Supplemental Information for "Attosecond MeV γ-Ray Pulse Compression via Radiation-Lifetime Shortening in a Longitudinal Magnetic Field"

This supplementary material provides detailed support for the attosecond γ-ray pulse compression mechanism presented in the main text. Its contents encompass a complete theoretical derivation of the method, illustrating how an external magnetic field shortens the radiation lifetime via physical formulations; comparative simulations of different laser modes, indicating the optimal performance of the right-handed circularly polarized Laguerre-Gaussian laser; and a specific analysis of the direction and uniformity of the external magnetic field, clarifying the critical role of the correct magnetic field orientation in pulse formation.