Study of channel formation and relativistic ultra-short laser pulse propagation in helium plasma

In this study, plasma channel formation in pure He plasma (ionization electron density 0.01–0.1n _c) interacting with ultra-short relativistic laser pulses (50 fs, >10¹⁹ W cm⁻²) was observed and analyzed. By appropriately selecting the laser pulse and gas backing pressure of the gas jet, a clear density channel longer than 300 μm and wider than 25 μm was achieved in less than 1.5 ps following the passage of the laser pulse, with a radial electron density gradient of ∼10²³ cm⁻⁴ at the channel walls. Numerical simulations for studying the affects of the plasma density, kinetic motion of electrons and ions, and nonlinear laser propagation on the plasma channel formation were carried out, which reproduced the experimental features. These density channels were mainly driven by the radial expulsion of plasma ions, with strong continuous laser self-focusing acting to improve the channeling efficiency. These channels can guide the propagation of ultra-intense laser pulses and supply several advanced applications in high-energy physics, including fast-ignition inertial confinement fusion, plasma-based particle accelerations, and sources of radiation. (paper)