Atomic Density Measurements Research Articles

Modelling and diagnostics of plasma chemical processes in mixed-gas arcs

Abstract

High fractional dissociation efficiency in H 2 and H 2N 2 gas mixtures in a helical resonator discharge

High fractional dissociation of H 2 in a helical resonator discharge, operated in inductive mode, has been measured using two-photon allowed laser induced fluorescence of H atom in H 2 and %H 2%N 2 gas mixtures. Absolute H atom density measurements yield 11% dissociation efficiency in H 2 at 1 Torr. The fractional dissociation efficiency of H 2 was found to increase (approaching 100%) with an increase in %N 2 ion H 2N 2 gas mixtures leading to a nearly constant H atom density over a wide range of %H 2%N 2 gas compositions.

Absorption spectroscopy measurements of atomic and molecular carbon population densities in an expanding thermal arc plasma

Absolute population densities of argon and atomic and molecular carbon are determined using the method of reabsorption in an expanding thermal arc plasma during the deposition of amorphous hydrogenated carbon (a-C:H) coatings. The reactor conditions were the following: background pressure 20–200 Pa, argon flow rate 58–116 cm 3 s −1 , arc current 45 A, arc voltage 70–80 V; hydrocarbons (CH 4 or C 2 H 2 ) with a flow rate of 3–6 cm 3 s −1 , were injected either into the nozzle of the arc or directly into the vessel. Depending on the argon-methane/acetylene gas mixture and the hydrocarbon injection (downstream or in the nozzle), the stationary positive or negative absorption between the quantum states of Ar(3p 5 4p → 3p 5 4s) (λ = 696.5 nm), C(2p 2 3s → 2p 2 1s) (λ = 247.9 nm) and C 2 ( d 3 Π g , v ′ = 0 → a 3 Π u , v ′' = 0) (band head at λ = 516.5 nm) is determined. From this absorption the absolute population densities of the radicals in the plasma are obtained. Depending on the plasma conditions, the density of the argon first excited state Ar(3p 5 4s, 3 P 2 ) was about 10 16 –10 17 m −3 , whereas the atomic and molecular carbon densities were of the order of 10 18 –10 19 m −3 .

Diagnostics for the spatial distribution of hydrogen atoms around the divertor region

For measurements of spatial distributions of hydrogen atoms around the divertor region, laser-induced fluorescence (LIF) based on various two-phonon excitation schemes from the ground level is discussed, and merits and demerits under different experimental conditions are compared. Excitation to the n=3 level using an ArF excimer laser (wavelength = 193 nm) and its Raman shifts in deuterium gas (205 and 219 nm) and observation at Balmer alpha transition (656 nm) was found to be the best for the above purpose. A design study of the fluorescence observation and estimation of expected signal to noise (SN) ratios were performed for measurements of atomic hydrogen densities on plasmas around the JFT-2M tokamak divertor, showing that such a measurement is possible.

Twilight rocket measurements of high-latitude atomic oxygen density during the DYANA campaign

During the DYANA ( DYnamics Adapted Network for the Atmosphere) campaign, a rocket containing a resonance fluorescence experiment for measurement of atomic oxygen concentrations was launched at twilight-dawn from ESRANGE, Kiruna, Sweden in March 1990. The measured atomic oxygen concentration rose very sharply near 80 km to about 10 11 atoms cm −3, achieved a peak value of 3 × 10 11 atoms cm −3 between 90 to 105 km, and decreased with increasing rocket altitude to about 155 km where the instrument's noise level was reached. In addition, ground-based, near-infrared radiometric and spectral measurements of the mesospheric and lower thermospheric hydroxyl (OH) airglow emissions were also obtained during the night up to the time of rocket launch. We have used least-squares spectral fitting procedures to obtain the OH Meinel (3-1) band intensities and accurate ( ± 2.5K) rotational temperatures from individual scans. The band intensities show considerable structure throughout the night, dropping sharply by about a factor of 3 during sunrise when the rocket was launched. The rotational temperatures were consistently around 225 K throughout the night and during the rocket flight. During the MAP/WINE campaign in February 1984, similar measurements using identical rocket-borne and ground-based techniques were made at night from ESRANGE. In this paper, the DYANA and MAP/WINE measurements are inter-compared and discussed and further compared with model calculations.

