Hybrid electric discharge plasma technologies for water decontamination: a short review Hot!
Electric discharge plasma (EDP) can efficiently degrade aqueous pollutants by its in situ generated strong oxidative species (·OH, ·O, H2O2, O3, etc) and other physiochemical effects (UV irradiation, shockwaves, local high temperature, etc), but a high energy consumptions limit the application of EDP in water treatment. Some adsorbents, catalysts, and oxidants have been employed for enhancing the degradation of pollutants by discharge plasma. These hybrid plasma technologies offer improved water treatment performance compared to discharge plasma alone. This paper reviews the water decontamination performance and mechanisms of these hybrid plasma technologies, and some suggestions on future water treatment technologies based on discharge plasma are also proposed.
Hysteresis characteristics of the initiating and extinguishing boundaries in a nanosecond pulsed DBD
The dielectric barrier discharge (DBD) is presently used in many fields, including plasma medicine, surface modification, and ozone synthesis; the influence of airflow on the DBD is a widely investigated topic. In this work, a hysteresis characteristic on the initiating and extinguishing boundaries is observed in a nanosecond pulsed DBD, which is sensitive to the variation in the airflow velocities and pulse repetition frequencies (PRFs). It is found that, at a certain airflow velocity, the initiating PRF is higher than the extinguishing PRF. This difference between the initiating PRF and the extinguishing PRF leads to a hysteresis phenomenon on the initiating and extinguishing boundaries. When the airflow velocity is increased, both the initiating and extinguishing PRFs are increased and the difference between the initiating PRF and the extinguishing PRF also increased. The hysteresis width between the initiating and extinguishing boundaries is enhanced. To explain these results, the physical processes involved with the seed particles and the mechanisms of forming discharge channels are discussed.
Numerical simulation and analysis of lithium plasma during low-pressure DC arc discharge
Alkali metal DC arc discharge has the characteristics of easy ionization, low power consumption, high plasma temperature and ionization degree, etc, which can be applied in aerospace vehicles in various ways. In this paper, we calculate the physical property parameters of lithium vapor, one of the major alkali metals, and analyze the discharge characteristics of lithium plasma with the magnetohydrodynamic (MHD) model. The discharge effects between constant current and voltage sources are also compared. It is shown that the lithium plasma of DC arc discharge has relatively high temperature and current density. The peak temperature can reach tens of thousands of K, and the current density reaches 6×10 7 A −2 . Under the same rated power, the plasma parameters of the constant voltage source discharge are much higher than those of the constant current source discharge, which can be used as the preferred discharge mode for aerospace applications.
Design and preliminary experiment of radial sheet beam terahertz source based on radial pseudospark discharge
To satisfy the demands for compact, inexpensive terahertz (THz) sources with power of hundreds of watts, a radial sheet beam THz source which does not require an external magnetic field and is driven by a radial pseudospark discharge plasma electron gun (PSDP-EGUN) is proposed. Radial design has been used in pseudospark switches, but in this paper the design of a PSDP-EGUN to drive a radial THz source is presented for the first time. Being different from the latest reported axial quasi-rectangular sheet beam THz sources driven by an axial PSDP-EGUN, a new design consisting of a circular plate-shaped sheet beam that is directly generated by the radial PSDP-EGUN is reported. As compared to an axial system, the radial configuration may result in a larger beam current and a larger beam-wave interaction area together with a higher potential of THz output power. Theoretical analysis and particle-in-cell simulation have been employed in the design of the radial sheet beam THz source. Output powers in the kilowatt range have been observed in the simulation of this 0.22 THz source. Preliminary experimental results of the radial PSDP-EGUN are also presented.
Plasma-assisted Ru/Zr-MOF catalyst for hydrogenation of CO2 to methane
As an important type of metal–organic framework (MOF), Zr-MOF shows excellent CO2 adsorption performance. In this work, a Zr-MOF was synthesized by a solvothermal method and adopted to support Ru through simple incipient-wetness impregnation. Then the Ru/Zr-MOF was applied for CO2 hydrogenation (VH2 : VCO2= 4:1) with the assistance of dielectric barrier discharge (DBD) plasma. The hydrogenation of CO2 results showed that methane was produced selectively under the synergistic effect between plasma and the Ru/Zr-MOF catalyst, and the selectivity and yield of methane reached 94.6% and 39.1%, respectively. The XRD and SEM analyses indicate that the basic crystalline phase structure and morphology of the Zr-MOF and Ru/Zr-MOF remained the same after DBD plasma treatment, suggesting that the catalysts are stable in plasma. The guest molecules in the pores of the Zr-MOF are removed and the Ru3+ ions are reduced to metallic Ru0 in the reduction atmosphere according to the BET and XPS results, which are responsible for the high performance of plasma with the Ru/Zr-MOF catalyst. In situ optical emission spectra of pure plasma, plasma with Zr-MOF, and plasma with Ru/Zr-MOF were measured, and the active species of C, H and CH for CO2 hydrogenation were detected. The plasma-assisted Ru/Zr-MOF exhibited high catalytic activity and stability in CO2 hydrogenation to methane, and it has great guiding significance for CO2 hydrogenation by using plasma and MOF materials.
