Influence of the cathode and anode jets on the properties of a high-current electric arc

A study is made of the effects related to the formation of electrode jets in discharges in hydrogen and air at a current of 10⁵–10⁶ A, a current growth rate of 10¹⁰ A/s, an initial pressure of 0.1–4.0 MPa, and a discharge gap length of 5–40 mm. After secondary breakdown, jets are observed in a semitransparent discharge channel expanding with a velocity of (4–7)×10² m/s. The formation of shock waves in the interaction of the jets with the ambient gas and the opposite electrode is observed by the shadowgraphy method. Seventy microseconds after the beginning of the discharge, the pressure of the metal vapor plasma near the end of the tungsten cathode amounts to 177 MPa. The brightness temperature in this case is T=59×10³ K, the average ion charge number is [`(m)] = 3.1\overline m = 3.1 , and the metal vapor density is n=5.3×10<sup>19</sup> cm<sup>−3</sup>. After 90 μs, the average ion charge number and the metal vapor density near the anode end are [`(m)] = 2.6\overline m = 2.6 and n=7.4×10¹⁹ cm⁻³, respectively. Based on the experimental data, possible reasons for the abnormally high values of the total voltage drop near the electrodes (up to ∼1 kV) are discussed.     

The profile of the Fe 372·00 nm line excited in a hollow cathode discharge

The profile of the Fe 372·00 nm line emitted in the negative glow of a hollow-cathode discharge is analyzed. The value of the discharge current was varied within the range 7–15 mA, the neon pressure being 1·22–4·40 Torr. The Lorenzian and Gaussian components of the recorded line profile were corrected for instrumental broadening. A correlation was found to exist between the Gaussian component and the cathode voltage drop. The half-width of the Lorentzian component shows the expected dependence on gas pressure. Under the operating conditions used, this component is approx. 10² times greater than the natural one. No dependence on the discharge current was observed. The emitted line profiles may be considered to be Voigt functions with the Voigt parameter varying in the range 3·2 × 10^-2 to 5.5 × 10^-2     

Measurement of Excited States Density and the VUV-Radiation in the Pulsed Xenon Medium Pressure Discharge

Time resolved absolute density measurements of the 1s5 (metastable) and 1s4 (resonance) level in the positive column of a pulsed xenon gas discharge for gas pressures between 1 and 40 Torr and currents between 100 and 300 mA are presented. The densities ranged from 1 × 10¹¹ to 5 × 10¹² cm^?3 and were found to be ten times larger in the afterglow than in the active part of the discharge. The enhanced radiation obtained in the afterglow in the near infrared regions as well as in the VUV region is caused by a dissociative recombination and formation of excimer states.     

The impact of the Ramsauer effect on the frequency of elastic collisions in inductive RF discharges in inert gases

This paper presents the results of investigating the power absorption mechanism of an inductive RF discharge plasma. Dependences of the frequency of elastic electron collisions with inert gas atoms (helium, neon, argon, and krypton) on the pressure are given. In the frequency range of 3 × 10⁶–3 × 10⁷ s^?1, an equivalent plasma resistance and the power input into the plasma are determined by the values of collision frequency and electron density within a skin layer and do not depend on the type of gas within the limits of experimental error. Upon reaching the electron temperature of ～1 eV, the energy of the main part of electrons lies in the range of Ramsauer’s minimum for elastic cross section. This leads to a decreasing elastic-collision frequency in heavy inert gases as compared to helium.     

常压下气体放电等离子体振荡的实验与理论研究

在实验上研究了高压交流电弧发生器电极间隙的气体放电及等离子体振荡, 观察到了气体放电过程中的纳秒脉冲.以电子的流体运动方程和麦克斯韦方程为理论基础, 利用δ函数来描述交变外电场作用下电极处的电子堆积现象,建立了常压下气体放电时等离子体在外电场中振荡的理论模型,通过Laplace变换求解出电极间的放电电压.理论与实验结果基本符合, 从而可估算出实验中等离子体的电子数密度为1.3× 10¹²/m³.     

