Diagnosing Magnetic Fields in Cylindrical Implosions with Oblique Proton Radiography

TL;DR

This paper uses oblique proton radiography and simulations to analyze magnetic fields in cylindrical implosions, revealing complex plasma features and assessing measurement techniques for better understanding magnetic field behavior.

Contribution

It introduces a method combining experimental radiography with 3-D HYDRA simulations to interpret magnetic field measurements in cylindrical implosions, addressing challenges posed by plasma features.

Findings

Synthetic radiographs match experimental features except small-scale structures.

Direct inversion works on synthetic data but is limited on experimental data.

Field coils partially block protons, complicating measurements.

Abstract

Two experiments on the OMEGA Laser System used oblique proton radiography to measure magnetic fields in cylindrical implosions with and without an applied axial magnetic field. Although the goal of both experiments was to measure the magnitude of the compressed axial magnetic field in the core of the implosion, this field was obfuscated by two features in the coronal plasma produced by the compression beams: an azimuthal self-generated magnetic field and small length scale, high-amplitude structures attributed to collisionless effects. In order to understand these features, synthetic radiographs are generated using fields produced by 3-D HYDRA simulations. These synthetic radiographs reproduce the features of the experimental radiographs with the exception of the small-scale structures. A direct inversion algorithm is successfully applied to a synthetic radiograph, but is only partially able to invert the experimental radiographs in part because some protons are blocked by the field coils. The origins of the radiograph features and their dependence on various experimental parameters are explored. The results of this analysis should inform future measurements of compressed axial magnetic fields in cylindrical implosions.