Low-Pressure Helicon-Plasma Discharge Initiation via Magnetic Field Ramping

Discharge initiation at low pressures and flow rates is investigated in the Madison Helicon Experiment flowing helicon source. At low pressures (below 14-sccm flow rate), a threshold magnetic field exists for discharge initiation which depends on RF power and gas flow rate. Above the threshold magnetic field, RF discharges start only after a significant delay (approximately seconds) and sometimes will not start at all. This threshold magnetic field is interpreted using electron multipactor arguments. A technique is described for initiating discharges at low flow rates and pressures $(lambda_{{rm en}, {rm iz}} ≫ L_{rm system})$ and high magnetic fields (above the threshold value). Without a static magnetic field present, the RF power is turned on, and a lower density $(≪ 10^{11} hbox{cm}^{-3})$ unmagnetized discharge occurs. The magnetic field is then applied, and the discharge transitions to the higher density (up to $10^{13} hbox{cm}^{-3}$) regime. Using this method, magnetized discharges can be started at flow rates as low as 1 sccm ( $1.8 times 10^{-4} hbox{torr}$ at $z = -91 hbox{cm}$ , $1.7 times 10^{-5} hbox{torr}$ at $z = 105 hbox{cm}$) at 500 W in a 1.04-kG magnetic field. This technique can be used to initiate low-pressure helicon discharges for basic plasma science experiments and other applications.      

Plasma-Pretreated Catalyst for Methanol Synthesis From Syngas

A new catalyst preparation method by combining the traditional coprecipitation method with an oxygen plasma pretreatment was developed in this paper. The ternary Cu/Zn/Al catalyst powders were first prepared by coprecipitation method, and then pretreated by the plasma in a fluidized-bed dielectric barrier discharge reactor with a carrier gas of a molar ratio of $hbox{O}_{2}/hbox{Ar} = 5/95$ . Then, the plasma-pretreated catalyst was conducted with the methanol synthesis test at the following conditions: $hbox{H}_{2}/hbox{CO}/hbox{N}_{2} = 64/32/4$, $Q = 900 hbox{sccm}$, $P = 4 hbox{MPa}$, and $T = 255 ^{circ}hbox{C}$. The plasma pretreatment could enhance the catalytic activity, e.g., the methanol selectivity increased from 28% to 72% and the methanol yield from 0.19 to 0.52 $hbox{g/g}_{rm cat.}cdothbox{h}$ , although the CO and $hbox{H}_{2}$ conversions were slightly decreased. On the other hand, the BET, inductively coupled plasma/optical emission spectroscopy, and X-ray diffraction analysis found that the catalytic properties did vary after plasma pretreatment. In short, this paper reveals that the plasma pretreatment is a viable process for preparation of Cu/Zn/Al catalyst in terms of methanol synthesis.      

Energy Efficiency Analysis of the Discharge Circuit of Caltech Spheromak Experiment

The Caltech spheromak experiment uses a size A ignitron in switching a 59-$ muhbox{F}$ capacitor bank (charged up to 8 kV) across an inductive plasma load. Typical power levels in the discharge circuit are $sim$200 MW for a duration of $sim!! 10 muhbox{s}$. This paper describes the setup of the circuit and the measurements of various impedances in the circuit. The combined impedance of the size A ignitron and the cables was found to be significantly larger than the plasma impedance. This causes the circuit to behave like a current source with low energy transfer efficiency. This behavior is expected to be common with other pulsed plasma experiments of similar size that employ an ignitron switch.      

Design and Numerical Optimization of a Cusp-Gun-Based Electron Beam for Millimeter-Wave Gyro-Devices

A novel thermionic cusp electron gun operating in the temperature-limited regime that produces a large-orbit electron beam through a nonadiabatic magnetic-field reversal was designed, analyzed, and optimized to give an electron-beam ideal for driving gyro-devices, particularly in the millimeter-to-submillimeter-wavelength range due to its small cross-sectional size. The annular-shaped axis-encircling electron beam had a beam current of 1.5 A at an acceleration potential of 40 kV, a tunable velocity ratio $alpha (= v_{ perp}/v_{z})$ between one and three, an optimized axial velocity spread $Delta v_{z}/v_{z}$ of $sim$ 8%, and a relative alpha spread $Deltaalpha/alpha$ of $sim$10% at an alpha value of 1.65.      

