Spectroscopical Observation of a Relaxing High Speed Nitrogen Plasma Jet

A near sonic nitrogen plasma jet operating at pressures between 100 Torr and 1 atm has been investigated spectroscopically. From the absolute emission coefficient of a NI spectral line, local values for the electron temperature T_e have been derived. For pressures above 200 Torr, T_g was found to coincide within the limits of experimental error with the gas temperature T_g. The latter quantity has been determined via the relative emission coefficient of selected rotational line components of the N₂⁺ molecular band at 3914 Å. The results of these measurements together with control data for the electron density derived from the continuous emission coefficient indicate that at pressures above 200 Torr the existence of a thermal equilibrium between the degrees of freedom corresponding to particle translation, electron excitation, and ionization can be accepted, at least for the inner zone of the plasma jet. To the contrary, the data for the absolute emission coefficient of N₂⁺ molecular band lines show that the degree of dissociation in the plasma jet is much in excess of that corresponding to equilibrium. This phenomenon can be explained as a result of the rapid temperature decay in the plasma from initially 13000 K in the arc heating zone to T ≦ 9000 K in the plasma jet zone proceeding in a time interval of 10^?5 s which is much shorter than the time necessary for adjustment of dissociation equilibrium. In the outer cool zone of the plasma jet, an unusual high intensity of the N₂⁺ radiation was found thus indicating the existence of a nonequilibrium excitation mechanism typical for a decaying nitrogen plasma. From the supernormal high degree of dissociation in the high-speed subatmospheric nitrogen plasma jet, conclusions are drawn with respect to its applicability as source of reactive particles in plasma-chemical experiments.     

Spatiotemporal spectroscopic diagnostics of a pulse-periodic strontium vapor laser

Results of spectroscopic investigations into plasma of a pulse-periodic strontium vapor laser operating in the superradiance mode on the infrared transition at λ = 6.45 μm are presented. The method of determining the electron temperature and concentration as well as the gas temperature – T _e , n _e , and T _g – based on measuring the absolute intensities of some SrI and SrII and buffer gas (helium or neon) spectral lines is used. Time dependences of the line intensities during a current pulse (τ = 150 ns) and near afterglow (up to 3 μs) are obtained under conditions of non-equilibrium plasma ionization and recombination. The optical system collects radiation from the entire length of the plasma column by means of separating radial volume zones, includingthe central zone and the zone closer to the walls, with the monochromator slit. The results obtained allow us not only to calculate T _e , n _e , and T _g values, but also to trace the spatiotemporal plasma evolution.     

Optical Emission Diagnostics of an U-Shaped Argon Stabilized d.c. Arc

The radial distribution of parameters has been measured by using the optical emission spectroscopy of an U-shaped argon stabilized low current arc at atmospheric pressure. All the measurements reported here were performed from a side-on observation direction by applying the Abel inversion routine. Radial distributions of apparent temperatures (T_exc., T_e, T_i, T_g) and of electron number density (n_e) for the plasma were measured, with and without presence of KCl (spectrochemical buffer). The measured data of n_e are compared to the theoretically calculated values of the equilibrium plasma composition. On the basis of the measured data, the validity of LTE concept is considered. It was found that deviation from LTE increases to the plasma periphery.     

Plasma dynamics from laser ablated solid lithium

Emission plasma plume generated by pulsed laser ablation of a lithium solid target by a ruby laser (694 nm, 20 ns, 3 J) was subjected to optical emission spectroscopy: time and space resolved optical emission was characterised as a function of distance from the target surface. Propagation of the plume was studied through ambient background of argon gas. Spectroscopic observations can, in general, be used to analyse plume structure with respect to an appropriate theoretical plasma model. The plume expansion dynamics in this case could be explained through a shock wave propagation model wherein, the experimental observations made were seen to fit well with the theoretical predictions. Spectral information derived from measurement of peak intensity and line width determined the parameters, electron temperature (T _e) and electron number density (n _e), typically used to characterise laser produced plasma plume emission. These measurements were also used to validate the assumptions underlying the local thermodynamic equilibrium (LTE) model, invoked for the high density laser plasma under study. Some interesting results pertaining to the analysis of plume structure and spatio-temporal behaviour of T _e and n _e along the plume length will be presented and discussed.     

