Thermal Nonequilibrium of a Free-Burning Argon Arc Plasma

The axial distributions of the electron temperature and number density of a free-burning atmospheric argon arc plasma at 15 Amps have been measured using a Thomson scattering technique. In addition, the excitation temperature of singly ionized argon has been found spectroscopically by relative line intensity method. The results are presented in terms of the degree of electron temperature nonequilibrium as compared with the calculated Saha equilibrium temperature and with the measured ionic excitation temperature along the axis of the arc. The observed temperatures are discussed with respect to the theoretical ion-like particle temperature and to the effect of electrode geometry.     

Intraexciton and Intracenter Terahertz Radiation from Doped Silicon under Interband Photoexcitation

Terahertz photoluminescence of boron- and phosphorus-doped silicon at low temperatures under interband photoexcitation is investigated. The lines of radiative transitions between free-exciton levels and between the levels of shallow impurity centers are observed. The intensities of these lines exhibit different dependences on temperature and excitation intensity. At temperatures near the temperature of liquid helium (T ~ 5 K), the terahertz radiation spectrum features a broad band (about 18–20 meV wide) with a peak at an energy of about 20–22 meV. This band is apparently associated with radiative transitions of nonequilibrium charge carriers from the states of the continuum to the state of an electron–hole liquid.     

Critical Review of Plasma Spectroscopic Diagnostics via MTE

The often used absolute, ratio, and off-axis peaking methods of LTE spectroscopy are examined using the Multithermal Equilibrium (MTE) multifluid model. The partial MTE model distinguishes between the upper level and total excitation temperatures of atom and ion (Tex?a or Tex?i and Texa or Texi, respectively) as well as different valued electron (Te) and heavy particle (Ta = Ti) temperatures. In complete MTE, all excitation temperatures are equal valued but may differ from kinetic temperature values. In this paper, the particular temperature approximated by common diagnostic methods is identified. The probable errors in the temperature values are presented in a figure as a function of probable errors in data, temperature and level energy difference. The results indicate that the ratio methods (neutral-neutral, ion-neutral, and continuumneutral) yield upper level excitation temperature values; the absolute neutral line and absolute continuum (at low Texa) yield total atom excitation temperatures; and the off-axis peaking method for the neutral line yields the total excitation temperature. Application of the methods are more complex than in LTE spectroscopy. Probable errors are smallest for the absolute and off-axis peaking methods. The electron temperature may be determined from the continuum-neutral ratio at very high excitation temperatures (Texa >> E?/10k). The paper also comments on MTE diagnostic methods.     

Ionization-Excitation of helium-like ions by high-energy electrons

The differential and total cross sections are calculated for the ionization of helium-like ions that is accompanied by the excitation of residual ions to ns states owing to an electron impact. Nonrelativistic perturbation theory in electron–electron interaction with Coulomb functions used for a zero-order approximation underlies these calculations. The expressions obtained in this way have a universal character. They are applicable at moderate values of the target charge number Z and high energies of incident electrons. A comparison with total cross sections calculated for the helium atom within various theoretical approaches is performed.     

Plasma and heat flux characteristics of a supersonic ammonia or nitrogen/hydrogen-mixture direct-current plasma jet impinging on a flat plate

Spectroscopic and electrostatic probe measurements were made to examine plasma characteristics with or without a titanium plate under nitriding for a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle in a low-pressure environment. Heat fluxes into the plate from the plasma were also evaluated with a Nickel slug and thermocouple arrangement. Ammonia and mixtures of nitrogen and hydrogen were used as a working gas. The NH/sub 3/ and N/sub 2/+3H/sub 2/ plasmas in the nozzle and in the downstream plume without a substrate plate were in thermodynamical nonequilibrium states. As a result, the H-atom electronic excitation temperature and the N/sub 2/ molecule-rotational excitation temperature intensively decreased downstream in the nozzle although the NH molecule-rotational excitation temperature did not show an axial decrease. Each temperature was kept in a small range in the plume without a substrate plate except for the NH rotational temperature for NH/sub 3/ working gas. On the other hand, as approaching the titanium plate, the thermodynamical nonequilibrium plasma came to be a temperature-equilibrium one because the plasma flow tended to stagnate in front of the plate. The electron temperature had a small radial variation near the plate. Both the electron number density and the heat flux decreased radially outward, and an increase in H/sub 2/ mole fraction raised them at a constant radial position. In cases with NH/sub 3/ and N/sub 2/+3H/sub 2/, a radical of NH with a radially wide distribution was considered to contribute to the better nitriding as a chemically active and non heating process.     

Spectroscopic measurements in nitrogen arcs with respect to local thermodynamic equilibrium

A wall-stabilized nitrogen arc at atmospheric pressure was studied spectroscopically in the current range from 20 to 70 A with the aim of detecting departures from LTE. Measurements of the relative intensity of several N(I) lines showed that the populations of the upper excited states are in equilibrium for currents greater than approx. 25 A. Comparisons of the excitation temperatures obtained from relative intensities with the electron temperature and with the temperature curves calculated for different departures from LTE show that deviations from equilibrium are small at the arc center for electron densities above approx. 4 × 10¹⁶ cm^-3. Deviations between the radial profiles of the electron and excitation temperatures were observed for currents between 35 and 55 A.     

