  We use analytical methods and particle-in-cell simulation to investigate the
origin of electrons accelerated by the process of direct laser acceleration
driven by high-power laser pulses in preformed narrow cylindrical plasma
channels. The simulation shows that the majority of accelerated electrons are
originally located along the interface between the channel wall and the channel
interior. The analytical model based on the electron hydrodynamics illustrates
the underlying physical mechanism of the release of electrons from the channel
wall when irradiated by an intense laser, the subsequent electron dynamics, and
the corresponding evolution of the channel density profile. The quantitative
predictions of the total charge of released electrons and the average electron
density inside the channel are validated by comparison with the simulation
results.
