EMISSION AND HOLOGRAPHIC-INTERFEROMETRY MEASUREMENTS IN A SUPERORBITAL EXPANSION TUBE

Free-piston-driven expansion tubes are capable of generating flaw cond itions over a wide range of enthalpies ranging from orbital up to supe rorbital velocities. Initial optical measurements aimed at investigati ng the flow in such a facility are presented. Emission studies were us ed to identify impurities in the how and to investigate spectral regio ns that are accessible by optical techniques. At moderate enthalpies, it was found that significant radiation resulted from metallic contami nants. At high enthalpies, the spectrum consisted of a number of atomi c lines together with a broadband background component indicative of t he presence of electrons. The presence of this radiation may limit the applicability of optical techniques that require spectral regions fre e from the influence of atomic transitions or background radiation. Em ission spectroscopy (through Stark broadened hydrogen lines) and two-w avelength holographic interferometry were used to measure the electron number density behind a bow shock on a blunt body at conditions where significant ionization was observed. They yielded average concentrati ons of (3 +/- 1) x 10(17) cm(-3) from the emission measurements and (3 .8 +/- 0.6) x 10(17) cm(-3) from the interferometry.