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12Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2019
X-RAY EMISSION AND ABSORPTION SPECTROSCOPY IN STUDIES OF ENERGY TRANSFER IN WARM DENSE LASER PLASMA
S.N. Ryazantsev1, I.Y. Skobelev1,2, D.D. Arich1,2, P.S. Bratchenko1,2, A.Y. Faenov1,3, T.A. Pikuz1,3, P. Durey4, L. Doehl4, D. Farley4, C.D. Baird4, K.L. Lancaster4, C.D. Murphy4, N. Booth5, C. Spindloe5, P. McKenna6, S.B. Hansen7, J. Colgan8, R. Kodama3,9,10, N. Woolsey4,
S.A. Pikuz1,2
Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia
e-mail: sergryaz@mail.ru 2National Research Nuclear University "MEPhI, ", Moscow, Russia 3Open and Transdisciplinary Research Initiative, Osaka University, Osaka, Japan 4York Plasma Institute, Department of Physics, University of York, York, UK 5Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot, UK 6Department of Physics, SUPA, University of Strathclyde, Glasgow, UK 7Sandia National Laboratories, Albuquerque, USA
o
Theoretical Division, Los Alamos National Laboratory, Los Alamos, USA
9Graduate School of Engineering, Osaka University, Osaka, Japan 10Institute of Laser Engineering, Osaka University, Suita, Osaka, Japan
During the interaction of high contrast sub-PW laser pulses with solid matter a considerable fraction of the laser energy is converted to relativistic electrons accelerated to MeV energies. These energetic electrons subsequently flood the target and heat up the remaining bulk electrons due to collisions and strong return currents on the time scale of picoseconds. As a result, in regions of a laser target, which are not directly exposed to the laser radiation a special state of matter with electron temperatures of few eV and near-solid electron density (generally labeled as Warm Dense Matter -WDM) is created. Study of such type of matter is pursued to increase the fundamental understanding of atomic and equations-of-state properties of material at high-energy density and for developing several important applications, such as fast ignition of thermonuclear fusion targets and the creation of bright radiographic sources.
For WDM investigations mass-limited targets like thin wires [1] can be used. In that case strong electrostatic sheath fields confine the electrons to the target which efficiently and homogeneously heat the target matter to high energy densities, which can be observed along the whole trajectory of high-energy electrons because of the target reduced cross-dimensions.
Another way of WDM investigation based on analysis of X-ray spectra registered from a rear side of a thin flat foil during irradiation by a powerful laser pulse. X-rays emitted by plasma created in a focal spot on a front side of the foil are partially absorbed by deep layers of the target which were isochorically heated by the fast electrons. Absorption properties of the heated matter in that case directly depends only on its electron temperature and thickness and can be obtained by steady-state collisional-radiative kinetics calculations. After that it becomes possible to determine the properties of WDM inside the target by comparing of the rear side spectrum experimental shape with shape, which has been theoretically simulated as a convolution of the front side spectrum with calculated
absorption function. The method was developed and applied to experiments involving thin silicon
20
foils irradiated by 0.5-1.5 ps duration ultrahigh contrast laser pulses at intensities between 0.5*10
20 2
and 2.5*10 W/cm . The electron temperature of the material at the rear side of the target is estimated to be in the range of 140-300 eV. The diagnostic approach enables the study of warm dense matter states with low self-emissivity.
References
[1] A. Schonlein et al. 2016 EPL 114 (4) 45002