Electron impact ionization and excitation rate coefficients in the negative glow region of a glow discharge

Some easy to use reasonable approximations for electron impact rate coefficients have been considered. The most important rate coefficients for electron collisions in noble gases are electron-neutral ionization and electron impact excitation. Electron-neutral ionization besides electron impact excitation of some states of the argon and helium atom in direct current (dc) glow discharge plasma has been calculated. The plasma parameters of electron are significant factors for computing the rate coefficients. We present first results of probe diagnostic that includes the double probe measurements of the plasma parameters, namely, electron temperature (T_e) and electron density (n_e). Electron properties obtained from the double probe characteristic curves including T_e and n_e as well as the calculated rate coefficients (ionization and excitation) were studied as a function of the axial distance from the cathode while the discharge operating parameters of voltage and pressure were varied. Two regions of the glow discharge were investigated: cathode fall region and negative glow. Particular emphasis was placed on the negative glow region.     

Anomalous Electric Fields Inside a Dense Plasma of a Current Sheet

For the first time there have been analyzed profiles of electric-field-sensitive n_a spectral lines of the Li-like impurity ions CIV, NV, OVI emitted by a hot dense plasma of a plane current sheet. Theoretical profiles of these lines have been calculated with an allowance for Anomalous Electric Fields (AEF). By comparing the experimental and theoretical profiles of the lines it has been established for the first time that in the plane current sheet there were developed AEF of rms strengths F₀ ≈ (100–120) kV/cm in the time interval from τ = ?0.2 μs to τ = 0 (the time τ was counted from the instant of the maximum intensity of the spectral line OVI 5291 Å). These results have been backed up by a similar analysis of the experimental profiles of the spectral line HeII 4686 Å of the primary gas of the discharge. The AEF reached these strengths at the same instant of time, τ = ?0.2 μs, when electron and ion temperatures reached their maxima: T_e ? 100 eV, T_i ≈ 300 eV. The instant τ = ?0.2 μs corresponds to a transition from the metastable stage to the explosive phase of the current sheet evolution. At this time the ratio of the AEF energy density to the thermal energy density of the plasma was § ≡ F²₀/[8π/N_eT_e + N_iT_i)] ≈ 10^?3, which exceeded a similar ratio for the thermal wave field by three orders of magnitude.     

Ionization Dynamics in a Pulse Disturbed Magnetically Confined Helium Plasma

The equilibrium of a magnetized Helium plasma is disturbed by a pulsed Trivelpiece-Gouldwave. The electrons obtain the energy by linear collisionless wave absorption. The relaxation phenomena of density and energy are explained in terms of two relaxation times τ_E, τ₁ and a quantity giving the additional ionization. These quantities are derived from a small signal fluid model based upon energy and particle balance equations. In the experiment they are taken from the transient curves of Langmuir-probe current, optical line radiation and the noise power at the electron cyclotron frequency. The experimental conditions are: Helium-gas, p = 1 …? 5 Pa, T_e = 4 eV, n = 1 …? 5 · 10¹⁰ cm^?3, B = 6,5 · 10^?2 T, 27 MHz rf plasma source, low frequency fluctuation level < 1%, classical losses. The energy relaxation time …?_E = 10 …? 15 μs is given by inelastic collision losses. The ionization time constant τ₁ is related to the instantaneous ionization frequency during the transient state. It shows a high value at the very beginning of the pulse which must be explained by a tail formation in the distribution function and enhanced radial losses becoming Bohm-like in the transition phase.     

双温度氦等离子体输运性质计算

获得覆盖较宽温度和压力范围内的等离子体输运性质是进行等离子体传热和流动过程数值模拟的必要条件.本文采用Saha方程计算等离子体组分, 采用基于将Chapman-Enskog方法扩展到高阶近似的方法, 计算获得了电子温度(T_e)不等于重粒子温度(T_h)的情形下, 在300 K到40000 K的温度范围内氦等离子体的黏性、热导率和电导率. 研究结果表明压力和热力学非平衡参数(θ =T_e/T_h)对氦等离子体的输运性质有较大的影响. 在局域热力学平衡条件下,计算获得的氦等离子体输运性质和文献报道的数据符合良好.     

