Acceleration
of plasma electrons by laser wake-filed is the most promising process
to produce the next generation of compact accelerators because the very
high acceleration gradient, as high as 100 GV/m, can be obtained through
the interaction of ultraintense laser fields with plasmas. We describe
here updated results of laser plasma cathode experiments at the University
of Tokyo. To achieve the efficient and stable mono-energetic acceleration
of plasma electrons, we studied the effects of a laser pre-pulse, the
plasma density gradient, and wave-breaking processes through single
shot measurements, simultaneously observing the spectrum of accelerated
electrons. Strong correlation between the laser pre-pulse forming the
plasma cavity and electron acceleration was directly observed.