Measurements of electron density and Stark width of neutral helium lines in a helium arc plasma

Electron densities in an atmospheric helium arc plasma have been measured with the Stark broadening parameters of helium spectral lines. The spatially distributed radiation intensities are converted to plasma emission coefficients at every wavelength by means of Abel inversion. From the inverted profiles of He I lines of 4713 ?, 5016 ?, and 6678 ? electron density has been calculated, which ranges from 0.5 ×10¹⁶ to 4 ×10¹⁶ cm^-3 for a helium arc with current 200 A. Stark widths of He I lines of 3889 ? and 7065 ? are determined based on the measurements and compared with existing data.     

Change of phonon dispersion curves due to interstitials in Al

An experimental investigation of the profiles of the 4471 Å and 4922 Å lines in the afterglow of an high density plasma of helium produced by laser is reported. The measured profiles of these two neutral helium lines and their forbidden components are given at electron densities between 5 · 10¹⁶ cm^?3 and 2 · 10¹⁷ cm^?3 and for electron temperature between 3 and 4 eV.     

Measurement of the Stark broadening parameters of some singly ionized argon lines

Half-widths of fifteen Stark broadened argon II lines have been measured in argon plasma behind the reflected shock wave produced in an electromagnetically driven “T” tube. The plasma electron density was determined by the laser interferometry at three different wavelengths, while the plasma temperature was measured from relative intensities of A II lines. Temperatures were in the range 8,500–16,500 °K; electron densities varied from 1.82 to 3.94 · 10¹⁷ cm^?3. The measured A II linewidths are compared with theoretical and other experimental results. It is shown that a) the broadening of A II lines is in good agreement with the theory, b) line broadening increases linearly with electron density, and c) the Stark broadened lines follow the dispersion profile to the distance of at least three halfwidths from the line center.     

Density dependence of electron drift velocities in helium and hydrogen at 77.6 K

Electron drift velocities have been measured in helium and hydrogen at 77.6 K and gas density of 6.6×10²¹ cm^?3 (approximately 80 atm). At these high densities the electron drift velocities do not depend only on the ratio of the electric field to gas density (E/N). At constantE/N the electron drift velocity decreases with increasing gas density. In helium a decrease was found down to 6.4% of the value at low density, in hydrogen down to 0.52%. The results are discussed in terms of theories of multiple scattering. Legler's theory fits our data in the lower density range, but at the highest densities predicts too small an effect. The percolation theory by Eggarter and Cohen gives no agreement with the experiment. Up to the highest densities we did not find bubbles; slow negative charge carriers could be identified as oxygen ions.     

Emission properties of apokamp discharge at atmospheric pressure in air,argon, and helium

Emission spectra of the plasma jet of apokamp discharge in air, helium, and argon are studied. Apokamp at atmospheric pressure is formed in the areas of strengthening of the electric field near the bends in the channel of the pulse-periodic discharge and is directed perpendicularly to the discharge channel. Apokamp consists of a bright narrow “appendage” connected with the discharge channel and with the diffuse jet emerging from the channel. It is shown that, in helium, the emission of the diffuse part of apokamp is dominated by N₂ and N₂ ⁺, while emission of the “appendage” display lines and bands of He, N₂, N₂ ⁺, O, and OH. In argon, emission spectra of the diffuse part of the plasma jets contain not only N₂ and N₂ ⁺, but also Ar lines. It is assumed that the surrounding air plays an important role in the formation of the diffuse part of apokamp in helium and argon.     

Parametric dependence of ion temperature and electron density in the summa hot-ion plasma using laser light scattering and emission spectroscopy

Hot-ion plasma experiments were conducted in the NASA Lewis SUMMA facility. A steady-state modified Penning discharge was formed by applying a radially inward d.c. electric field of several kilovolts near the magnetic mirror maxima. Results are reported for a hydrogen plasma covering a wide range in midplane magnetic flux densities from 0.5 to 3.37 T. Input power greater than 45 kW was obtained with water-cooled cathodes. Steady-state plasmas with ion kinetic temperatures from 18 to 830 eV were produced and measured spectroscopically. These ion temperatures were correlated with current, voltage, and magnetic flux density as the independent variables. Electron density measurements were made using an unusually sensitive Thomson scattering apparatus. The measured electron densities ranged from 2.1 x 10¹¹ to 6.8 x 10¹²1/cm³.     

Isotope shifts for the P,Q, R lines in indium-doped silicon

We have measured the isotope shift of the P and R luminescence lines in Si doped with In and diffused with isotopically enriched Fe. We find no measurable shift in the R line. Similar experiments on the luminescence lines attributed to an isolated Fe complex show no shift in the previously identified “phonon replicas.” Theoretical calculations of defect phonon resonances near 9 meV for interstitial and substitutional Fe predict an isotope shift of 0.17 meV for ⁵⁶Fe and ⁵⁴Fe. This value is in disagreement with the observed lack of a shift.     

