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This paper presents fully kinetic particle simulations of plasma charging at lunar craters with the presence of lunar lander modules using the recently developed Parallel Immersed-Finite-Element Particle-in-Cell (PIFE-PIC) code. The computation model explicitly includes the lunar regolith layer on top of the lunar bedrock, taking into account the regolith layer thickness and permittivity as well as the lunar lander module in the simulation domain, resolving a nontrivial surface terrain or lunar lander configuration. Simulations were carried out to study the lunar surface and lunar lander module charging near craters at the lunar terminator region under mean and severe plasma environments. The lunar module’s position is also investigated to see its effect on the plasma charging relative to the craters. Differential surface charging was clearly resolved by the simulations. For the charging of a lunar lander module made of conducting materials, the results show a near-uniform potential close to that of its surrounding environment and moderate levels of local electric fields. Additionally, the risks associated with charging and discharging increase significantly under a more severe plasma charging environment as shown in the severe plasma environment cases.

This article presents the derivation of semianalytic solutions to a new 1-D photoelectron sheath model near the lunar surface. The plasma species include the cold solar wind protons, drifting Maxwellian solar wind electrons, and Maxwellian photoelectrons emitted from the surface. The semianalytic model is then numerically solved to obtain profiles of quantities of interest as functions of the vertical distance from the surface. A fully-kinetic 3-D finite-difference (FD) particle-in-cell (PIC) code is then utilized to simulate the 1-D photoelectron sheath and the results agree well with the numerical solution to the semianalytic model. A $\kappa $ -distribution for solar wind electrons is also implemented to the FD-PIC code to compare with the Maxwellian distribution. Results show that photoelectron sheath may reach as high as close to 100 m above the illuminated flat lunar surface near the terminator region and up to about 50 m near the equator region. Our results show that under average solar wind condition, the photoelectron sheath profiles obtained with Maxwellian and $\kappa $ -distribution (with $\kappa = 4.5$ ) are very close for 1-D numerical results.

This paper presents a recently developed particle simulation code package PIFE-PIC, which is a novel three-dimensional (3-D) parallel immersed finite element (IFE) particle-in-cell (PIC) simulation...

This paper presents a 3-D, fully kinetic particle-in-cell (PIC) simulation study of plasma charging for small, irregularly shaped, composite asteroids in the solar wind. Three types of asteroid internal structures are considered: 1) a homogeneous composition of regolith; 2) a single hard rock; and 3) a composite of hard rock core covered with a regolith outer layer. The simulation model resolves the plasma flow field, asteroid surface differential charging, and asteroid internal electric field self-consistently. The results show that while the ambient plasma condition and surface property determine surface charging, the internal composition substantially affects the subsurface electric field inside an asteroid.

This paper presents a simulation study of the dynamics of dust grains around a small asteroid. Plasma-asteroid interactions and asteroid charging are obtained from fully kinetic particle-in-cell simulations. Dust transport simulation takes into account the electrostatic force, solar radiation pressure, and the gravitational force. Both the result derived from laboratory measurements of regolith surface charging and the single dust charging model are considered for dust charging. For the small asteroid considered in this paper, we find that dust transport is mostly determined by the competition between the electrostatic force and the solar pressure and dust distribution is sensitive to both dust charging and dust grain size. For dust grains in the tens of micrometer size range, the solar radiation pressure dominates dust transport at small dust charge-to-mass ratio, whereas the electrostatic force dictates dust transport at large dust charge-to-mass ratio.

This paper presents an adaptive Kriging based method to perform uncertainty quantification (UQ) of the photoelectron sheath and dust levitation on the lunar surface. The objective of this study is to identify the upper and lower bounds of the electric potential and that of dust levitation height, given the intervals of model parameters in the one-dimensional (1D) photoelectron sheath model. To improve the calculation efficiency, we employ the widely used adaptive Kriging method (AKM). A task-oriented learning function and a stopping criterion are developed to train the Kriging model and customize the AKM. Experiment analysis shows that the proposed AKM is both accurate and efficient.

