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 (1010) laser pulses with thin Al foil targets at 30 angle of laser incidence is studied. Proton maximum energies of 30 and 18 MeV are measured along the target normal rear and front sides, respectively, showing intensity scaling as Ib. For the target front bf ront ¼ 0.5–0.6 and for the target rear brear ¼ 0.7–0.8 is observed in the intensity range 1020–1021 W/cm2 . The fast scaling from the target rear I0:75 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 I0:5 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.