Proton Radiography Inversions with Source Extraction and Comparison to Mesh Methods

TL;DR

This paper compares proton radiography inversion techniques, introduces a method for source profile determination, and emphasizes the importance of boundary conditions for accurate electromagnetic field reconstructions in laser-produced plasmas.

Contribution

It presents a novel approach to determine the unperturbed proton source profile and compares fluence-based and mesh-based inversion methods with boundary condition considerations.

Findings

Good agreement between methods when nonzero boundary conditions are used

The embedded fluence inversion improves source profile accuracy

A scheme to quantify uncertainty from source errors is proposed

Abstract

Proton radiography is a central diagnostic technique for measuring electromagnetic (EM) fields in high-energy-density, laser-produced plasmas. In this technique, protons traverse the plasma where they accumulate small EM deflections which lead to variations in the proton fluence pattern on a detector. Path-integrated EM fields can then be extracted from the fluence image through an inversion process. In this work, experiments of laser-driven foils were conducted on the OMEGA laser and magnetic field reconstructions were performed using both "fluence-based" techniques and high-fidelity "mesh-based" methods. We implement nonzero boundary conditions into the inversion and show their importance by comparing against mesh measurements. Good agreement between the methods is found only when nonzero boundary conditions are used. We also introduce an approach to determine the unperturbed proton source profile, which is a required input in fluence reconstruction algorithms. In this approach, a fluence inversion is embedded inside of a mesh region, which provides overconstrained magnetic boundary conditions. A source profile is then iteratively optimized to satisfy the boundary information. This method substantially enhances the accuracy in recovering EM fields. Lastly, we propose a scheme to quantify uncertainty in the final inversion that is introduced through errors in the source retrieval.