Electron Beam Fluorescence Technique Research Articles

The electron beam diagnostic of the clustered supersonic nitrogen jets

Axial and radial distributions of the rotational temperature and density of N2 molecules in supersonic nitrogen jets formed with conic nozzles (critical diameters dcr of 0.17 and 0.21 mm) were studied using the electron beam fluorescence technique at stagnation pressures P0 of 0.1-0.6 MPa. A rotational temperature Tr, equaling a gas temperature Tg owing to fast RT relaxation, was obtained using the rotational line relative intensity distribution in (0-1) vibrational band of the N2 first negative system. Gas density profiles in the jets were obtained using the integral intensity of the band. It is found, Tr at the nozzle outlet is of the order of a few tens of Kelvin and at further expansion Tr drops up to 15-20K at distance of (100-200) dcr. The gas temperature and density distributions in the studied supersonic nitrogen jets are not similar to the isentropic distributions. It is shown that the lower is the stagnation pressure the faster the gas density and temperature decrease with distance from the nozzle. Increase in P0 leads to elevating Tg in the jets. A reason for this effect may be cluster formation in the jets. Estimations of cluster mean sizes in the jets using Hagena’s parameter show presence of large clusters (M≥200) at P0 = 0.4-0.6 MPa.

Wave processes in a viscous shock layer and control of fluctuations

Generation and development of disturbances in a hypersonic viscous shock layer on a flat plate is studied both experimentally and numerically. The study is performed at the Mach number M∞ = 21 and the Reynolds number ReL = 1.44 × 105 and is aimed at elucidating the physical mechanisms that govern the receptivity and instability of the shock layer at extremely high hypersonic velocities. The experiments are conducted in a hypersonic nitrogen-driven wind tunnel. An electron-beam fluorescence technique, a Pitot probe and a piezoceramic transducer are used to measure the mean density and Mach number contours, as well as density and pressure fluctuations, their spectra and spatial distributions in the shock layer. Direct numerical simulations are performed by solving the Navier–Stokes equations with a high-order shock-capturing scheme in a computational domain including the leading and trailing edges of the plate, so that the bow shock wave and the wake behind the plate are also simulated. It is demonstrated that computational and experimental data characterizing the mean flow field, intensity of density fluctuations and their spatial distributions in the shock layer are in close agreement. It is found that excitation of the shock layer by external acoustic waves leads to generation of entropy–vortex disturbances with two maxima of density fluctuations: directly behind the shock wave and on the external edge of the boundary layer. At the same time, the pressure fluctuations decay inward into the shock layer, away from the shock, which agrees with the linear theory of interaction of shock waves with small perturbations. Thus, the entropy–vortex disturbances are shown to dominate in the hypersonic shock layer at very high Mach numbers, in contrast with the boundary layers at moderate hypersonic velocities where acoustic modes are most important. A parametric numerical study of wave processes in the shock layer induced by external acoustic waves is performed with variations of frequency, amplitude and angle of propagation of external disturbances. The amplitude of generated disturbances is observed to grow and decay periodically along the streamwise coordinate, and the characteristics of these variations depend on the frequency and direction of incident acoustic waves. The hypersonic shock layer excited by periodic blowing and suction near the leading edge is also investigated; in the experiments, this type of excitation is obtained by using an oblique-cut whistle. It is shown that blowing/suction generates disturbances resembling those generated by external acoustic waves, with similar spatial distributions and phase velocities. This result paves the way for active control of instability development in the shock layer by means of destructive interference of two types of disturbances. Numerical simulations are performed to show that instability waves can be significantly amplified or almost entirely suppressed, depending on the relative phase of blowing/suction and acoustic disturbances. Wind-tunnel experiments completely confirm this numerical prediction. Thus, the feasibility of delaying instability development in the hypersonic shock layer has been demonstrated for the first time.

Rocket-borne instrument for measuring vibrational-rotational temperature and density in the lower thermosphere

We applied the electron beam fluorescence (EBF) technique to measure the vibrational temperature, rotational temperature, and number density of atmospheric molecular nitrogen (N2) in the altitude range of 100–150km. The atmospheric N2 is excited and ionized by electron beam of the energy of 1keV and the subsequent fluorescence of the N2+ first negative band is detected by a sensitive spectrometer, which covers the 360–440nm wavelength range. In contrast to previous rocket-borne photometric measurements using the EBF technique, the spectrometric measurement provides the above three parameters simultaneously and more accurately. Preflight laboratory experiments were carried out in order to test the spectral sensitivity over the whole spectral range, to calibrate the number density from the band intensity, and to know the accuracy of the rotational temperature measurement. Finally, we tried to calibrate the vibrational temperature measurement by using heated N2 gas which is ejected from the small nozzle.

