Mitigation of the detrimental role of the longitudinal laser electric field during direct laser acceleration of electrons

Next generation laser facilities around the world are rapidly increasing the power capabilities of their lasers. In research studying high-intensity laser-plasma interactions, much of the focus has been to leverage this promising power to significantly enhance the field intensities of the laser at focus. Notably, the peak energy of an electron in vacuum irradiated by a high-intensity plane electro-magnetic wave scales linearly with the laser’s peak intensity. For a tightly focused laser however, the longitudinal components of the laser fields necessarily increase and can prove to be detrimental. By using particle-in-cell (PIC) kinetic simulations, test-particle models, and theoretical results, we show that a less tightly focused laser of the same power can indeed generate noticeably higher energy electrons. In particular, we apply this analysis to a high-intensity laser-plasma setup where plasma electrons have prolonged interactions with the laser through a 1cm long plasma target. In doing so, we highlight the importance of balancing the benefits of high-intensity laser fields with the detriments of the longitudinal components of the laser fields in direct laser acceleration of plasma electrons.