Messung und Deutung des Leitwertabklingens zylindrischer Bögen

The decay of the electrical conductance of the column of a cascaded arc (100 A) after current interruption is measured for the gases N₂, SF₆, CO₂ and Ar. Numerical computations of temperature and conductance decay are in good agreement with experiment for Ar but for N₂ and SF₆ there are drastic deviations. An interpretation of the decay processes is given by the aid of a simple model for the energy transfer including nonequilibrium processes with regard to recombination in the wall near arc regions. Introducing the model by a formal manner in the computer program leads to good agreement with experiment.     

High power self-controled volume discharge based molecular lasers

An investigation was made of the characteristics of the formation of a selfcontroled volume discharge for the pumping of CO₂ lasers, i.e. self-sustained volume discharge (SSVD), which involved a preliminary filling of a discharge gap by an electron flux from an auxiliary-discharge plasma. We have found that this method was suitable for large interelectrode gaps, that distortion of the electric field in the gap by the space charge of the electron flux played an important role in the formation of the discharge and that the electrodes could be profiled dynamically during propagation of an electron flux through the discharge gap and a SSVD could form in systems with a strongly inhomogeneous field. High power SSVD based CO₂ laser systems have been created and investigated. Another type of self-controled volume discharge without pre-ionization, i.e. a selfinitiated volume discharge (SIVD), in nonchain HF lasers with SF₆−C₂H₆ mixtures was investigated as well in our review. We have established that, after the primary local electrical breakdown of the discharge gap, the SIVD spreads along the gap in directions perpendicular to that of the electric field by means of the successive formation of overlapping diffuse channels under a discharge voltage close to its quasi-steady state value. It is shown that, as new channels appear, the current flowing through the channels formed earlier decreases. The volume occupied by the SIVD increases with increase in the energy deposited in the plasma and, when the discharge volume is confined by a dielectric surface, the discharge voltage increases simultaneously with increase in the current. The possible mechanisms which explain the observed phenomena, namely the dissociation of SF₆ molecules and electron attachment SF₆ molecules, are examined. A simple analytical model, which makes it possible to describe these mechanisms at a qualitative level, was developed. High power SIVD based HF(DF) lasers have been developed and tested.     

Mass spectrometry of arcs in SF6 circuit breakers

Today, SF₆ is used to a great extent as insulating and arc-quenching medium in high-voltage gas-blast circuit breakers. The arcing in SF₆ during current interruption forms decomposition products. These can influence the arc-quenching properties of the circuit breaker. Furthermore, they can cause corrosion of the circuit breaker housing. In this comprehensive study we present results obtained for the first time from a direct mass spectrometric investigation of the exhaust gases of a high pressure SF₆ arc in a model circuit breaker. Our mass spectrometric system consists of a time-of-flight mass spectrometer (TOFMS) equipped with a molecular beam sampling systems. This device allows us to measure mass spectra of high pressure sources with a time resolution of up to 10,000 spectra per second. We have determined the formation rate of the most abundant decomposition products in a SF₆ arc at 1 bar. These products are SF₄, CF₄, WF₆, SOF₂, SO₂, CS₂ S₂F₂ and HF. The fast detection time inherent to our system permits also the determination of the formation of SF₄, which is 0.45–0.50 Vol. %/(kJ/1SF₆). In addition, we have studied the influence of water and oxygen impurities which are responsible for the production of highly corrosive HF. Finally, we have considered the influence of the thermal degradation of teflon (P.T.F.E.), which is used as nozzle and insulating material in circuit breakers. On this occasion we have demonstrated that CF₄, which exhibits dielectric properties similar to SF₆, is the main decomposition product formed from teflon. However, we have found that besides CF₄ also excess carbon is formed, which is deposited on insulators of the model circuit breaker.Our time-resolved mass spectra reveal that the CF₄ production from teflon is delayed by a few milliseconds with respect to the SF₆ dissociation in the arc. This delay can influence the interrupting process of the circuit breaker by changing the plasma composition during the arcing period. Although our experiments have been performed on a model circuit breaker we claim that the results presented in this study can be applied to real circuit breakers, since the arc current density and the energy dissipated per liter SF₆ are of the same order of magnitude in both devices.     

