Radiation Reaction Effects on Particle Dynamics in Intense Counterpropagating Laser Pulses

The radiation-dominated regime, in which charged particles lose a significant fraction of their energy to their own radiation, will be within the reach of next-generation high-power laser facilities. In this work, we identify a new signature of radiation reaction in all-optical experiments beyond the 10 PW scale. Using one- and two-dimensional particle-in-cell simulations, we study the dynamics of a low-density plasma target struck by counterpropagating circularly polarized laser pulses. In particular, we consider the case where one pulse has both a shorter wavelength and a lower normalized vector potential. When radiation reaction is neglected, the longitudinal particle motion is typically biased in the direction of the shorter-wavelength pulse. However, we find a regime where radiation reaction reverses the dominant direction such that the particles move with the higher-intensity pulse. Through single-particle considerations, we find three inequalities that approximately bound this regime and depend only on the wavelength, intensity, and pulse duration ratios of the two lasers as well as the product of a dimensionless radiation reaction parameter with the pulse duration. The bounds agree with our simulations. Finally, we suggest an experimental procedure by which this effect could be investigated.