激光冷却获得高亮度的亚稳态惰性气体原子束和原子阱

高强度的亚稳态惰性原子束流在原子分子物理实验研究中具有广泛的应用.使用射频电离方法和激光横向冷却技术制备了高强度的亚稳态氪原子束流,并使用数值模拟方法对横向冷却激光场中的原子径迹进行了分析.通过激光诱导荧光光谱方法测量原子束的束流特性,结果显示,横向冷却后在束流源下游230 cm处的原子束流强度达1.6atoms/(s*sr),束流强度提高了两个量级.利用这种高强度原子束流,我们成功囚禁了1.3×10¹⁰个亚稳态⁸⁴Kr原子,同时冷原子装载速率达到了3.0×10¹¹atoms/s;并利用该装置成功地实现了高亮度的亚稳态氩原子束和原子阱. 关键词： 横向冷却 原子束 原子阱 惰性气体     

Measurement of atomic hydrogen density in non-thermal H<Subscript>2</Subscript> plasmas via threshold ionisation-molecular beam mass spectrometry

Determinations of radical density are essential to investigate the physical-chemical processes in plasmas and setup the related theoretical models. This paper presents the experimental measurement of atomic hydrogen near grounded electrode in dielectric barrier discharge medium-pressure hydrogen plasma via threshold ionisation-molecular beam mass spectrometry. After investigating the possible influences from parent molecules in excited states, background component and space-charge, evolution of atomic hydrogen density as functions of discharge parameters are investigated utilising the signal of H₂ molecule beam as the reference. At fixed gas pressure of 6.0 torr and a discharge voltage of 24 kV, atomic hydrogen density increases monotonously from 1.1×10¹⁴ to 2.0×10¹⁵ cm^-3 as the discharge frequency increases from 9 to 26 kHz. Similarly the rising discharge voltage also lead to enhancement of atomic hydrogen density.     

Effects of discharge current and voltage on the high density of metastable helium atoms

Both hollow-cathode and Penning-type discharges were adopted to excite helium atoms to a metastable state. Experimental data indicate that Penning discharge is more suitable for generating high fractions of metastables in a low-density helium beam for laser-induced fluorescence technique in measuring electric fields at the edge of a plasma. The metastable density increases with increasing helium gas pressure in the range of 1.33×10^{-2}－66.7Pa. The highest metastable density of 3.8×10^{16}m^{-3} is observed at a static gas pressure of 66.7Pa. An approximately linear relationship between the density of metastable helium atoms and the plasma discharge current is observed. Magnetic field plays a very important role in producing a high density of metastable atoms in Penning discharge.     

Probe measurements in a discharge with liquid nonmetallic electrodes in air at atmospheric pressure

Discharges with liquid nonmetallic electrodes of much interest for applications are investigated. It is found that a dc discharge between two streams of tap water in air at atmospheric pressure is stable at a currentof 40≤I≤100 mA. The discharge exists in the diffuse (volume) form with a relatively low current density (∼0.2 A/cm²) and a high (above one kilovolt) voltage drop across the air gap (∼1 cm) between the water electrodes. The current density and voltage depend only slightly on the discharge current. Probe measurements show that three regions can be distinguished in the discharge: two electrode regions (1–2 mm in length) and a discharge column with a constant electric field of ≈0.8 kV/cm (i.e., E/N≈20 Td, because the gas in the discharge is heated up to 1500–2000 K). The average electric field strength near the electrodes is E≈2–3×10³ V/cm (E/N≈60–80 Td). The charged particle density in the column is n ∼ 10¹² cm⁻³. The probe measurements of n agree with the previous microwave absorption measurements. The water vapor concentration in the column is also estimated from probe measurements.     

Discharge in the Atmosphere in a Gaussian Beam of Subthreshold Millimeter Waves

The propagation velocities of a subthreshold discharge excited in air at atmospheric pressure by a pulsed microwave beam with a Gaussian field distribution, a wavelength of 4 mm, and an intensity up to 30 kW/cm² have been measured by means of optical and microwave diagnostics. It has been shown that the motion of a discharge front along the path of the beam toward the region of an increasing microwave field is accompanied by an increase in the velocity from subsonic (~10^–4 cm/s) to supersonic (~(6–8) × 10⁴ cm/s). At the same time, motion toward the decreasing field region is accompanied by a decrease in the velocity from supersonic to subsonic. It has been found that the maximum temperature of the gas in the discharge at velocities of ~10⁴ cm/s reaches ~5.3 kK.     

