High-speed two-frame shadowgraphy for velocity measurements of laser-induced plasma and shock-wave evolution

Abstract

We describe a high-speed, two-frame shadowgraph method for the two-dimensional visualization of an expanding laser-induced plasma and shock wave in two time instances. The developed experimental method uses a 30 ps, green-laser, polarized pulse for the direct and delayed illumination separated by a variable time delay in the range from 300 ps to 30 ns. Since the exposed images of a single event are captured with two CCD cameras, the established method enables velocity measurements of the fast laser-induced phenomena within the nanosecond excitation-laser pulse as well as at later times-when the excitation-laser radiation has already ended.

Laser-Induced Plasma Measurements Using Nd:YAG Laser and Streak Camera: Timing Considerations

“…Laser-induced breakdown (LIB) is induced by focusing an intense laser beam on a gas, liquid, or solid target. Studying the plasma formation with a high temporal, spectral, and spatial resolution is of a great interest, and the formation of laser-induced breakdown of plasma in air has been studied by many researchers [1][2][3][4][5][6], including the references presented herein.…”

“…After the initial breakdown, plasma plume propagates towards the laser beam focusing lens [1,3]. The bright plasma core of the LIB in the open air is surrounded by a layer of cold, moderately ionized gas called the sheath [1].…”

High-speed photography of shock waves with an adaptive illumination

“…An alternative approach is to modify the illumination part of the illumination-detection visualization system, while, if needed, preserving the benefits of fast-camera acquisition. This is a more affordable approach, because it is often sufficient to capture the object of interest, e.g., a shock wave [11] or a cavitation-bubble boundary [12], multiple times during a single exposure, where it is sufficient to use a simple still camera.…”

“…Different light sources [lasers or laser diodes and lightemitting diodes (LEDs)] and various pulse-generation strategies have been employed for this purpose. Multiple pulses can be generated by splitting a single pulse into multiple pulses and delaying them either with optical fibers of different lengths [13] or by using different free-space paths [11]. These techniques lack adaptability, due to mechanical realization of the intrapulse delays.…”

“…A drawback of using a typical laser source like Q-switched [13] or mode-locked lasers [11,14] as the light source is the inflexibility of the pulse modulation and the speckled, nonhomogeneous spatial profile. The illumination can be switched on and off relatively rapidly with LEDs [3,12,15], but to achieve high-brightness pulses with durations shorter than 100 ns, laser diodes are preferred, especially when unwanted light sources at other wavelengths have to be filtered out.…”