  Imposing an external magnetic field in short-pulse intense laser-plasma
interaction is of broad scientific interest in related plasma research areas.
We propose a simple method using a virtual current layer by introducing an
extra current density term to simulate the external magnetic field, and
demonstrate it with three-dimensional particle-in-cell simulations. The field
distribution and its evolution in sub-picosecond time scale are obtained. The
magnetization process takes a much longer time than that of laser-plasma
interaction due to plasma diamagnetism arising from collective response. The
long-time evolution of magnetic diffusion and diamagnetic current can be
predicted based on a simplified analytic model in combination with simulations.
