Abstract
Cavitation bubbles can be seeded from a plasma following optical breakdown, by focusing an intense laser in water. The fast dynamics are associated with extreme states of gas and liquid, especially in the nascent state. This offers a unique setting to probe water and water vapor far-from equilibrium. However, current optical techniques cannot quantify these early states due to contrast and resolution limitations. X-ray holography with single X-ray free-electron laser pulses has now enabled a quasi-instantaneous high resolution structural probe with contrast proportional to the electron density of the object. In this work, we demonstrate cone-beam holographic flash imaging of laser-induced cavitation bubbles in water with nanofocused X-ray free-electron laser pulses. We quantify the spatial and temporal pressure distribution of the shockwave surrounding the expanding cavitation bubble at time delays shortly after seeding and compare the results to numerical simulations.
Highlights
Cavitation bubbles can be seeded from a plasma following optical breakdown, by focusing an intense laser in water
We demonstrate near-field holographic imaging of cavitation bubbles with single X-ray free-electron laser (XFEL) pulses
The experiment is performed at the MID (Materials Imaging and Dynamics) instrument[33,34] of the European XFEL35
Summary
Cavitation bubbles can be seeded from a plasma following optical breakdown, by focusing an intense laser in water. We demonstrate near-field holographic imaging of cavitation bubbles with single X-ray free-electron laser (XFEL) pulses This experimental approach offers a quasi-instantaneous high resolution structural probe at different stages after seeding, useful to investigate extreme states of bubble generation and collapse. The method offers higher resolution and penetration depth than ultra-fast optical microscopy, and importantly a unique direct sensitivity to the electron density profile, which is not accessible by the aforementioned optical methods Such experimental data are required to assess the validity and limits of current numerical models and theoretical hypotheses and improve our basic physical understanding of these processes. Compared to the recently demonstrated X-ray microscopy of laser-induced dynamic processes with parallel beam optics[29,30] or an incoherent plasma Xray source[31], the present method offers higher spatial resolution and sensitivity, not limited by the detector pixel size. Density and pressure distributions are evaluated for more than 3000 individual cavitation events, which can be used to compute histograms of physical properties beyond simple ensemble averages
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