The dependence of the mean kinetic energy of laser-accelerated electrons on the laser intensity, so-called ponderomotive scaling, was derived theoretically with consideration of the motion of a single electron in oscillating laser fields. This scaling explains well the experimental results obtained with high-intensity pulses and durations shorter than a picosecond; however, this scaling is no longer applicable to the multi-picosecond (multi-ps) facility experiments. Here, we experimentally clarified the generation of the super-ponderomotive-relativistic electrons (SP-REs) through multi-ps relativistic laser-plasma interactions using prepulse-free LFEX laser pulses that were realized using a plasma mirror (PM). The SP-REs are produced with direct laser acceleration assisted by the self-generated quasi-static electric field and with loop-injected direct acceleration by the self- generated quasi-static magnetic field, which grow in a blowout plasma heated by a multi-ps laser pulse. Finally, we theoretically derive the threshold pulse duration to boost the acceleration of REs, which provides an important insight into the determination of laser pulse duration at kilojoule- petawatt laser facilities.
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