Novel calibration-free seedless velocimetry using laser-induced shockwave

Abstract

Shockwave propagation from focused-laser-induced plasmas was characterized for implementing calibration-free seedless velocimetry. An emission-free probe beam deflection (EF-PBD) technique and short-gated shadowgraph/schlieren imaging were used for the investigation. Nanosecond laser pulses at 532 nm were focused to induce plasmas generating shockwaves from the focal point at atmospheric condition. A continuous-wave (CW) laser beam was set up to pass nearby for detecting the shock arrival at the beam location; the shock diverts the CW-laser beam, which is sensed by the photodiode. The EF-PBD was devised to minimize the influence of the plasma emission that affects the probe beam intensity measured by the photodiode. The probe beam was moved from the focal point to consecutive locations very close to the plasma (every 0.2 mm from 0 to 1.8 mm) for accurately measuring the shock speed in the region adjacent to the plasma. A point-explosion model was employed for predicting the shockwave propagation phenomenon. It was found that the ambient flow velocity around the plasma affects the shockwave arrival time on the probe beam, which suggests the feasibility of a seedless velocimetry technique. When the distance between the probe beam and the plasma is known, the velocimetry does not require calibration. The proposed velocimetry technique was tested successfully in a range of 30–180 m/s ambient flow.