Optimum conditions of the plasma density for the generation of the maximum bunch charge in a laser wakefield accelerator

The effects of plasma density on the bunch charge in a laser wakefield accelerator were investigated by using experiments and simulations. We used an energy spectrometer and Integrating Current Transformer (ICT) to measure the energy and the charge of a quasi-monoenergetic electron bunch. Also, the plasma density was measured using a biprism interferometer. The measured electron energy showed that the electron energy decreased with increasing plasma density due to the dephasing effect. The results of the bunch charge measurement showed that the effect of the plasma density on the bunch charge could be separated into two different density regions. In the low-density region (cτ/λp < 2), the bunch charge increased with increasing plasma density. In the high-density region (cτ/λp > 2), the bunch charge decreased with increasing plasma density. Thus, an optimum plasma density to generate a high bunch charge exists. Particle-in-cell simulations show that multi-cavitation of the plasma wake wave results in maximum generation of the bunch charge in a laser wakefield accelerator.