Two-dimensional models of expanding vacuum arc plasmas

Multifluid equations describing the plasma of vacuum arcs expanding anisotropically from a cathode spot are given and discussed. Some first and preliminary results from an approximate analytical integration of such a system of equations, based on the representation of all plasma parameters by asymptotic power series, are presented and evaluated. Though the dependence of the plasma parameters on the direction (angle ϑ) is weak, the inclusion of angular terms and angular equations changes some results significantly, compared with the experiences from usually 1-D models. While the basic radial dependence (r) of the terms ~r^-2k/5, k=0,1,2..., remains the same as in 1-D theories, we now get an angular dependence ~ϑ² of the parameters in a first approximation, with positive factors, i.e., a flattening of the whole plasma distribution. However, with analytical solutions of higher flexibility, cos-like distributions are possible as well. The final kinetic energy of the plasma ions in the case of Cu arc spots is caused by the electric field within the plasma (contributing approximately 27%), the ion pressure gradient (~24%), and the electron-ion friction (plasma resistivity, ~49%) in directions near to the discharge axis. Again, similar to the results from 1-D models. With an approach to the cathode surface (ϑ→π/2), the plasma flow becomes ambipolar. Finally, some further possibilities which are available with the help of this model are discussed     

Electron-temperature evolution in expanding ultracold neutral plasmas

We have used the free expansion of ultracold neutral plasmas as a time-resolved probe of electron temperature. A combination of experimental measurements of the ion expansion velocity and numerical simulations characterize the crossover from an elastic-collision regime at low initial Gamma(e), which is dominated by adiabatic cooling of the electrons, to the regime of high Gamma(e) in which inelastic processes drastically heat the electrons. We identify the time scales and relative contributions of various processes, and we experimentally show the importance of radiative decay and disorder-induced electron heating for the first time in ultracold neutral plasmas.     

Coulomb crystallization in expanding laser-cooled neutral plasmas

We present long-time simulations of expanding ultracold neutral plasmas, including a full treatment of the strongly coupled ion dynamics. Thereby, the relaxation of the expanding laser-cooled plasma is studied, taking into account elastic as well as inelastic collisions. It is demonstrated that, depending on the initial conditions, the ionic component of the plasma may exhibit short-range order or even a superimposed long-range order resulting in concentric ion shells. In contrast to ionic plasmas confined in traps, the shell structures build up from the center of the plasma cloud rather than from the periphery.     

Relaxation to nonequilibrium in expanding ultracold neutral plasmas

We investigate the strongly correlated ion dynamics and the degree of coupling achievable in the evolution of freely expanding ultracold neutral plasmas. We demonstrate that the ionic Coulomb coupling parameter Gamma(i) increases considerably in later stages of the expansion, reaching the strongly coupled regime despite the well known initial drop of Gamma(i) to order unity due to disorder-induced heating. Furthermore, we formulate a suitable measure of correlation and show that Gamma(i) calculated from the ionic temperature and density reflects the degree of order in the system if it is sufficiently close to a quasisteady state. At later times, however, the expansion of the plasma cloud becomes faster than the relaxation of correlations, and the system does not reach thermodynamic equilibrium anymore.     

New developments in processing cathodic arc plasmas

Filtering of plasmas by curved solenoidal ducts is well established as a method of removing macroparticles. By analyzing the interactions of planar probes with the drifting plasma of the cathodic arc, new insights have been obtained into the operation of these ducts. Theoretical modeling of these interactions suggests, and experiment confirms, that the use of a separate biased electrode on the inside of the duct gives enhanced transmission without drawing excessive electron current. Theoretical modeling of a negatively biased planar electrode lying parallel to the drift velocity as well as experiment both show that ions are captured effectively onto the electrode producing a macroparticle free film at good deposition rates. The application of pulsed high voltage to the substrate placed at the exit of the duct is treated theoretically, and a model is proposed which gives a good agreement with the experimental concentration profile for a silicon surface coated and simultaneously implanted with titanium     

Quasi-monochromatic measurements of homogeneous arc plasmas

The refined diagnostic information obtainable by high-order spectrometry is illustrated by the results of quantitative measurements of a few rotational lines of OH in the ultraviolet spectrum of water-vapor plasmas generated in a wall-stabilized arc. Because of the high spectral and spatial resolution achieved in end-on measurements, the emission and also the absorption coefficients pertaining to homogeneous arc regions were obtained directly from measured line spectra—although the absorption was not measured explicitly—leading to the occupation of the upper and the lower state for the transition. The gas temperature was determined from the halfwidth of the Doppler-broadened rotational lines. The measured resolving power of the spectrometer was of the order of 400,000 in these measurements.     

