Sheath Boundary Effects on the Stability of Hall Plasmas
Date
2018-09-19
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0001-8671-2356
Type
Thesis
Degree Level
Masters
Abstract
Plasma devices based on the ExB- field con figuration (where the externally applied electric
and magnetic fields are everywhere perpendicular) are used for a variety of applications
such as electric propulsion (i.e., Hall thrusters), diagnostic tools (e.g., Penning trap) and
plasma processing of materials (i.e., magnetrons). Transverse electron current due to the
electric and magnetic forces as well as diamagnetic
flows due to the presence of pressure
and magnetic field gradients are the sources of gradient-drift instabilities which result in
turbulence and anomalous transport (the transport of particles, momentum and energy that
cannot be explained by theory). Most investigations into plasma instabilities for various
configurations of the fields and plasma parameters have been done under the assumption
that the plasma is unbounded and, therefore, in neglect of any boundary effect. However,
the presence of physical boundaries may significantly alter the dynamics of the plasma by restricting
the parallel electron dynamics and introducing plasma sheath regions (regions where
quasi-neutrality is violated) near the walls. Previous works have shown that conservation of
charge at the boundary of the plasma affects the instability criteria for the gradient-drift
modes as well as result in new instabilities. Effects of the sheath boundary conditions on
the instabilities of partially magnetized plasmas are further investigated in this project. It
is shown, for the first time, that sheath dissipation results in the instability of the anti-drift
mode (i.e. the Simon-Hoh instability) due to the plasma density gradient. It is also shown
that sheath dissipation results in a strong instability in conditions where the criterion for the
standard Simon-Hoh instability (which results from the difference in electron and ion drifts
together with a density gradient) is not satis ed; this result is important as it may provide
an explanation for anomalous transport when the Simon-Hoh instability is absent. Then,
including the electric potential induced by the sheath as well as the sheath boundary condition,
one arrives at a set of two nonlinear ordinary differential equations with a non-local
integral condition. The boundary conditions are finally derived consistent with the sheath
to fully define the boundary-value problem.
Description
Keywords
Plasma, instability, transport, Hall thruster, Penning trap, magnetron, sheath
Citation
Degree
Master of Science (M.Sc.)
Department
Physics and Engineering Physics
Program
Physics