Laser induced fluorescence spectroscopy in flames

A diagnostic has been developed to measure velocity and translational temperature in the plume of a 1-kW-class arcjet thruster operating on hydrogen. Laser-induced fluorescence with a narrow-band cw laser is used to probe the Balmer α transition of excited atomic hydrogen. The velocity is determined from the Doppler shift of the fluorescence excitation spectrum, whereas the temperature is inferred from the lineshape. Analysis shows that although Doppler broadening is the only significant broadening mechanism, the fine structure of the transition must be taken into account. Near the exit plane, axial velocities vary from 4 to 14 km/s, radial velocities vary from 0 to 4 km/s, and swirl velocities are shown to be relatively small. Temperatures from 1000 to 5000 K indicate high dissociation fractions.

Laser-induced fluorescence diagnostic for temperature and velocity measurements in a hydrogen arcjet plume

A particle-based Monte Carlo numerical method is developed for computation of flow in a low-thrust hydrogen arcjet. The method employs the direct simulation Monte Carlo technique to compute the nonequilibrium fluid mechanics and thermochemical relaxation. Simulation of plasma effects is included at a simple level that ensures charge neutrality. A new model is developed to include Ohmic heating in the simulation. Results are presented for a flow condition that has been studied experimentally with a number of different diagnostic techniques. A strong degree of thermal nonequilibrium is found in the flow. It is demonstrated that Ohmic heating is important, and that the Monte Carlo model captures the important physics very well. The predicted results are in very good agreement with most of the experimental data.

Monte Carlo simulation of nonequilibrium flow in a low-power hydrogen arcjet

We report on quantitative, spatially resolved density, temperature, and velocity measurements on ground-state atomic hydrogen in an expanding thermal Ar–H plasma using two-photon excitation laser-induced fluorescence (LIF). The method’s diagnostic value for application in this plasma is assessed by identifying and evaluating the possibly disturbing factors on the interpretation of the LIF signal in terms of density, temperature, and velocity. In order to obtain quantitative density numbers, the LIF setup is calibrated for H measurements using two different methods. A commonly applied calibration method, in which the LIF signal from a, by titration, known amount of H generated by a flow-tube reactor is used as a reference, is compared to a rather new calibration method, in which the H density in the plasma jet is derived from a measurement of the two-photon LIF signal generated from krypton at a well-known pressure, using a known Kr to H detection sensitivity ratio. The two methods yield nearly the same result, which validates the new H density calibration. Gauging the new “rare gas method” by the “flow-tube reactor method,” we find a krypton to hydrogen two-photon excitation cross section ratio σKr(2)/σH(2) of 0.56, close to the reported value of 0.62. Since the H density calibration via two-photon LIF of krypton is experimentally far more easy than the one using a flow-tube reactor, it is foreseen that the “rare gas method” will become the method of choice in two-photon LIF experiments. The current two-photon LIF detection limit for H in the Ar–H plasma jet is 1015 m−3. The accuracy of the density measurements depends on the accuracy of the calibration, which is currently limited to 33%. The reproducibility depends on the signal-to-noise (S/N) ratio in the LIF measurements and is orders of magnitude better. The accuracy in the temperature determination also depends on the S/N ratio of the LIF signal and on the ratio between the Doppler-width of the transition and the linewidth of the excitation laser. Due to the small H mass, the current linewidth of the UV laser radiation is never the accuracy limiting factor in the H temperature determination, even not at room temperature. Quantitative velocity numbers are obtained by measuring the Doppler shift in the H two-photon excitation spectrum. Both the radial and axial velocity components are obtained by applying a perpendicular and an antiparallel excitation configuration, respectively. The required laser frequency calibration is accomplished by simultaneously recording the I2 absorption spectrum with the fundamental frequency component of the laser system. This method, which is well-established in spectroscopic applications, enables us to achieve a relative accuracy in the transition frequency measurement below 10−6, corresponding to an accuracy in the velocity of approximately 200 m/s. This accuracy is nearly laser linewidth limited.

