Precursor Ionization and Propagation Velocity of a Laser-Absorption Wave in 1.053 and 10.6- $\mu{\rm m}$ Wavelengths Laser Radiation

A propagation model of a laser-absorption wave was proposed and validated using the measured propagation velocity. The model describes the propagation mechanism in terms of avalanche ionization through an inverse Bremsstrahlung process and photoionization by UV radiation from bulk plasma behind the wave. Using plasma spectroscopy, the electron temperature and density at the head of laser-absorption wave were estimated as 2 eV and (1.5-2.6) × 10
²⁴
m
^-3
, respectively, at 10.6-μm laser wavelength and 5 eV and (2.6-3.3) × 10
²⁴
m
^-3
at 1.05 μm when the laser intensity was near the laser-supported detonation threshold in the air and argon atmosphere. Using the measured plasma properties, we estimated UV photon flux radiated by the Bremsstrahlung, which contributes the photoionization ahead of the laser-absorption wave. The resulting propagation velocity of the laser-absorption wave was 10
³
m/s, which showed good agreement with the velocity measured using a high-speed camera.