Visualizing plasmons and ultrafast kinetic instabilities in laser-driven solids using X-ray scattering

communications physics Article https://doi.org/10.1038/s42005-024-01776-6 Visualizing plasmons and ultrafast kinetic instabilities in laser-driven solids using X-ray scattering Check for updates 1234567890():,; 1234567890():,; 1,2 1 3 1 1 Paweł Ordyna , Carsten Bähtz , Erik Brambrink , Michael Bussmann , Alejandro Laso Garcia , Marco Garten 1,6, Lennart Gaus 1,2, Sebastian Göde3, Jörg Grenzer1, Christian Gutt 4, Hauke Höppner 1, Lingen Huang 1, Uwe Hübner5, Oliver Humphries 3, Brian Edward Marré1,2, Josefine Metzkes-Ng1, Thomas Miethlinger1,2, Motoaki Nakatsutsumi3, Özgül Öztürk 4, Xiayun Pan 1,2, Franziska Paschke-Brühl1, Alexander Pelka1, Irene Prencipe1, Thomas R. Preston 3, Lisa Randolph3,4, Hans-Peter Schlenvoigt 1, Jan-Patrick Schwinkendorf1,3, Michal Šmíd 1, Sebastian Starke 1, Radka Štefaníková 1,2, Erik Thiessenhusen1,2, Toma Toncian 1, Karl Zeil 1, Ulrich Schramm 1,2, Thomas E. Cowan1,2 & Thomas Kluge 1 Ultra-intense lasers that ionize atoms and accelerate electrons in solids to near the speed of light can lead to kinetic instabilities that alter the laser absorption and subsequent electron transport, isochoric heating, and ion acceleration. These instabilities can be difficult to characterize, but X-ray scattering at keV photon energies allows for their visualization with femtosecond temporal resolution on the few nanometer mesoscale. Here, we perform such experiment on laser-driven flat silicon membranes that shows the development of structure with a dominant scale of 60 nm in the plane of the laser axis and laser polarization, and 95 nm in the vertical direction with a growth rate faster than 0.1 fs−1. Combining the XFEL experiments with simulations provides a complete picture of the structural evolution of ultrafast laser-induced plasma density development, indicating the excitation of plasmons and a filamentation instability. Particle-in-cell simulations confirm that these signals are due to an oblique two-stream filamentation instability. These findings provide new insight into ultra-fast instability and heating processes in solids under extreme conditions at the nanometer level with possible implications for laser particle acceleration, inertial confinement fusion, and laboratory astrophysics. Visualizing, understanding, and controlling laser absorption, isochoric heating, particle acceleration, and other relativistic non-linear physics that occur at the interaction of powerful lasers with solids is important for applications ranging from next-generation laser ion accelerators (LIA) for medical use1 to high-energy density physics including laboratory astrophysics2 and inertial confinement fusion3,4. Only recently, (proton) fast ignition for inertial confinement fusion has gained renewed interest as a viable path towards commercialization of Inertial Fusion Energy5–7 after the breakthrough fusion ignition achievements at the National Ignition Facility (NIF)8,9. Of special relevance is the understanding and control of plasma instabilities that can occur on largely different spatial, temporal, and plasma density scales. For example, compression and ignition of fusion targets in indirect-drive experiments carried out e.g. at the NIF nanosecond laser rely on the conversion of the laser energy into a homogeneous radiation field by laser-self-generated grating structures at the hohlraum entrance10, and stabilizing instability growth that happens on the scale of 10s of microns during the capsule compression11. Small fluctuations in the radiation pressure on the pellet surface or in the particle heater pulse would otherwise drive instabilities there, inhibiting maximum compression or heating12,13. Here we focus on ultrafast relativistic instabilities that grow over a few or tens of femtoseconds that are important e.g. in fusion fast ignition scenarios (FIS)14–16 that could potentially allow for a much better efficiency. A highintensity short pulse laser generates a dense particle beam that heats the 1 Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany. 2Technical University Dresden, Dresden, Germany. 3European XFEL, Schenefeld, Germany. Universität Siegen, Siegen, Germany. 5Leibniz Institute of Photonic Technology, Jena, Germany. 6Present address: Lawrence Berkeley National Laboratory, e-mail: ; Berkeley, CA, USA. 4 Communications Physics | (2024)7:296 1 Article https://doi.org/10.1038/s42005-024-01776-6 high-density core and ignites the thermonuclear reaction. On their path, the relativistic electrons are susceptible to beam-plasma instabilities that occur typically on scales of the skin depth, i.e. 10s of nanometers. On the path from laser particle acceleration to application as an ignitor such instabilities are amongst the long-standing crucial physics processes that need to be understood in simulations or experimental test-beds17. Theories for instabilities in relativistic high-intensity laser interaction with solids fall into two categories: (i) hydrodynamic instabilities growing at interfaces between two fluid-like plasma or photon ensembles, or (ii) kinetic instabilities that occur e.g. when one plasma streams through the other. Whether one or the other dominates depends on the detailed laser and solid properties. For example, in solids with a structured surface, or driven by lasers with a shallow rising edge, laser absorption to relativistic electron currents reaches up to nearly 100%, emphasizing the kinetic streaming instabilities at the front surface18,19 or at the rear of the target20–22, e.g. two-stream instability (TSI), Weibel instability (WI), or current filamentation instability (CFI). On the other hand, strong hydrodynamic Rayleigh-Taylor-like instabilities (RTI) following two-plasmon decay or parametric instabilities at the front of plasmas can be dominant for materials consisting of light ions, or driven by ultra-short high-contrast laser pulses, and can break up the laser to electron coupling and inhibit streaming instabilities23–29. The physics of these fast few femtosecond, few nanometer plasma instability dynamics and merging to the micron-scale after a few picoseconds in high-intensity laser-driven solids is one of the large unsolved issues in high-intensity laser plasma science, but its direct observation has previously not been possible because of the small time and few nanometer length scales involved. Microscopic interpretations were therefore primarily based on simulations24,26,27,30–33 and indirect measurements, e.g. via optical microscopy34, interferometry35,36, spectroscopy37, Faraday rotation38, or radiography20,21,39 or the impact on the divergence of the electron beam40. Here, we demonstrate experimentally that such instabilities indeed exist also in the hot solid density plasmas, quantify the strength, and give limits to the growth rate. Recent advances in the time-resolved diffraction, based on ultra-fast X-ray pulses from X-ray free-electron lasers (XFELs), now enable us to investigate laser-produced plasmas (...truncated)