Experimental realization of near-critical-density laser wakefield acceleration: Efficient pointing 100-keV-class electron beam generation by microcapillary targets

We experimentally demonstrated the generation of a pointing stable, low-divergence, low-energy electron beam driven by near-critical-density laser wakefield acceleration using a moderate low-intensity laser pulse. Electron beams with a half-beam divergence angle of similar to 30 mrad were generated at laser intensities of 4 x 10(16)-1 x 10(18) W/cm(2) from a microcapillary hole. The pointing fluctuation of the electron beam was 1.8 mrad (root-mean-square) at the maximum laser intensity of 1 x 10(18) W/cm(2). The energies of the electron beam were up to 400 keV at 1 x 10(18) W/cm(2) and 50 keV even at 1 x 10(16) W/cm(2). We confirmed that the peak energy of the hump or cutoff energy of the electron beams was reproduced in particle-in-cell simulation. Such low divergence electron beam generation at sub-relativistic intensity (10(16) to 10(17) W/cm(2) order) will lead to various applications of laser-driven keV-class electron beams, such as advanced radiotherapy.