Comparison of the first negative corona current pulses in N2 + SF6 and CO2 + SF6 mixtures

The current waveforms of the first negative corona pulses in a small point-to-plane gap have been measured with a nanosecond time resolution in N₂ + SF₆ and CO₂ + SF₆ mixtures at a pressure 50 kPa for various contents of SF₆ as a function of applied gap voltages. The physical mechanism for the pulses in these mixtures with low concentration of SF₆ has been described using the streamer-based theory. The influence of changing admixtures of SF₆ in N₂ and CO₂ has been compared. Differences in the pulse waveforms observed in N₂- and CO₂-based gas mixtures are explained by differences in the first and second Townsend ionization coefficients. This work was supported by the Grant Agency VEGA from the Ministry of Education of Slovak Republic under contracts 1/1011/04 and 1/2017/05.     

The influence of vibrational excitation on the burning voltage of a pulsed electric discharge in HF laser working media

The burning voltages of an intermediate pressure self-sustained volume discharge (SSVD) in SF₆ and SF₆-C₂H₆ mixtures irradiated by a 10.6 μm pulse TEA CO₂ laser, have been measured on varying the laser fluences over a wide range. The delay between the voltage application and the laser pulse onset is 4 μs, and the laser pulse lasts ∼3 μs. The considerable rise observed in the discharge voltages with increasing absorbed specific laser radiation energy, is due to electron attachment to vibrationally excited molecules of SF₆. Different processes of relaxation of the vibrational energy stored in SF₆ molecules are analyzed and the relevant characteristic times are numerically assessed. The gas heating process owing to vibration-translation energy exchange is qualitatively described in terms of the “thermal explosion”. The relation between the “explosion” and delay times determines the peculiarities of electron attachment to vibrationally excited SF₆ molecules. The burning voltages of a submicrosecond non-irradiated SSVD in the above-mentioned media versus the specific electric energy deposited are also measured. They are compared to those of a laser-illuminated SSVD at commensurable specific laser energy depositions. It is concluded that electron attachment to the discharge-produced vibrationally excited SF₆ molecules is not capable of noticeably affecting the discharge voltages of a submicrosecond non-irradiated SSVD. PACS 42.55 Ks; 52.80     

Abnormal polarity effect in nanosecond-pulse breakdown of SF6 and nitrogen

The breakdown of gas gaps in an inhomogeneous electric field at subnanosecond and nanosecond voltage pulse rise times are studied, and the famous polarity effect in point-to-plane gaps is investigated. It is shown that at a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect takes place not only in electronegative gases such as SF₆, but also occurs in electropositive nitrogen. The inversion of polarity effect is related to a delay of electron emission from the plane cathode on arrival of the ionization wave front anode to the cathode. It is found that with a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect occurs at SF₆ and SF₆–N₂ pressures of 0.25 MPa and lower, and with a voltage pulse rise time of 15 ns, at a SF₆ pressure lower than 0.12 MPa.     

Dielectric properties of SF6 mixtures containing oxygen and other gases

Conclusion This paper has explored the influence of an oxygen addition on the dielectric strength of the SF₆ by numerically solving the Boltzmann equation with given cross section data. The linear variation of the dielectric strength with a mole fraction of oxygen was obtained. Similar calculations were performed for SF₆ + air for which only approximative estimates had existed in the literature. To check the results and for other practical reasons the method of the Boltzmann equation was applied to the SF₆ + He and SF₆ + N₂ mixtures, the dielectric strength of which had already been explored by other authors. In comparison with other mixtures currently studied the SF₆ + O₂ mixture reveals rather poor dielectric properties. In spite of the fact that the oxygen is an attaching gas, its addition to SF₆ and its mixtures spoils the dielectric strength. This numerical analysis exactly qualified this behaviour and it is in agreement with the few available experiments.     

Impulse breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF6

This paper presents the impulse pre-breakdown and breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF₆ gas. The breakdown voltage–time (V–t) characteristics and the breakdown voltage–gas pressure (V–p) characteristics of a highly non-uniform SF₆ gas gap under positive and negative lightning impulse voltages are investigated in the pressure range between 0.1 and 0.5 MPa. The pre-breakdown developments are examined by the corona current and light emission measurements with high time resolution. As a result, the dielectric strengths versus time-to-breakdown of SF₆ gas gap under positive lightning impulse voltages were nearly independent of the gas pressure. The first streamer corona was initiated at the tip of the needle electrode, and the streamer corona pulses developed with a stepwise propagation. The discharge paths were zigzag, and the branches of the discharge channel for positive polarity were created. On the other hand, the leader channel in the negative polarity was thicker and brighter than that in the positive polarity.     

