Petr Cagas

Continuum kinetic and multi-fluid simulations of classical sheaths

The kinetic study of plasma sheaths is critical, among other things, to understand the deposition of heat on walls, the effect of sputtering, and contamination of the plasma with detrimental impurities. The plasma sheath also provides a boundary condition and can often have a significant global impact on the bulk plasma. In this paper, kinetic studies of classical sheaths are performed with the continuum code, Gkeyll, that directly solves the Vlasov-Poisson/Maxwell equations. The code uses a novel version of the finite-element discontinuous Galerkin (DG) scheme that conserves energy in the continuous-time limit. The electrostatic field is computed using the Poisson equation. Ionization and scattering collisions are included, however, surface effects are neglected. The aim of this work is to introduce the continuum-kinetic method and compare its results to those obtained from an already established finite-volume multi-fluid model also implemented in Gkeyll. Novel boundary conditions on the fluids allow the sheath to form without specifying wall fluxes, so the fluids and fields adjust self-consistently at the wall. The work presented here demonstrates that the kinetic and fluid results are in agreement for the momentum flux, showing that in certain regimes, a multi-fluid model can be a useful approximation for simulating the plasma boundary. There are differences in the electrostatic potential between the fluid and kinetic results. Further, the direct solutions of the distribution function presented here highlight the non-Maxwellian distribution of electrons in the sheath, emphasizing the need for a kinetic model.

Nonlinear saturation of the Weibel instability

The growth and saturation of magnetic fields due to the Weibel instability (WI) have important implications for laboratory and astrophysical plasmas, and this has drawn significant interest recently. Since the WI can generate a large magnetic field from no initial field, the maximum magnitudes achieved can have significant consequences for a number of applications. Hence, an understanding of the detailed dynamics driving the nonlinear saturation of the WI is important. This work considers the nonlinear saturation of the WI when counter-streaming populations of initially unmagnetized electrons are perturbed by a magnetic field oriented perpendicular to the direction of streaming. Previous works have found magnetic trapping to be important [Davidson et al., Phys. Fluids 15, 317 (1972)] and connected electron skin depth spatial scales to the nonlinear saturation of the WI [Califano et al., Phys. Rev. E 57, 7048 (1998)]. The results presented in this work are consistent with these findings for a high-temperat...

Fluid and kinetic simulations of plasma instabilities

Recent advances in plasma modeling have allowed development of sophisticated plasma fluid and kinetic simulation tools for a wide range of parameter regimes. This short paper presents some illustra...

Recent advances in plasma modeling for space applications

ABSTRACT This paper presents a brief overview of the application of advanced plasma modeling techniques to several space science and engineering problems currently of significant interest. Recent advances in both kinetic and fluid modeling provide the ability to study a wide variety of problems that may be important to space plasmas including spacecraft–environment interactions, plasma–material interactions for propulsion systems such as Hall thrusters, ionospheric plasma instabilities, plasma separation from magnetic nozzles, active space experiments, and a host of additional problems. Some of the key findings are summarized here.

Simulations of plasma sheaths using continuum kinetic models

Summary form only given. When plasma interacts with a surface, a plasma sheath forms at the interface, which is typically a region of net positive space charge. Ions, accelerated by the electric field in the sheath region, and hot electrons are known to cause emission from the surface. This can have consequences for devices such as Hall thrusters as electron emissions can increase the rate of erosion of the electrodes affecting performance and longevity of the thrusters - an important concern for space-bound missions. The length-scale of sheaths is small in comparison to the undisturbed plasma (on order of the Debye length, λD) yet the sheath has a global effect on plasma and needs to be included self-consistently in computer simulations. This usually means resolving the Debye length and the plasma oscillation frequency, which makes global and complex simulations extremely demanding in terms of the computational cost.We will present simulations of classical sheaths using a continuum kinetic model where we directly solve the Boltzmann equation for each of the ion and electron species using the discontinuous Galerkin method. Furthermore, we will present results of sheaths in the presence of magnetic fields. We will build on the work of Loizu et al. on the magnetic presheath in the cold ion limit, which uses relatively small magnetic field incident angles due to its application to tokamaks. In our work we plan to focus on Hall thruster relevant geometries with magnetic field perpendicular to the surface. We will also extend previous studies of magnetized sheaths and presheaths with warm ions.