Production of a large area diffuse arc plasma with multiple cathode

An arc channel at atmospheric pressure tends to shrink generally. In this paper, a non-transferred DC arc plasma device with multiple cathode is introduced to produce a large area arc plasma at atmospheric pressure. This device is comprised of a 42-mm diameter tubular chamber, multiple cathode which is radially inserted into the chamber, and a tungsten anode with a nozzle in its center. In argon/helium atmosphere, a large area and circumferential homogenous diffuse arc plasma, which fills the entire cross section surrounded by the cathode tips, is observed. Results show that the uniformity and stability of diffuse arc plasma are strongly related to the plasma forming gas. Based on these experimental results, an explanation to the arc diffusion is suggested. Moreover, the electron excitation temperature and electron density measured in diffuse helium plasma are much lower than those of constricted arc column, which indicates the diffuse helium plasma probably deviates from the local thermodynamic equilibrium state. Unlike the common non-transferred arc plasma devices, this device can provide a condition for axial-fed feedstock particles. The plasma device is attempted to spheroidize alumina powders by using the central axis to send the powder. Results show that the powder produced is usually a typical hollow sphere.     

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.     

Electron temperature determination in LTE and non-LTE plasmas

A method is presented for calculating electron temperatures (T_e) in dense plasmas, which does not assume equivalence with the excited level distribution temperatures (T_ex). The method involves the upper-level Saha ionization equation at the ionization limit, the limiting weighted population density (N_I/g_I) obtained from measured population densities and the experimentally obtained electron density. Electron temperatures calculated for 0.1-bar hydrogen and 1-atm helium and argon arcs are found to be up to twice as large as excited level distribution temperatures. For subatmospheric argon arcs, the calculated T_e are equivalent to the excitation temperature of the middle levels, but are two to three times smaller than the quoted T_ex for the highest levels. Reasons are discussed for the apparent invisibility of true electron temperatures and for differences between them and the excitation temperatures.     

大气压低温氦等离子体射流的诊断研究

为了加快低温氦气等离子体射流的工程化进程,通过自主设计的同轴式介质阻挡放电等离子体射流发生器,在放电频率10 kHz,一个大气压条件下产生了稳定的氦气等离子体射流。通过分析不同工况下的电压电流波形可以发现单纯增加氦气体积流量只能小幅的增加电流脉冲幅值,而对放电时间、电流脉冲数的影响不大。增加放电峰值电压时电流脉冲幅值会得到较大幅度增加。通过发射光谱法对大气压氦气等离子射流的活性粒子种类、电子激发温度、电子密度进行了诊断。结果表明,大气压氦气等离子体射流中的主要活性粒子为He Ⅰ原子、N₂第二正带系、N+₂的第一负带系、羟基（OH）,H原子的巴尔末线系（H_α和H_β）与O原子,这表明虽然该试验中使用的氦气纯度已达99.99%,但其中仍残留有少量的空气,同时放电时大气中的空气会被卷吸到放电空间发生电离。还可以发现,主要活性粒子的相对光谱强度随氦气体积流量的增加及放电峰值电压的增大均呈现上涨的趋势。选用He Ⅰ原子的四条谱线对不同试验工况下的电子激发温度进行了计算,得到大气压氦气等离子体射流的电子激发温度在3 500~6 300 K之间,电子激发温度随放电峰值电压与氦气体积流量的增大总体上呈现上升的趋势。但由于反向电场的存在,某些峰值电压可能会出现电子激发温度下降的情况;根据Stark展宽原理对大气压氦气等离子体射流的电子密度进行了计算,发现电子密度的数量级可达1015 cm-3,同时增大峰值电压与氦气体积流量均可有效的提高射流中的电子密度。这些参数的研究对氦气等离子体射流在工程实际中的应用具有重要意义。     

Two-temperature channel model of a direct current arc

A relatively simple method is proposed for computing the gas and electron temperatures in an arc plasmotron channel within the framework of the self-consistent two-temperature channel model of an arc discharge. This method affords the possibility of obtaining the gas and electron temperature distribution with good enough accuracy for given discharge parameters (current intensity in the discharge, power inserted in the discharge, etc.) as a function of the radial coordinate in both nonequilibrium (T_e T_ai) and quasi-equilibrium (T_e = T_ai within the current conducting channel) cases. The results obtained can be utilized in model computations to estimate the gas and electron temperatures as well, possibly, as in a number of engineering computations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 80–86, July, 1990.The author is grateful to L. A. Rachevskii for useful discussion of results of the research.     

