Space and time analysis of the N2 vibrational non-equilibrium in the N2 and air nanosecond discharge afterglow
2019 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Knowledge of the aero-optical effects accompanying hypersonic flight conditions is of critical importance to ensure reliable operation of optical-based instrumentation and air date system. However, such effects dependent on the scalar polarizability are scarcely known in literatures under the thermal non-equilibrium and molecule internal excitation conditions. In this paper, nanosecond pulsed discharges in N2 and air at a low-pressure condition (i.e. 90 Torr) have been created to reproduce such non-equilibrium condition and to provide a test bed for quantitative study the scalar polarizability under non-equilibrium. Spontaneous Raman spectroscopy (SRS) and optical emission spectroscopy (OES) have been used to investigate the non-equilibrium process in the discharge afterglow. The rotational and vibrational temperatures of both the ground state N2 (X2 g+)and the electronically excited state N2 (C2 ) are measured by the spectral fitting and the Boltzmann plot. The ratio between the first-and higher-level vibrational temperatures of the ground state has been used to investigate the nitrogen vibrational non-equilibrium as the function of space and time. Meanwhile, the averaged vibrational quanta have been applied to understand the energy loading and transfer in the ground state vibration modes. The electronically excited state provides an extra path to study the energy transfer among states through the e-V, V-V, and V-T processes at different timescales. The work provides the scheme to calibrate the non-equilibrium states which have been further used to measure the scalar polarizability under the same condition and study the dependence of the polarizability on the gas temperature as well as the vibrational non-equilibrium states.