Stabilizing direct laser acceleration with long-scale-length plasma targets

We investigate the influence of plasma scale length on electron beam stability in direct laser acceleration (DLA) using a series of experiments on multiple laser systems with peak power spanning from 20 to 140 TW. An ultrashort, relativistic-intensity laser pulse interacts with a pre-expanded near-critical-density plasma formed by a nanosecond pre-pulse. We show that plasma expansion times of tens of nanoseconds, corresponding to long, shallow density gradients, result in electron beam pointing stability below 1° (RMS). Two-dimensional particle-in-cell simulations reveal that extended scale lengths suppress laser-driven filamentation and promote sustained self-focusing, leading to a stable acceleration channel. These results establish long-scale-length plasma targets as a robust route to improving beam stability in DLA, specifically when applied as laser-driven electron and neutron sources. Using this electron source, we demonstrated photoneutron generation with up to 9.1 × 10⁷ neutrons per shot.