Motivated by the need to handle complex boundary conditions efficiently and accurately in particle-in-cell (PIC) simulations, this paper presents a three-dimensional (3D) linear immersed finite element (IFE) method with non-homogeneous flux jump conditions for solving electrostatic field involving complex boundary conditions using structured meshes independent of the interface. This method treats an object boundary as part of the simulation domain and solves the electric field at the boundary as an interface problem. In order to resolve charging on a dielectric surface, a new 3D linear IFE basis function is designed for each interface element to capture the electric field jump on the interface. Numerical experiments are provided to demonstrate the optimal convergence rates in L 2 and H 1 norms of the IFE solution. This new IFE method is integrated into a PIC method for simulations involving charging of a complex dielectric surface in a plasma. A numerical study of plasma-surface interactions at the lunar terminator is presented to demonstrate the applicability of the new method.

Ion beam neutralization during operation of ion sources requires current coupling between beam ions and electrons from the neutralizer. The current and charge density equalization during neutralization is accomplished in practice, however, the exact process has not been adequately simulated/understood due to the wide range of conditions. In order to resolve the role of electrons in the beam neutralization process, full particle-in-cell (PIC) simulations are carried out using a high-fidelity electrostatic particle-in-cell code EUPIC1. In this talk we extend our previous work2-6 and present a broader view of ion beam neutralization process by electron emission. We consider an idealized 1 D configuration following a theoretical ion beam model7, where ions and electrons are injected from the same area into vacuum. Charge and current neutralization is assumed to take place at a certain distance downstream of the injection plane. The theoretical model includes dimensionless Beam Kinetic Energy (KEi) normalized by electron thermal energy (kTe) and predicts a dimensionless neutralization distance as a function of beam kinetic energy. Our simulation results demonstrates the current coupling of both fast and mesothermal ion beams as well as neutralization distances.

In this talk, we will report development progresses of PIFE-PIC, a 3-D Parallel Immersed Finite Element Particle-in-Cell code for particle simulations of plasmas1. This code is based on a recently developed non-homogeneous electrostatic IFE-PIC algorithm2, which is designed to handle complex plasma-material interface boundary conditions associated with irregular geometries using a Cartesian-mesh-based PIC. 3-D domain decomposition is used in both electric field-solve and particle-push to distribute the computation among multiple processors. A validation cases simulating the charging/sheath of a dielectric sphere immersed in a stationary plasma is carried out to demonstrate the capability of code. Timing benchmark and parallel efficiency will be investigated and discussed.

We present recent developments of a multi-weight collision method for the direct simulation Monte Carlo (DSMC) and collisional particle-in-cell (PIC) methods that have been widely used to simulate gases and partially-ionized plasmas. Single-weight methods in DSMC and collisional PIC are not suited for simulation of mixtures with large density differences. The multi-weight particle collision techniques developed so far have been largely unable to conserve energy and momentum during each collision. Our new multi-weight collision method conserves momentum and energy in each collision event. The method introduces a real collision counter (RCC) scheme for particle collision pair selection that extends the existing No-Time Counter (NTC) method for single-weight collisions. In addition, our method uses the concept of particle velocity spread that allows to conserve energy and momentum by splitting and merging colliding particles during each collision. The multi-weight collision method is verified by considering relaxation processes of monatomic gases, a one dimensional Couette flow problem, and a heat transfer problem.

The direct simulation Monte Carlo (DSMC) and particle-incell (PIC) methods have been widely used to model the kinetics of gases and plasmas. In both DSMC and PIC, the Boltzmann equation is solved by representing distribution functions with a number of macro-particles that correspond to volumes in a phase space. Typically the weight of the macro-particles i.e., the number of real particles is kept the same for all species. However, in many problems of interest, there are multiple species of very different densities that undergo collisions. Collisions of particles with different weights is very challenging problem because split\merge algorithms developed so far don't allow to conserve energy and momentum in the same time per each collision event. In this talk, we present the theory, development, and preliminary results of a novel multi-weight collision algorithm. The new algorithm is capable of handling collisions between species of different densities using particle-dependent weights, while conserving both momentum and energy in each collision event. Preliminary results for elastic collisions demonstrate the concept of the new multi-weight collision algorithm and show the relaxation process of monatomic gases.

Ion beam neutralization during operation of ion sources requires current coupling between beam ions and electrons from the neutralizer. The current and charge density equalization during neutralization is practically accomplished in practice, however the exact process has not been adequately simulated due to the wide range of conditions. In order to resolve the role of electrons in the beam neutralization process, full particle-in-cell (PIC) simulations are carried out using a high-fidelity electrostatic particle-incell code (EUPIC).