An investigation of the transfer of orbital angular momentum in the two-plasmon decay instability*

The two plasmon decay (TPD) instability is the parametric decay of laser photons into electron plasma waves (EPWs). It occurs in plasmas where the plasma density is near the quarter critical density for the incident laser light and when the intensity of the laser light is sufficiently high. While of significant fundamental interest, it is also a major preheat concern for inertial confinement fusion (ICF). For parameters relevant for directly driven ICF, this talk describes the qualitative and quantitative effects of orbital angular momentum (OAM), carried by the incident light, on the decay process for linearly polarized beams. This is performed by numerically simulating the decay of Laguerre-Gauss beams using the Laser Plasma Simulation Environment (LPSE); the LPSE code solving the coupled wave equations responsible for TPD. The resulting transfer of angular momentum from the electromagnetic field to the mechanical angular momentum carried by the EPWs is observed, analyzed and quantified using a new LPSE angular momentum diagnostic. A geometrical optics (ray tracing) approach is found to qualitatively describe the results of angular momentum transfer and the effect of laser beam angular momentum on the observed growth rates. On this basis, the viability of mitigating TPD through the introduction of laser beam OAM with contemporary beam shaping techniques is assessed.