Measurements of N$_{\mathrm{2}}$(A$^{\mathrm{3}}\Sigma _{\mathrm{u}}^{\mathrm{+}}$,v$=$0) Populations in a Nonequilibrium Supersonic Flow Wind Tunnel

Absolute time-resolved population of N$_{\mathrm{2}}$(A$^{\mathrm{3}}\Sigma _{\mathrm{u}}^{\mathrm{+}}$,v$=$0) electronic state has been measured in the plenum of a Mach 5 blowdown wind tunnel by Tunable Diode Laser Spectroscopy (TDLAS). N$_{\mathrm{2}}$(A) is generated in the plenum of the wind tunnel using a ns pulse generator (30 kV, FWHM 10 ns), operated at a pulse repetition rate from 4 to 100 kHz. The wind tunnel is operated at the plenum pressure of P$_{\mathrm{0}}=$227 Torr, with the flow expanding to a static pressure of P$=$1.15 Torr in the test section, corresponding to the Mach number of 4.2. During the run, the laser wavelength is tuned to the peak absorption of the overlapping transitions Q$_{\mathrm{1}}$(18) and Q$_{\mathrm{3}}$(8) in the N$_{\mathrm{2}}$(B,v$=$2þþ$\leftarrow $A,v$=$0) band, at 771.417 nm, and the absorption signal is measured during the discharge burst and in the afterglow. At 4 kHz pulse repetition rate, N$_{\mathrm{2}}$(A,v$=$0) population peaks at 1.6x10$^{\mathrm{13}}$ cm$^{\mathrm{-3}}$ after each discharge pulse, , and decays between the pulses almost completely. At 100 kHz pulse repetition rate, N$_{\mathrm{2}}$(A,v$=$0) population increases during the first 5 pulses, peaking at 4x10$^{\mathrm{13}}$ cm$^{\mathrm{-3}}$, and then begins to decay before leveling off after 100 pulses. Comparison with kinetic modeling is expected to provide insight into the mechanism of N$_{\mathrm{2}}$(A) excitation and decay at these conditions.