Observation and modeling of interspecies ion separation in inertial confinement fusion implosions via imaging x-ray spectroscopy

Abstract

We report first direct experimental evidence of interspecies ion separation in direct-drive ICF experiments performed at the OMEGA laser facility via spectrally, temporally and spatially resolved imaging x-rayspectroscopy data [S. C. Hsu et al., EPL 115, 65001 (2016)]. These experiments were designed based on the expectation that interspecies ion thermo-diffusion would be strongest for species with large mass and charge difference. The targets were spherical plastic shells filled with D 2 and a trace amount of Ar (0.1% or 1% by atom). Ar K-shell spectral features were observed primarily between the time of first-shock convergence and slightly before neutron bang time, using a time-and space-integrated spectrometer, a streaked crystal spectrometer, and two gated multi-monochromatic x-ray imagers fielded along quasi-orthogonal lines of sight. Detailed spectroscopic analyses of spatially resolved Ar K-shell lines reveal deviation from the initial 1% Ar gas fill and show both Ar-concentration enhancement and depletion at different times and radial positions of the implosion. The experimental results are interpreted with radiation-hydrodynamic simulations that include recently implemented, first-principles models of interspecies ion diffusion. The experimentally inferred Aratom fraction profiles agree reasonably with calculated profiles associated with the incoming and rebounding first shock.

Progress on observations of interspecies ion separation in inertial-confinement-fusion implosions via imaging x-ray spectroscopy

“…The theory also predicts that the ion thermo-diffusion ratio maximizes, for a D/Ar plasma, with an Ar atomic concentration of around 1%, 34 thus motivating us to conduct experiments at and near this value. As in our previous publications, 34,35 we interpret observations of any deviation of the Ar ion number fraction from the spatially uniform target pre-fill value as proof of interspecies ion separation in the implosion core. The main objectives of this campaign were to (a) confirm the observation of interspecies ion separation reported in our previous work, (b) investigate the effect of shell-fuel mixing on the experimental inference of interspecies ion separation, (c) compare experimental observations of interspecies ion separation with radiation hydrodynamic simulations that incorporate a multi-ion-species transport model, 16,29,[36][37][38] and (d) explore the dependence of interspecies ion separation on the relative concentration of the heavier species to further validate the theory.…”

“…Thus, we had four different types of targets based on the combinations of various pressures of D 2 /Ar gas. The core fuel pressures in the targets of this campaign were higher than those in the previous campaign 34,35 with a goal of observing slightly lowertemperature and more-symmetric implosions compared to the previous campaign. Higher fill pressure for otherwise the same target and laser drive increases collisionality and suppresses the interspecies ion separation (see Appendix B).…”

“…Similarly, space-resolved spectra were extracted from the annular regions on the core images across the MMI data. 35 Figure 1(a) shows the MMI data from shot 86684 TIM 2 which consists of data recorded by four strips, and Fig. 1(b) shows the Ar Heb and Lyb narrow-band images constructed from 80 eV wide spectral ranges across the photon energies of the respective lines from the Frame-4, i.e., bottom frame.…”

“…Our earlier experimental campaign on interspecies ion separation in ICF implosions 34,35 provided direct experimental evidence for interspecies ion separation in an ICF implosion via detailed analysis of imaging x-ray spectroscopy data. In this paper, we provide the results from the analysis of imaging x-ray spectroscopy data from our second interspecies ion separation experimental campaign.…”

Observation of shock-front separation in multi-ion-species collisional plasma shocks

“…Prior experiments, simulations, and theoretical work explored multiion-species effects in the context of inertial confinement fusion (ICF), for which species separation in the fusion fuel potentially leads to neutron yield degradation [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Interspecies ion separation and velocity separation were experimentally observed [19][20][21][22][23]. Additional simulation and theoretical research on multi-ion-species plasmas examined how ion species diffusion causes species separation [24][25][26][27][28][29][30].…”