Afterglow of an e-beam discharge in several atmospheres of helium

Spectral observations of recombining helium afterglows at neutral pressures in the range 1 to 7 atmospheres and initial electron densities of 5 × 10¹⁴ cm^?3 are presented.     

The nature of excitonic complexes in ZnTe

Uniaxial stress experiments were used to investigate the nature of the luminescence lines observed at low temperatures in ZnTe in the vicinity of the absorption edge. The single crystals used in this experiment were grown from solution of ZnTe in tellurium. Both “as-grown” crystals and crystals annealed in Zn vapour were investigated. The most intense line in “as-grown” crystals is attributed to an exciton bound to a neutral acceptor. The binding energy of the exciton in this center is 6 meV. After annealing a new center appears in the same spectral region. Stress experiments as well as the temperature dependence of the intensity of the luminescence indicate that this center is a complex consisting of an exciton and an ionized donor. Splitting of J = 1 (Γ₅) and J = 2 (Γ₃ + Γ₄) levels was found to be 1.2 meV.     

The Broadening of He I lines including ion dynamic corrections,with application to λ4471Å

A theory is developed for the broadening of He I lines with forbidden components which is valid at low densities (where the forbidden line is reasonably well isolated). This theory takes into account the effects of dynamic ion broadening. Application to λ4471Å gives good agreement with experiment and with a recent calculation of Lee; reasons for agreement with Lee are discussed in detail. Comprehensive tables for 4471Å are presented. These have been extended further into the line wings than previous tabulations. Static ion theory is found to be adequate for densities greater than ～ 5 × 10¹⁵ cm^?3. A simple numerical profile for allowed lines, taking into account dynamic ion effects, is also presented.     

Stark broadening of the Paschen-beta line of hydrogen in a linear discharge

Experimental profiles of the hydrogen Paschen-beta line have been measured in a helium plasma with 0.85% hydrogen. Electron densities were deduced from profiles of the He(I) line (λ = 4471.5 Å) and are in the range 10¹⁴–10¹⁵ cm^-3. Our measurements are compared with theoretical calculations.     

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.     

Monte Carlo Simulation of Helium Transport in the Vicinity of Divertor Plate

A Monte Carlo code “1DHET” has been utilized for simulating the transport of neutral helium near divertor flat pumping target plate. The dominant atomic processes of electron impact ionization and elastic scattering by plasma ions are included. The thermal and streaming motion of the ions along the magnetic field is taken into consideration. Calculations show that the probability of neutral helium turning back to the target plate will increase by 45% if the divertor edge plasma density is smoothly raised up to 4.7 times higher at the divertor plate than that at 7cm apart inward as compared with the ordinary outward decreasing density profile while keeping temperature profile fixed. The temperature decreases smoothly from 40 eV at z = 7 cm to 3 eV at the plate. The effects of angle between the magnetic field and the target plate, the temperature of plate, edge plasma sheath potential and different target materials on the probability of helium particles scattered back to the plate are studied.     

Low frequency phonon spectra of mixed crystal CdxHg1−xTe

Far infrared absorption spectra for the mixed crystal Cd_xHg_1?xTe with x=0.18 to 0.45 in the wavenumber region of 10 to 400cm^?1 and temperature region of 4.5 to 300K are reported. A low frequency absorption band has been found between 20 and 50 cm^?1 for all the samples which have been measured in addition to the two-phonon bands in both sides of the reststrahlen absorption bands of the material. By comparison with the phonon densities of states estimated from the phonon frequencies of CdTe, we have attributed this band to TA band modes induced by the disorder and “impurity” effects.     

Spectroscopic Mapping of the Nonequilibrium between Electron and Excitation Temperatures in a 1 ATM Helium ARC

The thermostatic states of a 100 amp, 1.016 bar, free-burning helium short arc with a 10 mm electrode gap are mapped from spectroscopic measurements at eight cross sections. The theoretical model used is a multifluids model extended to consider nonequilibrium between electron and excitation temperatures, as well as simple nonequilibrium among excited electronic levels. Seven helium lines are used to determine population densities and upper level excitation temperatures. The electron density is calculated from continuum intensity measurements at C4690. Electron temperatures are found from an astrophysical method suggested by Athay and Menzel. The effective total excitation temperature is obtained by iteration using the multifluids model. The results indicate total excitation temperature values close to the usually calculated "LTE" temperatures, but electron temperatures up to three times larger than the total excitation temperature on the arc centerline near the electrodes. The ratio is approximately 1.5 in the middle of the arc. The heavy particle kinetic temperatures appear to follow the electron temperature, except near the anode, where they drop to values smaller than the total excitation temperatures.     

