High-spectral resolution PLIF imaging of compressible flows and plasmas

Pulsed energy deposition using laser is an effective method of local flow control. Therefore, careful investigation of the laser-induced plasma and the resulting flow field is required. Although some properties of the resulting plasma and flow field have been reported, there is still a need for detailed investigation of the flow field. We report the development of high-spectral-resolution planar laser induced fluorescence (PLIF) of nitric oxide (NO) for simultaneous measurement of pressure, temperature, and velocity in compressible flow field generated by laser-induced optical breakdown. This molecular-based technique eliminates the difficulties associated with particle lag related to traditionally used particle-based techniques such as particle image velocimetry. An optical parametric oscillator (OPO) coupled with pulsed dye amplifiers (PDA) was used as a radiation source to excite electronic transitions of NO in the A-X (0, 0) system. The output from injection-seeded OPO near 452 nm was amplified to ∼13 mJ/pulse using two PDA stages, while maintaining the spectral linewidth of ∼250 MHz full-width-half-maximum (FWHM). The OPO/PDA output was frequency-doubled and a laser sheet of the 226-nm UV light was produced to perform PLIF imaging in a plasma generated by focusing the second harmonic output (∼532 nm) of another injection-seeded Nd:YAG laser. The spectrally narrow output from the OPO/PDA system gives the unique capability for simultaneous measurement of pressure, temperature, and velocity over a wide range of conditions in the investigated flow field.