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 μm thickness were used as targets. In foams, the NCD-plasma was produced by a mechanism of super-sonic ionization when a well-defined separate ns-pulse was sent onto the foamtarget forerunning the relativistic main pulse. The application of sub-mm thick low density foam layers provided a 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. The performed experiments on the electron heating by a 100 J, 750 fs short laser pulse of 2-5×10 19 W cm −2 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 a strong increase of the mean electron energy and the number of ultra-relativistic laseraccelerated electrons is reinforced by the results of gamma-yield measurements that showed a 1000fold increase of the measured doses. The experiment was supported by 3D-PIC and FLUKA simulations, which considered the laser parameters and the geometry of the experimental set-up. Both, measurements and simulations showed a high directionality of the acceleration process, since the strongest increase in the electron energy, charge and corresponding gamma-yield was observed close to the direction of the laser pulse propagation. The charge of super-ponderomotive electrons with energy above 30 MeV reached a very high value of 78 nC.

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