In the laser wakefield accelerator (LWFA) a short intense laser pulse, with a
duration of the order of a plasma wave period, excites an unusually strong
plasma wake wave (laser wakefield). Recent experiments on laser wakefield
acceleration [Nature (London) 431, p.535, p.538, p.541 (2004)] demonstrated
generation of ultra-short (with a duration of a few femtoseconds) relativistic
electron bunches with relatively low energy spread of the order of a few
percent. We have studied the dynamics of such bunches in vacuum and in laser
wakefield. The results show strong bunch dynamics already on a few millimeters
propagation distance in both cases. In vacuum, the bunch radius and emittance
quickly grow. The latter worsens the focusability of the bunch. We found that
when a femtosecond bunch is accelerated in a channel-guided laser wakefield,
for realistic bunch lengths, the bunch length is approximately conserved.
However, the spread in betatron frequencies leads to fast betatron phase mixing
in the bunch envelope for on-axis injection. When bunch is injected in a laser
wakefield off-axis, the bunch decoherence results in considerable increase in
the normalized bunch emittance, and, in some cases, in increase in the energy
spread, after acceleration. We also discuss a possible two-stage laser
wakefield accelerator.Comment: 34 pages, incl. 11 figs. and 1 table, submitted for publicatio