  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.
