Abstract
The interaction of intense laser beams with plasmas created on solid targets involves a rich nonlinear physics. Because such dense plasmas are reflective for laser light, the coupling with the incident beam occurs within a thin layer at the interface between plasma and vacuum. One of the main paradigms used to understand this coupling, known as the Brunel mechanism, is expected to be valid only for very steep plasma surfaces. Despite innumerable studies, its validity range remains uncertain, and the physics involved for smoother plasma-vacuum interfaces is unclear, especially for ultrahigh laser intensities. We report the first comprehensive experimental and numerical study of the laser-plasma coupling mechanisms as a function of the plasma interface steepness, in the relativistic interaction regime. Our results reveal a clear transition from the temporally periodic Brunel mechanism to a chaotic dynamic associated to stochastic heating. By revealing the key signatures of these two distinct regimes on experimental observables, we provide an important landmark for the interpretation of future experiments.
6 More- Received 5 September 2018
- Revised 21 December 2018
DOI:https://doi.org/10.1103/PhysRevX.9.011050
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Intense laser pulses can quickly deposit large amounts of energy into solid material, thus creating dense plasma and subjecting matter to extreme temperature and pressure. This is useful in a variety of scientific applications, such as reproducing astrophysical phenomena in a lab, accelerating particles to high energies over short distances, or generating light pulses lasting just attoseconds. Yet, the mechanisms through which laser energy is deposited at such extreme intensities are extremely complex and far from understood. Here, we present a combination of highly controlled experiments and state-of-the-art numerical simulations that shed light on these mechanisms and the scenarios in which they are valid.
To gain insight into the dynamics of the plasma surface electrons in the laser field, we measure correlations between the high-energy electrons expelled from the target and the high-order harmonics of the laser light generated by the interaction. Combining these measurements with numerical simulations, we demonstrate that the coupling mechanism between the laser field and the plasma depends on the steepness of the density gradient at the target surface, which results from the plasma expansion into vacuum. As this gradient is increased, the electron dynamics switches from periodic to chaotic behavior, and we identify clear experimental evidence of this transition.
This better understanding of the laser-plasma coupling mechanism should help researchers optimize particle and light sources produced in such interactions.