From the Rose–DuBois ansatz of hotspot fields to the instanton solution: A pedestrian presentation

This paper gives a pedestrian presentation of some technical results recently published in mathematical physics with non-trivial implications for laser–plasma interaction. The aim is to get across the main results without going into the details of the calculations or offering a specialist's user guide but by focusing conceptually on how these results modify the commonly held description—in terms of laser hotspot fields—of backscattering instabilities with a spatially smoothed laser beam. The intended readers are plasma physicists as well as graduate students interested in laser–plasma interaction. No prior knowledge of scattering instabilities is required. Step by step, we explain how the laser hot spots are gradually replaced with other structures, called instantons, as the amplification of the scattered light increases. In the amplification range of interest for laser–plasma interaction, instanton–hotspot complexes tend to appear in the laser field (in addition to the expected hot spots), with a non-negligible probability. For even larger amplifications and systems longer than a hotspot length, the hotspot field description is clearly invalidated by the instanton takeover.