High-speed time-resolved visualisation of laser-induced plasma explosions

The early evolution of laser-induced plasma explosions has been investigated by means of a high-speed time-resolved schlieren visualisation. Images were obtained with a high-speed video camera yielding frame rates of up to 1 million frames per second at a frame resolution of 312 by 260 pixels. With this setup it was possible to resolve the temporal development of the ionised plasma kernel and its associated shock wave. The plasma is formed by focusing a pulsed ruby laser beam, with pulse energies of up to 4.5 J. The time-resolved visual data have been used to yield shock speeds, from which, together with direct energy measurements, one can determine the portion of energy released by the plasma explosion to drive the shock. Shock sphericity as well as plasma growth and emission lifetimes have also been evaluated. The location of longest emission lifetime was found to change as a function of laser pulse energy: for high energy pulses, the longest-living plasma luminosity was located ahead of the focal spot, i.e. closer to the laser source, while with lower energy pulses the longest-living plasma luminosity was located behind the focal spot. This behaviour was also observed for double-pulsed plasma explosions, when a second laser pulse was generated with a delay time of 50 μs. The experiments show that for single pulses, more than 50 percent of the laser energy is expended in generating the shock wave.