Laser-induced fluorescence measurements of velocity in arc jet flows

Abstract

We have been working on a technique for the measurement of critical flowfield properties in arc jets using laser-induced fluorescence, which makes use of a nascent seed particle, atomic copper. The copper atoms are produced via sputtering from the arc anode. Our technique is based on Doppler-shift methods for the determination of velocity and turbulence and Doppler absorption widths for the measurement of temperature. The technique uses absorption from the ground s1/2 to the excited P1/2 state of copper at 327.5 nm. Fluorescence is observed on the P1/2 - D3/2 transition at 578 nm. We have used both CW ring dye lasers and narrow bandwith pulsed dye lasers to characterize the copper atom absorption lineshape and measure velocity in a supersonic free jet of atomic copper seeded in helium. The shift in the absorption spectra obtained with the cw ring dye laser gives the freestream flow velocity, and the Doppler width gives the velocity distribution. When probed with a frequency doubled, pulsed Nd:YAG pumped dye laser, the absorption lineshape is now determined by the laser profile, but the Doppler shift due to the flow is identical (within error limits) to the shift obtained in the cw laser measurements. The same experiments performed with the pulsed laser have now been replicated at the NASA/JSC 10 MW arc driven test facility. Doppler shift measurements have been performed in the facility as a function of specific anthalpy. An asymptotic behavior of the velocity as a function of specific anthalpy is obtained due to the increasing degree of molecular oxygen dissociation at higher anthalpies.