R. A. Hill

Focused, Multiple-Pass Cell for Raman Scattering

A simple optical system is described that makes use of a unique property of ellipsoidal mirrors, viz., light brought to one focus will be reflected alternately through the two foci and collapse to the major axis. This system consists of an on-axis ellipsoidal mirror facing a coaxial flat-spherical mirror assembly that is positioned at the minor axis. Calculations indicate that gains of the order of 500 in the light flux at the point of observation should be attainable with low-eccentricity ellipsoids. Raman-scattered light from atmospheric N(2) was obtained with a system employing a 0.2 eccentricity ellipsoid. An experimental gain of 93 was determined by the ratio of the scattering with the system to the scattering obtained with one beam. This result is in good agreement with the theory.

Retroreflecting multipass cell for Raman scattering

A retroreflecting multipass cell consisting of two lenses and both on-axis and off-axis retroreflecting mirror assemblies has been constructed and tested. A gain in Raman scattered signal intensity of 20 has been attained in a focal volume 1.1 mm x 0.3 mm x 0.3 mm. A system employing off-axis paraboloids should provide somewhat higher gain and a diffraction limited focal volume. These systems are applicable to Raman diagnostics of various gas dynamic processes, including flame gases, or the characterization of the gas mixing process in gas-dynamic or chemical laser nozzle arrays.

Ignition-delay times in laser initiated combustion

A single pulse from a TEA CO(2) laser is used to heat 1:7:14 mixtures of SF(6):CH(4):O(2) to temperatures near 1000 K. A short- or long-duration pulse (one-half the energy deposited in 0.25 or 0.82 microsec, respectively) from a second TEA CO(2) laser is used to ignite the mixture. At comparable values of absorbed energy from the second laser, ignition-delay times for the long-duration secondary pulse are approximately twice those for the short-duration pulse. Ignition of the hot mixture requires about 10% less absorbed energy with the short-duration pulse than with the long-duration pulse. These results indicate the short-duration pulse is more effective in producing a high population density of reactive species that initiate the reactions necessary for ignition.

Pulsed spontaneous Raman scattering technique for luminous environments.

A gated photon counting system, cavity-dumped argon-ion laser, and a multipass retroreflecting light cell have been combined in a system to enhance spontaneous Raman scattering in luminous background situations. Signal-to-background ratio (SBR) has been improved a factor of 450 over a single-pass cw system with cw power equal to the pulsed average power. The improvement attributable to the gating and cavity dumping the laser is a factor of 30. A factor of 15 improvement is due to the retroreflecting light cell. Temperature and density data obtained from N(2)Q-branch spectra of a luminous methane-air flame are presented to illustrate the utility of the system. A passband fitting technique is introduced to analyze the data.

Raman spectroscopic study of a laminar hydrogen diffusion flame in air

Abstract Spontaneous Raman spectroscopy has been employed for time-averaged, spatially-resolved measurements of temperature and species concentration in an axisymmetric, laminar hydrogen diffusion flame in quiescent air. Temperatures were obtained from vibrational Q-branch raman spectra of N2, O2, and H2 and the rotational Raman spectra of N2 and H2, and concentrations of H2, and N2 were determined. The results are compared to existing numerical nonequilibrium calculations for the conditions of this experiment. Significant differences between experimental and predicted temperature and concentration profiles are observed. In particular, the flame is larger in both diameter and length and the flame zone is thicker than predicted. Some possible sources of the discrepancies are discussed.

TEMPERATURES FROM ROTATIONAL-VIBRATIONAL RAMAN Q-BRANCHES

Abstract An iterative, least-squares analysis scheme, based on the differential of the intensity, dI = dτ∂I/∂τ + dϱ ∂I/∂ϱ has been developed for obtaining the temperature θ = 1/τ and density ϱ from a set of rotational-vibrational Raman, Q-branch intensities. Spectral locations that cover the maximum variation in ∂I/∂τ were determined from a set of computed intensities and derivatives. The results for the Stokes (anti-Stokes) bands indicate for temperatures less than 2100K (2700K) that the maximum extent of ∂I/∂τ can be spanned by recording intensities only in the first two vibrational bands, v(1-0) and v(2-1) and, due to the multivalued nature of ∂I/∂τ, equally spaced values for ∂I/∂τ can be obtained only by programming the spectrometer to select an appropriate set of non-uniformly spaced passbands.

A Rapid Scan Spectrograph for Plasma Spectroscopy

An ultrarapid scanning spectrograph, having a spectral scan speed of from 10 to 200 A/μsec and a scanning range of 210 A, is described. This spectrograph was employed to obtain time-resolved Hα, Hβ, and Hγ line profiles, and line-to-continuum ratios, in a shock-heated hydrogen plasma. Electron densities were determined at various times behind the shock front from the Stark-broadened lines and the plasma temperature was obtained from the line-to-continuum ratio. The electron densities obtained from the three lines were found to compare remarkably well. In addition, it was possible to utilize the Saha equation and correlate the time-behavior of the electron density with the temperature and atom density measurements.

A New Plane Grating Monochromator with Off-Axis Paraboloids and Curved Slits

A design for a new plane grating monochromator is described. The system employs a symmetrical configuration of off-axis paraboloids for both the collimator and camera mirrors. The entrance and exit slits are mounted on the same side of the grating and are curved in order to eliminate wavelength errors due to spectral line curvature. There is, in fact, a remarkable coincidence between the curvature which is required to avoid wavelength errors and the curvature which is required to obtain equal curvatures in the object and image planes. Ray tracing calculations for a symmetrical configuration of off-axis paraboloids show that the image of a curved slit with a length equal to 0.05 times the focal length of an f/5 mirror is diffraction limited for wavelengths lambda > 0.2 . micro.

RAPID SCAN SPECTROMETERS FOR THE DIAGNOSTICS OF TRANSIENT PLASMAS.

An observation that a symmetrical configuration of off-axis paraboloidal mirrors produces an excellent image of an extended slit, while an unsymmetrical configuration produces poor images, has led to a modification of the original Sandia rapid scan spectrometer and to the design of a new wide range, rapid scan spectrometer. The new instrument employs a high speed rotating mirror in a Littrow arrangement to obtain a spectral scan speed double that of the original spectrometer; e.g., with a 600-lines/mm grating, a spectral scan speed of ~410 A/microsec is obtained at 1000 rps. Techniques for obtaining an accurate calibration of the spectral scan speed are described, and some examples of spectra which were recorded from a xenon and a hydrogen plasma are shown.

A Multiple-Scan, Rapid-Scan Spectrograph for Electron Density Measurements in Transient Plasmas

Our rapid-scan spectrograph has been modified to provide a means of recording many time-resolved profiles of one emission line from a transient plasma. This is accomplished by scanning an emission line across an array of closely spaced exit slits. Post-optics have been designed which direct the radiation from each slit to the same photomultiplier tube. At a spectral scan speed of 120 A/μsec, an emission line profile has been scanned every 1.4 μsec; however, with closer spaced slits, this time could be easily halved. This device has been used to obtain Stark-broadened Hα and Hβ line profiles from hydrogen heated in an impulse tube. Electron densities obtained by fitting the line wings of Hα compare very well with values obtained from the half-width of Hβ.