New class of violation of local constant field approximation in intense crossed laser pulse scenarios

Understand

It is commonly assumed that in ultrastrong laser fields, when the strong field parameter of the laser field $\xi$ is larger than one, the electron radiation is well described by the local constant field approximation (LCFA).

• We discuss the failure of this conjecture, considering radiation of an ultrarelativistic electron interacting with strong counterpropagating laser waves.
• A deviation from LCFA, in particular in the high-frequency domain, is shown to occur even at $\xi\gg 1$ because of the appearance of an additional small time scale in the trajectory.
• Moreover, we identify a new class of LCFA violation, when the radiation formation length becomes smaller than the one via LCFA.