Boosting electron energy in direct laser acceleration through longitudinal gradients of plasma magnetic field

In ultra-high intensity laser-plasma interactions, direct laser acceleration of electrons is a critical mechanism for depositing laser energy into the plasma and generating ultra-relativistic electrons. During propagation, the laser beam tends to drive a longitudinal plasma current that creates and sustains a quasi-static azimuthal plasma magnetic field. This field facilitates energy transfer from the transverse laser electric field to plasma electrons by inducing betatron oscillations. Energy gain is particularly efficient when the betatron frequency matches the frequency of laser field oscillations at the electron location. However, the differing dependence of these two frequencies on electron energy ultimately terminates the energy gain. We found that longitudinal gradients of the plasma magnetic field can offset this frequency difference, significantly boosting electron energy gain compared to the commonly considered case where there is no longitudinal dependence.