  A new method for diagnosing the temporal characteristics of ultrashort
electron bunches with linear energy chirp generated from a laser wakefield
accelerator is described. When the ionization-injected bunch interacts with the
back of the drive laser, it is deflected and stretched along the direction of
the electric field of the laser. Upon exiting the plasma, if the bunch goes
through a narrow slit in front of the dipole magnet that disperses the
electrons in the plane of the laser polarization, it can form a series of
bunchlets that have different energies but are separated by half a laser
wavelength. Since only the electrons that are undeflected by the laser go
through the slit, the energy spectrum of the bunch is modulated. By analyzing
the modulated energy spectrum, the shots where the bunch has a linear energy
chirp can be recognized. Consequently, the energy chirp and beam current
profile of those bunches can be reconstructed. This method is demonstrated
through particle-in-cell simulations and experiment.
