Temperature And Pressure Dependence Of Signal Amplitudes For Electrostriction Laser Induced Thermal Acoustics

The relative signal strength of electrostriction-only (no thermal grating) laser-induced thermal acoustics (LITA) in gas-phase air is reported as a function of temperature T and pressure P. Measurements were made in the free stream of a variable Mach number supersonic wind tunnel, where T and P are varied simultaneously as Mach number is varied. Using optical heterodyning, the measured signal amplitude (related to the optical reflectivity of the acoustic grating) was averaged for each of 11 flow conditions and compared to the expected theoretical dependence of a pure-electrostriction LITA process, where the signal is proportional to the square root of [P*P /( T*T*T)]. Herring, Gregory C. Langley Research Center NASA/TM-2015-218762, L-20562, NF1676L-21439

Laser-induced thermal acoustics (LITA) is a promising optical four- wave mixing technique for gasdynamic measurement. The X(3) nonlinear process is a sequence of two opto-acoustic effects, electrostriction and absorption/rapid- thermalization, and the acousto-optic effect. The evolution of the laser-induced acoustic structures temporally modulates X(3) and thereby the LITA signal. Time resolution of the signal provides the sound speed, thermal diffusivity, and acoustic damping rate, along with information about atomic or molecular energy transfer rates. LITA can also measure spectra of both the real and imaginary gas susceptibility. The physics of LITA is discussed and the derivation is sketched of a simple analytical expression that accurately describes both the magnitude and time history of the LITA signal. Early experimental results are presented. Sound speeds accurate to 0.5% and transport properties accurate to 30% have been measured in a single-shot without calibration. More realistic modeling should dramatically improve transport-property measurement. LITA spectra have been taken of weak spectral lines of NO2 in concentrations less than 50 ppb. Signal reflectivities as high as 0.0001 have been estimated. New applications of LITA, including velocimetry, are suggested. LITA, Four-wave mixing, Thermal grating, Single-shot measurement, Velocimetry.

The Fifth International Conference on Laser Spectroscopy or VICOLS, was held at Jasper Park Lodge, in Jasper, Canada, June 29 to July 3, 1981. Following the tradition of the previous conferences in Vail, Megeve, Jackson Lake, and Rottach-Egern, it was hoped that VICOLS would provide an opportunity for act ive scientists to meet in an informal atmosphere for discussions of recent developments and applications in laser spectroscopy. The excellent conference facilities and remote location of Jasper Park Lodge in the heart of the Canadian Rockies, amply fulfilled these expectations. The conference was truly international, with 230 scientists from 19 countries participating. The busy program of invited talks lasted four days, with two evening sessions, one a panel discussion on Rydberg state spectro scopy, the other a lively poster session of approximately 60 post-deadline papers. We wish to thank all of the participants for their outstanding contribu tions and for preparation of their papers, now available to a wider audience. Our thanks go to the members of the International Steering Committee for their suggestions and recommendations. We are especially pleased to have held this conference under the auspices of the International Union of Pure and Applied Physics. VICOLS would not have been possible with out the financial support of the Natural Sciences and Engineering Research Council of Canada, and the Office of Naval Research and Air Force Office of Scientific Research of the United States* of America.