  A method based on laser wakefield acceleration with controlled ionization
injection triggered by another frequency-tripled laser is proposed, which can
produce electron bunches with low energy spread. As two color pulses
co-propagate in the background plasma, the peak amplitude of the combined laser
field is modulated in time and space during the laser propagation due to the
plasma dispersion. Ionization injection occurs when the peak amplitude exceeds
certain threshold. The threshold is exceeded for limited duration periodically
at different propagation distances, leading to multiple ionization injections
and separated electron bunches. The method is demonstrated through
multi-dimensional particle-in-cell simulations. Such electron bunches may be
used to generate multi-chromatic X-ray sources for a variety of applications.