Computational Studies of Atmospheric-Pressure Methane–Hydrogen DC Micro Glow Discharges

Atmospheric-pressure methane–hydrogen micro glow discharges were computationally investigated using a 2-D hybrid model. The plasma model was solved simultaneously with a model for the external circuit. Simulations were conducted for a pin-to-plate electrode configuration with an interelectrode separation of 400 ${rm mu}hbox{m}$. The spatiotemporal evolutions of electrons, species densities, electric field, and electron and gas temperatures were studied. A total of 81 reactions were considered, which included electron–neutral, electron–ion, ion–neutral, and neutral–neutral reactions. An 84-step reaction mechanism consisting of 15 surface species and four deposited bulk species was considered. A time-stepping technique was employed to address the time scales of plasma transport (in microseconds) and neutral and fluid transport (in milliseconds) in 2-D simulations with detailed volume and surface chemistry. The simulations indicated $hbox{H}_{3}^{+}$ and $hbox{CH}_{5}^{+}$ ions to be the most prominent hydrogen and hydrocarbon ions. The gas temperature predictions suggested the discharge to be operating as a nonthermal glow discharge. The effect of discharge current on both plasma and deposition characteristics was studied. The simulations predicted a flat voltage–current characteristic, indicating the discharge to be operating in normal glow mode. The predicted voltage–current characteristic was found to be in favorable agreement with the experimental measurements. With an increase in discharge current, the deposition rate profile expanded in the lateral direction, suggesting that deposition occurred at the cathode spot.      

Proof of Principle Experiments on Direct Generation of the $hbox{TE}_{11}$ Mode in a Coaxial Vircator

Experiments on a coaxial vircator with a sectioned emitter are reported. The emitting area is sectioned to form two opposing emitters in order to favor growth of the $hbox{TE}_{11}$ mode and inhibit growth of the $hbox{TM}_{01}$ mode that is usually excited in a coaxial vircator. Experiments are performed using a compact 320-J 400-kV Marx generator and a compact coaxial vircator built in a standard 8$^{primeprime}$ vacuum tube. The radiated magnetic-field strength is measured by means of four free-field (B-dot) probes, and experiments show that sectioning the emitter does, in fact, lead to generation of the $hbox{TE}_{11}$ mode.      

Preliminary Study on Applications of an Atmospheric-Pressure Argon Plasma Discharge With a Single-Electrode Configuration

An atmospheric-pressure (AP) argon plasma discharge generated in a single-electrode configuration with the power supply operating at a frequency of 45 kHz is employed to perform some applications, such as the treatment of the interior surface of a medical infusion tube, hydrophilic modification of insulator surface, hardening of metal surface, and acidification treatment of water. It is shown that the active gases ( $hbox{O}_{2}$ and $hbox{N}_{2}$) mixed in a carrier gas (Ar) play a significant role in carrying out such applications. A preliminary study on its applications is presented to demonstrate the promising potential of AP single-electrode-configuration plasma jet.      

Dynamics of the Current Distribution in a Discharge of the PF-3 Plasma Focus Facility

In this paper, results are presented from studies of the dynamics of the plasma-current sheath and current distribution in the PF-3 facility, one of the largest plasma focus machines in the world. The experiments were done at input energy of $W = 290 hbox{kJ}$ and discharge current of $I sim 2 hbox{MA}$, with the chamber being stationary filled with the working gas. The current sheath parameters were measured with absolutely calibrated magnetic probes installed at different distances from the system axis and at different heights above the anode plane. The possibility is demonstrated of the formation of closed current loops due to the development of shunting breakdowns in the insulator region. The maximum residual plasma density at which the electrode gap remains magnetically self-insulated is estimated.      

Ferrite-Enhanced U-Shaped Internal Antenna for Large-Area Inductively Coupled Plasma System

A Ni–Zn ferrite-enhanced U-shaped internal inductive antenna (240 mm $times$ 2300 mm) operated at 2 MHz was used as a linear plasma source for an ultralarge-area plasma, and its plasma and electrical characteristics were investigated and compared with those of the antenna operated at 13.56 MHz without the ferrite. By the magnetic field enhancement, the operation of the source showed higher power transfer efficiency, lower antenna impedance, and lower RF rms voltage compared to that operated at 13.56 MHz without the ferrite. When photoresist etch uniformity was measured by etching the photoresist using a 40-mtorr $hbox{Ar/O}_{2} (7:3)$ mixture at 2 MHz by locating three U-shaped antennas in parallel, the etch uniformity less than 11% could be obtained on the substrate size of 2300 mm $times$ 2000 mm.      

Generation of Plasma Wave at Pump-Wave Frequency and Second Harmonic Generation at Ultrarelativistic Laser Power

In this paper, the influence of ponderomotive and relativistic nonlinearities on the filamentation of an ultraintense laser pulse is investigated in three dimensions within the paraxial ray approximations. Generation of electron plasma wave (EPW) structure at pump-wave frequency and the second harmonic generation in these filamentary structures are reported. The generation of the plasma wave is due to intensity gradient (in the transverse direction of the laser beam in filamentary structure) and density gradient (due to ponderomotive-force effect). For typical laser–plasma parameters: The $hbox{laser intensity} = 2.5timesbreak 10^{20} hbox{W/cm}^{2}$; the $hbox{particles density} = 1.9 times 10^{19} hbox{cm}^{-3}$; and it is found that the maximum intensity of EPW is in the range of $2.0 times 10^{13} hbox{W}/hbox{cm}^{2}$. Interaction of the plasma wave with the incident laser beam leads to second harmonic generation, and the yield comes out to be $approx! 2.1 times 10^{-7}$.      

