Theoretical research on the transport and ionization rate coefficients in glow discharge dusty plasma

The electron kinetic model for investigating the transport and ionization rate coefficients of argon glow discharge dusty plasma is developed from the Boltzmann equation.Both of the electron-neutral and electron-dust collisions are considered as collision terms in the kinetic equation.The kinetic equation is simplified by employing the local approximation and nonlocal approach under the same discharge conditions,and the corresponding simplified kinetic equations are known as local and nonlocal kinetic equations respectively.Then the electron energy distribution function(EEDF)is obtained by numerically solving the local and nonlocal kinetic equations and the dust charging equations simultaneously.Based on the obtained EEDFs,the effective electron temperature,electron mobility,electron diffusion coefficient and ionization rate coefficient are calculated for different discharge conditions.It is shown that the EEDFs calculated from the local kinetic model clearly differ from the nonlocal EEDFs and both the local and nonlocal EEDFs are also clearly different with Maxwellian distributions.The appearance of dust particles results in an obvious decrease of high energy electrons and increase of low energy electrons when axial electric field is low.With the increase of axial electric field,the influence of dust particles on the EEDFs becomes smaller.The electron mobility and diffusion coefficients calculated on the basis of local and nonlocal EEDFs do not differ greatly to the dust-free ones.While,when dust density nd=10^6 cm^?3,the electron mobility increases obviously compared with the dust-free results at low axial electric field and decreases with the increasing axial electric field until they are close to the dust-free ones.Meanwhile,electron diffusion coefficients for dusty case become smaller and decrease with the increasing axial electric field.The ionization rate coefficients decrease when dust particles are introduced and they approach the dust-free results gradually with the increasing axial electric field.     

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 kT_e/e for argon.     

Oxide Coated Cathode Plasma Source of Linear Magnetized Plasma Device

Plasma source is the most important part of the laboratory plasma platform for fundamental plasma experimental research. Barium oxide coated cathode plasma source is well recognized as an effective technique due to its high electron emission current. An indirectly heated oxide coated cathode plasma source has been constructed on a linear magnetized plasma device. The electron emission current density can reach 2 A/cm 2 to 6 A/cm ² in pulsed mode within pulse length 5–20 ms. A 10 cm diameter, 2 m long plasma column with density 10 ¹⁸ m ⁻³ to 10 ¹⁹ m ³ and electron temperature T_e ≈ 3–7 eV is produced. The spatial uniformity of the emission ability is less than 4% and the discharge reproducibility is better than 97%. With a wide range of the plasma parameters, this kind of plasma source provides great flexibility for many basic plasma investigations. The detail of construction and initial characterization of oxide coated cathode are described in this paper.     

Ionization process and distinctive characteristic of atmospheric pressure cold plasma jet driven resonantly by microwave pulses

In the present study, a coaxial transmission line resonator is constructed, which is always capable of generating cold microwave plasma jet plumes in ambient air in spite of using argon, nitrogen, or even air, respectively. Although the different kinds of working gas induce the different discharge performance, their ionization processes all indicate that the ionization enhancement has taken place twice in each pulsed periods, and the electron densities measured by the method of microwave Rayleigh scattering are higher than the amplitude order of 10¹⁸ m⁻³. The tail region of plasma jets all contain a large number of active particles, like NO, O, emitted photons, etc, but without O3. The formation mechanism and the distinctive characteristics are attributed to the resonance excitation of the locally enhanced electric fields, the ionization wave propulsion, and the temporal and spatial distribution of different particles in the pulsed microwave plasma jets. The parameters of plasma jet could be modulated by adjusting microwave power, modulation pulse parameters (modulation frequency and duty ratio), gas type and its flow rate, according to the requirements of application scenarios.     

