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Compressed ultrafast photography of plasmas formed from laser breakdown of dense gases reveals that internal processes dominate evolution at early times
Phys. Rev. E 113, 015209 – Published 30 January, 2026
DOI: https://doi.org/10.1103/3vw9-h2g5
Abstract
Compressed ultrafast photography (CUP) is applied to laser breakdown in argon and xenon under pressures up to 40 bars to obtain two-dimensional images of the plasma dynamics of single events with a spatial resolution of pixels and an equivalent frame rate of 500 GHz. Light emission as a function of position and time is measured through red, green, blue, and broadband filters. The spatially encoded and temporally sheared image normally used in CUP is now enhanced by the introduction of a constraint given by a spatially integrated and temporally sheared unencoded signal. The data yield insights into the temperature, opacity, the plasma formation process, and heat flow within the plasma and to the surrounding ambient gas. Contours of constant emission indicate that plasmas formed from sufficiently dense gas contract rather than expand despite having a temperature of a few eV. Plasmas formed from relatively low pressure gases such as 7-bar argon can radiate with emissivity near unity. Modeling transport and opacity as arising from inverse Bremsstrahlung requires a degree of ionization that strongly exceeds expectations based on Saha's equation even as customarily modified to include density and screening. According to this model, both electrons and ions are strongly coupled with a plasma coefficient . During the first few nanoseconds after formation, Stefan-Boltzmann radiation and thermal conduction to ambient gas are too weak to explain the observed cooling rates, suggesting that transport within the plasma dominates its evolution. Yet, thermal conduction within the plasma itself is also small as indicated by the persistence of thermal inhomogeneities for far longer timescales. The fact that plasma is isolated from the surroundings makes it an excellent system for the study of the equation of state and hydrodynamics of such dense plasmas via the systems and techniques described.
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Focus
Ultrafast Movie Reveals Unexpected Plasma Behavior
Using a camera with 2-picosecond time resolution, researchers show that the atoms in a laser-induced plasma are more highly ionized than theory predicts.
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