Preionization layer and its contribution to advancement mechanism of laser-produced plasma

Summary form only given. Laser-induced plasma in a gaseous form has attracted great interest for use in such new devices as laser propulsion. In the laser-supported detonation (LSD) regime, the energy conversion achieves higher efficiency in the initial stage rather than after LSD termination. Understanding LSD regime transitions and its propagation mechanism is a key to improving the performance of propulsion system. Previous study shows that pre-ionization layer plays an important role in laser-supported detonation wave propagation. Since the mean free path of photon contributing photoionization of O₂ (in air) is much longer than that of electron and Debye length, photoionization due to photon emission is the dominant process in generating precursor-electrons ahead of shock wave. These electrons might be a source of avalanche ionization. The theoretical predictions of cascade ionization in laser-induced plasma have been verified by experiments and simulations. Much effort has been spent on models for the breakdown threshold. The advancement mechanism of optical detonation wave has not yet been investigated. The present study investigated on the advancement mechanism of LSD, experimentally and simply theoretically, in the same manner as in microwave and streamer discharge. The LSD propagation velocity was taken by laser-shadowgraph images. Plasma parameters were investigated by emission spectroscopy. Theoretical velocity was estimated using the net ionization frequency and mean free path of photons. The net ionization frequency considers the cascade ionization, photo-ionization by UV emission, attachment, and diffusion from heating layer. As a result, the number of UV photon emission from plasma is 10¹⁵ m³, assuming Bremsstrahlung radiation is dominant. The ionization frequency is in the order of 10^-7 s, both experimentally and theoretically. Results reveal that the thickness of pre-ionization layer corresponds to the or- er of 10^-4 m as the mean free path of photo at O₂ ionization threshold.