Dosed extraction of hydrogen from a layered thin-film system

Radiation-stimulated interfacial gas release in an Ag-glass thin-film system is investigated. It is established that under proton irradiation hydrogen accumulates in bubbles, which are the interfacial gas reservoirs, at the interface. The gas bubbles formed are studied and their parameters are determined by optical microinterferometry. It is shown that 15–50 μm in radius bubbles contain 2×10⁹–2×10¹¹ hydrogen molecules. Hydrogen is extracted from the reservoir by rupturing the thin-film dome of a bubble with a 2×10¹³ W·cm⁻² laser beam. Zh. Tekh. Fiz. 69, 112–115 (February 1999)     

Advanced high-pressure plasma diagnostics with hairpin resonator probe surrounded by film and sheath

The hairpin probe using microwave resonance in plasma is applicable to high pressure 1.33×10 3-1.01×10 5 Pa)) as developed recently.In this work,an analytic model of the hairpin resonator probe surrounded by a thin dielectric layer and a sheath layer is proposed.The correction factor due to these surroundings is analytically found and confirmed by electromagnetic field finite difference time domain simulation,thus enabling the accurate measurement of electron density in a high-pressure non-equilibrium uniform discharge.     

Performance of a ten-liter electron avalanche-discharge XeCl laser device

This paper describes the laser performance of an X-ray preionized ten-liter volume, electron-avalanche discharge XeCl system, operated at dc charging voltages of up to 70 kV both with and without X-ray preionization. An output energy of more than 11J uniformly distributed over a (7×10) cm² spot size was achieved using a multichannel spark-gap as the main discharge switch. The discharge was also successfully operated in the switchless mode, yielding a 4.8 J output energy and a 0.9% efficiency. The time-resolved gain measurements provided information on the laser discharge evolution.ENEA GuestENEA Guest on leave from the Shanghai Institute of Optics and Fine Mechanics, the Chinese Academy of Sciences, Shanghai, China     

Parametric study of radiofrequency helium discharge under atmospheric pressure

The parameters of radiofrequency helium discharge under atmospheric pressure were studied by electrical and optical measurements using high voltage probe, current probe and optical emission spectroscopy. Two discharge modes α and γ were observed within certain limits. During α to γ mode transition, a decrease in voltage (280–168 V), current (2.05–1.61 A) and phase angle (76^°–56^°) occurred. The discharge parameters such as resistance, reactance, sheath thickness, electron density, excitation temperature and gas temperature were assessed by electrical measurements using equivalent circuit model and optical emission spectroscopy. In α mode, the discharge current increased from 1.17 to 2.05 A, electron density increased from 0.19 × 10¹² to 0.47 × 10¹² cm^?3 while sheath thickness decreased from 0.40 to 0.25 mm. The excitation temperatures in the α and γ modes were 3266 and 4500 K respectively, evaluated by Boltzmann’s plot method. The estimated gas temperature increased from 335 K in the α mode to 485 K in the γ mode, suggesting that the radiofrequency atmospheric pressure helium discharge can be used for surface treatment applications.     

Determination of the OH radical in atmospheric pressure dielectric barrier discharge plasmas using near infrared cavity ring-down spectroscopy

The hydroxyl radical (OH) plays an important role in combustion systems, atmospheric chemistry and the removal of air pollutants by non-thermal plasmas. The present work reports the determination of the hydroxyl radicals in atmospheric dielectric barrier discharge plasmas via near infrared continuous wave cavity ring-down spectroscopy. The P-branches of OH X²Π_i (ν' = 2 ←ν^′′ = 0) bands were used for its number density measurements. The minimum measurable absorption coefficient is about 3 × 10^-8 cm^-1 in DBD plasmas. At certain experimental conditions (a.c. frequency of 70 kHz, 6700 ppm H₂O in He, 1 atm), when the peak-to-peak discharge voltage varied from 6 kV to 10.4 kV, the determined OH radical concentration increased from (2.1 ± 0.1) × 10¹³ molecules cm^-3 to (3.7 ± 0.1) × 10¹³ molecules cm^-3. The plasma gas temperature, derived from the Boltzmann plots of OH rotational population distributions, ranged from 312 ± 10 K to 363 ± 10 K when the discharge voltage was raised in the above range. The influences of O₂ and N₂ addition on the production of OH radicals have been also investigated.     