Die Erzeugung von Seriengrenzkontinua mit Hilfe von Gleitfunken zur Absolutmessung im Vakuum-Ultraviolett

A lithium plasma is produced by discharging a 40 kV, 0,3 μF capacitor through a lithium-hydride-capillary (diameter 2 mm, length 20 mm) in vacuum (p～10^?4 Torr). During the first half-cycle (0,6 μs) Bremsstrahlung of Li III is observed in the visible and infrared, and the Lyman series of Li III together with the recombination continuum in the vacuum UV (100 Å). The high members of the Lyman series are broadened by Stark effect giving an electron density of about 6 · 10¹⁸ cm^?3. In the infrared the radiation is emitted from an optically thick plasma at a time when the free-free continuum in the visible is emitted from an optically thin plasma. Temporal development of electron temperatureT _e and electron density N_e has been measured from the absolute intensity in these spectral regions. Typical values ofT _e=230 000° K andN _e 5 · 10¹⁸ cm^?3 e.g. have been obtained. For these values the relaxation time for an ionisation equilibrium is short compared to the observation time. The complete ionisation of Li III has been checked by absorption measurements near 100 A. Thus the absolute intensity of the recombination continuum could be calculated. An experimental arrangement was built to measure simultaneously the time history of the intensity in the vacuum ultraviolet, in the visible, and infrared spectral regions.     

Excited-state populations of hydrogenlike and heliumlike ions and diagnostics of dense transient plasmas from the relative intensities of spectral lines

A collisional-radiative model of H- and He-like ions is constructed. The model allows a determination of excited-state population densities for arbitraryN _e ,T _e , and ion composition. The coefficientsr ₀ andr ₁, the collisional-radiative ionization and recombination coefficients for H-like ions and for the He-like ions F VIII and Mg XI are calculated. The methods of plasma diagnostics from the relative intensities of spectral lines are substantiated. It is indicated that the possibility of measuringN _e from the resonance-to-intercombination line intensity ratio of a [He] ion is determined by the knowledge of the ion composition andT _e . Translated from Trudy Fizicheskogo Instituta im. P. N. Lebedeva, Lebedev Physics Institute, Russian Academy of Sciences, Moscow, Vol. 195, pp. 158–178, 1989.     

Electron cooling in three‐dimensional Dirac fermion systems at low temperature: Effect of screening

Hot electron cooling rate P, due to acoustic phonons, is investigated in three‐dimensional Dirac fermion systems at low temperature taking account of the screening of electron–acoustic phonon interaction. P is studied as a function of electron temperature T_e and electron concentration n_e. Screening is found to suppress P very significantly for about T_e < 0.5 K and its effect reduces considerably for about T_e > 1 K in Cd₃As₂. In Bloch–Grüneisen (BG) regime, for screened (unscreened) case the T_e dependence is P ～ T_e⁹(T_e⁵) and the n_e dependence gives P ～ n_e^–5/3 (n_e^–1/3). The T_e dependence is characteristic of 3D phonons and n_e dependence is characteristics of 3D Dirac fermions. The plot of P /T_e⁴ vs. T_e shows a maximum at temperature T_em which shifts to higher values for larger n_e. Interestingly, the maximum is nearly same for different n_e and T_em/n_e^1/3 being nearly constant. More importantly, we propose, the n_e dependent measurements of P would provide a clearer signature to identify 3D Dirac semimetal phase. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Velocity distribution of plasma electrons in the negative H2- and He-glow with superimposed longitudinal magnetic field

The negative glow plasma has been found nearly field free in axial direction. Therefore plasma electrons in the stationary glow can thermalize down to the temperature of the neutral gas, whenever their diffusion—and recombination—lifetime is high enough. Applying Boltzmann's equation to this problem, the conditions of thermalization of plasma electrons are derived as a function of the outer parameters of the plasma: vessel diameter 2R, neutral gas pressurep and longitudinal magnetic fieldB. — If plasma electrons have a too short diffusion—and recombination—lifetime to be in thermal equilibrium with the neutral gas, the electron energy increases. For this case the distribution function of plasma electrons is derived using Boltzmann's equation. Approximating the calculated energy distribution by a Maxwellian distribution function, the electron temperature in the glow is obtained as a function of the parameters:R, p, B. OurT _e -measurements carried out in the H₂- and He-glows of different tube diameters, neutral gas pressures and magnetic fields agree closely with the theoretical results. TheT _e -measurements have been performed with Langmuir probes and by the method of reversal of the radial ambipolar electric field in a longitudinal magnetic field.     

