Fyzikální ústav Akademie věd ČR

Unique combination of plasma temperature and density has been achieved with X-ray free-electron laser

In the first February issue of Nature, a Letter [1] appeared attracting an attention of specialists in areas of high-energy-density physics, laboratory astrophysics, inertial confinement fusion (ICF) and plasma physics. An international team of researchers headed by young Oxford physicist Sam Vinko created a unique state of matter by isochorically heating a thin aluminium foil using a tightly focused X-ray beam produced by the LCLS (Linac Coherent Light Source) free-electron laser in California. Solid-density plasmas at a temperature > 106 K were produced under these irradiation conditions.

Matter with a high energy density (>10⁵J/cm³) is prevalent throughout the Universe, being present in all types of stars and towards the centre of the giant planets; it is also relevant for inertial confinement fusion. Its thermodynamic and transport properties are challenging to measure, requiring the creation of sufficiently long-lived samples at homogeneous temperatures and densities. The LCLS facility provides the most intense X-ray source on the planet and understanding the detailed interactions process of intense X-ray radiation with matter is important from a fundamental viewpoint as well as for applications.

Although dense and hot systems can be generated in alternative ways, for example using intense optical lasers or particle beams, these never interact with a system at well-defined density. In the case of optical radiation this is because of the presence of a critical surface for the absorption, while for particle beams the pulse lengths are generally too long to justify neglecting hydrodynamic expansion. These intrinsic density gradients make the accurate study of dense plasma states extremely challenging. By using the LCLS pulse, the authors have been able to study extremely well-defined hot-dense plasma states for the first time, with unprecedented detail.

Detailed simulations of the X-ray-matter interaction process conducted with a radiative-collisional code further showed good qualitative agreement with the experimental results, providing additional insight into the evolution of the charge state distribution of the system, the electron density and temperature, and the timescales of collisional processes. These results should feed back into future high-intensity X-ray experiments involving dense samples, such as X-ray diffractive imaging of biological systems, material science investigations, and the study of matter in extreme conditions.

Nomarski (DIC – Differential Interference Contrast) images of ablation imprints created in lead tungstate (PbWO₄) by the focused beam of the X-ray free-electron laser LCLS built and operated at SLAC, Menlo Park, CA. The top-left inset contains a dependence of the ration of threshold and peak fluence f = E_th/E_pulse on the damaged area of the PbWO₄ surface, that serves to the determination of an effective area of the focused beam and other interaction parameters - for more information, please, see refs [2], [3].

L. Juha, V. Hájková, J. Chalupský, T. Burian, and L. Vyšín, i.e., IoP-ASCR affiliated co-authors of the Letter, contributed to the experiment and an interpretation of its results by an analysis of the focused beam of the X-ray laser. They reconstructed intensity distribution in the beam investigating its ablation imprints in a suitable material applying their original methods developed earlier [2], [3]. Monocrystalline PbWO4 was used for imprinting the beam. Lead tungstate strongly absorbs X-rays, also exhibiting suitable ablation properties. This material has been prepared and studied at IP-ASCR in Prague systematically for several decades as a scintillator in groups of M. Nikl and P. Boháček. These earlier studies made possible an efficient testing of this material for X-ray ablation.

[1] S. M. Vinko, O. Ciricosta, B.-I. Cho, K. Engelhorn, H.-K. Chung, C. Brown, T. Burian, J. Chalupsky, R. Falcone, C. Graves, V. Hajkova, A. Higginbotham, L. Juha, J. Krzywinski, H. J. Lee, M. Messerschmidt, C. Murphy, Y. Ping, A. Scherz, W. Schlotter, S. Toleikis, J. J. Turner, L. Vysin, T. Wang, B. Wu, U. Zastrau, D. Zhu, R. W. Lee, P. A. Heimann, B. Nagler, J. S. Wark: Creation and diagnosis of solid-density hot-dense matter with an X-ray free-electron laser, Nature 482, 59 (2012).