Electromagnetic Spokes in Laser-Solid Interactions

For more than 15 years proton radiography has been used to observe electromagnetic spokes protruding radially from the center of a beam interacting with a solid target. Despite this phenomenon being observed across a large range of laser intensities and target materials, a rigorous explanation for these fields has been lacking.



This talk presents, for the first time, 3D MHD simulations that reproduce the spoke structure. To obtain these results, small-scale magnetic field loops are seeded in the laser absorption region. These loops are then reorganized by a combination of plasma and Nernst advection to form the eye-catching pattern observed in proton radiographs. In particular, it is the loops generated at the ‘Nernst layer’ that play an important role. The Nernst effect acts as an anchor, holding one side of the loop close to the foil, while plasma advection stretches the loop from the other end.



There are a number of feasible sources for the small-scale magnetic field loops. This talk will focus on laser speckles, as investigated through kinetic simulations.



Of course, the next question is ‘what impact do these magnetic fields have on the plasma conditions and laser coupling?’ There is some evidence from interferometry that the fields are imprinting a modulation onto the plasma density. Analysis of the 3D MHD simulations will attempt to shed light on this hypothesis, leading to a discussion of the ramifications of these formerly-anomalous electromagnetic fields on hohlraums and direct-drive ablation.