  Experimental measurements using the OMEGA EP laser facility demonstrated
direct laser acceleration (DLA) of electron beams to (505 $\pm$ 75) MeV with
(140 $\pm$ 30)~nC of charge from a low-density plasma target using a 400 J,
picosecond duration pulse. Similar trends of electron energy with target
density are also observed in self-consistent two-dimensional particle-in-cell
simulations. The intensity of the laser pulse is sufficiently large that the
electrons are rapidly expelled from along the laser pulse propagation axis to
form a channel. The dominant acceleration mechanism is confirmed to be DLA and
the effect of quasi-static channel fields on energetic electron dynamics is
examined. A strong channel magnetic field, self-generated by the accelerated
electrons, is found to play a comparable role to the transverse electric
channel field in defining the boundary of electron motion.
