Depletion of intense fields

TL;DR

This paper investigates how intense electromagnetic fields interacting with charged particles can cause significant energy depletion of the laser pulse, challenging the common external field approximation at high intensities.

Contribution

It provides estimates showing when laser pulse depletion becomes significant, highlighting the limits of the external field approximation in high-intensity regimes.

Findings

Depletion becomes relevant at field amplitudes around a0 ~ 10^3.

Significant laser energy depletion occurs with electron bunches of about 10 nC.

External field approximation breaks down under these conditions.

Abstract

The interaction of charged particles and photons with intense electromagnetic fields gives rise to multi-photon Compton and Breit-Wheeler processes. These are usually described in the framework of the external field approximation, where the electromagnetic field is assumed to have infinite energy. However, the multi-photon nature of these processes implies the absorption of a significant number of photons, which scales as the external field amplitude cubed. As a result, the interaction of a highly charged electron bunch with an intense laser pulse can lead to significant depletion of the laser pulse energy, thus rendering the external field approximation invalid. We provide relevant estimates for this depletion and find it to become important in the interaction between fields of amplitude $a_0 \sim 10^3$ and electron bunches with charges of the order of 10 nC.