Ion emission from arc discharge plasmas

Results are presented from studies of the ion-emission properties of the anode plasmas of low-pressure contracted arc discharges and vacuum arcs. It is shown that creating a longitudinal magnetic field in the anode region of a discharge changes the plasma parameters significantly and facilitates a large increase in the ion current. Space charge limited ion current in a vacuum arc leads to a reduction in the noise level of the total ion current and of its components with charges of up to +3, while creating Penning discharge conditions ensures that ions of different gases can be generated in this discharge system at fractions as high as 90%, depending on the type of plasma forming gas.Institute of High-Current Electronics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vuzov, Fizika, No. 3, pp. 24–33, March, 1994.     

Fluorescence measurements of expanding strongly coupled neutral plasmas

We report new detailed density profile measurements in expanding strongly coupled neutral calcium plasmas. Using laser-induced fluorescence techniques, we determine plasma densities in the range of 10(5) to 10(9) cm(-3) to with a time resolution limit as small as 7 ns. Strong coupling in the plasma ions is inferred directly from the fluorescence signals. Evidence for strong coupling at late times is presented, confirming a recent theoretical result.     

Electrical characterization of atmospheric pressure arc plasmas

The properties of atmospheric pressure arcs are investigated by means of electric exploration of plasma column and anode region. For the electrostatic probe technique, where the level of collisionality distorts the characteristic curve, data interpretation is difficult because no comprehensive underlying theory exists for the non-homogeneous electric arcs used in industry. Results are presented from an extended study of Langmuir probes applied to short, point-plane arcs. A multi-wire apparatus, operating for arc currents in the range 50-200 A is described and ion current densities and temperature maps are shown. The reduction of the probe determined temperature with respect to emission spectroscopy values is discussed and the cooling is ascribed to ion-electron recombination within the perturbation region formed around the probe. This region is investigated by means of emission spectroscopy and the extension found agrees both with numerical estimations and fast speed camera photographs. Diamond Like Carbon (DLC) partially coated wires can address data inversion problems and the role of arc flow directionality on charge capture and preliminary observations are shown. Charge capture and anode fall structure can be investigated using a split-anode technique. A prototype of a modified apparatus is described and preliminary results on the collected current are given.Received: 20 August 2003PACS: 52. Physics of plasmas and electric discharges - 52.80.Mg Arcs; sparks; lightning; atmospheric electricity - 52.70.-m Plasma diagnostic techniques and instrumentation     

A hydrodynamic model of vacuum arc plasmas

The properties of plasmas expanding from cathode spots of vacuum arcs are calculated with a one-dimensional two-fluid model. The system of simplified hydrodynamic equations can be solved under stationary conditions using asymptotic power series. Although necessarily only an approximation, such analytical solutions prove to be advantageous compared with numerical integrations. All the plasma parameters are functions of (I/r)^2/5 (current, I: distance, r). The three forces accelerating the ions to high kinetic energies are quantitatively calculable: the electric field, the ion pressure gradient, and the electron-ion friction. The potential is decreasing towards the anode, and the residence of the plasma is negative. The ion temperature reaches only about 35% of the electron temperature. Although only asymptotic, the solution is suited to describe the arc plasma in a sufficient manner all over the expansion region     

Macroparticle filtering of high-current vacuum arc plasmas

The transport of vacuum arc plasmas through a 90° curved magnetic macroparticle filter was investigated using a high-current pulsed arc source with a carbon cathode. The peak arc current was in the kiloampere range, exceeding considerably the level of what has been reported in the literature. The main question investigated was whether magnetic macroparticle filters could be scaled up while maintaining the transport efficiency of small filters. In front of the cathode, we found that arc current dependent total ion saturation currents were in the range from 10% to 23% of the arc current. The best relative transmission was 25% (time integrated output/time integrated input) at a duct wall bias of 12.5 V and at an axial magnetic field of about 100 mT. The measured relative transmission of the used high-current arrangement is comparable to what has been observed with other low-current filters. The absolute measurable ion saturation currents at the filter exit reached 70 A at an arc current of about 1000 A     

