Relativistic Electron Acceleration with Ultrashort Mid-IR Laser Pulses*

We report the first results of laser plasma wakefield acceleration driven by ultrashort mid-infrared laser pulses ($\lambda =$ 3.9 $\mu m$, pulsewidth 100 fs, energy \textless 20 mJ, peak power \textless 1 TW)), which enables near- and above-critical density interactions with moderate-density gas jets. We present thresholds for electron acceleration based on critical parameters for relativistic self-focusing and target width, as well as trends in the accelerated beam profiles, charge and energy spectra which are supported by 3D particle-in-cell simulations. These results extend earlier work with sub-TW self-modulated laser wakefield acceleration using near IR drivers [1] to the Mid-IR, and enable us to capture time-resolved images of relativistic self-focusing of the laser pulse. [1] 1. A.J. Goers \textit{et al}., Phys. Rev. Lett. \textbf{115}, 194802 (2015)