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Experiments were performed to study electron acceleration by intensesub-picosecond laser pulses propagating in sub-mm long plasmas of near criticalelectron density (NCD). Low density foam layers of 300-500 um thickness wereused as targets. The NCD-plasma was produced by a mechanism of a super-sonicionization when a well-defined separate ns-pulse was sent onto the foam-targetforerunning the relativistic main pulse. The effect of the relativistic laserpulse channeling and creation of quasi-static azimuthal magnetic and radialelectric fields that keeps electrons in the channel ensured effective couplingof the laser energy into energetic electrons. Application of sub-mm thick lowdensity foam layers provided substantial increase of the electron accelerationpath in a NCD-plasma compared to the case of freely expanding plasmas createdin the interaction of the ns-laser pulse with solid foils. Performedexperiments on the electron heating by a 100J, 750 fs short laser pulse of(2-5)x1019 W/cm2 intensity demonstrated that the effective temperature ofsupra-thermal electrons increased from 1.5-2 MeV, in the case of therelativistic laser interaction with a metallic foil at high laser contrast, upto 13 MeV for the laser shots onto the pre-ionized foam. The observed tendencytowards the strong increase of the mean electron energy and the number ofultra-relativistic laser-accelerated electrons is reinforced by the results ofgamma-yield measurements that showed a 1000-fold increase of the measureddoses. The experiment was supported by the 3D-PIC and FLUKA simulations madefor used laser parameters and geometry of the experimental set-up. Bothmeasurements and simulations show high directionality of the accelerationprocess close to the direction of the laser pulse propagation. The charge ofsuper-ponderomotive electrons with E > 30 MeV reaches a high value of 78nC.

Interaction of relativistically intense laser pulses with long-scale  near critical plasmas for optimization of laser based sources of MeV  elec-trons and gamma-rays