamber was developed. Ion energies and fluxes were measured for 40–50 MHz discharges in argon and helium at 1–10 Pa with and without the notch filter for various power transmission schemes. Powers in plasma were increased to 300–400 W to achieve ion fluxes suitable for cleaning. 40 MHz discharges were used for cleaning as being more stable. In helium at 5 Pa the ion flux of 1.3·1019 ions·m−2 s−1 and the ion energies of 120–140 eV were considered for cleaning. Sputtering rates of metal layers were measured at 4–5 nm h−1 for W/WO3 films. Sputtering rates were non-uniform over the electrode and were lower than 0.5 nm h−1 at the edges. At 40–50 MHz, two independent discharges could be ignited at the First and the Second mirrors in argon and helium, and were maintained with minimum interference. Redeposition rate on the walls was estimated as 1–1.5 nm h−1, mainly consisting of the chamber construction materials. Parasitic discharges were observed at powers above 200 W in plasma and influenced plasma stability at pressures 1–2 Pa. The results are important for a number of ITER optical diagnostics where plasma cleaning of front-end water-cooled diagnostic mirrors shall be used.

Improving spacecraft payload system performance through low-pressure discharge process

3D model simulates low-pressure discharge in microwave circuits to clarify the relevant transformation between secondary and ionized electrons that impact spacecraft microwave loads

Characterization of hexamethyldisiloxane plasma polymerization in a DC glow discharge in an argon flow

This work aims to study hexamethyldisiloxane (HMDSO) plasma polymerization in a low-pressure glow discharge in a gas flow. HMDSO was activated in a plasma-chemical reactor with a DC glow discharge in an argon flow. The argon flow (the mass flow rate was 230 mg/min) was injected in the direction of the anode from the cathode. HMDSO vapors were injected into the plasma-chemical reactor either through the hollow cathode or through the inlet located between the cathode and the anode. Polymer coatings were deposited on the substrates located in a vacuum chamber. Plasma polymerization was characterized based on the mass of coatings deposited under varying external conditions: the HMDSO mass flow rate (1–10 mg/min), the average discharge current (6–60 mA), and the discharge power (6–30 W). The operation modes of the plasma-chemical system were determined. The chemical structure of the coatings was analyzed using the infrared spectroscopy. The processes occurring in different regions of the glow discharge and at the interface near the substrate surface are proposed. A DC glow discharge in a gas flow can be used for local deposition of polymer coatings on the surface of dielectric or conductive materials.

Revealing pre-breakdown processes of a single-gap pseudospark switch triggered from the anode side

Pseudospark switch is a special kind of low-pressure (typically 1–100 Pa) high-voltage pulsed discharge switch with the hollow electrode configuration. This paper experimentally investigates the pre-breakdown processes of a single-gap pseudospark switch triggered from the anode side. The species and contents of impurities on copper electrode surface are firstly analyzed, which mainly include C, N and O. Then, at the same conditions, the trigger delay and jitter of triggering from the anode side are significantly higher than the cathode side, as well as the minimum voltage and pressure that the switch can be triggered. The evolution of the discharge region of the pseudospark switch triggered from the anode side is observed by the ICCD camera for the first time in the literature. The time-resolved images show that the luminescence firstly appears in the hollow anode where the trigger discharge happens and then in the hollow cathode, the cathode hole, the main gap and the anode hole successively, which is completely different from triggering from the cathode side. Finally, a model considering the migration and collisions of ions and fast atoms is established, and it points out that the ion flux from the hollow anode plays an important role in triggering from the anode side. The key processes also include the formations of the positive space charge column inside the hollow cathode and the intense secondary electron emission as an electron source for the main gap.

Research on thermal vacuum reliability of pulse high-power solid-state power amplifier (SSPA) for satellite use

This paper investigates the abnormal phenomenon of local blackening of the internal circuit substrate caused by low-pressure discharge in the thermal vacuum test of an L-band high-power pulsed solid-state power amplifier for satellite use, i.e., the residual flux inside the product volatilizes under vacuum conditions. Thus, the low-pressure discharge causes a rise in local temperature, and under high local temperature, the silicone rubber is thermally decomposed. There are four proposed engineering improvements, which are proved to be feasible and effective through simulation and re-test verification. The blackening problem's observation, analysis, and resolution are crucial to improve the reliability of high-power microwave solid-state power amplifiers used in spacecraft in high orbit.

On the correlation between the TiN thin film properties and the energy flux of neutral sputtered atoms in direct current magnetron discharge

In this study, the energy flux of sputtered atoms on a substrate was correlated to the properties of titanium nitride (TiN) films deposited using direct current magnetron sputtering (dcMS) under mixed Ar and N2 atmospheres. The neutral titanium sputtered atoms velocity distribution functions (AVDFs) were measured by tunable diode-laser induced fluorescence (TD-LIF), and the flux of particles and their energy were derived. Mass spectrometry was used to characterize the energy-resolved flux of the ions. It was found that the neutral sputtered atoms flux and deposition rate were in good agreement, indicating that the flux of the neutral titanium ground state represents the number of deposited atoms. Moreover, TiN films were deposited at different gas pressures and at various Ar/N2 gas mixtures close to the conditions where stoichiometric TiN was formed, without bias voltage and heating of the substrates. The energy flux of the sputtered neutral Ti into the substrate was calculated from TD-LIF measurements. At a relatively low magnetron discharge pressure of 0.4 Pa, we demonstrated that the energy of sputtered neutral Ti impinging on the substrate is higher than the energy flux of ionized particles corresponding mainly to Ar+. Thus, the influence of the energy flux of the sputtered atoms on the texture and microstructure of the films is revealed. The (200) texture was obtained at 0.4 Pa when the energy flux of the sputtered atoms was higher than the ion energy flux. At 1.3 Pa where the sputtered atoms energy flux is one order lower compared to 0.4 Pa the (111) texture was obtained. The high-energy flux of the ground state of Ti sputtered atoms seems to allow stress removal in the films.

Characteristics of electron evolution during initial low-pressure discharge stage upon microwave circuits

High-power microwave-induced low-pressure discharges seriously threaten the reliability of space payload systems. Under extremely low-pressure conditions, the evolution of ionized and secondary electrons at the initial stage of discharge is crucial to figure out the discharge process. Therefore, this paper investigates the development of multiple electrons in the discharge process under a highly low-pressure environment using numerical simulation. A three-dimensional simulation model based on the Monte Carlo algorithm is established by considering various electron-gas collisions and secondary electron emissions from different material surfaces. The evolution characteristics of various electrons' populations, energy, and distribution patterns during the discharge process are analyzed. In addition, the influence of the critical conditions at different air pressures on the electron evolution during the discharge process and the intrinsic causes are also investigated. This study is significant in revealing the transition characteristics between multipactor and low-pressure discharge and exploring their inherent mechanisms.