Numerical investigation on the effect of gas parameters on ozone generation in pulsed dielectric barrier discharge

Pulsed dielectric barrier discharge is a promising technology for ozone generation and is drawing increasing interest. To overcome the drawback of experimental investigation, a kinetic model is applied to numerically investigate the effect of gas parameters including inlet gas temperature, gas pressure, and gas flow rate on ozone generation using pulsed dielectric barrier discharge. The results show that ozone concentration and ozone yield increase with decreasing inlet gas temperature, gas pressure, and gas flow rate. The highest ozone concentration and ozone yield in oxygen are about 1.8 and 2.5 times higher than those in air, respectively. A very interesting phenomenon is observed: the peak ozone yield occurs at a lower ozone concentration when the inlet gas temperature and gas pressure are higher because of the increasing average gas temperature in the discharge gap as well as the decreasing reduced electric field and electron density in the microdischarge channel. Furthermore, the sensitivity and rate of production analysis based on the specific input energy (SIE) for the four most important species O₃, O, O(1D), and O₂(b¹∑) are executed to quantitatively understand the effects of every reaction on them, and to determine the contribution of individual reactions to their net production or destruction rates. A reasonable increase in SIE is beneficial to ozone generation. However, excessively high SIE is not favorable for ozone production.     

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.     

A low power 50 Hz argon plasma for surface modification of polytetrafluoroethylene

The characteristics of a low power 50 Hz argon plasma for surface treatment of polytetrafluoroethylene (PTFE) film is presented in this article. The current–voltage behavior of the discharge and time-varying intensity of the discharge showed that a DC glow discharge was generated in reversed polarity at every half-cycle. At discharge power between 0.5 and 1 W, the measured electron temperature and density were 2–3 eV and ∼10⁸ cm⁻³, respectively. The optical emission spectrum of the argon plasma showed presence of some 'impurity species' such as OH, N₂ and H, which presumably originated from the residual air in the discharge chamber. On exposure of PTFE films to the argon glow plasma at pressure 120 Pa and discharge power 0.5 to 1 W, the water contact angle reduced by 4% to 20% from the original 114° at pristine condition, which confirms improvement of its surface wettability. The increase in wettability was attributed to incorporation of oxygen-containing functional groups on the treated surface and concomitant reduction in fluorine as revealed by the XPS analysis and increase in surface roughness analyzed from the atomic force micrographs. Ageing upon storage in ambient air showed retention of the induced increase in surface wettability.     

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 T_e ≈ 2–4 eV and n_e ≈ 1×10 ¹⁷– 8×10¹⁷ m⁻³ in front of plasma grid.     

氚灯亮度的Monte-Carlo模拟研究

本文基于氚灯物理模型，运用Monte-Carlo方法研究氚气压强和氚灯几何条件对氚灯亮度的影响，得到与文献实验值吻合较好的理论结果。该方法为氚灯工程设计和优化提供了有力的手段。     

Numerical simulation of atmospheric pulse-modulated radio-frequency glow discharge ignition characteristics assisted by a pulsed discharge

A one-dimensional self-consistent fluid numerical model was developed to study the ignition characteristics of a pulse-modulated(PM) radio-frequency(RF) glow discharge in atmospheric helium assisted by a sub-microsecond voltage excited pulsed discharge. The temporal evolution of discharge current density and electron density during PM RF discharge burst was investigated to demonstrate the discharge ignition characteristics with or without the pulsed discharge. Under the assistance of pulsed discharge, the electron density in RF discharge burst reaches the magnitude of 1.87 × 10~(17) m~(-3) within 10 RF cycles, accompanied by the formation of sheath structure. It proposes that the pulsed discharge plays an important role in the ignition of PM RF discharge burst. Furthermore, the dynamics of PM RF glow discharge are demonstrated by the spatiotemporal evolution of the electron density with and without pulsed discharge. The spatial profiles of electron density, electron energy and electric field at specific time instants are given to explain the assistive role of the pulsed discharge on PM RF discharge ignition.     

