Two-dimensional Grid-based Vlasov Simulations of Electric Propulsion Plume*

In this work, a parallel, multi-dimensional grid-based Vlasov solver (Vlasolver) developed at USC is used to study the electric propulsion plume expansion. A comparison of the results from the Vlasov and PIC simulations shows that, while both models agree well in overall plume density and electric potential, the Vlasov model is able to resolve local electron density, temperature, heat flux, and electron velocity distribution without the effects of statistical noise. A core region and a fan region can be found in the electron heat flux. The core region remains a quasi-constant magnitude and shape, the fan region shape grows with time. Near-Maxwellian features are found in the v_x direction while top-hat shape features are found in the v_y direction for electron velocity distribution function (eVDF). The eVDF in the v_y direction at different locations can be mapped to the semi-analytical relations in the 1D finite-size plasma expansion by Mora et. al. This work also extends our recent study to investigate the excitation and propagation of electron-scale perturbations in the plume expansion. The grid-based Vlasov approach, though computationally more expensive than PIC, can be advantageous in applications requiring accurate eVDF and accurate small-scale physical properties.