  Experiments were performed to study electron acceleration by intense
sub-picosecond laser pulses propagating in sub-mm long plasmas of near critical
electron density (NCD). Low density foam layers of 300-500 um thickness were
used as targets. The NCD-plasma was produced by a mechanism of a super-sonic
ionization when a well-defined separate ns-pulse was sent onto the foam-target
forerunning the relativistic main pulse. The effect of the relativistic laser
pulse channeling and creation of quasi-static azimuthal magnetic and radial
electric fields that keeps electrons in the channel ensured effective coupling
of the laser energy into energetic electrons. Application of sub-mm thick low
density foam layers provided substantial increase of the electron acceleration
path in a NCD-plasma compared to the case of freely expanding plasmas created
in the interaction of the ns-laser pulse with solid foils. Performed
experiments on the electron heating by a 100J, 750 fs short laser pulse of
(2-5)x1019 W/cm2 intensity demonstrated that the effective temperature of
supra-thermal electrons increased from 1.5-2 MeV, in the case of the
relativistic laser interaction with a metallic foil at high laser contrast, up
to 13 MeV for the laser shots onto the pre-ionized foam. The observed tendency
towards the strong increase of the mean electron energy and the number of
ultra-relativistic laser-accelerated electrons is reinforced by the results of
gamma-yield measurements that showed a 1000-fold increase of the measured
doses. The experiment was supported by the 3D-PIC and FLUKA simulations made
for used laser parameters and geometry of the experimental set-up. Both
measurements and simulations show high directionality of the acceleration
process close to the direction of the laser pulse propagation. The charge of
super-ponderomotive electrons with E > 30 MeV reaches a high value of 78nC.
