Plasma current and magnetic field generation during direct laser acceleration

Direct Laser Acceleration (DLA) enables the generation of ultra-relativistic forward-moving electrons in high-intensity laser–plasma interactions through direct energy transfer from the laser’s electric field. A quasi-static azimuthal magnetic field, generated by a laser-driven plasma current, is critical for sustaining energy gain, as it mitigates the oscillatory nature of the laser field and enables prolonged acceleration via transverse electron deflections. The detailed structure of this non-oscillating current — and how it is driven by the laser — is not yet fully established. In this work, we examine the process using particle-in-cell simulations and find that the plasma current responsible for generating the magnetic field is carried by two distinct electron populations: forward-moving electrons undergoing betatron oscillations near the laser propagation axis and electrons farther from the axis executing cyclotron-like orbits. Together, these populations produce an approximately uniform current density across the channel.