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.