Enhancement of laser-driven electron acceleration in an ion channel

A laser beam with duration longer than the period of plasma oscillations propagating through an underdense plasma produces a steady-state positively charged channel in the electron density. We consider a test electron in the two-dimensional plane channel under the combined action of the laser field and the transverse static electric field of the channel. At ultrarelativistic laser wave amplitude (a >> 1), the electron is pushed primarily forward. As the electron gradually dephases from the wave, the field it samples and its relativistic gamma-factor strongly oscillate. The natural frequency of electron oscillations across the channel (betatron frequency) depends on gamma, which couples the betatron oscillations to the longitudinal motion induced by the wave. We show that the modulation of the natural frequency makes the oscillations unstable. The resulting amplification of the oscillations across the channel reduces the axial dephasing between the electron and the wave, leading to a considerable electron energy enhancement well above the ponderomotive energy. We find that there is a well-pronounced laser amplitude threshold a(*), above which the enhancement takes place, that scales as a(*) alpha 1/root n(0), where n(0) is the ion density. The presented mechanism of energy enhancement is robust with respect to a longitudinal variation of the density, because it relies on a threshold phenomenon rather than on a narrow linear resonance. (C) 2014 AIP Publishing LLC.