Rear-side escape of energetic electrons fromnear-critical-densityplasma targets driven by an intense laser

The efficient escape of energetic electrons from laser-irradiated near-critical-density (NCD) plasmas is crucial for applications such as radiotherapy. Electron escape dynamics in NCD plasmas play a key role in determining beam performance. Here, using particle-in-cell (PIC) simulations, we demonstrate an important confinement mechanism. Strong self-generated magnetic fields at the target rear, combined with intense lateral scattering and the longitudinal electric field, effectively trap electrons. This combined suppression limits the fraction of escaping electrons to approximately 2% of the total accelerated population. To overcome such strong confinement, our results indicate that increasing the laser pulse duration effectively reshapes the rear-side plasma profile and modifies the electromagnetic field configuration, which suppresses beam branching and enlarges the magnetic-null region. As a result, the escaping electron charge is enhanced by a factor of 30, offering an effective strategy for generating high-quality electron beams suited for applications including radiotherapy.