Two-photon laser-induced fluorescence measurements of absolute atomic hydrogen densities and powder formation in a silane discharge

Absolute and spatially resolved hydrogen densities were measured in a silane radio frequency discharge by two-photon laser induced fluorescence spectroscopy. The excitation wavelength was 205 nm and the observation wavelength was 656 nm (Balmer alpha). Titration in a flow tube reactor was used for calibration of the detection system, enabling for the first time the determination of reliable absolute hydrogen densities in a silane discharge. In addition, spontaneous as well as laser induced powder formation was observed.

Absorption spectroscopic measurement of atomic density in laser-induced vapor plume

Laser absorption spectroscopy employing a tunable continuous wave dye laser has been used to measure the density of atomic niobium vapor in a laser ablation plume near the target surface. Population densities for one atomic level were measured to be ∼2×1015 cm−3 near the center of the plume.

Bias in emission absorption measurements of temperature and atomic density

Presented here is an analysis of the bias arising from two fundamental difficulties hi applying emission absorption methods to large-scale combustion systems: 1) maintaining clean optical windows; and 2) maintaining the optical alignment. The impetus for the study was an anomalously large temperature measurement uncertainty for potassium D-line measurements hi a coal-fired magnetohydrodynamic (MHD) flow, while no increased uncertainty was observed for the measured seed atom density. The large uncertainty can be explained by a rapid, oscillatory misalignment. This misalignment biases the measured temperature hi the same manner as that for a fouled lamp-side window. Expressions for the biases for fouled optical windows are presented. Expressions are developed for the bias and uncertainty caused by a harmonic misalignment. These expressions provide a consistent explanation of the temperature results. In contrast, the robustness of a two-wavelength emission absorption method for atomic density measurements is demonstrated.

Atomic hydrogen behaviour in Heliotron E

In order to understand atomic hydrogen behaviour and particle confinement properties in Heliotron E plasmas, techniques of laser induced fluorescence (LIF) have been extensively used, combined with measurements of absolute Balmer alpha emissions and density fluctuations. The results revealed that for an average electron density n¯ e > 1.5 × 10 19 m −3, Hα fluorescence allowed the measurements of atomic hydrogen densities and yielded the recycled Franck-Condon neutrals (~ 3 eV) to penetrate into core plasmas, whereas for n¯ e < 1.0 × 10 19 m −3, such measurements were perturbed by dissociative-excitation of H 2 molecules into the n = 2 state. Also, the global confinement times showed a clear correlation with electron density fluctuations.

Atomic hydrogen density measurements in the Tara tandem mirror experiment

Neutral and plasma density have been measured in the north well of the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982)]. The electron plasma density and temperature on the magnetic axis were measured by Thomson scattering to be about 3×1012 cm−3 and 70 eV, respectively. The corresponding axial neutral hydrogen density was found to be 1 ×109 cm−3, while near the plasma edge at r=15 cm it reached 1×1010 cm−3. The fill gas density at r≥22.5 cm was ≊1011 cm−3. Additional information from secondary electron detectors was used to estimate the radial ion temperature distribution, which was found to have about the same width, 12 cm, as the plasma density. The resulting ion pressure profile is peaked compared to the electron pressure profile. Charge exchange losses in the well are found to have a maximum at a radius equal to half the e-folding distance of the plasma density and ion temperature distributions.

