Transition Radiation Field Enhanced Laser Proton Acceleration Employing Near-Critical-Density Foam

Laser-driven protons with ultrafast temporal properties attract great interest in fields ranging from flash radiation oncology to compact accelerators. High-efficiency energy coupling of protons from laser-induced accelerating fields is complex, hybrid acceleration mechanisms that combine multiple field contributions prove critical for optimizing proton energy. Here, we report a laser proton acceleration scheme in which proton energy can be enhanced by a transition radiation field (TRF) built by high-energy and large-charged electron bunches. Using near-critical-density plasmas, we experimentally produce electron beams with charges up to ~30 nC (>13 MeV). As these electrons exit the target, they emit intense TRF with energy up to 0.6 J in 0.1-15 THz range, corresponding to an acceleration field of 10^12-13 V m^-1. When superposed with the charge-separation field (CSF), proton cut-off energy is boosted by more than a factor of two, reaching 90 MeV. The resulting spectra exhibit a distinctive plateau-shaped feature in the high-energy regime. Multi-dimensional kinetic simulations confirm the synergistic role of the TRF and CSF in both enhancing the proton energy and shaping the spectral structure. This scheme provides new insights into the coupling between relativistic electron beams and acceleration fields and facilitates more efficient laser-driven proton acceleration.