Investigations of gradient-drift and two-stream instabilities with analytical models and Particle-in-Cell simulations
Date
2021-09-15
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0001-6423-0778
Type
Thesis
Degree Level
Masters
Abstract
Plasmas with drifting electrons in crossed electric E and magnetic B field are used as ion sources in several applications including space propulsion and material processing. Despite long history, the nature of plasma instabilities in specific systems remains obscure. Gradient drift modes driven by combinations of the electron E×B drift and density gradient have been considered as one of the primary sources of fluctuations. In this work, we have verified the linear instability criteria for three possible regimes: ion-acoustic, modified two-stream, and electron cyclotron drift modes without the effect of the density gradients. For plasma parameters of interest for electric propulsion, we have studied the effects of finite values of the wave vector along the magnetic field and investigated the broadening and overlap of the cyclotron resonances and the transitions toward the ion-acoustic regime.
Studying the influence of density gradient on the Electron Cyclotron Drift Instability (ECDI), we show that for purely azimuthal modes, instabilities are enhanced for negative gradient density, while the positive gradients reduce growth rates. The lower-hybrid modes, which are a special case of more general ECDI, are then studied by eigenfrequencies analysis from the kinetic theory and comparison with an advanced fluid model, verifying the validity of the fluid model for different limits. The results indicate that in linear stage, the growth rates from the fluid model agree well with the kinetic theory.
This research also looks at the linear and nonlinear aspects of Buneman instability in magnetized and unmagnetized plasma, as a limit of ECDI. In this regard, the 1D (one-dimensional) particle in cell (PIC) simulations in the limit of cold electrons for the magnetized case are performed. The linear stage of the instability agrees well with the theoretical prediction. In the case of unmagnetized Buneman instability, it is found that in the regime of low drift velocity, the growth rate of the linear stage of the instability in 1D-PIC simulations differs significantly from the theoretical results. Hereof a series of highly resolved PIC simulations are performed with two different PIC codes. The initial noise due to particle discreteness is identified as a cause for discrepancies. The results of the performed simulations reveal that, although a quiet start scheme does not entirely solve the noise issue in PIC simulation, it improves the accuracy of linear growth rates.
Description
Keywords
Plasma, Plasma instabilities, Density gradient drift, lower-hybrid modes, particle in cell simulations, Buneman instability
Citation
Degree
Master of Science (M.Sc.)
Department
Physics and Engineering Physics
Program
Physics