Kinetic modeling of low temperature plasmas: space and laboratory plasmas

Nuwal, Nakul

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https://hdl.handle.net/2142/120334 Copy

Description

Title

Kinetic modeling of low temperature plasmas: space and laboratory plasmas

Author(s)

Nuwal, Nakul

Issue Date

2023-01-26

Director of Research (if dissertation) or Advisor (if thesis)

Levin, Deborah A

Doctoral Committee Chair(s)

Levin, Deborah A

Committee Member(s)

• Kaganovich, Igor D
• Rovey, Joshua L
• Ruzic, David N

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Kinetic, Plasma

Abstract

Kinetic modeling is an important aspect of predictive and diagnostic modeling of low-temperature plasmas, where such a fundamental treatment of physics leads to valuable insights and more accurate predictions. A fully kinetic treatment of plasma has been a challenge in the past because of the computational memory and expense required in modeling plasma problems in more than one or two dimensions. Recent improvement in the computational architecture has enabled the development of a CPU-GPU based computational code CHAOS which has provided a multi-GPU scalability that is required to kinetically model plasmas in three dimensions. While the code was developed originally to model ion thruster plumes, it has been modified to be applicable to a wider range of problems as a part of this thesis. A novel multi-GPU approach to perform Monte Carlo Collisions (MCC) between charged particles and neutral gas has been appended to the existing collision module of the code where, for the first time, ionization reactions were implemented in a multi-GPU framework. Other additions in CHAOS include the implementation of surface-charging boundary conditions, which enable the self-consistent modeling of surfaces interacting with plasma. Plasma surface interactions are an important aspect of spacecraft design. Interactions of a spacecraft surface with the plasma in space may cause surface charging, sputtering, and power losses, which need to be accounted for during the design process. In this work, interactions of spacecraft surfaces with the plasma environment generated by an ion thruster plume in a vacuum are modeled to quantify the surface damage due to ion impingement. Since ion thrusters usually are used for deep-space propulsion, the ambient plasma environment is not dense enough to affect the spacecraft surfaces. While most of the ions from the thruster move in the opposite direction to the spacecraft, a few ions generated due to charge exchange reactions can be influenced by the local electric fields that are generated near the spacecraft surfaces. Using a fully kinetic approach, ion flux and their kinetic energy distribution when impinging on the surface were quantified for solar panel and other unbiased surfaces. Based on the empirical relations, surface sputtering rates were also estimated for silicon and aluminum spacecraft materials. For spacecrafts in low earth orbits, the plasma density is high enough to interact with the solar panel surfaces on the spacecraft. In this work, ambient plasma interaction with the exposed interconnect and dielectric surfaces of a spacecraft solar panel is kinetically modeled. While previous work has applied a fluid and hybrid approach to resolve the physics near interconnects, a fully kinetic modeling has not been performed before. In this work, with a particle treatment of electrons, both the electron and ion sheaths are quantified and compared with previous theoretical, empirical, and numerical work. The electron plasma sheaths near the exposed interconnects lead to a non-linear parasitic current loss called snapover. Kinetic modeling of this phenomenon led to the discovery that an electron avalanche occurs on the dielectric surfaces of solar panels that lie in the regions close to the interconnect. Electron avalanche-based first order estimates were computed for the parasitic current, which showed reasonable agreement with the fully kinetic simulation result. Other than plasma-based ion thrusters, neutralized ion beams also find their applications in manufacturing processes. An economical way of neutralization is the use of an external electron source which provides better control but is shown to result in a poor beam neutralization profile. In this work, neutralization rates of an ion beam in 2D and 3D geometries were compared, and their divergence along the beam axis was quantified. A kinetic modeling of such a setup showed the excitation of electrostatic solitary waves (ESWs) along both 2D planar and 3D geometry beams. These waves are robust and dissipate slowly in the beam. In this work, we also quantify the movement, collisions, shape, and sizes of these ESWs and show comparisons with the existing theory. Using a theoretical analysis supported by the kinetic results, the existence of long ESWs in beams was discovered in the presence of non-Maxwellian beam electrons. Further, kinetic modeling in 3D beams showed a surprising formation of surface waves that are only excited in cases with sufficiently large beam radii. A comparison of kinetic results with the newly derived 2D and the original 3D dispersion relations showed the stark differences in the phase speeds required to excite these waves. In the pursuit of dissolving the ESWs that become excited in a neutralizing beam, the neutralization of a 2D ion beam in a low-pressure background gas is studied kinetically. The newly added MCC routine was used to quantify the rate of neutralization, throughput current loss, and plasma waves in a beam passing through a background gas of neutral particles of different pressures. Nuances of ESW excitation in cases with different background pressures are discussed quantitatively in this work, and the criteria for the locations and possibility of ESW excitation are identified. Criteria for a quantifiable degree of neutralization are discussed where parameters of beam potential profile, ESWs, ion throughput current, and ion energy distribution are compared for the cases of different background pressures. The collisionless kinetic modeling framework of CHAOS is also used to model the kinetic response of plasma when a linear pulse of electron time-scale is applied to it. Plasma sheath expansion is studied, and the plasma oscillations are quantified based on their frequencies and amplitudes. A comparison with the experiments conducted on a similar setup showed agreement in oscillations frequency and a qualitative agreement in the amplitude variation with the pulse-width of the applied pulse. The problems discussed in this thesis demonstrate the capability of the CHAOS framework in kinetically modeling cutting-edge problems in the field of low-temperature plasmas. The added capabilities to the code may be used to apply this framework to problems, such as modeling of emissive sheaths inside boundary layers, discharge-driven neutralization, and modeling of plasma chamber facility effects.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Nakul Nuwal

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