Two-dimensional self-consistent numerical simulation of the whole discharge region in an atmospheric argon arc

A 2D self-consistent numerical model of the whole argon-arc discharge region that includes electrodes is developed in this work to facilitate analysis of the physical processes occurring in atmospheric arc plasma. The 2D arc column model contains the ionization and thermal non-equilibrium, which is coupled with a 1D electrode sheath model. The influence of plasma-species diffusion near the electrode region is investigated based on Maxwell–Stefan equations and the generalized Ohm's law. The numerical results of argon free-burning arcs at atmospheric pressure are then investigated. The simulation shows that the plasma is obviously in the state of thermal and ionization equilibrium in the arc core region, while it deviates from thermal and ionization equilibrium in the arc fringe region. The actual electron density decreases rapidly in the near-anode and near-cathode regions due to non-equilibrium ionization, resulting in a large electron number gradient in these regions. The results indicate that electron diffusion has an important role in the near-cathode and near-anode regions. When the anode arc root gradually contracts, it is easy to obtain a positive voltage drop of the anode sheath (I = 50 A), while it remains difficult to acquire a positive anode sheath voltage drop (I = 150 A). The current–voltage characteristics predicted by our model are found to be identical to the experimental values.     

Measurement of the first Townsend's ionization coefficient for atmospheric pressure neon by a dielectric barrier discharge

Fast photography and optical emission spectroscopy are implemented in a 5 mm neon gap dielectric barrier discharge (DBD) at atmospheric pressure with quartz glass used as the dielectric layer. Results show that it starts with a Townsend discharge and ends at a sub-normal glow discharge in neon DBD. Based on the Townsend discharge, the first ionization coefficient of neon is measured. The measurements are consistent with those at low pressure. Optical emission spectroscopy indicates that the spectra are mainly composed of atomic lines of neon, molecular bands and molecular ion bands originating from inevitable gas impurities (mainly nitrogen). Moreover, spectral lines emitted from atomic neon corresponding to the transitions (2p⁵ 3p → 2p⁵ 3s) are predominant. Although the second positive system of N₂(C³Π_u → B³Π_g) is observed, their intensities are too weak compared with neon's spectrum. The molecular nitrogen ion line of 391.4 nm is observed. It reveals that Penning ionization between high energy neon excited states and the inevitable gas impurities plays an important role in the value of the α coefficient.     

Influence of water vapor concentration on discharge dynamics and reaction products of underwater discharge within a He/H_2O-filled bubble at atmospheric pressure

In this study, a two-dimensional fluid model is proposed to simulate the underwater discharge in a He/H_2O-filled bubble at atmospheric pressure. The molar fraction of water vapor is varied in the range of 0.01%–1% to investigate the dependence of discharge dynamics and reaction products on water vapor concentration(WVC). The numerical results show that most properties of the discharge sensitively depend on the WVC. The increase of WVC leads to an increase in the electron density and discharge propagation velocity, which is attributed to Penning ionization between He*and H_2O. The main positive ion switches from He+to H_2O~+, while the WVC increases from 0.01% to 1%. The dominant reactive oxygen species is OH, whose peak density is about two orders of magnitude higher than that of O. Besides, the densities of OH and O radicals increase with the increasing WVC. It is shown that the formation mechanism of O radicals is significantly affected by the WVC. The dominant reaction creating O radicals changes from the charge exchange between He+2 and H_2O to the electron impact dissociation of H_2O as the WVC increases from 0.01% to 1%. This study is helpful for better understanding the application of non-thermal plasmas discharges in water, such as biomedical, environmental engineering.     