Electron beam fluorescence temperature measurements of N2 in a semiconductor plasma reactor

The rotational temperature of nitrogen molecules in inductively coupled plasma (ICP) discharge has been measured using the electron beam fluorescence (EBF) technique. The neutral gas temperature is an important parameter in understanding the energy balance and neutral radical uniformity in plasma processing. The EBF technique has been used to study rarefied flows, but has not been applied previously to characterize a semiconductor plasma reactor. In this work an electron beam was integrated into an inductively coupled semiconductor plasma reactor, and was used to excite neutral nitrogen molecules from the N2X 1Σg+ state into the excited molecular ion N2+B 2Σu+ state. The electron beam excitation process maps the rotational population distribution of the original ground state into the excited molecular ion state following a dipole model with ΔK=±1. The rotational temperature, which is thought to be equivalent to the neutral gas translational temperature under plasma reactor conditions, can be determined from the rotational intensity distribution of the fluorescence spectrum from the N2+B 2Σu+ state. For the range of neutral gas temperatures found in the plasma reactor a dipole model of the excitation-emission process provides good agreement with the measured spectrum. A least-square fitting procedure comparing the model and measured emission spectra is used to determine the rotation temperature. In this article the first EBF measurements of gas heating in an ICP reactor at different gas pressures (20, 50 mTorr) and plasma source input powers (250, 500, and 1000 W) are reported.

Evolution of Controlled Disturbances in the Shock Layer on the Compression Surface

Results of an experimental study of density-wave characteristics in the shock layer are presented for the case of a hypersonic nitrogen flow around a model with a two-dimensional compression surface, which is an arc of a circle, and a sharp leading edge. Controlled periodic disturbances developed on the streaky structure are registered by the electron-beam fluorescence technique. The streaky structure of the type of two vortices rotating in the opposite directions is generated in the shock layer by an oblique gas-dynamic whistle.

Experimental investigation on the chaotic phenomena in the wake of a natural thermal convection flow

Chaotic phenomena in the wake of thermal convection flow fields above a heating flat plate were investigated experimentally. A newly developed electron beam fluorescence technique (EBF) was used to simultaneously measure density fluctuation at 7 points in a cross section above the plate. Correlation dimensions, intermittence coefficients, Fourier spectrum have been obtained for different Grashof numbers. Spatial distribution of correlation dimensions are presented. The experimental result shows that there is a certain relationship between the density fluctuation and the Gr number. And time-spacial characteristic of chaos evolution is also given.

Rotational relaxation time in CO free jets

The electron beam fluorescence technique was applied to measure the rotational energy relaxation cross-sections in carbon monoxide free jets. The data were obtained in the low temperature region from 20 to 200 K and compared with the results of ultrasound, thermal conductivity, free jet and line broadening experiments. The overall scatter of the experimental data runs to one order of magnitude, and their temperature dependence differs. This fact calls for further theoretical and experimental investigations of rotational relaxation rates in carbon monoxide.

Non-intrusive optical diagnostic experiments for high-speed flow generator flowfield characterization

Two non-intrusive optical diagnostic methods, electron beam fluorescence (EBF) and focused schlieren (FS) have been set up and utilized for high-speed flow characterization and visualization of the High-speed Flow Generator (HFG) at NASA LaRC. Using the EBF technique, we were able to visualize and calculate several parameters of the low-density, free jet expansion of N 2 gas through a 5.0 mm nozzle. We also developed a high-sensitivity FS system to see shock structure in front of a spherical barrier and roughly determine the lower limit of its capability to visualize the N 2 flowfield.

The electron beam technique in carbon monoxide

Abstract The electron beam fluorescence technique was developed to measure the rotational level population in the ground state of a carbon monoxide molecule. The data on rotational transition probabilities under ionization–excitation of CO (X 1 Σ + ) into the CO + (B 2 Σ + ) state by high energy electrons are presented, which noticeably differ from those predicted by the dipole excitation model. A flow in a free jet was used to obtain the experimental data in a low temperature rotationally equilibrium gas target. The probability matrix is represented in a compact form corresponding to the sudden approximation model.

Propagation of medium energy electrons (10–20 keV) in carbon dioxide

Propagation of medium energy electrons (10–20 keV) in carbon dioxide is studied for application to using the Electron Beam Fluorescence (EBF) technique with CO2 as a rarefied gas dynamic diagnostic tool. A collimated (1 mm nominal diameter) electron beam at energies of 10, 15, and 20 keV was injected into a vacuum chamber containing ambient CO2 maintained at pressures of 50, 100, 200, 400, 600, and 800 mTorr. Through deconvolution of recorded two-dimensional images of the emitted fluorescence, the growth of the root-mean-square (rms) radius of the beam was obtained as a function of axial distance. An empirical form of the differential scattering cross section which includes the contributions of both elastic and inelastic collisions between fast electrons and CO2 molecules is used in single scattering, multiple scattering, and scattering envelope models developed for describing the electron distribution along the electron beam. Using these models, the rms radius of the electron beam is calculated and compared with experimental results obtained from imaging of the electron beam fluorescence. It is found that the multiple scattering and envelope models significantly overestimate experimental rms radius data. The single scattering model underestimates radius data at target thickness higher than ∼2 and 4 Torr cm for the 10 and 15 keV beams, respectively. The single scattering model matches well with the 20 keV beam data.

