  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.
