Fig. 1. Raw images of electron beam energy spectra for 15 shots divided into 5 groups, each group is for a fixed gas mixture concentration. (a) 3 typical spectra for beams generated from laser-driven pure He gas jet, (b) results for 0.1% $\text{N}_{2}$ mixed in 99.9% of He, (c) results for 0.3% $\text{N}_{2}$ mixed in 99.7% of He, (d) 0.5% $\text{N}_{2}$ mixed in 99.5% of He, and (e) 1% $\text{N}_{2}$ mixed in 99% of He. For (a–e), the unmatched laser–plasma parameter $k_{p}w_{0}$ is 11.2, 11.8, 13.6, 11.8, and 10.8, respectively. The laser power for all the shots is 30 TW level, and the helium electron density is shown for each group.

Fig. 2. Monoenergetic peak energy and FWHM energy spread of electron beams as a function of laser power for four different concentrations of nitrogen–helium gas mixture targets: (a) 0.1% $\text{N}_{2}$ mixed in 99.9% of He, (b) 0.3% $\text{N}_{2}$ mixed in 99.7% of He, (c) 0.5% $\text{N}_{2}$ mixed in 99.5% of He, and (d) 1% $\text{N}_{2}$ mixed in 99% of He. The helium plasma density is $5.0\times 10^{18}~\text{cm}^{-3}$ in all plots, expect for the case of (d) where the density range is slightly different. The unmatched laser–plasma parameters for all points in this graphs are in the range of $k_{p}w_{0}\sim 10.8{-}12.1$ and $2(a_{0})^{1/2}\sim 1.9{-}2.2$.

Fig. 3. 3D-PIC simulation results using OSIRIS code. Panels (a–c) are results from ionization injection, while (d–f) are from self-injection in pure helium, detailed parameters are shown in the text. (a) and (d) Evolution of the maximum laser electric field and pseudopotential difference; (b) and (e) injected electron charge along the propagation; (c) and (f) electron energy spectra.