Plasma Processes of DC Ion Thruster Discharge Chambers

Abstract

This study has advanced state-of-the-art discharge modeling and revealed important aspects of discharge plasma processes. These extensions of existing ion thruster technology and understanding are necessary to fulfill the needs of future space missions. A multi-component hybrid 2-D computational Discharge Chamber Model (DCM) was developed to help identify important ion thruster discharge processes and investigate miniaturization issues. The model is designed to integrate thruster component (cathode and grid) wear models to allow the determination of thruster life and long-term performance. The model accounts for the five major chamber design parameters (chamber geometry, magnetic field, discharge cathode, propellant feed, ion extraction grid characteristics) and self-consistently tracks the effects of the four plasma species (neutral propellant atoms, secondary electrons, primary electrons, and ions). Results from the model show good agreement with experimental data at two operating points for the 30cm NSTAR ion thruster. A thruster design sensitivity performed with DCM suggests that NSTAR thruster performance is greatly enhanced by increasing the strength of the middle magnet ring. The model analyses show that the peak observed in the NSTAR beam profile is due to double ions that are created by overconfinement of primary electrons on the thruster axis. Design sensitivity results show that, at the NSTAR thruster scale, efficient confinement of primary electrons is relatively easy to achieve; therefore, efforts to improve thruster performance should focus on effectively utilizing the primary electrons to minimize double ion production and maximize the number of single ions extracted to the beam. DCM results also show that non-classical effects are important for predicting the perpendicular mobility of secondary electrons in ion thruster discharges. Good agreement with experimental data was found by weighting the influence of Bohm-type diffusion by considering the non-uniform levels of ionization in the discharge. It was found that ion thrusters operate in an intermediately ionized plasma regime that is between fully and weakly ionized approximations. The observations from this study have furthered the understanding of discharge processes and should improve future ion thruster design and modeling efforts. DCM advances state-of-the-art ion thruster modeling and provides a framework for a complete thruster model that can be used for long-life performance assessment and life validation.

Magnetic Field Aligned Mesh for Ring-Cusp Discharge Chambers

“…More specifically, the off-diagonal terms in mobility tensor (M B in Eq. (20)) vanish for the MFA mesh, so the gradient along the facet, ∂Q/∂s, does not need to be evaluated. If the near-boundary mesh quality is acceptable such that it does not introduce additional error in the solution, the MFA mesh can only be better compared to the uniform mesh.…”

“…33 In this formulation, the conservations of mass and momentum equations for singly charged ions and electrons are combined, assuming the same diffusion rates for singly charged ions and electrons due to the ambipolar field. 20,21 It also assumes a quasi-neutral (n i = n e ), diffusive (u · ∇u = 0), and ideal plasma (∇p = k∇(nT )) where n is the density, u is the velocity, p is the pressure, T is the temperature, k is the Boltzmann constant, and the subscripts i and e denote ions and electrons, respectively. Further assumptions are made such that ions dominate the mass and momentum diffusion, and the perpendicular electric field is "short-circuited," yielding an approximation of E ⊥ = 0.…”

“…In later simulations, they switched to a uniform mesh due to the inability of their MFA mesh algorithm to quickly adapt to changing magnetic field structures. 20 The objective of this investigation is to develop a 2-D axisymmetric MFA mesh generation method and solve the plasma diffusion equation using the MFA mesh to evaluate its accuracy in relation to a regular mesh. The method provided herein is intended to be integrated in the discharge model developed by Wirz and Katz 20,21 and used for simulations to address the design and optimization challenges of miniature to micro discharges on the order of 3 cm to 1 cm in diameter.…”

Three‐dimensional immersed finite‐element method for anisotropic magnetostatic/electrostatic interface problems with nonhomogeneous flux jump

“…In this way, the material properties along the mesh lines will be consistent and thus easy to be described. Noncharacteristic line mesh method: In this method, the mesh lines are not the characteristic lines any more. Therefore, this method allows the use of structured meshes, such as Cartesian meshes . However, the material properties along the characteristic lines need to be transferred onto the mesh lines by coordinate transformation.…”

“…The characteristic line mesh method has its own advantages for studying some specific physical phenomena, but it may not be suitable for certain types of problems, such as the optimization of an ion thruster discharge chamber, which is an anisotropic diffusion problem. In many types of anisotropic problems, the material properties are usually very sensitive to the external environment.…”