Wave Processes in the Hypersonic Shock Layer on a Flat Plate

An electron-beam fluorescence technique is used to investigate the properties of density waves in the shock layer on a flat plate placed in a hypersonic stream (Mx=21) at zero incidence; the Reynolds number based on the longitudinal coordinate and the freestream parameters is Rex=(2.7–3)·105. Transverse profiles of the mean density and the overall and spectral levels of the density fluctuations are obtained, the longitudinal and lateral phase velocities of the waves and the correlation scales are determined, and the longitudinal increments of the waves are derived. The data are compared with the experimental results obtained at Rex=(2.6–7)10·4.

Spectroscopic modeling of the CO2+A2Πu→ X2Πg (110-010) Renner-Teller bands in electron beam fluorescence of carbon dioxide

Improvements have been made to a spectroscopic model of the electron beam fluorescence (EBF) spectra from the CO 2 + A 2 Π u→ X 2 Π g electronic transition of the (100-000), (200-100) and (300-200) bands through the inclusion of the (110-010) Renner-Teller bands. The (010) vibrational state of the CO 2 molecule is thermally excited at temperatures near 300 K based on an analysis of the vibrational partition function for this molecule. The excitation and emission processes for these Renner-Teller bands were modeled with dipole moments. The inclusion of these Renner-Teller bands into the spectroscopic model produces an improved fit to the measured fluorescence spectrum between 337 nm and 339 nm. This improved model provides for a more accurate measurement of the rotational temperature using the EBF technique.

Initial experimental study of pulsed electron beam fluorescence

The pulsed electron beam fluorescence technique for the measurement of gas specie densities along with rotational and vibrational population distributions is being developed for use in high enthalpy, chemically reacting flows. The fluorescence generated by a very high current, short duration pulse of energetic electrons is spectroscopically analyzed sufficiently early in the excitation-emission sequence to minimize the effects of collisional quenching and to avoid the subsequent large energy deposition in the gas caused by the beam's electrons. We present the results of both rotational temperature and gas density measurements in static, room temperature nitrogen at pressures between 5 and 150 mtorr. Using a pulsed electron gun as a source, 150-A electron beams with durations of tens of nanoseconds were used to excite fluorescence from nitrogen's first negative system (N 2 + B 2 Σ u - →N 2 + X 2 Σ g + ). The (0,0) band near 391 nm was observed with a 0.85-m spectrometer for determination of nitrogen rotational temperature and density. The spectrometer's exit slit was modified to enable simultaneous emission intensity measurements in portions of the (0, 0) band's P and R branches.

Flow-field properties under deposition of films from low-density jets

Statistical methods in the direct simulation Monte Carlo procedure and the electron-beam fluorescence technique were applied to investigate the distribution of density and temperature in a monatomic jet. Initial conditions of investigated flows corresponded to ones under deposition of films from low-density jets. In calculations, the variable hard-sphere model for molecular interaction potential and the diffuse reflection model with complete thermal accommodation were used. Distribution of density of Ar atoms in the jet and in the vicinity of the substrate were measured by the spontaneous radiation of Ar i 5912 Å (transition 4p[1/2]←4d[3/2]0,1←1′) and Ar ii 4610 Å (4s′2D←4p′2F0,5/2←7/2) induced by the electrons. The temperature was evaluated with the rotational temperature of N2, a small portion (5%) of which was added into Ar. The quantities of rotational temperature were found by measuring the intensity of the rotational lines of the first negative system N+2 band. The experimental data provide information about profiles of density along and across the center line of a jet over a range of 1×10−3≤Kn*≤4×10−2. On the whole, the computed profiles of density are in good agreement with the measurements. The calculated properties in the vicinity of the substrate allowed the performance of a detailed analysis of the gas-surface interaction process. A good comparison of the computed profile of the flux of molecules, incident on the substrate, and data on the film thickness of a-Si:H were obtained.

Low-density nozzle flow by the direct simulation Monte Carlo and continuum methods

Two different approaches, the direct simulation Monte Carlo (DSMC) method based on molecular gasdynamics, and a finite-volume approximation of the Navier-Stokes equations, which are based on continuum gasdynamics, are employed in the analysis of a low-density gas flow in a small converging-diverging nozzle. The fluid experiences various kinds of flow regimes including continuum, slip, transition, and free-molecular. Results from the two numerical methods are compared with Rothe's experimental data, in which density and rotational temperature variations along the centerline and at various locations inside a low-density nozzle were measured by the electron-beam fluorescence technique. The continuum approach showed good agreement with the experimental data as far as density is concerned. The results from the DSMC method showed good agreement with the experimental data, both in the density and the rotational temperature. It is also shown that the simulation parameters, such as the gas/surface interaction model, the energy exchange model between rotational and translational modes, and the viscosity-temperature exponent, have substantial effects on the results of the DSMC method.

