Boundary driven unconventional mechanism of macroscopic magnetic field generation in beam-plasma interaction

The all pervading magnetic field in nature has aroused great curiosity and spawned many efforts to understand its generation. We propose, simulate, and experimentally demonstrate another mechanism of long-scale magnetic field generation in the context of a laser-plasma interaction. It relies on two realistic features, namely the finite size of the laser generated electron beam and an initial current imbalance. It is shown that magnetic fields of scale lengths comparable to the transverse beam dimension, are generated much before the onset of conventional instabilities associated with the beam-plasma system. This is due to radiative leakage at the boundaries of the finite beam, wherein even a small but finite current imbalance plays the crucial role of a radiative antenna. These features have been absent in simulations and theoretical analyses using the periodic boundary condition.