Unfolding the spatial structure and thermal transport details of high temperature implosions at OMEGA

Inertial confinement fusion and high-energy density science experiments based on implosion platforms that produce plasmas with temperatures larger than 2keV render Ar tracer spectroscopy no longer useful to extract plasma conditions.¹ Yet, measuring spatially resolved plasma conditions is central to the physics of ignition as well as fundamental high-energy density phenomena such as ion stopping power², electron-ion energy transfer³, and thermal transport. Recent applications of x-ray tracer spectroscopy at NIF and OMEGA have employed Kr as a spectroscopic tracer of hot implosion cores^1,5. However, only spatially averaged conditions have been reported with Kr so far. We discuss the first spatially resolved measurements in hot implosion cores at OMEGA extracted from the observation of Kr K-shell line emission. To this end, two separate x-ray imaging spectrometers were simultaneously fielded in the experiments: a slit- and a pinhole-based imagers. The data recorded with these instruments complement each other and enable consistency checks. The pinhole imager combines a pinhole array with a Ge Bragg crystal to record (gated) arrays of spectrally resolved core images in the photon energy range from 12keV to 16keV covering K-shell lines in He- and H-like Kr ions; this is a multi-monochromatic x-ray imager.⁴ From the data, narrowband and broadband x-ray images of the core plasma as well as spatially integrated and resolved x-ray intensity distributions in the core can be extracted. The latter provides the key to extract spatially resolved plasma conditions. Atomic and radiation physics models have been developed to analyze the spectra that account for non-equilibrium atomic kinetics and radiation transport as well as detailed Stark-broadened line shapes. The results show the spatial distributions of temperature, density and pressure in the core, and their correlation with target details, laser pulse parameters, and implosion asymmetries. Furthermore, thermal transport can be determined from the temperature gradients thus enabling an assessment of energy balance in the core.

¹E. Gallardo, et al, PoP (submitted)

²J. Frenje, et al, PRL 122, 015002 (2019)

³P. J. Adrian, et al, PRE 106, L053201 (2022)

⁴E. Gallardo, et al, RSI 93, 113525 (2022)

⁵L. Gao, et al, PRL 128, 185002 (2022)