Emission from a sulfur-hexafluoride volume discharge induced by attachment instability

Characteristics of a low-pressure volume discharge in SF₆ are investigated. The discharge with a spherical anode-plane cathode electrode system was induced by attachment instability. It is found that applying a dc voltage (U _ch≤1.3 kV) to the anode in the absence of a confining dielectric wall results in the ignition of a repetitive discharge (f=0.1–120 kHz). The spatial, frequency, and current-voltage characteristics of the volume discharge; plasma emission in the spectral range 200–700 nm; and the waveforms of the discharge voltage, current, and plasma emission are investigated. It is shown that the plasma under study exists in the form of a domain (autosoliton) and that the volume discharge is self-induced because, during attachment instability, the plasma itself acts as a switch enabling repetitive pulsed operation. The results obtained can be applied to studying the physics and technology of electric-discharge chemical HF (DF) lasers based on nonchain reactions and to developing high-aperture low-pressure repetitive Ar-, Kr-, and Xe-fluoride lamps with low-corrosive working media (low-pressure mixtures of Ar, Kr, and Xe with SF₆).     

The application of a high pulse repetition rate CO2 laser with high average power for isotope separation by molecular dissociation in a strong IR field

Considering a SF₆ molecule we demonstrate feasibility of using high pulse repetition rate CO₂ laser for isotope separation by selective molecular dissociation in a strong IR field. Dependences of dissociation efficiency as well as separation selectivity on pulse repetition rate up to 150 Hz are investigated. The inherent thermal effects are discussed.     

Folded tandem ion accelerator facility at trombay

The folded tandem ion accelerator (FOTIA) project at BARC has been commissioned. The analysed carbon beams of 40 nA(3⁺) and 25 nA(4⁺), at terminal voltage of 2.5 MV with N₂ + CO₂ as insulating gas, were obtained. The beams were characterized by performing the Rutherford back scattering (RBS) on gold, tin and iron targets. The beam energy of 12.5 MeV for ¹²C⁴⁺ was consistent with the terminal voltage of 2.5 MV. The N₂ + CO₂ mixture is being replaced by SF₆ gas in order to achieve 6 MV on the terminal. In this paper, some of the salient features of the FOTIA and its present status are discussed.     

Optically pumped 15.90 μm SF6 laser

Optically pumped vibrational transition lasing has been achieved for the first time in a nonlinear molecule. Laser radiation at 628.74 ± 0.02 cm^?1 was generated from SF₆ using CO₂ TEA laser excitation. The SF₆ pumping is shown to be via absorption of two photons.     

电激励脉冲HF激光SF_6/C_2H_6工作气体的放电特性

研究了电激励脉冲HF激光工作介质SF6/C2H6混合气体的放电特性。通过对放电等离子体荧光图像和放电波形的测量,分析比较了不同条件下放电稳定性、剩余电压、能量沉积效率等特性参数的变化情况。实验结果表明:混合气体的放电过程存在主放电、剩余电压维持和电弧放电3个阶段,各阶段的放电特性有所差异;提高充电电压有利于放电能量的有效沉积,也会使不稳定的电弧放电提前;增加C2H6原子分数能起到抑制电弧放电的作用;混合气体总压的增加会导致剩余电压的提高以及辉光放电的能量沉积效率的降低;最佳的能量沉积出现在电弧放电阶段与辉光放电阶段即将融合的临界状态。     

Spectral characteristics of IR-multiphoton excitation in supersonic molecular beams

The multiphoton absorption of SF₆ was investigated in supersonic molecular beams in dependence on the fluence and the wavelength of the CO₂ laser. The temperatures of the SF₆ molecules have been reduced using seeded beams of different concentrations. The experimental results are discussed on the basis of the known spectroscopic data of SF₆ and provide some novel information about the spectral characteristics of the ir-multiphoton excitation of strongly cooled molecules in the collision-free case.     

