Electric microfields in dense carbon-hydrogen plasmas

Abstract

Classical molecular dynamics is used to investigate stationary and time-dependent properties of microfields in hot, solid density, electron-ion plasmas. Even at the high temperatures considered here, such simulations require quantum statistical potentials (QSPs) to mimic the essential effects of diffraction and exchange symmetry for electrons. Fortunately, key results relevant to microfield distributions are found to be insensitive to different, plausible QSP choices. Atomic processes in plasmas will depend on the time average of the microfields. It is not clear, a priori, what the time duration of this average should be. The question of how best to extract the quasistatic (low-frequency) microfield from a classical molecular dynamics simulation is explored in some detail, and the time-averaging approach we adopt involves both plasma and atomic time scale constraints. One of the major findings described in the paper is that for a large time interval, the time-averaged microfield does not significantly change. Our discussion of this suite of large simulations for plasma mixtures focuses on understanding various features and trends revealed by data for C-H plasmas having carbon fractions ranging from 0.01 to 1, and different temperatures well above TFermi.

Microfield dynamics in dense hydrogen plasmas with high-Zimpurities

“…Therefore, we employed a multi-species HNC code to determine pair functions for plasmas having ions and free electrons interacting via our standard QSPs (all-particle HNC), and for plasmas having Yukawa-screened ions (Yukawa HNC). It is known that such HNC calculations are in good agreement with results of MD simulations when both approaches employ the same potentials [8,24], and this has been confirmed for our code ddcMD [10,27].…”

“…F scr to be extracted from the time-dependent total field of allparticle MD? We followed earlier researchers [6,7,10] and determined this slow field from a running, short time ( τ ) average of ! F tot (t) at each ion.…”

“…Here, we continue to investigate the important question featured in our study of C +6 -H + plasmas [10]: How should τ be chosen so that , t ion…”

Linac Coherent Light Source: The first five years

“…A broad literature discusses the expected signal and noise levels as well as the impacts of radiation damage, with only one clear conclusion at this time -that shorter pulses are better for a given number of photons per pulse. There is a clear advantage for having pulses shorter than the Auger lifetimes of the relevant atoms in the sample (Fung et al, 2009;Hau-Riege, 2012;Hau-Riege et al, 2007;Loh and Elser, 2009;Maia et al, 2009;Son et al, 2011). Continued effort will be required to systematically resolve the technical challenges involved in single-particle imaging.…”

Classical molecular dynamics simulations of hydrogen plasmas and development of an analytical statistical model for computational validity assessment

“…On the upside, however, collective behaviors (e.g., ion dynamic effects) emerge in a natural way, the range of validity extends to strongly coupled plasmas and, with the lack of experimental data, MD results are often considered as a reference to reveal model deficiencies and provide valuable guidance for theory improvement. Classical MD simulations have been applied to the study of diverse statistical properties, particle correlation effects, and in particular to the investigation of plasma electric microfield distributions [17][18][19][20][21][22][23][24][25]. Although mainly performed in the context of fully ionized two-component plasmas, all this work enabled the study of electric microfield issues beyond the capability of most theoretical methods.…”