Performance Analysis of a Diagonal MHD Accelerator for Space Propulsion

The performance of a diagonal magnetohydrodynamic (MHD) accelerator has been numerically investigated. Studies were carried out using air plasma as a working gas in an equilibrium condition based on the MHD Augmented Propulsion Experiment channel designed by NASA. The MacCormack scheme is employed in order to solve the set of differential equations with MHD approximations. The fundamental performance of a diagonal MHD accelerator considering both flow performance along the channel and propulsion performance has been evaluated under various applied input currents and magnetic fields. The optimum performance is dominated by ${bf j} times {bf B}$ Lorentz body force acceleration, while it is increased with Joule heating and the ${bf u} times {bf B}$ term's contribution, which are detrimental to the propulsion performance. Moreover, friction forces resist the flow performance, particularly near the channel exit.      

Titanium Oxide Film Deposition on Acrylic Resin by Atmospheric TPCVD

A dye-sensitized solar cell (DSC) is a solar cell that uses an anatase film as a photovoltaic device. Since the anatase film and dye play the roles of electron carrier and electron generator, respectively, in the DSC, porous anatase films are desirable. In this paper, in order to develop a low-cost fabrication process for the photovoltaic device of the DSC, photocatalytic titanium oxide film deposition was carried out by atmospheric thermal plasma CVD. Ar gas, which served as the working gas for the plasma jet and substrate, and a 20 $times$ 40 $times$ 3 mm transparent acrylic resin plate were used. Titanium tetra iso butoxide was used as feedstock. Consequently, by cooling the substrate, an anatase-dominant film could be deposited at 773 K in deposition temperature without meltdown of the substrate on the condition of 100 mm in deposition distance, even in the case of acrylic resin substrate use. By wettability and methylene-blue decoloration tests, it was confirmed that the film showed hydrophilic and decolored methylene blue perfectly by 8-h UV irradiation. Furthermore, the DSC in which the titanium oxide film deposited by this technique was included as a photovoltaic device generated a photoelectromotive force of 25 mV. From these results, these thermal plasma processes were found to have high potential for DSC fabrication.      

Modeling of the Dynamic Plasma Pinch in Plasma Focus Discharges Based in Von Karman Approximations

Dynamic plasma pinches occur in a variety of devices, as $Z$ -pinches and plasma focus. In this paper, a lumped parameter model of a dynamic plasma pinch produced in a plasma focus discharge is presented. The model is based in Von Karman approximations of the radial density and velocity profiles, which leads to the reduction to a system of ordinary differential equations describing the dynamic evolution of the pinch compression and expansion. The model was coupled with a fusion kernel to produce an estimate of the neutron yield per pulse. The calculations were tested against available data of the pressure–yield curve of seven experimental devices ranging from 1 to 250 kJ, showing excellent agreement, particularly regarding the curvature of the pressure–yield curve.      

Optimization of Sustain Electrode Gap for High-Efficiency Plasma Display by Using a Counter-Electrode Structure

A counter-electrode structure as a microscaled dielectric barrier discharge for an application to the alternating-current plasma display has been investigated with six in-test panels fabricated by using a thick-film ceramic-sheet method. Based on observations of infrared (IR) emission, the optimum sustain electrode gap to generate an efficient positive-column-like microdischarge plasma was suggested. The IR emission as well as the firing and sustain voltages increases with the increasing electrode gap, and the positive-column-like microdischarge plasma was built up for electrode gaps over 400 $muhbox{m}$, which can be used for an intense vacuum ultraviolet source. Furthermore, the luminance and luminous efficiency, as well as the panel capacitance of the counter-electrode structure, were compared with those of the conventional reference structure.      

Studies on the Measuring Method of Number Density of Ions in the High Speed Flow Field

At present, the method of using a parallel-plate probe to measure the number density of ions at atmosphere has some limitations such as big size of probe, low velocity of gas flow allowed to measure, and low accuracy. In this paper, a detector with a spherical probe is fabricated and is used to measure the number density of ions. The diameter of probe can be reduced to 6 mm. The highest velocity that can be measured is extended to 28 m/s, and the collecting rate is up to 100%. The spatial distribution diagram of ion number density is also given, with a cross section of 0.36 $hbox{m}^{2}$. Then, an effective method is provided to measure the number density of ions generated from the miniature ion source or in the gas flow with high velocity.      

