Positron production in a laser plasma for measuring superhigh laser intensities

Quantum electrodynamics (QED) predicts the phenomenon of electron-positron pair production via the decay of a high-energy photon in the presence of strong external fields. Such photons can be generated due to nonlinear Compton scattering involving electrons propagating in an intense external background. Here we investigate the possibility of extracting superhigh laser intensities by measuring the positron yield. Our numerical simulations based on the QED rates of the two aforementioned processes provide the total number of positrons as a function of the laser intensity for two different setups: a single focused laser pulse and a combination of two counterpropagating pulses. As seed particles, we consider a free-electron gas and a gas of neutral Xe atoms. In this study, special focus is placed on taking into account the cascade process of pair production, which can occur in the case of superhigh laser intensities. It is demonstrated that the contribution of the QED cascade is not important in the vicinity of the positron generation threshold; i.e., an extended intensity domain where the laser field produces a substantial number of positrons but does not yet launch the cascade exists. Within this range, the intensity diagnostics is particularly accurate. By adjusting the geometry of the experimental setup, one can change the corresponding threshold value of the laser intensity in order to maximize the accuracy of the diagnostic scheme.