Evaluation of Direct Inversion of Proton Radiographs in the Context of Cylindrical Implosions*

Three publicly available direct inversion routines for proton radiography have been applied to laser-driven cylindrical implosions. The inversion routines used were a Monge-Ampere solver [1], a power-diagram method [2] and a probabilistic method that does not require a specified source profile [3]. The routines were first applied to four different test problems generated using specified fields in proton-tracing routines, three of them cylindrical and one spherical. Two of the test problems included field discontinuities that generate caustics, representative of ion fronts, shock fronts, and material interfaces. Only the power-diagram method successfully inverted test problems with caustics. The power-diagram method was then applied to proton radiographs obtained from cylindrical implosions on the OMEGA laser. In the experiments, proton trajectories intersected one another so there was not a unique solution for the path-integrated transverse Lorentz force on the protons. Nonetheless, the power-diagram method was able to find a solution that minimized proton deflection. Another solution was found by proton tracing using PlasmaPy [4] in fields from 3-D magneto-hydrodynamic simulations with the self-generated magnetic field reduced until adequate agreement was obtained. The direct inversion shows that the self-generated magnetic field could be even lower than indicated by the scaled-simulation results.

 

[1] M. F. Kasim, Invert Shadowgraphy and Proton Radiography, Accessed 8 July 2021, https://github.com/mfkasim1/invert-shadowgraphy/tree/fast-inverse.

[2] M. F. Kasim, Invert Shadowgraphy and Proton Radiography, Accessed 8 July 2021, https://github.com/mfkasim1/invert-shadowgraphy.

[3] M. F. Kasim et al., Phys. Rev. E 100, 033208 (2019).

[4] PlasmaPy Community et al., PlasmaPy (Version 0.6.0), Zenodo, Accessed 14 March 2021, http://doi.org/10.5281/zenodo.4602818.