Enhanced direct laser acceleration by superluminal phase velocities demonstrated in particle-in-cell simulations

Direct Laser Acceleration (DLA) in plasma enables compact generation of high-energy electrons and photons. While theoretical models predict that superluminal laser phase velocities can significantly enhance DLA [Khudik et al., Phys. Plasmas 23, 103108 (2016)], this has not been clearly demonstrated in fully self-consistent kinetic simulations. Using particle-in-cell (PIC) simulations with a moderate-intensity laser ($a_0 = 25$) and a long structured plasma channel target, we show that superluminosity enables electrons to reach $10$ GeV energies, in agreement with DLA test-electron models. Further, we find that variations in the plasma and laser parameters, as the laser propagates through the target, generally promote energy gain. Though the fields are modestly intense, the resulting high energy electrons undergo strong gamma-ray emission and radiation recoil, which significantly alters the electron dynamics. These results establish superluminosity as a key factor in DLA and highlight its importance for optimizing electron acceleration in realistic plasma-laser configurations.