Heating structure of laser-supported detonation and its oscillation motion using half self-emission half shadowgraph visualization

Summary form only given. Understanding laser absorption and its heating structure is a key to improving the performance of myriad applications, especially laser propulsion thruster. The laser heating structure is known to ZND model in the same manner as chemical detonation. Much effort has been spent on models for Raizer´s theory. However, experimental investigation on previous models has not yet been explained completely. To investigate the heating structure of laser-supported detonation (LSD), half self-emission and half shadowgraph (HSHS) visualization provides the self-emission image from the plasma on the top half and the shadowgraph image of the induced shock wave on the bottom half, simultaneously. Spectroscopic study enables to estimate the laser absorption depth I_ab behind the shock wave using inverse Bremsstrahlung coefficients. A TEA CO₂ laser was used at 7 and 10 J incident energy. The location of both the shock front (S-front) and ionization wave front (I-front) were detected from the brightness distribution of the HSHS images with spatial resolution of 62.5 μm per pixel and temporal resolution of 30 ns. Results reveal that the shock wave propagates in the heating layer during LSD regime. The propagation of I-front precedes that of S-front by the order of 10^-4 m in both laser energy cases. From spectroscopic study, the depth I_ab behind the S-front is by the order of 10^-4 m during LSD regime. Furthermore, the distance between I-front and S-front has an oscillating motion. Amplitude of oscillation decreases as time goes by. Similar behavior has been observed in chemical detonation wave front.