Experimental study of the hypersonic turbulent boundary layer on a cold slender cone

This paper presents measurements of both the mean and time-resolved properties of a naturally turbulent boundary layer on a highly cooled sharp cone in hypersonic flow. The experiments were carried out on a 5 deg half-angle cone at zero angle of attack in a Mach 9.26 flow in a gun tunnel. The unit Reynolds number was 4.4 x lOVm and the wall-to-stagnation temperature ratio was 0.273. Conventional surface and flow field instrumentation, including a new compact fast responding total temperature probe, was used to acquire time- averaged data. Measurements of the mean and the fluctuating density within the boundary layer are included that were obtained using the electron beam fluorescence technique. Comparisons are made between the measured data and several laws applicable to low-Mach-numb er zero-pressure -gradient boundary layer flows. It is shown that measured data are correlated by the van Driest extension to the Crocco relationship and agree with the velocity defect law modified by Fernholz.1 As expected, the mixing length concept is shown to be too simple to predict the density fluctuation correctly for this flow. Fluctuation spectra differ from the result from incompressible flows. The spatial correlation of the fluctuations emphasizes that there is considerable energy in the small eddies.

Gas neutralizer flowfield modeling and experiments

A gas neutralizer internal flowfield is computed using the Navier-Stokes code VNAP2 to assess the code's value as a gas neutralizer modeling tool and to provide design calculations. The steady-state, viscous, continuum field is produced by injecting argon gas radially into a cylindrical tube at the midplane. Inflow conditions are selected in accordance with design requirements to optimize the conversion of ions to neutral particles. Results from the calculations include the velocity field, contours of Mach number and density, axial and radial profiles of density, and axial profiles of the argon target thickness parameter. In addition, measurements inside a V3-scale gas neutralizer model are performed to benchmark the code. Density field measurements are obtained using the electron-beam fluorescence technique, and limited pressure measurements are obtained from wall static pressure taps and a free molecular probe in the tube end. The two-dimensional code predictions are in reasonably good agreement with the three-dimensional experimental results, showing that VNAP2 is useful for future qualitative gas neutralizer design calculations.

Diffusive separation of binary mixtures of CO2–H2 in a sonic-orifice expansion

The separation of binary gas mixtures of carbon dioxide and hydrogen in sonic-orifice-generated free jets is investigated over a source Reynolds number range of 100 to 3000 using the well-known electron beam fluorescence technique. The lighter species, hydrogen, is chosen as the minor species in order to examine the extent of the validity of Sherman’s first-order diffusive separation theory for large separation. The diffusive separation of mixtures containing approximately 2%, 5%, and 20% hydrogen in carbon dioxide is investigated by experimentally determining the relative number densities of the two species as a function of centerline axial distance within the free jets. Sherman’s diffusive separation theory is applied to the jets using axial Mach number profiles which were a combination of experimental and analytical results for γ=1.40 expansions. The γ=1.40 expansion is shown to approximate the CO2–H2 expansion quite well, and the calculated separation agrees remarkably well with the experimentally determined values.

Rotational temperatures in nonequilibrium free jet expansion of nitrogen

Rotational nonequilibrium was investigated in hypersonic free jet expansions of nitrogen using the electron beam fluorescence technique. The results confirm the conclusion of previous investigations that a dipole excitation model, with an assumed Boltzmann energy distribution, is not consistent with the measured line intensities. This discrepancy was examined quantitatively and found to be independent of density and source Knudsen number (except at number densities greater than 1016 cm−3). The effect was attributed to interactions with the ejected (ionized) electron and a new excitation model was developed and shown to be consistent with the measurements throughout the flow conditions explored. The resulting rotational temperatures were compared to a simplified relaxation model of the jet and indicate a rotational collision number of 1.9.

Measurement of translational temperature in nitrogen with the electron beam fluorescence technique

The electron beam fluorescence technique was applied to nitrogen for the purpose of measuring the velocity distribution function in a nonequilibrium free jet expansion of a molecular species. This was accomplished by measuring the Doppler broadening of a single rotational line in the N+2 first negative band with a Fabry–Perot interferometer. It was found that the rotational lines of this band have an inherent line width equivalent to about 70 °K translational temperature. Because of the magnitude of this additional broadening, accurate determination of the translational temperature was not possible at the low temperatures in the free jet. A correction for the additional broadening is proposed which allows application of the technique to flows at higher temperature.