Calculation of radiative heat transfer in argon arc plasmas

Radiation emission and absorption in arc plasmas are important energy transfer processes. Exact calculations, though possible in principle, are usually impossible in practice because of the need to treat a large number of spectral lines and also the continuum radiation in the whole spectrum range. Recently, we have used an approximate method of partial characteristics to evaluate the radiation intensities, radiation fluxes and the divergence of radiation fluxes for SF₆ arc plasma with cylindrical symmetry. In this paper, we have extended our calculations toargon arc plasmas for the plasma pressures of 0.1, 0.5 and 1.0 MPa. We have calculated the coefficients of absorption for Ar plasmas at temperatures from 300 to 35 000 K, and have used these coefficients to calculate the partial characteristics. Both the continuum and the line spectra have been included in calculations. We have taken into account the radiative photo-recombination and bremsstrahlung for the continuous spectrum, and over 500 spectral lines for the discrete spectra.The method of partial characteristics has been applied to three-dimensional calculations of radiative heat transfer — i.e. radiation intensity, radiation flux and its divergence — in simplified temperature profiles. Conclusions have been made concerning validity and utilization of the method of partial characteristics in general gas dynamics problems.     

Analytic solutions to the Vlasov equations for expanding plasmas

The renormalization-group approach is applied to derive an exact solution to the self-consistent Vlasov kinetic equations for plasma particles in the quasineutral approximation. The solutions obtained describe three-dimensional adiabatic expansion of a plasma bunch with arbitrary initial velocity distributions of the electrons and ions. The solution found is illustrated by the examples on ion acceleration in a plasma with hot electrons and in a plasma with light impurity ions.     

Radiation emission coefficients for sulfur hexafluoride arc plasmas

Calculations are made of the total spectral absorptivities of plasmas of sulfur hexafluoride for wavelengths of 100 to 15,500 Å. Both line and continuum radiation are considered. For the contributions from line radiation, we have calculated the strengths, widths and shifts of both neutral, singly and doubly ionized atoms of S and F, except that we used experimental line strengths where they are available. The theory used was that of Griem, which assumes LS coupling. Curves are given for the emission coefficient of radiation appropriate to the arc center for isothermal cylindrical plasmas of various radii for pressures of 1 and 10 atm and temperatures from 5000 to 35,000°K. It is found that at 1 atm line radiation can be an order of magnitude higher than continuum radiation and radiation >2000 Å is less than 10% of the total radiation for temperatures greater than 15,000°K. Predictions are given of volt-ampere characteristics and central temperatures for arcs of various radii in SF₆ at pressures at 1 and 10 atm.     

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利用自行设计的快速移动静电探针对大气压下以氩气为工质的大尺度磁分散电弧等离子体进行诊断,分析了探针电压与电流的波形,绘制探针伏安特征曲线(U-I曲线),得到了发生器轴线方向等离子体波动特性以及电子温度沿轴线的变化趋势.发现等离子体发生器中心具有回流区,得到了电弧等离子体的大致体积.     

Ion charge state distributions of alloy-cathode vacuum arc plasmas

Ion charge state distributions (CSD's) of alloy-cathode vacuum arc plasmas have been calculated under the assumption that thermodynamic equilibrium is valid in the vicinity of the cathode spot, and equilibrium CSD's “freeze” when the plasma is rapidly expanding. In this way, experimental data of titanium-hafnium alloy-cathode plasmas have been simulated using a system of Saha equations generalized for multiple elements. It was found that the CSD's of titanium and hafnium freeze approximately at the same plasma temperature and density. The freezing parameters depend slightly on the plasma composition. For the example considered, freezing occurs at temperature of T=3.0-3.8 eV and a total heavy particle density of order 10²⁵m^-3     

A revised theoretical model of vacuum arc spot plasmas

The quasi-stationary hemispherical expansion of the cathodic plasma in vacuum arcs can be modeled with hydrodynamic two-fluid equations. In any case, the state of the plasma is determined by the only variable (I/r)^2/5 (with current I , distance r). In order to avoid some deficiencies of the model (as published) and to investigate more carefully the dependence of the plasma parameters on the arc current, the known analytic solution to the problem is improved by taking into consideration the variability of the Coulomb logarithm and the dependence of the boundary conditions on I. These effects are treated separately. Examples are used to illustrate the new results, with particular emphasis on ion acceleration. The influence of the above factors turns out to be rather unimportant. Quantitatively, they cause some shifts, but no qualitative change of the basic behavior of the plasma is seen