Radiation reaction effects on particle dynamics in intense counterpropagating laser pulses

In high-intensity laser–plasma interactions, particles can lose a substantial fraction of their energy by emitting radiation. Using particle-in-cell simulations, we study the impact of radiation reaction on the dynamics of an underdense plasma target struck by counterpropagating circularly polarized laser pulses. By varying the relative wavelengths and intensities of the pulses, we find a range of parameters where radiation reaction can detrap electrons from the interference beat wave. The resulting charge separation field and the dominant direction of ion expulsion are thus reversed by radiative effects. Based on the electron dynamics during the interaction, we estimate the bounds on the parameter regime where the reversal occurs. The bounds take the form of three simple inequalities that depend only on the wavelengths, normalized vector potentials, and pulse duration ratios of the two lasers, as well as the product of the pulse duration with a dimensionless radiation reaction parameter. Our estimates, which predict whether radiation reaction will change the final ion direction for a given set of laser parameters, broadly agree with the simulated results. Finally, we outline an experimental procedure by which the reversal could be used to observe the transition to radiation-dominated dynamics.