Impact of 3D effects on the characteristics of a multi-monochromatic x-ray imager.

The multi-monochromatic x-ray imager instrument records arrays of time-gated spectrally resolved images of inertial confinement fusion targets. In the past, the image characteristics and spectral resolution of the instrument were estimated using a paraxial approximation and ignoring the substrate thickness of the pinhole array (PHA). In this work, we remove these two approximations to derive new analytic approximations and test them using a 3D ray tracing model. We show that variations in spatial resolution and transmission through the PHA are realized once the two approximations are removed simultaneously. A combination of the variation in pinhole transmission and solid angle gives rise to an image brightness modulation within the detector area. We also find a limit on the spectral resolution caused by an effective 2D dispersion from the multi-layered mirror and a spectral broadening related to spatial resolution. These findings enable better data processing and synthetic data for testing, and give further knowledge of the instrument's characteristics and limitations.

X-ray line polarization of He-like Si satellite spectra in plasmas driven by high-intensity ultrashort pulsed lasers.

We present a modeling study of x-ray line polarization in plasmas driven by high-intensity, ultrashort duration pulsed lasers. Electron kinetics simulations of these transient and nonequilibrium plasmas predict non-Maxwellian and anisotropic electron distribution functions. Under these conditions, the magnetic sublevels within fine structure levels can be unequally populated which leads to the emission of polarized lines. We have developed a time-dependent, collisional-radiative atomic kinetics model of magnetic sublevels to understand the underlying processes and mechanisms leading to the formation of polarized x-ray line emission in plasmas with anisotropic electron distribution functions. The electron distribution function consists of a thermal component extracted from hydrodynamic calculations and a beam component determined by PIC simulations of the laser-plasma interaction. We focus on the polarization properties of the He-like Si satellites of the L y(alpha) line, discuss the time evolution of polarized satellite spectra, and identify suitable polarization markers that are sensitive to the anisotropy of the electron distribution function and can be used for diagnostic applications.