Attosecond MeV γ-ray pulse compression via radiation-lifetime shortening in a longitudinal magnetic field

Mega-electron volt-scale attosecond γ-ray pulses open unprecedented avenues for interrogating ultrafast electron and nuclear dynamics. However, their generation remains hampered by complex experimental configurations and limited tunability. Here, we propose and numerically validate a compact scheme where a relativistic right-handed circularly polarized Laguerre–Gaussian laser irradiates a thin plasma foil in the presence of a co-propagating aligned magnetic field. The magnetic field significantly reduces the synchrotron radiation lifetime, enabling an efficient temporal compression of γ-ray pulses to approximately 57% of their original duration, while simultaneously enhancing the stability of the photon number, radiation energy, and control over the orbital angular momentum (OAM) of the emitted γ-rays. Under strong magnetic fields, a reversal of the γ-ray OAM is observed. The compression exhibits a nonmonotonic dependence on field strength. This mechanism offers a robust pathway toward compact, structure-tunable attosecond γ-ray sources with controllable OAM, suitable for high-field quantum electrodynamics studies and next-generation light sources.