Laser-driven proton beams attract great interest in fields ranging from flash radiation oncology to compact accelerators. Increasing the proton energy is a critical prerequisite for advancing the practical applications of laser-driven proton sources. This study aims to elucidate the energy coupling mechanism between a direct-laser-acceleration electron beam and the proton accelerating field, with the goal of achieving enhanced proton acceleration. Utilizing an ultra-intense laser to irradiate a near-critical-density foam target, measurements are conducted of electrons, transition radiation, and protons. The experiments yielded a high-energy, high-charge electron beam, which triggers transition radiation in the terahertz band with an energy of up to 0.6 mJ. Concurrently, high-energy protons with a cutoff energy of 90 MeV is observed. Calculations reveal that in the near-field region behind the target rear surface, the energy associated with the transition radiation can generate an accelerating electric field of 10¹²–10¹³V/m, capable of facilitating efficient proton acceleration. Further PIC simulations indicate that the relativistic electron beam provides the accelerating field predominantly via transition radiation. This electromagnetic wave field, superimposed on the plasma field, collectively drives proton acceleration. Based on these findings, a novel hybrid proton acceleration scheme, distinct from pure charge-separation field mechanisms, is proposed.

laser proton acceleration,transition radiation field acceleration,relativistic electron beam,near critical density foam