Simple reactor for the synthesis of silver nanoparticles with the assistance of ethanol by gas–liquid discharge plasma
Atmospheric pressure plasma technology is gaining increasing importance because it is a simple and tunable synthesis process for the production of metallic nanoparticles. In addition to the development of the power supply, improving the reactor is also one of the main strategies to enhance the utility. In this study, a simple reactor for the gas–liquid discharge plasma induced by argon gas was applied to synthesize silver nanoparticles from silver nitrate (AgNO3) in solution. An AC power supply with a peak voltage of 3.5 kV was used. The frequency and on-time were set to 50 kHz and 2.5 μs, respectively. The oscilloscope showed that the rising time was approximately 2 μs. The ethanol was used as the source for the reactive reducing agent. No more additional components existed in the solution during the discharge and neither of the electrodes was in contact with the treated solution. The temperature increased by 10 °C within 1min without a cooling system. Carbon was the main impurity and was expected to be produced from the decomposition of the organics under the plasma. The elevated temperature decreased the organic by-products by evaporation and could also decrease the production of carbon. Transmission electron microscopy showed that the spherical silver nanoparticles with a size of approximately 10 nm were synthesized with a crystal structure and that a low concentration of ethanol prefers the production of the mono-dispersed colloid.
Influence of gas atmosphere on synergistic control of mercury and dioxin by nonthermal plasma
In this paper, narrow-pulse power discharge is used to study the synergistic control of mercury and dioxins, in which 1,2,4-trichlorobenzene (TCB) was used as a dioxin analog, by using a self- designed experimental system. The competitive effects of NO, SO2 and HCl on the TCB removal by non-thermal plasma are discussed. The influence of acid gas on TCB degradation is reflected in the competitive effect. NO has the greatest influence on TCB degradation efficiency. The oxidation efficiency of Hg0 decreased by about 10% in all three acidic gas atmospheres, and the effect of each gas component on Hg0 oxidation is complex. In the flue gas atmosphere of ‘acid gas+Hg0 +TCB’, the mechanism of the synergistic control of Hg0 and TCB by the non- thermal plasma is different, which has competition and promotion relationship between each other. The contribution of various flue gas components to the results was complicated, but the overall experimental results show that the synergistic control effect of the system can continue to improve. According to the generated product backstepping, ·OH plays an important role in the synergistic control of the degradation of Hg0 and TCB. Through this study, we hope to provide basic research data for the collaborative control of flue gas in the incineration industry.
E-beam generation in discharges initiated by voltage pulses with a rise time of 200 ns at an air pressure of 12.5–100 kPa
The effect of air pressure (12.5, 25, 50, and 100 kPa) on the generation of runaway electron beams in a non-uniform electric field when applying voltage pulses (≈35 kV) with a rise time of ≈200 ns has been studied. The results show that the discharge has various stages: streamer, diffuse, and spark. Initially, a wide streamer develops in the gap and a diffuse discharge is formed. A spark is formed ≈100 ns after the breakdown. The current pulse of a supershort avalanche electron beam (SAEB) was measured with a collector at various pressures of air. Experiments show that there are two modes of generation of runaway electrons. At an air pressure of 25–100 kPa, a single SAEB current pulse with a full width at half-maximum (FWHM) of 120–140 ps is observed. At the air pressure of 12.5 kPa, two current pulses of the electron beam are observed. FWHM of the first and second current pulses are ≈140 ps and ≈300 ps, respectively. The current pulse amplitude of the second electron beam is higher than that of the first one, but the electron energy is less.
Measurement and analysis of the extreme ultraviolet emission band of laser-produced antimony plasmas
The temporal evolution of extreme ultraviolet (EUV) emission spectra of laser-produced antimony (Sb) plasmas has been measured in the 7–16 nm wavelength region using spatio- temporally resolved lase-produced plasma spectroscopy technique. The spectral profiles involve an intense quasi-continuous band with superimposed intense characteristic radiation and are different with the increase of delay time. The spectral structures were also analyzed according to Hartree–Fock calculations with configuration interaction effects and contributed from 4d–4f, 4d–4p, and 4d–5f unresolved transition arrays of Sb7+ – Sb13+. A steady-state collisional- radiative model was used to estimate the electron temperature and density range of Sb plasmas. This work would enrich the spectral data of highly-charged ions and provided a possible selection for developing EUV light sources.
