Laser absorption in plasmas: from nano-targets to near-QED regime

As the laser technology continues its spectacular development, ever higher field intensities and power levels become accessible in laboratories. The ELI project opens new horizons for laser applications in ultra-bright sources of short wavelength radiation. At the same time, the laser pulse quality -- like the contrast ratio -- is greatly improved so that fine structured targets maintain their structure till the main pulse arrival. This opens new and unexpected possibilities for laser-plasma engineering towards new physics. In the talk, we consider laser pulse interaction with nano- and micro-structured targets like nano-grass [1,2] in the intensity range 10$^{\mathrm{18}}$-10$^{\mathrm{20}}$ W/cm$^{\mathrm{2}}$. At intensities higher than 10$^{\mathrm{22}}$ W/cm$^{\mathrm{2}}$, the radiation damping force becomes important and can exceed the Lorentz force acting on an electron [2]. The $\gamma $-ray emission is then the major channel of laser energy absorption [3,4]. When a micro-plasma waveguide (MPW) is coupled with a readily available 2J laser, it may serve as a novel compact x-ray source. Electrons are extracted from the walls and form a dense helical bunch inside the channel. These electrons are efficiently accelerated and wiggled by the waveguide modes in the MPW, which results in a bright, well-collimated emission of hard x rays in the range of 1$\sim $100 keV [5].\\ \\References: \begin{enumerate} \item MA Purvis, VN Shlyaptsev, R Hollinger, C Bargsten, A Pukhov, et al. Nature Photonics 7, 796 (2013) \item Vural Kaymak, Alexander Pukhov, Vyacheslav N. Shlyaptsev, Jorge J. Rocca, ``Nano-scale ultra-dense Z-pinches formation from laser-irradiated nanowire arrays'', arXiv:1604.06330 (2016) \item LL Ji, A Pukhov, IY Kostyukov, BF Shen, K Akli Physical review letters 112, 145003 (2014) \item LL Ji, A Pukhov, EN Nerush, IY Kostyukov, BF Shen, KU Akli Physics of Plasmas 21, 023109 (2014) \item Longqing Yi, Alexander Pukhov, Phuc Luu-Thanh, and Baifei Shen, Phys. Rev. Lett. \textbf{116}, 115001 (2016) \end{enumerate}