Production of Capacitively Coupled Atmospheric Plasma Jet With Multiring Electrodes for the Medical Plasma Tool

An atmospheric capacitively coupled plasma jet has been developed by using multiring-electrode configurations for medical applications. It was found that the discharge conditions for plasma-jet production expanded with the increase of the number of pairs of multiring electrodes. The length of the plasma jet increased with the increase of the injected power and helium-gas flow rate. The temperature of the plasma jet, measured by a thermocouple, decreased with the increase of gas flow rate and was then attained to be around 50 $^{circ}hbox{C}$ at a helium flow rate of 30 L/min. Optical measurements of plasma-jet emission showed various optical emissions from helium atoms, nitrogen atoms, hydroxyl radicals, and metal atoms originating from the electrode material.      

空心针-板电极大气压氩气辉光放电的特性研究

大气压均匀放电等离子体在工业领域具有非常广泛的应用前景,它是利用直流电源激励的空心针-板放电装置,以氩气为工作气体在大气压空气中产生均匀稳定的放电。对氩气流量和气隙间距对辉光放电发光特性的关系进行了研究,结果表明放电所产生的等离子体柱连接两个电极,发光较为均匀(观察不到放电丝)。在板电极附近放电等离子体柱直径最大,最大直径随着电流和气流的增大而增大。放电伏安特性研究发现,与低气压辉光放电相类似,两电极间的电压随着电流的增大而减小,并且随气流和气隙间距的增大而增大。对该大气压直流均匀放电在扫描范围为330～450 nm的光学发射光谱进行分析,获得了放电等离子体的分子振动温度和谱线强度比I_391.4/I_337.1随氩气流量和气隙间距的变化关系。I_391.4/I_337.1均随流量和气隙间距的增大而降低。对等离子体柱的I_391.4/I_337.1沿气流方向(等离子体柱轴向)进行了空间分辨测量,并进行了定性分析,结果表明,振动温度及电子平均能量随着远离空心针口距离的增大而增大。这些结果对大气压辉光放电在工业中的应用具有重要意义。     

介质阻挡放电点燃大气压直流均匀放电的光谱研究

由于大气压均匀放电等离子体在工业领域具有广泛的应用前景,为了获得大尺寸的大气压均匀等离子体,采用氩气作为工作气体,在大气压空气环境中利用同轴介质阻挡放电点燃了针-板电极间的大气隙(气隙宽度达到5 cm)直流均匀放电。研究发现,同轴介质阻挡放电能够有效降低针-板电极间的击穿电压。该均匀放电由等离子体柱、等离子体羽、阴极暗区和阴极辉区组成。其中等离子体柱和阴极辉区都是连续放电。而等离子体羽不同位置的放电是不同时的。事实上,等离子体羽放电是由从阴极向着等离子体柱移动的发光光层(即等离子体子弹)叠加而成。利用电学方法测量了放电的伏安特性曲线,发现其与低气压正常辉光放电类似,均具有负斜率。采集了放电的发射光谱,发现存在N₂第二正带系、氩原子和氧原子谱线。通过Boltzmann plot方法对放电等离子体电子激发温度进行了空间分辨测量,发现等离子体柱的电子激发温度比等离子体羽的电子激发温度低。通过分析放电机制,对以上现象进行了定性解释。这些研究结果对大气压均匀放电等离子体源的研制和工业应用具有重要意义。     

大气压电解液阴极辉光放电发射光谱检测水体中的金属残留

水环境中的金属残留严重威胁人类的健康安全,急需快速、高效的金属残留检测技术。文章报道了自行建立的大气压电解液阴极辉光放电发射光谱装置。利用待测液体作为放电阴极进行大气压辉光放电实现了水体中金属离子的痕量检测。对配制的标准样品进行了定量测量,基于背景发射光谱的3σ计算,获得了大气压电解液阴极辉光放电光谱装置对Na, Li, Cu, Pb和Mn等5种金属元素的检测限,分别为0.008, 0.005, 1.1, 2.06和1.95 mg·L-1。该装置在金属残留的实时在线检测领域具有应用前景。     

直流大气压辉光放电高能电子密度的空间分布

利用针阴极和水阳极,在6 mm的空气隙产生了大气压空气辉光放电。该大气压辉光放电具有明显的负辉区、正柱区和阳极辉区等明亮的发光区。通过研究放电的电压电流特性,发现该放电处于亚辉光放电到正常辉光放电阶段。由于氮分子第二正带系337.1 nm的光谱强度反映高能电子密度,对337.1 nm谱线的强度进行了空间分辨测量。结果发现高能电子在针尖附近密度最大,而其他区域相差不多。随电压升高,高能电子密度减少。增大限流电阻,高能电子密度也减少。氧原子对杀菌消毒具有重要作用,利用发射光谱法对氧原子谱线强度的空间分布进行了测量,发现氧原子谱线强度与高能电子的空间分布及其随参数的变化关系一致。