Relativistic laser interaction with solids and its probing with Xray pulses at HIBEF, European XFEL
The interaction of ultra-intense lasers with solids creates extreme states of matter: dense, hot, non-equilibrium transient plasmas.
Their application to some of the most exciting and important questions - How can we produce enough sustainable energy? How can treat cancer better and make the best treatments available for everybody? How do plasmas form and behave in the extreme conditions of distant space objects? - can give new and surprising answers. We study how to accelerate ions with the most powerful lasers ever constructed, e.g. for pre-clinical studies of novel tumor radiation concepts; how to heat plasmas hot enough to fuse atoms and generate the energy we all need to power our modern live; and simulate plasma flows and instabilities in astrophysical jets to learn more about our universe.
We use the most advanced computing codes (PIConGPU), most powerful supercomputers (HYPNOS, TITAN and others) and search for patterns, laws and sometimes find surprisingly simple explanations of the wonderful physics that is seen in laser plasma experiments.
|SAXS @ XFEL|
|Foto: Thomas Kluge|
However, often the dynamics are elusive, since light cannot penetrate solids and the processes are happening on nanometer and femtosecond scales. We apply a novel approach to obtain unprecedented insight in dense and relativistic plasmas. While currently experiments rely on intensive and complex simulations of the underlying dynamics to interpret results and verify deduced explanations and models, we turn that around and now can take ultra-short snapshots of the interior of dense laser-heated plasmas using European XFEL pulses. Combining the world's most intense optical lasers and most advanced X-ray sources - X-ray free electron lasers - at European XFEL (HIBEF) gives detailed information that for the first time allows for a direct glimpse on the femtosecond, nanoscale plasma physics involved in ultra-short ultra-intense laser-solid interaction: plasma density oscillations, kinetic instabilities, opacity, strong magnetic fields, plasma expansion - to name just a few. We apply and develop innovative experimental schemes, such as (Resonant) (Grazing Incidence) Xray (Small Angle) Scattering, Faraday Rotation, Phase Contrast Imaging and Xray Thomson Scattering. Eventually those experiments can be used to test models, develop new explanations for some of the elusive complex relativistic plasma physics dynamics in solids, and benchmark our simulations and numerical models and eventually drive their predictive capabilities further.
When a UHI laser hits a solid foil it quickly ionizes the front surface. The electrons move in the combined laser and plasma electromagnetic fields. Due to the strong fields the electrons rapidly become highly relativistic. This can give rise to rich physics, such as instabilities (e.g. Rayleigh-Taylor (RT) like, Parametric, Weibel, Buneman, Resistive and Ionization instabilities), shock formation, buried layer heating by internal ambipolar expansion. These effects are important for example for an understanding of almost all fundamental questions in laser-solid plasma physics, such as laser absorption, electron and ion acceleration (RPA, TNSA, BOA), filamentation of electron and ion beams at the foil surface, inside the foil or behind the foil, and the generation of high harmonics. However, there does not yet exist any direct experimental observation of any of the mentioned processes (though e.g. indirect observations of hole boring via Doppler shift of reflected light exist). The reason is that the timescale of the physics is ultra-short, on the order of the pulse duration (few 10 femtoseconds), UHI laser period (typically few femtoseconds) or the plasma period (below 0.1 femtoseconds) and the relevant spatial scales are generally in the range of a few microns and below. Moreover, solid foils are not penetrable by IR, visible, or UV light.
Developing a predictive understanding of these, together with their interactions, and also far from equilibrium is a grand challenge of modern-day plasma physics, which will require significant advances in theory and numerical simulation of non-equilibrium and non-linear processes. At the same time it is of the utmost importance to obtain much better experimental data with high spatial and temporal resolutions. Probing the solid-density plasma with XFELs will open entirely new ways to directly observe these extreme conditions, and will provide fundamentally new data for developing improved models, as well as validate or falsify present theoretical treatments. This is the prerequisite to drive advanced applications such as fast ignition of nuclear fusion or ion acceleration to > 100 AMeV energies to cure deep seated tumors. Key observables include the local electron density, current density, quasi-static magnetic fields, and ionization state, as well as the growth rate of the various perturbations and instabilities.