Sign reversal in magnetic field amplification by relativistic laser-driven microtube implosions

TL;DR

This paper investigates the unexpected sign reversal of magnetic fields amplified by laser-driven microtube implosions, revealing how target shape and initial conditions influence magnetic field direction in high energy density plasma.

Contribution

It uncovers the phenomenon of magnetic field sign reversal in laser-driven microtube implosions and links it to surface magnetic field stability during the process.

Findings

Sign reversal occurs when target shape changes from square to circular.

Surface magnetic field stability determines the sign of the amplified magnetic field.

Target, laser, and seed field conditions influence magnetic field sign reversal.

Abstract

We demonstrate and explain the surprising phenomenon of sign reversal in magnetic field amplification by the laser-driven implosion of a structured target. Relativistically intense laser pulses incident on the outer surface of a microtube target consisting of thin opaque shell surrounding a $\mu$m-scale cylindrical void drive an initial ion implosion and later explosion capable of generating and subsequently amplifying strong magnetic fields. While the magnetic field generation is enhanced and spatially smoothed by the application of a kilotesla-level seed field, the sign of the generated field does not always follow the sign of the seed field. One unexpected consequence of the amplification process is a reversal in the sign of the amplified magnetic field when, for example, the target outer cross section is changed from square to circular. Using 2D particle-in-cell simulations, we demonstrate that sign reversal is linked to the stability of the surface magnetic field of opposite sign from the seed which arises at the target inner surface during laser irradiation. The stability of the surface magnetic field and consequently the sign of the final amplified field depends sensitively on the target, laser, and seed magnetic field conditions, which could be leveraged to make laser-driven microtube implosions an attractive platform for the study of magnetic fields in high energy density plasma in regimes where sign reversal either is or is not desired.