Quantitative two dimensional shadowgraphy for low density plasma characterization

Summary form only given. In plasma physics, shadowgraphy has always been considered as an important but a qualitative diagnostic. For optically thin plasmas quantitative data can also be extracted from shadowgrams. However, in contrast to other fields of physics, quantitative shadowgraphy can rarely be found in plasma physics. Density profiles are usually retrieved by means of interferometric techniques, which can give accurate results over a wide range of plasma densities. However, they are not suitable for real-time diagnostics as the processing of the interferogram requires phase unwrapping algorithms which often demand manual intervention. The aim of our investigation is to show that quantitative shadowgraphy can be a simpler alternative to interferometric methods for two dimensional, automated, real-time characterization of low density plasmas generated by intense laser pulses in gases. We verified the accuracy of the shadowgraphic method by retrieving density profiles of several test objects (optical fibers, gas jets, plasma filaments in air) and comparing the results with interferograms. Agreement has been found for samples satisfying the Rayleigh-Gans condition. An algorithm has been developed for the retrieval of the optical path using a multigrid solver that processes in real-time a 256times256 pixel frame in about 1 second on an AMD Turion 64-bit processor. According to our numerical simulations, the technique will be able to monitor in green light plasma samples up to a thickness of 100 micrometers with an electron density of up to 10¹⁸ cm ^-3