Meter-scale non-axismmetric waveguides for laser wakefield acceleration

We present a new method for forming long plasma waveguides using modified Bessel beams and an extended gas jet. In recent work [1], we demonstrated a two pulse method for forming plasma waveguides: a $J_0$ Bessel beam ionizes a plasma column via optical field ionization which expands outwards, then a high order $J_q$ Bessel beam follows at a delay, ionizing a ring at the edge of the expanded plasma, forming a plasma guiding structure with on axis densities as low as $5\times10^{16}~ cm^{-3}$ and attenuation lengths up to several meters. High-power guiding was demonstrated in a $5~cm$ long gas jet, but for longer jets the guide quality was significantly limited by obstruction of the focusing $J_q$ beam by the gas nozzle. Here, we demonstrate mitigation of this problem using a modified ‘binary’ $J_q$ formed with a $\pi$-step phase plate [2], which generates a non-axisymmetric Bessel-like focal line with resistance to focal deterioration in the presence of beam obstructions. Using such a beam in our two-pulse waveguide formation technique can generate plasma waveguides in gas jet targets several tens of $cm$ long. [1] Miao, B., et al. arXiv preprint arXiv:2005.14389 (2020). [2] Shutova, M., et al., J. Opt. Soc. Am. B 36, 1313-1319 (2019)