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Creation and diagnosis of a solid-density plasma with an X-ray free-electron laser


Matter with a high energy density (>105 joules per cm3) is prevalent throughout the Universe, being present in all types of stars1 and towards the centre of the giant planets2,3; it is also relevant for inertial confinement fusion4. Its thermodynamic and transport properties are challenging to measure, requiring the creation of sufficiently long-lived samples at homogeneous temperatures and densities5,6. With the advent of the Linac Coherent Light Source (LCLS) X-ray laser7, high-intensity radiation (>1017 watts per cm2, previously the domain of optical lasers) can be produced at X-ray wavelengths. The interaction of single atoms with such intense X-rays has recently been investigated8. An understanding of the contrasting case of intense X-ray interaction with dense systems is important from a fundamental viewpoint and for applications. Here we report the experimental creation of a solid-density plasma at temperatures in excess of 106 kelvin on inertial-confinement timescales using an X-ray free-electron laser. We discuss the pertinent physics of the intense X-ray–matter interactions, and illustrate the importance of electron–ion collisions. Detailed simulations of the interaction process conducted with a radiative-collisional code show good qualitative agreement with the experimental results. We obtain insights into the evolution of the charge state distribution of the system, the electron density and temperature, and the timescales of collisional processes. Our results should inform 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.

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Figure 1: Spectrally resolved Kα emission as a function of the X-ray FEL excitation photon energy.
Figure 2: Experimental and theoretical spectra, and charge-state emission pathways.
Figure 3: Temperature, density and charge state distribution at peak intensity.
Figure 4: Rates for atomic and collisional processes.


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Portions of this research were carried out on the SXR instrument at the LCLS, a division of SLAC National Accelerator Laboratory and an Office of Science user facility operated by Stanford University for the US Department of Energy. The SXR instrument and the Resonant Coherent Imaging (RCI) endstation are funded by a consortium whose membership includes the LCLS, Stanford University through the Stanford Institute for Materials Energy Sciences (SIMES), Lawrence Berkeley National Laboratory (LBNL), University of Hamburg through the BMBF priority programme FSP 301, and the Center for Free Electron Laser Science (CFEL). S.M.V., O.C. and J.S.W. thank the UK EPSRC for funding (EP/F020449/1 and EP/H035877/1). B.I.C., K.E., R.W.F. and P.A.H. acknowledge US DOE Basic Energy Science contract DE-AC03-76SF00098 and SSAA programme contract DE-FG52-06NA26212. T.B., J.C., L.J. and L.V. appreciate funding by grants LC510, LC528, LA08024, ME10046, P108/11/1312, P205/11/0571, IAAX00100903 and KAN300100702. U.Z. thanks the German Ministry for Education and Research (BMBF) for funding under FSP 301. C.D.M. was supported by UK EPSRC (EP/G007187/1). We also thank G. Gregori (Oxford University) for discussions.

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S.M.V., B.I.C., K.E., C.R.D.B., A.H., H.J.L., C.D.M., Y.P., S.T., U.Z., P.A.H., B.N. and J.S.W. performed the experiment and acquired the data. B.I.C., K.E., R.W.F. and P.A.H. analysed the data. S.M.V., O.C., H.-K.C., R.W.L. and J.S.W. performed the theoretical work. S.M.V., R.W.L. and J.S.W. wrote the paper. M.M., W.S. and J.J.T. operated the SXR beamline and the LCLS diagnostics. C.G., A.S., T.W., B.W. and D.Z. operated the RCI endstation. T.B., J.C, V.H., L.J., J.K. and L.V performed the spot-size characterisation and analysis. All authors contributed to the work presented here and to the final paper.

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Correspondence to S. M. Vinko.

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Vinko, S., Ciricosta, O., Cho, B. et al. Creation and diagnosis of a solid-density plasma with an X-ray free-electron laser. Nature 482, 59–62 (2012).

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