The instability of terahertz plasma waves in cylindrical FET
In this paper, the Dyakonov–Shur instability of terahertz (THz) plasma waves has been analyzed in gated cylindrical field effect transistor (FET). In the cylindrical FET, the hydrodynamic equations in cylindrical coordinates are used to describe the THz plasma wave in two- dimensional electronic gas. The research results show that the oscillation frequency of the THz plasma wave is increased by increasing the component of wave in the circumferential direction, but instability increment of the THz plasma wave are increased by increasing the radius of channel.
Spontaneous magnetic field multipolar structure in toroidal plasmas based on 2D equilibrium
The multipolar magnetic field structure is investigated by the momentum conservation equation with self-consistent 3D sheared flows during transition of plasma properties from local paramagnetic to diamagnetic fields. Numerical results show that the traditional poloidal magnetic field (BP) is one part of equilibrium magnetic fields. The non-zero-order quantities are originated from the higher-order terms of 2D equilibrium treatment based on a Fourier expansion of ψ (r, θ). The distributions of magnetic field vectors of the order of 1, 2, and 3 terms are presented respectively in two, four, and six polar fields with the local vortex structures (spontaneous magnetic connection). The excitation mechanisms of the magnetic vortices are the coupling effects of the magnetic fluid structure pattern and the toroidal effects. These results can help us understand the physical mechanism of the interaction between the external perturbation fields and control tearing modes, as well as the radial plasma flow and magnetic vortices.
Enhancement of proton collimation and acceleration by an ultra-intense laser interacting with a cone target followed by a beam collimator
A special method is proposed of a laser-induced cavity pressure acceleration scheme for collimating, accelerating and guiding protons, using a single-cone target with a beam collimator through a target normal sheath acceleration mechanism. In addition, the problems involved are studied by using two-dimensional particle-in-cell simulations. The results show that the proton beam can be collimated, accelerated and guided effectively through this type of target. Theoretically, a formula is derived for the combined electric field of accelerating protons. Compared with a proton beam without a beam collimator, the proton beam density and cut-off energy of protons in the type II are increased by 3.3 times and 10% respectively. Detailed analysis shows that the enhancement is mainly due to the compact and strong sheath electrostatic field, and that the beam collimator plays a role in focusing energy. In addition, the simulation results show that the divergence angle of the proton beam in type II is less than 1.67 times that of type I. The more prominent point is that the proton number of type II is 2.2 times higher than that of type I. This kind of target has important applications in many fields, such as fast ion ignition in inertial fusion, high energy physics and proton therapy.
Solitary kinetic Alfvén waves in dense plasmas with relativistic degenerate electrons and positrons
Propagation of solitary kinetic Alfvén waves (KAWs) is investigated in small but finite β (particle-to-magnetic pressure ratio) collisionless dense plasma whose constituents are nondegenerate warm ions, and relativistic degenerate electrons and positrons. Through the use of reductive perturbation technique, Kortweg–de Vries equation is derived to obtain small amplitude localized wave solution of KAWs. The effects of plasma β, positron concentration, electron relativistic degeneracy parameter, ion thermal temperature and obliqueness parameter on solitary KAWs are studied. The results of this theoretical investigation are aimed at elucidating characteristics of kinetic Alfvén solitary waves in relativistic degenerate e–p–i plasmas found in dense astrophysical objects specifically neutron stars and white dwarfs.
A comparison of emissive and cold floating probe techniques for electric potential measurements in rf inductive discharge
A cold floating probe method was compared with the emissive floating probe method in terms of a low-pressure radio-frequency inductive discharge. The dependences of difference between the plasma potential and the floating potential on the electron temperature 1–8 eV, plasma density 109 –1012 cm−3 and magnetic field 100–650 G were obtained. It was demonstrated that the difference between the potentials that obtained by these two methods can differ significantly from the expected value of 5.2 kTe/e for argon.
Effects of electron temperature on the ion
extraction characteristics in a decaying
plasma confined between two parallel plates
Reduction of turbulent boundary layer drag through dielectric-barrier-discharge plasma actuation based on the Spalding formula
Abstract It is a very difficult task to develop a method of reducing turbulent boundary layer drag. However, in recent years, plasma flow control technology has demonstrated huge potential in friction drag reduction. To further investigate this issue, a smooth plate model was designed as a testing object arranged with a bidirectional dielectric-barrier-discharge (DBD) plasma actuator. In addition, measurement of skin friction drag was achieved by applying hot wire anemometry to obtain the velocity distribution of the turbulent boundary layer. A method of quantifying the friction drag effect was adopted based on the Spalding formula fitted with the experiment data. When plasma actuation was conducted, a velocity defect occurred at the two measuring positions, compared with the no plasma control condition; this means that the DBD plasma actuation could reduce the drag successfully in the downstream of the actuator. Moreover, drag reduction caused by backward actuation was slightly more efficient than that caused by forward actuation. With an increasing distance from plasma actuation, the drag-reduction effect could become weaker. Experimental results also show that the improvement of drag-reduction efficiency using a DBD plasma actuator can achieve about 8.78% in the local region of the experimental flat model.
