Engineering topics

Albright, B. J. (ORCID:0000000277896525)

Publications and source records attributed to Albright, B. J. (ORCID:0000000277896525).

4 records

DOE OSTI2024

MeV x-ray production from a petawatt laser in the regime of a relativistically transparent preplasma, with applications to radiography

Bright sources of mega-electron volt (MeV) x-rays have many unique applications, including nuclear physics, radiation oncology, and imaging high areal density systems. High intensity lasers (>1018 W cm−2) incident on mm-thick metal targets can deliver MeV x-rays via the bremsstrahlung process, providing sources with ultrashort duration (∼ps) and small source size (∼100 μm). Here, we report on a reproducible regime of laser-driven MeV x-ray sources, where the x-ray dose can be further increased by 60% by coating the metal target with micrometers of plastic. High fidelity numerical simulations indicate that the interaction is a result of relativistic transparency in the preplasma. Though relativistic transparency is present in both cases, the greater sound speed and smaller ion inertia of the plastic target allow the laser to more deeply penetrate and couple more efficiently to electrons. Radiography with this system demonstrates a resolving power < 300 μm, important for imaging applications.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2024

Advances in laser-based bremsstrahlung x-ray sources. I. Optimizing laser-accelerated electrons

In this work, we have performed a suite of kinetic simulations of relativistic laser–plasma interaction under settings relevant to recent and planned experiments on a variety of laser systems. The goal of the study is to illuminate the physics of laser–target coupling and to provide guidance for how to optimize these sources for applications. It is shown that the production of relativistic electrons is maximized when conditions of relativistic induced transparency (RIT) in dense plasmas can be achieved over a large interaction volume at the time of arrival of most intense part of the laser pulse. RIT is shown to enhance both the numbers of relativistic electrons and the energies of the electrons, leading to an increased x-ray dose. A variety of approaches to enhancing laser–target coupling are considered. These include optimizing the effects of low-density pre-plasma (arising either from finite laser pedestal or from the use of foam coatings) and of modifying the laser focusing geometry to reduce effects of filamentation and self-focusing. Evidence of a novel approach to achieving stable laser propagation over distances of tens of micrometers in a plasma gradient is also presented. These conditions coincide with plasma and laser conditions explored in recent experiments on the Omega EP laser system and compare favorably with an analytic criterion for stable laser propagation in relativistically underdense plasma obtained from a nonlinear Wentzel–Kramers–Brillouin analysis.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2024

Advances in laser-based bremsstrahlung x-ray sources. II. Laser pulse propagation and guiding in nonuniform plasma media in the presence of self-focusing

An analytic Wentzel–Kramers–Brillouin model is presented of Gaussian laser pulse propagation through plasma with a quadratic transverse density profile and an arbitrarily varying, longitudinal density gradient under conditions of nonlinear self-focusing. From these solutions, it is shown that in the absence of nonlinear self-focusing and transverse nonuniformity, for exponential pre-plasma density profiles, the use of a low density coating of the laser target with electron density n0∼11 ncr (e.g., a CH foam of density 35 mg/cm3 for 1-micron laser light) maximizes laser intensity at best focus. Also, under laser and plasma conditions relevant to recent experiments on high-power laser systems, conditions are obtained for a Gaussian laser pulse to propagate stably through the pre-plasma medium. Such conditions would be expected to enhance the production of relativistic electrons from laser-target coupling, providing a possible explanation for the observed increase in MeV photon dose and enabling applications such as laser-based MeV X-ray radiography.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2024

Simulated signatures of ignition

Ignition on the National Ignition Facility (NIF) provides a novel opportunity to evaluate past data to identify signatures of capsule failure mechanisms. We have used new simulations of high-yield implosions as well as some from past studies in order to identify unique signatures of different ignition failure mechanisms: jetting due to the presence of voids or defects, jetting due to the capsule fill tube, interfacial mixing due to instabilities or due to plasma transport, radiative cooling due to the presence of contaminant in the hot spot, long-wavelength drive asymmetry, and preheat. Many of these failure mechanisms exhibit unique trajectories that can be distinguished through variations in experimental observables such as neutron yield, down-scattered ratio (DSR), and burn width. Our simulations include capsules using both plastic and high-density carbon ablators and span all high-yield designs considered since the beginning of the National Ignition Campaign in 2011. We observe that the variability in trajectories through the space of neutron yield, DSR, and burn width varies little across capsule design yet are unique to the failure mechanism. The experimental trajectories are most consistent with simulated preheat and jetting due to voids and defects, which are the only failure mechanisms that are indistinguishable in our analysis. This suggests that improvements to capsule compression due to improved capsule quality or reduced preheat have played a primary role in enabling high yields on NIF. Furthermore, our analysis suggests that further improvements have the potential to increase yields further.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