Probing the role of kinetic effects in simulations of collisionless plasmas by varying velocity distributions with shared low-order moments

We conduct numerical experiments where we vary the velocity distribution function to better understand the importance of wave particle interactions on the evolution of fundamental collisionless plasma processes. Specifically, we conduct particle-in-cell (PIC) simulations initialized with different velocity distributions that share identical lower order moments (up to order 3), but differ in their underlying shape, such as Maxwellian, Waterbag, and Lorenz distributions. We first illustrate this approach on the problem of Raman scattering in the kinetic regime, where we show the significant dependence of reflectivity on the shape of the initial velocity distribution. Next, we apply the technique to simulations of collisionless magnetic reconnection and demonstrate a weak sensitivity to the details of the distribution function. This technique allows us to isolate whether the observed dynamics in both Raman scattering and magnetic reconnection stem from kinetic effects or from low-order moment fluid behavior (velocity structure agnostic). Our improved understanding of the kinetic effects and how they couple to the large-scale plasma fluid behavior informs the development of reduced plasma models, crucial for multi-scale modeling of collisionless plasmas relevant in fusion devices and astrophysical environments.