| OCR Text |
Show flyash particles in which temperatures in excess of 2500 K are encountered. This medium is turbulent and represents perhaps the harshest environment in which CARS temperature measurements have been attempted to date. Since the conductivity of the gas stream in the MHD process is a strong function of peak temperature experienced by the alkali-seed (10), it is useful to measure temperature profiles to accurately model the performance of an MHD generator. This consideration precludes the use of more conventional line-of-sight optical temperature techniques (i.e. , sodium line reversal) and requires a more sophisticated technique such as CARS capable of point measurements. Several excellent reviews of the theory and application of CARS are available in the literature (11, 12, 13) and only a brief discussion is presented here. CARS is a non-linear optical process so described because theoretically the signal is created through the third order susceptibility tensor which is the coefficient of the cubic term in the electric field expansion of the dipole moment density of a medium. It is an example of four-wave mixing in which three photons, usually two at a pump frequency OK and one at a Stokes frequency co9 mix through the susceptibility to produce a fourth or anti-Stokes photon at frequency o)„. The intensity of the anti-Stokes beam 1^ can be written where 1^ and I2 are the pump and Stokes laser beam intensities, respectively, I is the length in which these beams coherently interact, X is the third order susceptibility of the medium which consists of a frequency dependent resonant part, X + iX , and a non-resonant part, Xnr and c is the speed of light. When the frequency difference 00, - co? approaches the frequency of a Raman activity energy resonance of a major species in the bulk media, the resor-pnt susceptibility becomes large compared to the non-resonant susceptibility. Because the intensity depends on the absolute value squared of the total susceptibility, the spectra appears as a resonant modulation on a non-resonant background. The resonant susceptibility is a function of the number density and 3 , 2 4TT (a. I2 I ,2 2 xl I (1) |
