We investigate the dynamics of convergent shock compression in the solid cylindrical 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 of ~ 10¹⁷A/m² and a lifetime of ~100 fs. The results show that the dominant compression mechanism is governed by the plasma β, i.e., the ratio of the thermal pressure to magnetic pressure. For small radii and low atomic number Z 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 theory is validated with the optical and X-ray Free electron lasers (XFEL) pump-probe experiments. This work could offer a novel platform to generate extremely high pressures exceeding Gbar (100 TPa) to study high-pressure physics using femtosecond J-level laser pulses, offering an alternative to the nanosecond kJ laser pulse-driven and pulse power Z-pinch compression methods.

convergent shock compression,return current,high-intensity laser solid interactions