Toward the Avogadro constant-preliminary results on the molar volume of silicon

The silicon molar volume has been measured through atomic weight and density measurements. Mass spectrometric determination of the isotopic composition was performed on two differently grown samples by comparison with the US National Bureau of Standards (NBS) reference material SRM 990. The density of specimens from the same crystals was obtained through direct mass and dimension measurements on one single-crystal sphere and through hydrostatic comparison for another sphere. Compatibility with previous measurements was checked through hydrostatic comparison with IMGC Zerodur density standards. The two molar volume determinations differ by 0.14 p.p.m. The preliminary value obtained is 12.058, 843+or-0.000, 029 cm/sup 3//mol at 22.5 degrees C and in vacuo.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurements of atomic hydrogen density in the plasma of FT-1 Tokamak by the resonance fluorescence method

The laser-induced fluorescence at Halpha line was used to study the neutral hydrogen behaviour in the FT-1 Tokamak. Absolute measurements of the hydrogen density profiles were carried out, and the influence of recycling and gas puffing on the particle balance was studied.

Ground‐based measurements of exospheric hydrogen density

A new observation technique, suggested by recent advance in exospheric theory, appears useful for timely measurement of exospheric atomic hydrogen density, [H]. Fabry‐Perot interferometer measurements of the geocoronal 6563 Å (Hα) emission in the plane of the solar azimuth are used to approximately isolate the singly scattered resonant fluorescent intensity of Hα (ISS) thereby simplifying the derivation of [H]. This initial use of the technique produces profiles of [H] from 400 to 10,000 km. The theoretical bulge of [H] in the early morning hours, previously escaping detection based on simple comparison of relative intensities, is now revealed with a magnitude exceeding the theoretical value. The measured ratio of [H] at 0430 local time to [H] at 2030 local time is 3.0−0.36+0.71 at 700 km, compared with a theoretical value of 1.72. At 2000 km we find no local time variation, in agreement with theory. Unlike nearly all previous studies, we find no arbitrary increase in the solar Lyman beta (Ly‐β) flux is necessary to reconcile derived ISS with current models of exospheric [H]. Deriving absolute [H] remains subject to large systematic uncertainties (on the order of 50%), but the new method substantially improves upon linear extrapolation of relative density variations from Hα intensity changes.

Interparticle interactions and structural changes of nucleosome core particles in low-salt solution

The structural behavior of the nucleosome core particles in the range of solvent Na+ concentration from 10.45 to 0.45 mM has been studied by small-angle neutron and synchroton radiation X-ray scattering, sedimentation, atomic absorption spectroscopy, density measurements, and circular dichroism. With decreasing salt concentration, the appearance of a scattering peak that is assignable to interparticle interactions, an intraparticle structural transition, a decrease in the sedimentation velocity of the particle, and a release of bound Na+ ions from the particle are all observed concurrently when the ratio of solvent Na+ ions per particle is below approximately 1000. These observations are interpreted to indicate that a release of bound Na+ ions from the particle brings about structural rearrangements and weakens the electrostatic shielding of the particle, and this introduces long-range repulsive ordering of the particle in low-salt solution. Analyses of the scattering data indicate that the rearrangement within the core particle in low-salt solution is slight, changing the particle's shape slightly from cylindrical to a more spherical form by moving the center of the mass of the DNA somewhat inward with accompanying small decreases in the radii of gyration of both the DNA and the histones.

Narrow Band, Near UV, High Repetition Rate Laser Induced Fluorescence System for use as an Edge Diagnostic in Fusion Machines*

A laser system for impurity diagnostics in the edge region of plasma devices is described. It consists of a scanning, single mode, cw dye laser followed by a 3-stage, fast flow dye cell amplifier, pumped by a high repetition rate excimer laser (60 mJ/pulse at 130 Hz, for 308 nm output). Substantial improvements are achieved over previous systems in scan speed (30 GHz/100 ms) and velocity resolution (now small relative to the widths of thermal distributions). The usefulness of high resolution is demonstrated by a model calculation for Fe velocity spectra involving the presence of thermal and sputtered flux, and spatial averaging. The high output pulse power (0.8 MW at 604 nm, 80 kW at 302 nm) allows efficient frequency doubling and can be used to vary the effective bandwidth by power broadening. Broadband operation (50 GHz FWHM) is also possible, for saturated measurements of atomic density. Laboratory velocity spectra for Fe atoms sputtered in the ground state demonstrate the capability for such measurements in a single Tokamak discharge at estimated densities of 108 atoms/cm3.

