Diagnosing ‘super-ponderomotive’ electron energy distribution with electron-positron pair production at LFEX*

Increasing the ‘super-ponderomotive’ electron population in laser-solid interactions would greatly benefit ion acceleration schemes and x-ray and positron generation. In this work we explore the possibility of using long laser pulse durations (~2 ps) than previous (~0.7 ps), large scale length plasmas (tens of microns) and high-laser intensities (10¹⁹-10²⁰ W/cm²) to increase the ‘super-ponderomotive’ electron population, at energies beyond the ponderomotive potential. Using the high-intensity laser LFEX we successfully generated ‘super-ponderomotive’ electron distributions by controlling the pre-plasma scale length (changing the laser pulse contrast) and target geometry. As a consequence of the increased ‘super-ponderomotive’ electron distribution, positron beams were generated and enhanced by one order of magnitude with a 9x10^-9 pulse contrast and a wedged target.