Measurement of nanoparticle temperature in a (CO2) N cluster beam using SF6 molecules as tiny probe thermometers

A temperature measurement technique using SF₆ molecules as tiny probe thermometers is described, and results are presented, for large (CO₂) _N van der Waals clusters (with N ≥ 10²) in a cluster beam. The SF₆ molecules captured by (CO₂) _N clusters in crossed cluster and molecular beams sublimate (evaporate) after a certain time, carrying information about the cluster velocity and internal temperature. Experiments are performed using detection of these molecules with an uncooled pyroelectric detector and infrared multiphoton excitation. The multiphoton absorption spectra of molecules sublimating from clusters are compared with the IR multiphoton absorption spectra of SF₆ in the incoming beam. As a result, the nanoparticle temperature in the (CO₂) _N cluster beam is estimated as T _cl < 150 K. Time-of-flight measurements using a pyroelectric detector and a pulsed CO₂ laser are performed to determine the velocity (kinetic energy) of SF₆ molecules sublimating from clusters, and the cluster temperature is found to be T _cl = 105 ± 15 K. The effects of various factors on the results of nanoparticle temperature measurements are analyzed. The potential use of the proposed technique for vibrational cooling of molecules to low temperatures is discussed.     

Improved separation of the rare sulfur isotopes by infrared multiphoton dissociation of SF6

The dissociation probabilities of³²SF₆ and some of³⁴SF₆ have been measured at a large number of CO₂ laser lines both at room temperature and at 140 K. The longwavelength wing of this dissociation spectrum is exponential in the wavenumber. Its logarithmic slope is proportional to the inverse temperature. Selectivities are high enough at 140 K, that the photons are consumed only for the rare isotope in the case of³⁴SF₆ and nearly so for³⁶SF₆. For³³SF₆ further improvement of the selectivity would be desirable.     

The viscosity and diffusion coefficients of eighteen binary gaseous systems

The paper reports accurate measurements of the viscosity of the eighteen binary gaseous systems: CF₄ with He, Ne, Ar, N₂, CO₂, CH₄; SF₆ with He, Ne, Ar, N₂, CO₂, CH₄, CF₄ and O₂ with He, Ne, CO₂, CF₄, SF₆. The measurements were performed in a high-precision oscillating-disk viscometer at atmospheric pressure and in the temperature range 25–200°C for the systems containing CH₄ or SF₆ and in the temperature range 25–400°C for the remainder. The reported viscosities are believed to be accurate to within ±0.1% at room temperature and to within ±0.2% at 400°C.It is shown that the data conform to the extended law of corresponding states developed by Kestin, Ro and Wakeham despite the complexity of some of the component gases. The standard deviation between the experimental values and those calculated from the law of corresponding states is only 0.3% which is commensurate with the uncertainty in the experimental results.Binary diffusion coefficients derived from the mixture viscosity data are also presented; they have an estimated uncertainty of ±2%.     

Measurement of the nonlinear refractive index of SF6 with a hollow prism

We measure the nonlinear refractive index of SF₆ at three different CO₂ laser lines. Higher terms than the cubic nonlinearity are found and the behavior of the refractive index as a function of the laser intensity depends strongly on the frequency used, reflecting the characteristic SF₆ absorption dynamics.     

IR multiphoton absorption of SF6 in flow with Ar at moderate energy fluences

6 in flow with Ar (SF₆: Ar=1:100) in conditions of a large vibrational/rotational temperature difference (T_V≃230 K, T_R≃60 K) was studied at moderate energy fluences from ≃0.1 to ≃100 mJ/cm², which are of interest for isotope selective two-step dissociation of molecules. A 50 cm Laval-type slit nozzle for the flow cooling, and a TEA CO₂-laser for excitation of molecules were used in the experiments. The laser energy fluence dependences of the SF₆ MPA were studied for several CO₂-laser lines which are in a good resonance with the linear absorption spectrum of the ν₃ vibration of SF₆ at low temperature. The effect of the laser pulse duration (intensity) on MPA of flow cooled SF₆ with Ar was also studied. The results are compared with those obtained in earlier studies. Received: 4 September 1995/Revised version: 15 February 1996