Electron energy boosting in laser-wake-field acceleration with external magnetic field B∼1 T and laser prepulses

Hundred-mega-electron-volt electron beams with quasi-monoenergetic distribution, and a transverse geometrical emittance as small as ∼0.02 π mm mrad are generated by low power (7 TW, 45 fs) laser pulses tightly focused in helium gas jets in an external static magnetic field, B∼1 T. Generation of monoenergetic beams strongly correlates with appearance of a straight, at least 2 mm length plasma channel in a short time before the main laser pulse and with the energy of copropagating picosecond pedestal pulses (PPP). For a moderate energy PPP, the multiple or staged electron self-injection in the channel gives several narrow peaks in the electron energy distribution.