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Electron confinement by laser-driven azimuthal magnetic fields during direct laser acceleration

A laser-driven azimuthal plasma magnetic field is known to facilitate electron energy gain from the irradiating laser pulse. The enhancement is due to changes in the orientation between the laser electric field and electron velocity caused by magnetic field deflections. Transverse electron confinement is critical for realizing this concept experimentally. We find that the phase velocity of the laser pulse has a profound impact on the transverse size of electron trajectories. The transverse size remains constant below a threshold energy that depends on the degree of the superluminosity and it increases with the electron energy above the threshold. This increase can cause electron losses in tightly focused laser pulses. We show using 3D particle-in-cell simulations that the electron energy gain can be significantly increased by increasing laser power at fixed intensity due to the increased electron confinement. This finding makes a strong case for designing experiments at multi-PW laser facilities.