Description
In Direct Laser Acceleration (DLA), energy is transferred directly from the fields of a laser pulse to electrons. Nonlinear propagation of the pulse, including the effects of self-focusing and filamentation, can interfere with DLA by distorting both the fields of the pulse and the quasi-static fields in the ion channel it produces. Here we show that a flying focus (FF) laser pulse can mitigate nonlinear propagation and generate a stable, uniform ion channel over the entire acceleration length, leading to higher electron energies and enhanced x-ray emission. The FF enables controlled focusing of the pulse, with a focal point that moves at an arbitrary sub- or super-luminal velocity over a distance far greater than a Rayleigh range. Three-dimensional particle-in-cell simulations of DLA in near-critical plasma indicate that a FF laser pulse with an optimal focal velocity yields a charge 80 times greater for electrons above 100 MeV and an electron cutoff energy 20% higher than a conventional Gaussian pulse with the same energy. Moreover, the resulting betatron radiation extends to higher energies, has a larger conversion efficiency, and is more collimated.