Ultrafast nonequilibrium ion and electron dynamics of a neon plasma produced by an ultra-intense x-ray pulse TaggedPD1X X Cheng GaoD2X X a, *, D3X X Yongjun LiD4X X a , D5X X Jiaolong ZengD6X X a , D7X X Jianmin YuanD8X X a,b TaggedP a College of Science, National University of Defense Technology, Changsha Hunan 410073, PR China b Graduate School, China Academy of Engineering Physics, Building 9, ZPark II, No. 10 Xiebeiwang Road, Beijing 100193, PR China TAGGEDPARTICLE INFO Article History: Received 11 May 2017 Accepted 15 May 2017 Available online 16 May 2017 TAGGEDPABSTRACT Ultrafast nonequilibrium ion and electron dynamics of a neon plasma produced in the interaction with an ultra-intense x-ray pulse is investigated theoretically. Electron energy distribution function (EEDF) is obtained by solving FokkerPlanck equation, which is implemented self-consistently in a time-dependent rate equation in the framework of detailed-level-accounting approximation. Evolution dynamics of EEDF are presented at a variety of ion density in interaction with x-ray pulses of different laser intensities. Ther- malization of free electrons is demonstrated after the x-ray pulses have turned off. The results are compared with two other simplified models, i.e., one is a relaxation model and the second uses the Maxwellian approach. Large discrepancies D9X X in the EEDF are found and the effects of detailed treatment of electron dynamics on population distributions are demonstrated and discussed. © 2017 Elsevier B.V. All rights reserved. TaggedPKeywords: Electron energy distribution function Rate equation X-ray laser-matter interaction 1. Introduction TaggedPThe investigation of light-matter interaction is expanding from the long-wavelength region into the short-wavelength region with the development of x-ray free electron lasers (XFEL) such as the Linac Coherent Light Source (LCLS) [1] and the Spring-8 Angstrom Compact free electron LAser (SACLA) [2]. Such ultra-intense and ultrafast x-ray pulses open new probabilities to study the ion and electron dynamics in the nonequilibrium plasmas produced. After the pioneering experimental work of Young et al. [3], considerable efforts have been devoted to understanding the interactions of ultra- intense x-ray pulses with atoms and molecules [39], clusters [10,11], and solid samples [1215]. TaggedPTo theoretically understand the interaction mechanism of an ultra-intense x-ray pulse with atoms, the population evolution of atomic states and energy distribution of free electrons are of funda- mental importance. At the current state of XFEL experiments, the coherent time of the x-ray pulses is usually very short [16] and therefore the Time Dependent Rate Equation (TDRE) approach is widely used to determine the population dynamic evolution [1723]. In the interaction with an x-ray pulse, the inner-shell elec- trons of atoms are photoionized or resonantly excited, resulting in core-hole states which relax by Auger decay with ejection of an elec- tron or by emitting a photon. For a dilute atom gas, the free electrons produced do not have pronounced effects on the ion and electron TaggedP dynamics. For a dense plasma, however, the free electrons would further interact with atoms by impact excitation or ionization and thus control the dynamic processes. In such cases, the free electron energy distribution function (EEDF) plays an important role on level population distribution for collision dominated plasmas. Most past work assumed that the free electrons instantaneously equilibrate, which is not valid for such ultrashort x-ray pulses. A precise treat- ment relies on the accurate determination of EEDF. The nonequilib- rium distribution of free electrons is reported in plasmas produced by electron beams [24], black-body radiative field [25], lasers in the optical regime [2629], VUV and EUV wavelength range [30,31]. Few research efforts with detailed and accurate treatment on EEDF are reported in the x-ray FEL-matter interaction in literature [32,33]. D10X X Abdallah, Colgan, and Rohringer [32] investigated nonequilibrium effects of EEDF on populations and radiative properties of neon plas- mas irradiated by an ultra-intense x-ray pulse by solving Fokker Planck equation. The simulation is carried out at an ion density of 1.6 £ 10 19 cm ¡3 . Varga et al. [33] calculated the EEDF of neon plas- mas at ion density of 1.0 £ 10 19 and 1.0 £ 10 22 cm ¡3 by utilizing a set of hydrogenic approximate formulas to obtain the rates of atomic processes within the framework of the super-configuration approxi- mation. Three-body recombination is excluded to simplify their calculation. These investigations deepened our understanding of nonequilibrium electron dynamics in ultra-intense x-ray pulse inter- acting with matter. TaggedPIn this paper, ultrafast dynamics of ion and electron is investi- gated for nonequilibrium plasmas produced in interaction with ultra-intense x-ray pulses by combining a TDRE with FokkerPlanck * Corresponding author. E-mail address: gaocheng@nudt.edu.cn (C. Gao), jlzeng@nudt.edu.cn (J. Zeng). http://dx.doi.org/10.1016/j.hedp.2017.05.008 1574-1818/© 2017 Elsevier B.V. All rights reserved. High Energy Density Physics 23 (2017) 217222 Contents lists available at ScienceDirect High Energy Density Physics journal homepage: www.elsevier.com/locate/hedp