Fig. 1. Illustration of a microtube implosion. Due to the laser-produced hot electrons with megaelectron volt energies, cold ions in the inner wall surface implode toward the central axis. By pre-seeding uniform magnetic fields of the kilotesla order, the Lorentz force induces a Larmor gyromotion of the imploding ions and electrons. Due to the resultant collective motion of relativistic charged particles around the central axis, strong spin currents of approximately peta-ampere/cm are produced with a few tens of nm size, generating megatesla-order magnetic fields.

Fig. 2. Perspective views of the normalized ion density and the -component of the magnetic field , respectively, observed at fs, which is obtained by a 3D EPOCH simulation. A cubic aluminum target with a size of is set at the center, which has a cylindrical cavity with a radius of μm and an axis overlapping the -axis. The seed magnetic field kT parallel to the -axis is uniformly set over the entire domain. The four faces of the target parallel to the -axis are normally irradiated by uniform laser pulses simultaneously, which are characterized by μm, W cm and fs.

Fig. 3. Temporal evolution of the central magnetic field, obtained from 2D EPOCH simulations, under four different laser intensities , which are labelled and applied to an aluminum target ( cm, and ). Other fixed parameters are μm, μm, μm, fs and kT. The target is assumed to be uniformly irradiated on the four sides. The laser peak time is fs. The four highlighted circles on the green curve correspond to the sampling times for the 2D patterns given in Figure 4.

Fig. 4. Snapshots of the 2D patterns for the magnetic field (upper row), the total current vectors (middle row) and the electron density (lower row) normalized by the initial value ( cm), corresponding to the four highlighted times on the green curve in Figure 3 ( W/cm). Generated magnetic fields are assumed to be positive if they are in the same direction as the seed magnetic field ( kT). Just after the collapse of the microtube cavity at around fs, the spin-structured plasma flow due to the seed magnetic field is formed, increasing the magnetic strength, as observed in the current patterns.

Fig. 5. Scaling for , and in terms of . A square target is used with parameters μm and μm. The laser is assumed to uniformly irradiate the four target surfaces. The fixed parameters for the 2D simulations are μm, ps and kT. Yellow shading denotes the model prediction, which is given in Equation (4), and the two bounding dashed curves correspond to the minimum and maximum laser absorption efficiencies () in Equation (3). To draw the model curve for and , an optimized aspect ratio is postulated.