Laser wakefield acceleration of electrons with ionization injection in a pure N5$^{+}$ plasma channel*

Preformed plasma channels have been successfully used in laser wakefield acceleration to accelerate electrons up to \textit{GeV} with modest laser intensity by eliminating the need for self-focusing. Here, we show that nitrogen is an excellent medium for ionization injection-based laser wakefield accelerators because of the extremely large ionization potential gap between the L-shell (98\textit{eV} to ionize N4$^{+}\to $N5$^{+})$ and K-shell electrons (552\textit{eV} to ionize N5$^{+}\to $N6$^{+})$. We have measured pure N5$^{+}$ plasma channels with a base density of $1\sim 5\times 10^{18}cm^{-3}$ and shock walls at $\sim 2\times 10^{19}cm^{-3}$ through hydrodynamic expansion of nitrogen cluster plasma. In this N5$^{+}$ plasma channel, we can decrease the laser intensity threshold for trapping and accelerating electrons by ionization injection and channel guiding. Particle-In-Cell simulations confirm trapping of electrons from N5$^{+}$ by tunneling ionization with initial laser intensity of $a_{0} =1$. Injection from the abundant N5$^{+}$ ionization source enables the space charge of the trapped electrons to stop further injection. The accelerated bunch can reach hundreds of \textit{pC} with energy gain of hundreds of \textit{MeV}. We will present preliminary results from corresponding acceleration experiments.