Simulations of low-density plasma channel formation through a range of collisionalities

This work presents one-dimensional, cylindrical, particle-in-cell (PIC-DSMC) simulations of low-density plasma channel formation. Several novel PIC techniques make these simulations more feasible including the use of energy-conserving PIC algorithms and an aggressive smoothing algorithm designed to combat particle noise when the Debye length is severely underresolved [1,2]. The formation of plasma channels with very low density on axis for use as waveguides is critical in laser wakefield acceleration (LWFA) because the maximum achievable particle energy gain in an LWFA stage scales as n₀^-3/2 where n₀ is the on-axis electron density. Thus, the challenge of coupling LWFA stages can be mitigated by moving to lower plasma densities (albeit at the cost of longer individual stages). Low-density channel formations have been demonstrated experimentally by using the hydrodynamic expansion of optically field ionized electrons (HOFI) to drive a shock into the surrounding neutrals [3]. However, at sufficiently low gas densities the neutral molecule mean free path becomes comparable to the channel diameter, and thus a shock may not be formed. The PIC simulations in this work range over the neutral gas density first matching experimental and hydrodynamic simulations results at neutral hydrogen density of 2 × 10¹⁸ / cm³ [4], and then reducing the neutral gas pressure until shock formation is no longer observed.

[1] L.C. Adams et al., arXiv:2503.13697

[2] G.R. Werner et al., arXiv:2503.05123

[3] R.J. Shalloo et al., Phys. Rev. Accel. Beams, 041302 (2019)

[4] B. Miao et al., Phys. Rev. Accel. Beams, 081302 (2024)