Description
Orbital angular momentum (OAM) transfer in laser-plasma interactions is a fundamental process far less understood than energy or linear momentum transfer. However, it has important direct applications, such as the production of strong magnetic fields.
Previous studies of angular momentum transfer in laser–plasma interactions have investigated both dissipative processes, as well as single electron motion in laser fields carrying OAM.
In this work, we present a novel ray-based approach of laser OAM transfer to plasmas. In this framework, refraction in plasmas with azimuthal density gradients enables non-dissipative transfer of OAM between the laser and plasma. These gradients can be pre-existing in the plasma, or self-generated by a spatiotemporally shaped laser pulse. This process is analogous to the mechanism behind orbital AM transfer in spiral phase plates. Our model provides a simple unified interpretation of OAM transfer in many laser-plasma interactions and can be used to explain previous results.
Utilizing recent breakthroughs in spatio-temporal shaping of high power laser systems, we show that light springs - lasers carrying multiple OAM modes in multiple frequency bands, can indeed drive plasma waves with helical density gradients, and self couple into the plasma driving strong axial magnetic fields. Using SMILEI particle-in-cell simulations, we validate this model and show that light springs are efficient at transferring the AM to electrons, leading to the generation of strong longitudinal magnetic fields (a few 100s of Tesla) at moderate laser power (a few 100s of GW to a few TW).