Ion acceleration in electrostatic field of charged cavity created by ultra-short laser pulses of 1020-1021 W/cm2

Ion acceleration resulting from the interaction of ultra-high intensity and ultra-high contrast (similar to 10(-10)) laser pulses with thin Al foil targets at 30 degrees angle of laser incidence is studied. Proton maximum energies of 30 and 18MeV are measured along the target normal rear and front sides, respectively, showing intensity scaling as I-b. For the target front b(f ront) = 0.5-0.6 and for the target rear b(rear) = 0.7-0.8 is observed in the intensity range 10(20)-10(21) W/cm(2). The fast scaling from the target rear similar to I-0.75 can be attributed enhancement of laser energy absorption as already observed at relatively low intensities. The backward acceleration of the front side protons with intensity scaling as similar to I-0.5 can be attributed to the to the formation of a positively charged cavity at the target front via ponderomotive displacement of the target electrons at the interaction of relativistic intense laser pulses with a solid target. The experimental results are in a good agreement with theoretical predictions. Published by AIP Publishing.