Extending LaITER Molecular Tagging Velocimetry to Subatmospheric Pressure

The potential application of laser ionization tagging and emission reexcitation (LaITER) at subatmospheric pressures relevant to supersonic and hypersonic flows is explored in this study. Gated imaging of emissions from laser-microwave interaction in low pressure cell was conducted. Air and N2 were investigated at room temperature and pressures of 1, 5, and 10 kPa. A femtosecond laser produced long-lived ionization in the gas cell that was re-excited by the time-delayed microwave pulse, leading to emission. At 1 kPa, laser-microwave interaction was observed for over 500 μs, whereas at 5 kPa, it was observed for 100 μs. An optimal reduced electric field was shown to lead to excitation of laser tag while inhibiting plasma growth. Decreasing the electric field strength of the microwave pulse inhibited plasma growth, and increasing the pulse width of the microwave field lowered the threshold of electric field strength required for re-excitation.