Investigation of the Beam Plasma Discharge in Electronegative Gases Using the Second Derivative of Langmuir Probe Characteristics

The distribution of the charge carriers is measured and the influence of the attachment processes on the electron energy distribution function is demonstrated for beam plasma discharges in SF₆, CF₄ and O₂ with the aid of the second derivative of Langmuir probe characteristics. The structuration of the plasma into regions of predominating negative ions and regions of predominating electrons is determined by the the established radial T_e-profile. The dimension of the quasi electronfree plasma changes significantly as well at transition from the turbulent heating mechanism into the electron impact plasma generation as by occuring low-frequency instabilities. With increasing n_?/n_e a deficit of low energetic electrons appears in the electron energy distribution parallel to the formation of the negative ion peak. The saturation currents at n_e/n_?=0 yield the mass ratios between negative and positive ions.     

VUV emission model of REB-heated plasma

VUV emission model of a hygrogen plasma with oxygen impurity (T _e=tens of eV,n _e 10¹⁴–10¹⁶ cm^–3,ⁿimp=1–3 % n_e) is constructed in order to judge different possibilities of plasma diagnostics (especiallyT _e measurements) in the REBEX experiments. Two sets of calculations based on the nonstationary corona model are performed: time dependent continuous and line spectra in the range 5 eV—5 keV in the constantT _e approximation (discussion ofT _e measurements by the filter-method) and time dependent intensities of selected spectral lines (2s-2p type) of ionsO ²⁺–O⁵⁺ at variableT _e (including plasma heating by REB and radiative cooling). A possibility of plasma energy content determination from radiation losses is shown.We would like to acknowledge many helpful discussions with dr. P. unka; we thank also dr. J. Ullschmied for comparing our results with diamagnetic measurements.     

Electron temperature (T e) measurements by Thomson scattering system

Thomson scattering technique based on high power laser has already proved its superoirity in measuring the electron temperature (T _e and density (n _e) in fusion plasma devices like tokamaks. The method is a direct and unambiguous one, widely used for the localised and simultaneous measurements of the above parameters. In Thomson scattering experiment, the light scattered by the plasma electrons is used for the measurements. The plasma electron temperature is measured from the Doppler shifted scattered spectrum and density from the total scattered intensity. A single point Thomson scattering system involving a Q-switched ruby laser and PMTs as the detector is deployed in ADITYA tokamak to give the plasma electron parameters. The system is capable of providing the parameters T _e from 30 eV to 1 keV and n _e from 5 × 10¹²cm⁻³−5 × 10¹³cm⁻³. The system is also able to give the parameter profile from the plasma center (Z=0 cm) to a vertical position of Z=+22 cm to Z=−14 cm, with a spatial resolution of 1 cm on shot to shot basis. This paper discusses the initial measurements of the plasma temperature from ADITYA.     

Theoretical studies on dielectronic recombination of neonlike gold and its effects on plasma ionization balance and radiation energy

The dielectronic recombination (DR) of neonlike gold ions is investigated employing the flexible atomic code based on the relativistic configuration interaction method, and its influence on the ionization balance and radiation energy in high-temperature plasma is also studied. The total resonance strength for LMM configuration complex is in a good agreement with the experimental measurement and other theoretical works. The DR rate coefficients are calculated and compared with the three-body recombination and radiative recombination rate coefficients. The DR process is the dominant recombination mechanism of Ne-like gold ions for plasma with temperature T_e≥6.5 keV and density n_e≤2×10²² cm^-3, which is close to the condition of X-ray conversion region in the inertial confinement fusion. Moreover, the DR satellite spectra of LMM, LMN and LMO resonances are simulated, and compared with the intensities of the corresponding resonance lines induced by the electron impact excitation. The intensity ratio of the satellite line 3D’ [(2p⁵_3/23d_3/23d_5/2)_J=5/2[(2p^{5}_{3/2}3d_{3/2}3d_{5/2})_{J=5/2}–(2p⁶3d_3/2) _J=3/2](2p^{6}3d_{3/2})_{ J=3/2}] and the resonance line 3D [(2p⁵_3/23d_5/2)_J=1[(2p^{5}_{3/2}3d_{5/2})_{J=1}–(2p⁶)_J=0](2p^{6})_{J=0}] is given, which can be applied for diagnostics of plasma temperature.     