Near-Resonant Rayleigh Scattering Method for Measurements of Alkali Atomic Density in Combustion Gas Plasmas

This paper reports the development of a non-intrusive, space-resolved method for density measurements of alkali atoms in combustion gas plasmas. A sodium density of over 1014 cm-3 in an air-acetylene flame was measured using a near-resonant Rayleigh scattering (NRRS) method, in which the wavelength of the incident laser was detuned from the resonance line of the sodium atoms in order to avoid reabsorption of the scattered signal light. The results agree fairly well with those obtained by the spectral absorption method and the Na reference cell method. The results show that the NRRS method described here can also be used to measure atomic densities of the order of 1016 cm-3, typical values in an open-cycle MHD power-generation plasma.

Laser absorption measurements of atomic metastable densities

Helium triplet metastable densities have been calculated from measured absorption of resonant, CW, UV laser radiation on the 23S-53P transition at 294.511 nm. Gas temperatures have also been deduced by measuring the Doppler width of this transition. A 3 mm bore capillary tube filled to pressures in the range 1-8 Torr and run at currents in the range 2.5-80 mA was studied. With constant discharge current (60 mA) at a maximum value for the triplet metastable density of 11*1012 cm-3 at about 2 Torr was obtained. The metastable densities show the expected saturation behaviour, saturating at currents above 40 mA. Experimental values are used in conjunction with the rate equation describing the metastable level population in order to estimate the rate coefficient for loss of metastables through electron collisions. Mention is made of the use of the visible laser radiation for measuring the UV laser wavelength in a Kowalski-type wavemeter.

Metastable atom density measurements in He and Ne positive column plasmas by an improved self-absorption method

An improved self-absorption method obtained from the parallel-beam modification of Harrison's method for measuring the metastable atom density in gas discharges has been proposed. This method has the advantage of high spatial resolution in measurement with a relatively simple apparatus. The metastable atom densities of He (21S0 and 23S1) and Ne (1s3 and 1s5) in the positive column plasmas have been measured using this improved self-absorption method by varying the gas pressure from 1 to 10 Torr and the discharge current from 10 to 50 mA. 'Constriction' of the radial metastable atom density distribution in neon plasma has been observed. All experimental results are given and discussed.

High‐altitude atomic nitrogen densities

Theoretical calculations of the seasonal and diurnal variations of atomic nitrogen are compared with measurements made by the open source neutral mass spectrometer on the AE‐C satellite. With the simultaneous measurements of molecular nitrogen and atomic oxygen densities as input, model calculations of odd nitrogen densities predict the same trends in atomic nitrogen as those observed. From these comparisons it is inferred that horizontal transport significantly reduces the diurnal variation of atomic nitrogen. Estimates are given of the sensitivity of atomic nitrogen densities to variations in the photoelectron flux, the neutral temperatures, and the neutral winds.

Erosion products from the cathode spot region of a copper vacuum arc

Combined electrical, optical, and spectroscopic measurements have been used to determine the total flux, ion flux, particle flux, and neutral atom flux emitted from the cathode spot region of a copper vacuum arc of 80 A total current and 2-sec duration. It is found that the cathode erosion products consist predominantly of ions and particles, the emission of neutral atomic flux from the cathode being less than 1% of the total flux. The total and particle flux distributions are peaked in the direction of the cathode plane, whereas the ion-flux distribution is forward peaked. However, both ions and particles are detected in the cathode shadow, a result which is contrary to the hypothesis of purely collisionless transport of cathode erosion products. The neutral atom density measurements are consistent with a model assuming the source of the vapor to be evaporation in flight from the hot particles emitted from the cathode spot region. The size distribution of the particles has a maximum for particles of diameter in the range 0–1 μ.