  We present an analytical theory that reveals the importance of the
longitudinal laser electric field in the resonant acceleration of relativistic
electrons by the tightly confined laser beam. It is shown that this field
always counterworks to the laser transverse component and effectively decreases
the final energy gain of electrons through direct laser acceleration mechanism.
This effect is demonstrated by carrying out the particle-in-cell simulations in
the setup where the wakefield in the plasma bubble is compensated by the
longitudinal laser electric field experienced by the accelerated electrons. The
derived scalings and estimates are in good agreement with numerical
simulations.

Plasma Physics