MPD arcs as plasma sources for recombination lasers

In this study the decay of argon plasmas ejected from a MPD thruster described in a previous paper is investigated. Various parameters, e.g. the electron densityN _e , the electron temperatureT _e , and the absorption coefficient of the ArII-488 nm transition are measured by different experimental techniques. Axial and radial profiles ofN _e andT _e are determined, and used to decide on the relevant recombination mechanism. In spite of the fact that the dominant three-body recombination favours the population of the high-lying energy levels, population inversions have been observed even with the most sensitive method only in plasmas ejected from a reduced aperture of the MPD thruster. The theoretical analysis shows that the ranges ofT _e andN _e , in which recombination-lasing may be expected, are narrow. In addition, the mechanisms that limit the population inversions in discharge tubes of conventional Ar⁺-lasers restrict the dimension of the plasma perpendicular to the resonator axis. From these facts and the described measurements on population inversions we conclude that the initial diameter of the plasma has to be reduced. We therefore propose a new discharge configuration where extended regions of constant plasma parameters can be expected. With this arrangement it should be possible to reach population inversions required for laser oscillations in ArII.Polish Academy of Sciences, Gdansk, Poland     

Zur Mikrowellendiagnostik stromstarker Entladungen mit Skineffekt. Teil II: Bestimmung der Plasmaparameter eines angeströmten Stickstoffbogens bei Atmosphärendruck

Microwave diagnostics of an inhomogeneous, dense, cylindrical plasma column may be used to determine the complex impedance and a mean noise temperature in the range of strong skin effect (theory see part I). This case is realized if the specific d.c. impedance E/I falls below a critical value, e.g. 20 Ω/cm for ω = 10¹⁰. It is shown, that for any radial conductivity profile the complex impedance is a function of E/I only. A formula is derived for calculating the noise temperature of the column as a weighted mean value due to the local plasma temperature and conductivity. In the case of strong skin effect this mean value equals the electron temperature of plasma regions near the boundary of the conducting diameter. These results in connection with spectroscopical diagnostics are use to determine some plasma parameters of an arc discharge in a streaming nitrogen atmosphere at 760 Torr. The temperature profil was obtained from the intensity of the 3371 Å molecular band and then the conductivity profile was calculated using an energy balance equation. The results show that even in the 10 A-case the arc plasma is not in thermal equilibrium, the difference T_e—T_g being about 1,500 ºK. This large difference is due to the flow of neutral gas streaming downward the discharge chamber. The length of the discharge (1·3 cm) is not sufficient for the neutral atoms to reach their equilibrium temperature.     

Anfachung und Dämpfung von Ionisationswellen in MHD-Kanälen und ihr Einfluß auf die effektive elektrische Leitfähigkeit,den effektiven Hall-Parameter und die mittlere Elektronentemperatur

The phase velocity, the amplification rate and the critical Hall parameter are theoretically determined for ionization waves in a weakly ionized plasma streaming across a strong external magnetic field and bearing a current flowing perpendicular to both the magnetic field and the stream velocity. The investigations hold for seeded rare gases at any degree of seed ionization. The critical Hall parameter β_c depends on the degree of ionization, the ionization energy and the temperatures of electron gas T₀ and neutral gas T_g · β_c is always greater than one, if 0 < T₀ — T_g ? T₀ holds. The three-dimensional treatment indicates the existence of waves with a nonvanishing wave vector component in the direction of the magnetic field. The influence of ionization waves on mean current density, mean Hall field intensity and mean electron temperature is determined up to second order terms in the relative fluctuations of the electron temperature. The amplification of ionization waves reduces the effective electric conductivity, the effective Hall parameter and the mean electron temperature compared to the undisturbed state. Similar results are also obtained for steady state homogeneous isotropic turbulence and a special case of axially symmetric turbulence. Furthermore, a component of the electric field in direction or in opposite direction to the magnetic field vector may be generated by non isotropic and non homogeneous turbulence.     

Appearance of a Hα Minimumat the Onset of Net Recombination in Hydrogen Plasmas

Hydrogen plasmas out of ionization equilibrium are either ionizing or recombining depending on the electron temperature T_e . Within the transition region between these two opposite states a minimum of the H_α emission is often experimentally observed. Simple cases were previously analyzed which could be interpreted assuming only a temperature variation, i.e the electron density was constant in the transition region. Here we discuss two examples in which both the density and the temperature vary at the transition. In the linear plasma generator PSI‐II a hydrogen plasma is cooled down by puffing additional gas. We find a minimum at T_min ≈ 1.1 eV. A second example is the effect of an ELM(edge localized mode) pulse propagating through a recombining divertor plasma in the tokamak ASDEX Upgrade. The H_α response shows a double peak which can be interpreted as a local minimum assuming a simultaneous rise of density and temperature during an ELM. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)