Generation of Electron Beams exceeding 100 keV by Direct Laser Acceleration using Longitudinal Electric Fields*

We report on the generation of electron beams exceeding 100 keV by direct laser acceleration in a low-density gas using longitudinal electric fields. The utilization of radially polarized laser modes to generate longitudinal electric fields is a promising research area in the laser-based electron acceleration community. The Advanced Light Laser Source (ALLS) provided a 3 mJ, 12 fs pulse, centered at 1.8 µm with a repetition rate of 100 Hz. A TM₀₁ laser mode is created using a polarization state convertor and focused with an on-axis parabola (NA = 0.4) in a low-density molecular oxygen environment. The large longitudinal electric field component at the focus is theoretically capable of accelerating electrons up to relativistic energies with sub-femtosecond bunch durations. Our results in the gigawatt regime show the ability to create an electron beam up to a measured energy of 180 keV by scaling of the incident laser pulse energy. At these high intensities (> 10¹⁷ W/cm²), factors such as carrier envelop phase, non-optimal electron injection, nonlinear propagation effects and space charge can all play an increasing complex role in the acceleration mechanism. Further optimization coupled with increasing the laser pulse energy provides a viable pathway towards the ability to generate an energetic, ultrashort electron beam.