Abstract
Ionization injection is a simple and efficient method to trap an electron beam in a laser plasma accelerator. Yet, because of a long injection length, this injection technique leads generally to the production of large energy spread electron beams. Here, we propose to use a shock front transition to localize the injection. Experimental results show that the energy spread can be reduced down to 10 MeV and that the beam energy can be tuned by varying the position of the shock. This simple technique leads to very stable and reliable injection even for modest laser energy. It should therefore become a unique tool for the development of laser-plasma accelerators.
Highlights
Note that the shock front is almost perpendicular to the optical axis within the plasma, Abel inversion can be performed using the optical axis as a symmetry axis
We demonstrated a controlled injection technique which gathers most of the advantages of ionization and shock front injections
Electron trapping is confined to a small region, leading to the injection of electron beams with rather low energy spreads
Summary
The experiment was performed at LOA using the salle jaune laser facility. The laser duration and focal spot were 28 fs and 12.2 × 15.7 μm[2] full width at half maximum (FWHM) respectively. The stability is much better in the gas mixture; the standard deviation of the electron beam energy and charge are 2.5% and 12% respectively, while in pure helium they are 7% and 24%. The energy spread of the tail is very similar to that of electron beams obtained without shock at the same density (Fig. 5d), confirming that this tail is due to regular ionization injection. A way to increase the beam charge while avoiding this detrimental effect is to rise the fraction of nitrogen in the gas mixture
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