Extremely high axial magnetic fields above the gigagauss level are supposed to exist in neutron stars

which may be a one of the critical parameters for their internal structures and be responsible for the x-ray and gamma-ray emission from these stars. Here

we show that such ultrahigh magnetic fields can be produced by multiple petawatt-class lasers interacting with a cuboid solid target with a cylindrical microtube in the middle. It is found that the obliquely incident intense lasers at the target surfaces enable the produced hot electrons to form an azimuthal current and subsequently induce a seed magnetic field along the cylindrical axis inside the microtube as the hot electrons transport into it. This current–field configuration is similar to a theta-pinch device. When the hot electrons and energetic ions produced via target normal sheath acceleration converge toward the microtube axis

the seed magnetic field is dramatically amplified. This process continues until the magnetic pressure near the axis becomes comparable to the thermal pressure contributed both by hot electrons and by energetic ions. Later on

as the plasma in the center starts to be expelled outward by the magnetic pressure

an electron current ring with extremely high densities is formed

leading to a further boost of the magnetic fields to well above the gigagauss level. A scaling of the magnetic field strength with laser intensities

pulse durations

incident angles

and target sizes is presented and verified by numerical simulations

which demonstrates the robustness of our scheme. Our scheme is well suited for experimental realization on 100-TW-class to PW-class femtosecond or picosecond laser facilities with multiple linearly polarized laser beams.