岭澳核电站灯具更换的电磁兼容性研究

目前岭澳一期核电站主控室采用的电感灯将逐步被三星LED灯替代,但三星LED灯配置的开关电源产生的电磁辐射可能影响其他核电设备的正常工作,带来电磁兼容性问题。论文首先根据文献调研情况提出了主控室内设备设施的辐射抗扰度水平,然后从试验和现场实测两个角度,对三星LED灯和电感灯的电磁辐射骚扰特性进行了分析;同时在试验室对上述两种类型灯具的低频传导骚扰电压和谐波电流进行了对比测试。结果表明:从电磁辐射骚扰水平大小来看,考虑最严重条件下,主控室更换为三星LED灯后,产生的电场辐射骚扰水平和低频磁场辐射骚扰水平均远小于前述提出的核电设备辐射抗扰度水平,低频传导骚扰电压和谐波电流测试结果也满足现有国家标准要求,因此三星LED灯可替换目前主控室照明的电感灯。     

Spatial Evolution Study of EEDFs and Plasma Parameters in RF Stochastic Regime by Langmuir Probe

An RF compensated cylindrical Langmuir probe system has been developed and used to characterize an RF capacitive two temperature plasma discharge in a stochastic mode. The novelty of the work presented here is the use of the driven electrode （cathode） without ground shield. Measurements of the electron energy distribution function （EEDF） and plasma parameters were achieved under the following conditions： 50 W of RF power and 5× 10-2 mbar of argon pressure. The probe measurements are performed at 3 cm above the electrode and the probe was shifted radially （r direction） from the center （r = 0 cm） of the inter-electrodes region towards the chamber wall （R = 10.75 cm）. The results show that the EEDF is bi-Maxwellian and its shape remains the same through the scanned region. The farther the probe from the central region, the lower the EEDF maximum. The plasma density is observed to decrease according to a Gaussian profile along the radial direction and falls to 50% of its maximum when close to the cathode edge （r = 5.5 cm）. At the same time the effective electron temperature remains constant for r〈4 cm and increases for r≥4 cm. The high-temperature and low-temperature electrons＇ densities and temperatures are also discussed in the article.     

Investigation of Stable Al2O3 Scale on Fe-Al Aluminized Coating for Tritium Barrier

It is well known that α-Al₂O₃ phase has stablility performance, high permeation reduction factor and good resistance performance in liquid LiPb, which is considered as the reference tritium barrier coating in future fusion reactor. In order to study the formation mechanism of stable α-Al₂O₃ scales on fusion structure material, the oxidation behavior of Fe-Al aluminized coating on China Low Activated Martensitic （CLAM） steel was investigated under the oxygen partial pressure from 1 Pa to 20 kPa at the temperature of 940-980 ℃. The Al₂O₃ scales were analyzed by thermogravimetric analysis meter, grazing angle X-ray diffractometer, glow discharge spectrometer, focused ion beam and transmission electron microscope. A single continuous Al₂O₃ scales with the maximum thickness of about 2 000 nm was formed on the diffusion Fe-Al aluminized layer. Thermogravimetric analysis results show that the higher oxidation rate constant is achieved while increasing the oxygen partial pressure, and then oxidation rate constant decreases. The phase transformation of Al₂O₃ scales on the surface of Fe-Al aluminized coating was studied during different oxidation time ranges from 3 min to 180 min. The metastable γ-Al₂O₃ and α-（Al0.948Cr0.052）₂O₃ phases is formed in the earlier oxidation process and finally transformed to stable α-Al₂O₃ phase. The features of the transient α-（Al0.948Cr0.052）₂O₃（113） and α-Al₂O₃（113） were detected by GXRD and then confirmed by focused ion beam and transmission electron microscope.     

Investigation on pulsed discharge mode in SF6-C2H6 mixtures

The non-chain chemical HF(DF)laser is one of the most powerful electrically-driven lasers operating in mid-infrared,in which SF6-C2H6 mixtures are often used as lasering media.Due to the electronegativity of SF6,the discharge in SF6-C2H6 presents a complicated discharge mode.To achieve reproducible pulsed laser output,pulsed discharge in SF6-C2H6 mixtures is investigated for discharge mode using plane electrodes assisted by array pre-ionization spark pins in cathode surface.Firstly,two modes can be distinguished.One mode is called the selfsustained volume discharge(SSVD),which is characterized by spatial uniformity in the discharge gap and pulse to pulse repeatability.On the contrary,another mode includes random arc passages in the discharge gap and therefore cannot conduct lasering.By varying discharge conditions(gap voltage,gas pressure,etc)two discharge modes are observed.Secondly,the holding scope of the SSVD mode is analyzed for the optimal mixture ratio of 20:1,and the boundary tend of the holding scope of SSVD indicates there exists maximum gas pressure and maximum charging voltage for SSVD.Finally,the peak current of SSVD relates positively to charging voltage,while negatively to gas pressure,from which it is drawn that synchronous electron avalanches initiated by the sliding array overlap spatially into SSVD and thus SSVD is essentially an α ionization avalanche.     

