Influence on Sensitivity Enhancement of Expansion Tube Window in Multi-Pass Laser Absorption Spectroscopy and Time-Resolved Flow Diagnostics

Abstract

Although expansion tubes have been used to simulate the atmospheric entry environment, the flow conditions have not been completely characterized. In our previous research, the sensitivity of laser absorption spectroscopy (LAS) was not high enough to diagnose ISAS/JAXA expansion tube flows. In this study, a multi-pass cell was formed to increase the sensitivity of LAS. Since the mirrors were set outside of the chamber, the window displacement by the shock arrival was found to decrease the sensitivity. From the ray trace calculation, the window angle should be less than 2.4 degree to keep the sensitivity enhancement of 44 times. As a result of multi-pass LAS diagnostics using the oxygen molecule line at 763.43 nm, it was found that the operation time of the expansion tube was 55 us, and the translation temperature was ranged from 4,400 K to 6,000 K.