Variation in electron emission time in weakly nonlinear laser wakefield acceleration

As an advanced accelerator concept, laser wakefield acceleration, with its attractive acceleration gradient, has invoked great interest worldwide. The electron beams from laser wakefield acceleration were assumed to have the advantage of ``jitter-free" or relatively small jitter, since the emergence of quasi-monoenergetic bunch structures were considered to result from the trapping of electrons into the first bucket of the plasma wave. However, the emission times of the electron bunches from laser wakefield acceleration have not been real time monitored in experiments up to now. To examine this, we introduced the electro-optic spatial decoding method into laser wakefield experiments. The relative emission times of the electrons were observed to have variations with different plasma densities when using a laser with moderate intensity. The study shows that timing issues should not be ignored in some occasions of laser wakefield acceleration experiments.