同轴空心阴极氦等离子体的电子激发温度研究

利用同轴空心阴极放电装置,产生氦低温等离子体。通过对等离子体的发射光谱进行测量和计算,研究放电功率以及氦气压强对等离子体的电子激发温度的影响。结果表明:氦低温等离子体的发射光谱主要由连续谱和原子谱线构成,放电功率和压强对谱线的强度具有明显影响。压强的变化不仅影响电子从电场中获得的能量,还会影响电子与原子的碰撞频率,从而导致电子激发温度随着氦气压强的增大,出现先上升后下降的变化趋势。     

Plasma parameters and pumping mechanism in a helium-cadmium laser

The rate of pumping by electron impact from the cadmium atom ground state and by the Penning process Cd II = 4416 å upper transition level are calculated on the basis of a measurement of the electron temperature, density of charged particles, and density of metastable and excited helium atoms as well as the known cross sections. The pumping rate to the upper level is determined independently from observations of the side spontaneous emission of the tube and measurements of the fundamental He-Cd lasing parameters. A comparison of the measured pumping rate with the calculated values shows that the Penning process, involving other excited helium atoms as well as metastables, is a dominant factor in the filling of the upper level under optimum conditions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 85–90, August, 1972.     

Temperature dependences of the electron-phonon coupling, electron heat capacity and thermal conductivity in Ni under femtosecond laser irradiation

The electron temperature dependences of the electron-phonon coupling factor, electron heat capacity and thermal conductivity are investigated for Ni in a range of temperatures typically realized in femtosecond laser material processing applications, from room temperature up to temperatures of the order of 10⁴ K. The analysis is based on the electronic density of states obtained through the electronic structure calculations. Thermal excitation of d band electrons is found to result in a significant decrease in the strength of the electron-phonon coupling, as well as large deviations of the electron heat capacity and the electron thermal conductivity from the commonly used linear temperature dependences on the electron temperature. Results of the simulations performed with the two-temperature model demonstrate that the temperature dependence of the thermophysical parameters accounting for the thermal excitation of d band electrons leads to higher maximum lattice and electron temperatures achieved at the surface of an irradiated Ni target and brings the threshold fluences for surface melting closer to the experimentally measured values as compared to the predictions obtained with commonly used approximations of the thermophysical parameters.     

Electron state density and electron diffusion coefficient in energy space in nonideal nonequilibrium plasmas

We suggest a model for a hydrogenic low-temperature nonequilibrium nonideal plasma that allows the kinetic parameters of the plasma to be calculated by the method of molecular dynamics by taking into account the interparticle interaction. The charges interact according to Coulomb’s law; for unlike charges, the interaction is assumed to be equal to a constant at a distance smaller than several Bohr radii. For a system of particles, we solve the classical equations of motion under periodic boundary conditions. The initial conditions are specified in such a way that the electrons have a positive total energy. We consider the temperatures 1-50 K and densities n = 10⁹?10¹⁰ cm^?3 produced in an experiment through laser cooling and resonant excitation. We calculate the electron state density as a function of the plasma coupling parameter and the electron diffusion coefficient in energy space for highly excited (Rydberg) electron states close to the boundary of the discrete and continuum spectra.     

Validation of collisional radiative modelling of emission line ratios for helium beam plasma diagnostic

A time-dependent collisional radiation model has been used to validate the use of a pulsed helium beam to measure electron temperature profiles in the H-1NF flexible heliac. The diagnostics technique is based on the measurement of helium spectral line ratios which are a strong function of electron temperature while being a weak function of electron density. The validation procedure involves comparing measured and calculated emissivities of spectral lines emitted by helium atoms as a function of position in the beam.     

Excitation of helium atoms in collisions with plasma electrons in an electric field

The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical electron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a self-consistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. Therefore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 2¹ P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.     

Level populations in plasmas RF discharges and sonic channel

An absolute intensity calibration has been made to previously reported spectrographic measurements in an extensively studied argon rf discharge downstream of the exciting coil, and sonic afterglow. This calibration allows calculation of neutral argon atom density and of individual level population. Neutral atom densities were calculated from the electron density, the electron temperature, and Saha's equation, assuming that the electron temperatures were equal to the excitation temperatures. The densities calculated by these methods were far higher than those obtained from the perfect gas law, the gas temperature, and the pressure. The observed under- population of the ground level is shown to correspond to the “terminal non-equilibrium” defined by PARK.⁽³⁾ The present data are compared with two existing experiments to show that agreement exists among seemingly inconsistent and confusing reports, provided the non-equilibrium phenomena are properly accounted for.     

Substrate influence in electron-phonon coupling measurements in thin Au films

Accurate understanding and measurement of the energy transfer mechanisms during thermal nonequilibrium between electrons and the surrounding material systems is critical for a wide array of applications. With device dimensions decreasing to sizes on the order of the thermal penetration depth, the equilibration of the electrons could be effected by boundary effects in addition to electron-phonon coupling. In this study, the rate of electron equilibration in 20 nm thick Au films is measured with the Transient ThermoReflectance (TTR) technique. At very large incident laser fluences which result in very high electron temperatures, the electron-phonon coupling factors determined from TTR measurements deduced using traditional models are almost an order of magnitude greater than predicted from theory. By taking excess electron energy loss via electron-substrate transport into account with a proposed three temperature model, TTR electron-phonon coupling factor measurements are more in line with theory, indicating that in highly nonequilibrium situations, the high temperature electron system looses substantial energy to the substrate in addition to that transferred to the film lattice through coupling.