Effect of plasma treatment on the seed germination and seedling growth of radish (Raphanus sativus)

The effect on the germination and seedling growth of radish (Raphanus sativus) seeds were examined employing a dielectric barrier discharge (DBD) at atmospheric pressure and room temperature for various treatment time. DBD plasma using argon gas of flow rate 2 l m−1 was employed in this study. Radish seeds were treated with DBD plasma for 1–5 min, respectively. Germination characteristics, seedling growth parameters, the contact angle of the seed coat, water uptake capacity, mass loss, the temperature of the seeds, chlorophyll, and carotenoid contents of the seedlings were measured before and after the DBD plasma treatments. Plasma treatment of radish seeds significantly increased germination-related characters, including germination percentage, fresh and dry weight, vigor index, and total carotenoids contents. However, the cumulative production rate was found to be decreased. Results from the experiment indicate an acceleration in the water uptake of the radish seeds and make the seed surface hydrophilic by plasma treatment. Scanning electron microscopy analysis showed that etching effects on the seed coat occurred after the argon plasma treatments, which affected the wettability of the radish seed. The experimental findings showed that seeds being treated by DBD plasma for 2 and 3 min had a positive effect on the germination and seedling growth of radish.     

Effects of discharge parameters on the micro-hollow cathode sustained glow discharge

The effects of parameters such as pressure, first anode radius, and the cavity diameter on the micro-hollow cathode sustained glow discharge are investigated by using a two-dimensional self-consistent fluid model in pure argon. The results indicate that the three parameters influence the discharge in the regions inside and outside of the cavity. Under a fixed voltage on each electrode, a larger volume of high density plasma can be produced in the region between the first and the second anodes by selecting the appropriate pressure, the higher first anode, and the appropriate cavity diameter. As the pressure increases, the electron density inside the hollow cathode, the high density plasma volume between the first anode and second anodes, and the radial electric field in the cathode cavity initially increase and subsequently decrease. As the cavity diameter increases, the high-density plasma volume between the first and second anodes initially increases and subsequently decreases; whereas the electron density inside the hollow cathode decreases. As the first anode radius increases, the electron density increases both inside and outside of the cavity. Moreover, the increase of the electron density is more obvious in the microcathode sustained region than in the micro cavity region. The results reveal that the discharge inside the cavity interacts with that outside the cavity. The strong hollow cathode effect and the high-density plasma inside the cavity favor the formation of a sustained discharge between the first anode and the second anodes. Results also show that the radial boundary conditions exert a considerably weaker influence on the discharge except for a little change in the region close to the radial boundary.     

Improvement on Diamond Nucleation Treated by Pulsed Arc Discharge Plasma

A technique of improvement on diamond nucleation based on pulsed arc discharge plasma at atmospheric pressure was developed. The pulsed arc discharge was induced respectively by nitrogen, argon and methanol gas. After the arc plasma pretreatment, a nucleation density higher than 10^10 cm^-2 may be obtained subsequently in chemical vapor deposition (CVD) on a mirror-polished silicon substrate without any other mechanical treatment. The effects of the arc discharge plasma on the diamond nucleation were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR) and Raman spectroscopy. The enhancement of nucleation is postulated to be a result of the formation of carbonlike phase materials or nitrogenation on the substrate surface without surface defect produced by arc discharge.