Design and performance of a 2-D multi-wire position sensitive X-ray detector

A 2-D multi-wire position sensitive detector for X-ray diffraction and small angle X-ray scattering studies is described. The detector has an active area of 100 mm × 100 mm and consists of an anode plane with 10 μm SS wires at 3 mm spacing and a pair of orthogonal cathode readout planes with 25 μm SS wires placed at 1.5 mm spacing. The position information is obtained using charge division method and recorded using a laboratory built data acquisition system. The resolution and gas gain was measured for 5.9 keV X-rays (⁵⁵Fe-source) as a function of the anode wire voltage and gas pressure. It was observed that the proportional region of the PSD at 100 kPa pressure extended up to a high voltage value of around 1.5 kV and it shifted to high values up to 2 kV for gas pressure of 300 kPa. The energy resolution improved from 18% (FWHM) to 12% with increase in pressure. The spatial resolution of the PSD also showed improvement, with a value of 1.2 mm × 1.4 mm at 300 kPa gas pressure. A maximum gain of 5 × 10⁴ is obtained.     

Wide-aperture electric-discharge XeCl lasers

Experimental investigations have been performed for aXeCl laser with the active region aperture equal to 6.5×7 and 15×15 cm. To pump the laser, two types of generator were used: a generator connected in a double-loop circuit with a peaking capacitor and a multichannel spark gap switch and a magnetic thyratron generator. The working mixtureNe−Xe−HCl was preionized with soft x rays and a low-current electron beam, providing an initial electron density ranging from 10⁶ to 5·10¹³ cm⁻³. Conditions in which a homogeneous space discharge is initiated have been studied. It has been investigated how the degree of ionization of the gas and the aperture of the active region affect the discharge and lasing characteristics of the laser. The laser efficiency reaches 4% with an energy of ≈ 6 J extracted from one liter of the active region. Institute of High-Current Electronics, Siberian Division of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 54–59, April, 2000.     

Effect of atmospheric pressure pulsed discharge spatial inhomogeneity on Ar I and H I lines: Electron number density study

The spatial inhomogeneity of pulsed atmospheric pressure discharge in argon is investigated using the electron number density N_e diagnostics procedure applied to asymmetrically broadened Ar I lines. A dedicated fitting procedure is used for describing Ar I 703.0 nm line shape recorded from argon gas discharge and H I (at 486.13 and 656.28 nm) lines recorded from Ar-H₂ gas mixture discharge. The results revealed the change in N_e in both axial and radial directions. The additional Ar I lines at 614.5, 710.7, 731.2, and 731.6 nm, recorded from integral spatial radiation, are analysed as well to confirm the results from the plasma column region. The possibility of using AlO (B²∑⁺–X²∑⁺) and CN (B²∑⁺–X²∑⁺) molecular bands for gas temperature T_g measurements in this type of gas discharge source is demonstrated and T_g used as an input parameter for the N_e diagnostics procedure. For the proper identification of molecular band spectral lines, the Fortrat parabolas are constructed. The results obtained from Ar I 703.0 nm line indicate three different N_e values, with N_e1 ≈ 0.6 × 10¹⁶ cm⁻³, N_e2 ≈ 3.6 × 10¹⁶ cm⁻³, and N_e3 ≈ 19 × 10¹⁶ cm⁻³ measured from the plasma column. These N_e values increase in the cathode and anode region.     

On the branching ratios of several pairs of He(II) lines

Branching ratios of He(II) lines have been measured for the principal levels 5–10. The observations have been made on an He plasma produced by a hollow cathode-arc discharge, having an electron density of 3×10¹⁹m^-3 and an electron temperature of 2×10⁵ K on the axis of the discharge.The observed branching ratios differ by about 30% from the ones following from theoretical transition probabilities, if the sublevels of the main levels were populated according to their statistical weights. The shape of the He(II) 4686Åemission line observed off the axis of the discharge using a Fabry-Pérot interferometer gave evidence of a non-statistical population distribution for the sublevels.     

High-purity F atom beam source

Fluorine atoms are generated by 2.45 GHz microwave dissociation of 99.99% F₂ in a synthetic sapphire single-crystal discharge tube. Typical F₂ gas flow and beam intensity are: 0.1 mbar ls^?1 and 9.5×10¹⁷ atoms sterad^?1 s^?1. The measured degree of dissociation into atoms isD>98%. The high total F atom flux together with the purity of the beam seem to make the source a very promising device for both reaction kinetic-and molecular beam-experiments