Direct asymmetry measurement of temperature and density spatial distributions in inertial confinement fusion plasmas from pinhole space-resolved spectra

Abstract

Two-dimensional space-resolved temperature and density images of an inertial confinement fusion (ICF) implosion core have been diagnosed for the first time. Argon-doped, direct-drive ICF experiments were performed at the Omega Laser Facility and a collection of two-dimensional spaceresolved spectra were obtained from an array of gated, spectrally resolved pinhole images recorded by a multi-monochromatic x-ray imager. Detailed spectral analysis revealed asymmetries of the core not just in shape and size but in the temperature and density spatial distributions, thus characterizing the core with an unprecedented level of detail.Inertial confinement fusion (ICF) is an approach that utilizes laser produced ablation pressure to compress a millimeter-sized spherical shell capsule containing fuel (e.g., deuterium and tritium) and drive the fuel temperature and density to conditions suitable for ignition [1]. The key is a spherically symmetric and stable compression. While state-of-the-art hydrodynamics simulations have been used to design ignition implosions, the challenge of achieving a symmetric implosion experimentally has thus far prevented ICF from reaching the conditions required for successful ignition [2]. Hence, measuring the spatial asymmetry in the temperature and density distributions in the implosion core is crucial for understanding how to make it more symmetric.Several diagnostics were developed in the last few decades in order to investigate implosion core conditions. K-shell line emission spectroscopy using Ar as a tracer has proved to be a powerful tool to extract spaceaveraged electron temperature, T e , and density, n e [3-5]. However, two-dimensional (2-D) space-resolved spectra have never been extracted to study the asymmetries of T e and n e structures in the implosion core. X-ray pinhole imaging of ICF implosion cores has been used to study the shape and size of the core and, in particular, to characterize deviations from spherical symmetry [6,7]. Nevertheless, these images do not reveal the implosion asymmetries in T e and n e distributions.This Letter describes a new spectroscopic method that combines the ideas of Ar tracer spectroscopy and pinhole imaging to extract implosion core images in T e and n e without making symmetry assumptions. These pinhole images are extracted by analyzing a collection of 2-D space-resolved spectra obtained from an array of spectrally resolved core images. The direct measurement of temperature and density asymmetries in the core provides stringent constraints on what actually happens in implosion experiments and can be used to benchmark hydrodynamic simulations. The discussion here focuses on the application to ICF implosion core conditions; nevertheless, the ideas are general. The extraction and analysis of space-resolved spectra from spectrally resolved pinhole images opens up new possibilities for x-ray spectroscopy of high-energy density plasmas.The spectroscopic data were recorded in a series of Ar-doped ICF implosion experiments performed at the Omega Laser ...

Measurements of Ion Stopping Around the Bragg Peak in High-Energy-Density Plasmas

“…However, as the plasma stopping power at v i ∼ v th is highly sensitive to T e , and that a direct measurement of T e is lacking, any definite conclusions about the modeling of the data at v i ∼ v th cannot be made with this data set. To further validate and elucidate stopping-power formalisms at the Bragg peak, measurements of n e ðr; tÞ and T e ðr; tÞ will be conducted in future experiments using x-ray imaging spectroscopy of a dopant such as argon in the D 3 He fuel [37].…”

Multiple-view spectrally resolved x-ray imaging observations of polar-direct-drive implosions on OMEGA

“…In this case, five bright spots can be seen in the image as well as an elongation of the shape of the image along the direction of the projection of the target's stalk on the image plane. We note in passing that a connection between stalk and core shape asymmetry has also been observed in 60-beam spherically symmetric illumination implosions at OMEGA [32]. The pentagonal pattern can be noted both in the array of spectrally resolved images recorded with MMI as well as in the narrowband and broadband reconstructed images.…”

An important criterion for reliable multi-monochromatic x-ray imager diagnostics and its impact on the reconstructed images

“…The given resolutions are typical values for our applications [10][11][12] . The spatial resolution depends on pinhole size and object-PHAdetector distances.…”

“…By processing MMI data, one can extract narrow-band images (i.e., intensity images of narrow spectral ranges), spaceintegrated spectra [7][8][9] and 2D space-resolved spectra [10] . Spectroscopic analysis of the images and spectra extracted from the MMI data can provide a variety of approaches to constrain the ICF plasma spatial structure in electron temperature, T e , and electron density, n e .…”

“…Previously, we extracted timeresolved 2D T e maps from two narrow-band images [11] . From a collection of space-resolved spectra, we extracted time-resolved 2D T e and n e maps [10] . Synthetic investigations also suggest that time-resolved 3D T e and n e spatial distributions of ICF cores can be extracted by simultaneously analyzing collections of 2D space-resolved spectra extracted from three MMI instruments fielded along quasi-orthogonal lines of sight [12] .…”