  We investigate the dynamics of convergent shock compression in the solid wire
targets irradiated by an ultra-fast relativistic laser pulse. Our
Particle-in-Cell (PIC) simulations and coupled hydrodynamic simulations reveal
that the compression process is initiated by both magnetic pressure and surface
ablation associated with a strong transient surface return current with the
density in the order of 1e17 A/m^2 and a lifetime of 100 fs. The results show
that the dominant compression mechanism is governed by the plasma $\beta$,
i.e., the ratio of the thermal pressure to magnetic pressure. For small radii
and low atomic number Z wire targets, the magnetic pressure is the dominant
shock compression mechanism. As the target radius and atomic number Z increase,
the surface ablation pressure is the main mechanism to generate convergent
shocks based on the scaling law. Furthermore, the indirect experimental
indication of the shocked hydrogen compression is provided by measuring the
evolution of plasma expansion diameter via optical shadowgraphy. This work
could offer a novel platform to generate extremely high pressures exceeding
Gbar to study high-pressure physics using femtosecond J-level laser pulses,
offering an alternative to the nanosecond kJ laser pulse-initiated and pulse
power Z-pinch compression methods.