Simulation study of the influence of leak electrons on the discharge characteristics of a cusped field thruster
Previous studies have shown that leak electrons in cusped field thrusters can move along the channel axis to the anode after crossing the magnetic cusp on the exit. In this paper, a one- dimensional fluid model is built along two typical electron paths to study the influence of leak electrons on the discharge characteristics of a cusped field thruster, considering the electron temperature equation. It is found that the frequencies of low-frequency oscillations increase with a decrease in the proportion of leak electrons, which is related to an increase in the ion speed in the channel. Simulation results show that the position of the peak electron temperature is near the magnetic cusp on the exit and the position of the peak electron density is located downstream from the middle magnetic tip. With a decrease in the proportion of the leak electrons, the peak electron temperature and peak electron density decrease and the position of the peak electron density moves away from the exit, which is related to a decrease in the potential fall on the exit and an increase in confinement of electrons to the middle magnetic cusp.
Operando FT-IR study on basicity improvement of Ni(Mg, Al)O hydrotalcite-derived catalysts promoted by glow plasma discharge Hot!
CO2 adsorption on the surface of hydrotalcite-derived mixed oxide catalysts was investigated under low pressure glow discharge plasma in operando conditions via FT-IR spectroscopy. Nickel catalysts were promoted with various transition metal species (Ce, Fe, La, Zr) to influence their physico-chemical properties. Fe and Zr species were successfully incorporated into hydrotalcite brucite layers. After calcination formed a single phase with Ni(Mg, Al)O mixed oxide, while La and Ce species formed separate phases. This had a consequence in the distribution of surface basic sites as well as in the affinity to CO produced upon CO2 dissociation in plasma. Plasma treatment activated the surface of prepared materials and changed their properties via the generation of strong basic sites associated with low coordinated surface oxygen anions. Moreover, the CO2 adsorption capacity of prepared materials increased after plasma treatment.
Relation between etching profile and voltage–current shape of sintered SiC etching by atmospheric pressure plasma
Sintered silicon carbide (SiC) was etched by a dielectric barrier discharge source. A high voltage bipolar pulse was used with helium gas for the plasma generation. One stable filament plasma was generated and could be used for SiC etching. As the processing gas (NF3) mixing rate increased, the width and depth of the etching profile became narrower and deeper. The differentiated V–Q Lissajous method was used for measuring the capacitances (Ceq) of the electrode after the plasma turned on. The width of the etching profile was proportional to Ceq. As the current peak value Ismx of the substrate current increased, the volume removal rate of SiC increased. The etch depth was proportional to the ratio of Ismx to Ceq. Additionally, because of the different characteristics of the plasma disks on SiC substrate by the voltage polarity, the etching profile was unstable. However, in high NF3 mixing process, the etching profile became stable and deeper.
Plume diagnostics of BUSTLab microwave electrothermal thruster using Langmuir and Faraday probes
This study presents the Langmuir and Faraday probe measurements conducted to determine the plume characteristics of the BUSTLab microwave electrothermal thruster (MET). The thruster, designed to operate at 2.45 GHz frequency, is run with helium, argon and nitrogen gases as the propellant. For the measurements, the propellant volume flow rate and the delivered microwave power levels are varied. Experiments with nitrogen gas revealed certain operation regimes where a very luminous plume is observed. With the use of in-house-built Langmuir probes and a Faraday probe with guard ring, thruster plume electron temperature, plasma density and ion current density values are measured, and the results are presented. The measurements show that MET thruster plume effects on spacecraft will likely be similar to those of the arcjet plume. It is observed that the measured plume ion flux levels are very low for the high volume flow rates used for the operation of this thruster.
Preliminary results of optical emission spectroscopy by line ratio method in the RF negative ion source at ASIPP
Optical emission spectroscopy (OES) using the trace rare gases of Ar and Xe have been carried out in a radio frequency (RF) driven negative ion source at Institute of Plasma Physics, Chinese Academy of Science (ASIPP), in order to determine the electron temperature and density of the hydrogen plasma. The line-ratio methods based on population models are applied to describe the radiation process of the excited state particles and establish their relations with the plasma parameters. The spectral lines from the argon and xenon excited state atoms with the wavelength of 750.4 and 828.0 nm are used to calculate the electron temperature based on the corona model. The argon ions emission lines with the wavelength of 480 and 488 nm are selected to calculate the electron density based on the collisional radiative model. OES has given the preliminary results of the electron temperature and density by varying the discharge gas pressure and RF power. According to the experimental results, the typical plasma parameters is Te ≈ 2–4 eV and ne ≈ 1×10 17– 8×1017 m−3 in front of plasma grid.