Observation of a quasimonoenergetic electron beam from a femtosecond prepulse-exploded foil

We report the first observation of a quasimonoenergetic electron beam emitted from a femtosecond prepulse-exploded foil. At 5ns earlier, a 7.5-μm-thick plastic foil is irradiated by a copropagating femtosecond prepulse of modest intensity (1.5×1017W∕cm2), which then explodes to become an underdense foil plasma. When a 70-fs high-intensity (2.5×1018W∕cm2) pulse arrives and interacts with it, a collimated electron beam of divergence angle ≲4° is emitted in the forward direction. The energy spectrum of the electron beam shows a quasimonoenergetic peak at 0.63MeV of energy spread ∼21%, within which about 50 pC of charge is contained.