Temperature measurements in a decaying laser-induced plasma in air at elevated pressures

This article discusses two measurement techniques for temperature determination of laser-induced plasmas in a gas at pressures relevant for combustion engines. Plasmas induced by laser breakdown in air at initial pressures ranging from 0.3 MPa to 2.5 MPa are investigated using optical spectroscopy. Results for 0.8 MPa, 1.2 MPa and 1.6 MPa are reported here. Due to the elevated pressure, a significant contribution from continuum radiation is apparent. The first temperature measurement technique relies on the interpretation of the continuum emission. The second technique is based on the line emissions from different elements and ionization stages in the plasma and is implemented with the multi-element Saha-Boltzmann plot method. The methodology may be applicable for temperature measurements under various conditions, e.g., for plasmas in high pressure gas environments such as in industrial applications of laser-induced breakdown spectroscopy or for plasma sources for illumination purposes. We investigate optimizations of laser-induced spark ignition. The energy released in the laser-induced plasma is determined based on temperature measurements.