Optimizing laser-accelerated electrons for laser-based X-ray radiography*

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 as well as 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 10s of microns 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.