The energy distribution of secondary ion-electron emission

This study deals with the energy distribution of secondary electrons emitted when 3 keV He⁺ and Ne⁺ ions bombard the (100) surface of a copper single crystal and the (0001) surface of a zinc single crystal. A “long magnetic lens” was employed in the energy analysis. A group of high-energy electrons (having an energy of approximately 35 eV) is found to appear in the case of bombardment with helium ions. The results are discussed on the basis of the model of autoionization of the incoming ions (helium and neon) embedded in layers at near the surface. The effect of temperature and time on the energy distribution of secondary electrons is also examined.     

Helium wetting: Theme and variations on inhomogeneous helium

The surprising discovery, in 1991, that liquid helium does not wet a cesium surface at low temperature has triggered an important activity both theoretical and experimental: helium has become a model system for the study of wetting transitions. After summarizing the main theme of helium wetting, I will focus on more recent studies, such as the structure and excitations of helium interfaces, experiments on the capillary rise, the “surfactant effect” of helium-3 impurities on liquid helium-4 and the “quantum prewetting transition” of pure helium-3. Unexpected consequences on the phase separation of³He?⁴He mixtures in restricted geometry will be drawn.     

Helium scattering and work function investigation of co adsorption on Pt(111) and vicinal surfaces

CO adsorption on Pt(111) and vicinal Pt(111) surfaces has been studied by means of work function variation and He scattering measurements. AES and LEED were used mainly for correlations with other work. Special attention has been paid to the low coverage regime (θ_co < 0.1) with emphasis on surface structural dependencies. The minimum of the work function versus CO exposure curve occurs at a coverage less than 11% on “kink-free” surfaces. This is much lower than the hitherto commonly accepted value of 33%, and does not relate to any observed LEED superstructure. The value of Δφ_min depends strongly on the surface structure. For an “ideal” Pt(111) surface with a step density less than 10^?3 at a temperature of 300 K, Δφ_min = ?240 meV. The scattering cross section Σ of CO adsorbed on Pt(111) for 63 meV He is typically > 250 Ų, i.e. much larger than expected from the Van der Waals radii of He and CO. For two nominal Pt(111) surfaces with step densities of 10^?2 and less than 10^?3, respectively, the measured Σ values varied by a factor of three. This can be explained by preferential CO occupation of defect sites, which are already not “seen” by thermal helium. By comparing results on a stepped (997) and a kinked (12 11 9) Pt surface with similar defect densities, the kinks are proven to play a decisive role. They probably form saddles in the recently proposed activation barrier for migration between terrace and step sites.     

Three stage cool flame droplet burning behavior of n-alkane droplets at elevated pressure conditions: Hot,warm and cool flame

Transient, isolated n-alkane droplet combustion is simulated at elevated pressure for helium-diluent substituted-air mixtures. We report the presence of unique quasi-steady, three-stage burning behavior of large sphero-symmetric n-alkane droplets at these elevated pressures and helium substituted ambient fractions. Upon initiation of reaction, hot-flame diffusive burning of large droplets is initiated that radiatively extinguishes to establish cool flame burning conditions in nitrogen/oxygen “air” at atmospheric and elevated pressures. However, at elevated pressure and moderate helium substitution for nitrogen (X_He?>?20%), the initiated cool flame burning proceeds through two distinct, quasi-steady-state, cool flame burning conditions. The classical “Hot flame” (～1500?K) radiatively extinguishes into a “Warm flame” burning mode at a moderate maximum reaction zone temperature (～ 970?K), followed by a transition to a lower temperature (～765?K), quasi-steady “Cool flame” burning condition. The reaction zone (“flame”) temperatures are associated with distinctly different yields in intermediate reaction products within the reaction zones and surrounding near-field, and the flame-standoff ratios characterizing each burning mode progressively decrease. The presence of all three stages first appears with helium substitution near 20%, and the duration of each stage is observed to be strongly dependent on helium substitutions level between 20–60%. For helium substitution greater than 60%, the hot flame extinction is followed by only the lower temperature cool flame burning mode. In addition to the strong coupling between the diffusive loss of both energy and species and the slowly evolving degenerate branching in the low and negative temperature coefficient (NTC) kinetic regimes, the competition between the low-temperature chain branching and intermediate-temperature chain termination reactions control the “Warm” and “Cool” flame quasi-steady conditions and transitioning dynamics. Experiments onboard the International Space Station with n-dodecane droplets confirm the existence of these combustion characteristics and predictions agree favorably with these observations.