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.
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