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Investigation of femtosecond collisional ionization rates in a solid-density aluminium plasma

S. M. Vinko1, O. Ciricosta1, T. R. Preston1, D. S. Rackstraw1, C. R. D. Brown2, T. Burian3, J. Chalupský3, B. I. Cho4,5, H. K. Chung6, K. Engelhorn7, R. W. Falcone7,8, R. Fiokovinini1, V. Hájková3, P. A. Heimann9, L. Juha3, H. J. Lee9, R. W. Lee8, M. Messerschmidt10, B. Nagler9, W. Schlotter9, J. J. Turner9, L. Vyšín3, U. Zastrau11, J. S. Wark1
Nature Communications 6 (2015) 6397, doi: 10.1038/ncomms7397

The rate at which atoms and ions within a plasma are further ionized by collisions with the free electrons is a fundamental parameter that dictates the dynamics of plasma systems at intermediate and high densities. While collision rates are well known experimentally in a few dilute systems, similar measurements for nonideal plasmas at densities approaching or exceeding those of solids remain elusive. Here we describe a spectroscopic method to study collision rates in solid-density aluminium plasmas created and diagnosed using the Linac Coherent light Source free-electron X-ray laser, tuned to specific interaction pathways around the absorption edges of ionic charge states. We estimate the rate of collisional ionization in solid-density aluminium plasmas at temperatures ~30 eV to be several times higher than that predicted by standard semiempirical models.

An experimentally determined emission spectrum of a dense aluminum plasma formed by a focused beam of free electron X-ray laser.

1Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK
2Department of Plasma Physics, AWE Aldermaston, Reading RG7 4PR, UK
3Institute of Physics ASCR, Na Slovance 2, Prague 8 18221, Czech Republic
4Center for Relativistic Laser Science, Institute for Basic Science (IBS), Gwangju 500-712, Korea
5Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea
6Atomic and Molecular Data Unit, Nuclear Data Section, IAEA, PO Box 100, Vienna A-1400, Austria
7Lawrence Berkeley National Laboratory, 1 Cyclotron Road, California 94720, USA
8Department of Physics, University of California, Berkeley, California 94720, USA
9SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
10National Science Foundation BioXFEL Science and Technology Center, 700 Ellicott Street, Buffalo, New York 14203, USA
11IOQ, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, Jena 07743, Germany