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Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma

DECONVOLUTION OF ION VELOCITY DISTRIBUTIONS FROM LASER-INDUCED FLUORESCENCE SPECTRA OF XENON ELECTROSTATIC THRUSTER PLUMES by Timothy B. Smith Chairperson: Associate Professor A.D. Gallimore This thesis presents a method for extracting singly-ionized xenon (Xe II) velocity distribution estimates from single-point laser-induced fluorescence (LIF) spectra at 605.1 nm. Unlike currently-popular curve-fitting methods for extracting bulk velocity and temperature data from LIF spectra, this method makes no assumptions about the velocity distribution, and thus remains valid for non-equilibrium and counterstreaming plasmas. The well-established hyperfine structure and lifetime of the 5dD7=2 6pP 0 5=2 transition of Xe II provide the computational basis for a Fourier-transform deconvolution. Computational studies of three candidate deconvolution methods show that, in the absence of a priori knowledge of the power spectra of the velocity distribution and noise function, a Gaussian inverse filter provides an optimal balance between noise amplification and filter broadening. 1 Deconvolution of axial-injection and multiplex LIF spectra from the P5 Hall thruster plume yields near-field and far-field axial velocity distributions. Near-field LIF spectra provide velocity distributions that cannot be measured by probe-based methods, while far-field LIF spectra provide a basis for comparison with mass spectrometer data. Transforming far-field ion axial velocity distributions to an ion energy basis reproduces all Xe II features found in mass spectrometer data taken at the same location and conditions. Axial profiles of ion axial velocity show a zone of increasing velocity extending 20 cm downstream of the thruster exit plane, with decreasing velocity from 20 to 50 cm, and demonstrate repeatabilities within 2%. Vertical-beam LIF reveals unexpectedly strong interactions between counterflowing streams in the inward divergence region at the thruster centerline. Deconvolution of multiplex LIF spectra taken from the FMT-2 ion engine plume provides beamwise velocity distributions from 1.4 mm to 30 cm. Axial profiles of axial velocity fail to disclose the location of the neutralization plane, while radial sweeps of axial velocity show no discernable trend. Radial profiles of radial velocity show increasing divergence with radial position.

Deconvolution of ion velocity distributions from laser-induced fluorescence spectra of xenon electrostatic thruster plumes

A diagnostic has been developed to measure velocity and translational temperature in the plume of an arcjet thruster. Laser induced fluorescence with a narrowband CW laser is used to probe the Balmer alpha transition of excited atomic hydrogen. The velocity is determined from the Doppler shift of the fluorescence excitation spectrum while temperature is inferred from its shape. Analysis shows that while Doppler broadening is the only significant broadening mechanism, the fine structure of the transition must be accounted for. Near the exit plane, axial velocities vary from 4 to 14 km/s; radial velocities vary from 0 to 4 km/s; and swirl velocities are shown to be relatively small. Temperatures from 1000 to 5000 K indicate high dissociation fractions.

Flow diagnostics of an arcjet using laser-induced fluorescence

This paper describes an experimental study of the plasma plume properties of a 1 kW class hydrogen arcjet thruster and the comparison of measured temperature and velocity field to model predictions. The experiments are based on laser-induced fluorescence excitation of the Balmer-alpha transition. The model is based on a single-fluid magnetohydrodynamic description of the flow originally developed to predict arcjet thruster performance. Excellent agreement between model predictions and experimental velocity is found, despite the complex nature of the flow. Measured and predicted exit plane temperatures are in disagreement by as much as 2000K over a range of operating conditions. The possible sources for this discrepancy are discussed.

A comparison of arcjet plume properties to model predictions

A new diagnostic developed to measure the axial velocity of atomic hydrogen in a 1-kW hydrogen-fueled arcjet thruster is reported. The technique is based on laser-induced fluorescence of the Balmer alpha transition (656 nm) in atomic hydrogen. A narrow-band CW ring dye laser scans the excitation spectrum permitting accurate determination of the absorption line shape and position. The velocity is derived from the Doppler-shifted line position. A spatially resolved velocity profile is presented with a peak velocity of 12.8 km/s at the arcjet centerline near the exit plane.

Velocity measurements in a hydrogen arcjet using LIF

A simple approach, for the purpose of evaluating slip in a hydrogen-fueled arcjet, is to seed the flow with a species that is accessible with the same laser used to probe atomic hydrogen.In the present study,helium was chosen as the seed species owing to its inertness,its relative mass and its convenient electronic transitions in the visible wavelength region.Velocity and temperature are measured by LIF of both helium and atomic hydrogen .Absence of slip between helium and atomic hydrogen would suggest that slip is not a dominant mechanism in the nozzle or exit plane vicinity of our hydrogen arcjet.

John G. Liebeskind

Papers

Laser-induced fluorescence diagnostic for temperature and velocity measurements in a hydrogen arcjet plume

Flow diagnostics of an arcjet using laser-induced fluorescence

A comparison of arcjet plume properties to model predictions

Velocity measurements in a hydrogen arcjet using LIF

Experimental investigation of velocity slip near an arcjet exit plane