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The importance of ion propulsion devices as an option for in-space propulsion of space
crafts and satellites continues to grow. They are more efficient than conventional chemi-
cal thrusters, which rely on burning their propellant, by ionizing the propellant gas in a
discharge channel and emitting the heavy ions at very high velocities. The ion emission
region of a thruster is called the plume and extends several meters axially and radially
downstream from the exit of a thruster. This region is particularly important for the effi-
ciency of a thruster, because it determines energy and angular distribution of the emitted
ions. It also determines the interaction with the carrier space craft by defining the electric
potential shape and the fluxes and energies of the emitted high energy ions, which are the
key parameters for sputter erosion of satellite components such as solar panels. Developing
new ion thrusters is expensive because of the high number of prototypes and testing cycles
required. Numerical modeling can help to reduce the costs in thruster development, but
the vastly differing length and time scales of the system, particularly the large differences of
scales between the discharge chamber and the plume, make a simulation challenging. Often
both regions are considered to be decoupled and are treated with different models to make
their simulation technically feasible. The coupling between channel and plume plasmas and
its influence on each other is disregarded, because there is no interaction between the two
regions. Therefore, this thesis investigates the physical effects which arise from this cou-
pling as well as models suitable for an integrated simulation of the whole coupled problem
of channel and plume plasmas. For this purpose the High Efficiency Multistage Plasma
Thruster (HEMP-T) ion thruster is considered.
For the discharge channel plasma, a fully kinetic model is required and the Particle-in-Cell
(PIC) method is applied. The PIC method requires very high spatial and temporal resolu-
tions which makes it computationally costly. As a result, only the discharge channel and the
near-field plume close to the channel exit can be simulated. In the channel, the results show
that electrons are magnetized and follow the magnetic field lines. The orientation of the
magnetic field there is mostly parallel to the symmetry axis and the channel walls which re-
sults in a high parallel electron transport and leads to a flat electric potential and a reduced
plasma-wall sheath. Only at the magnetic cusps, which are characteristic of HEMP-Ts the
electrons are guided towards the wall, with ions following due to quasineutrality, where a
classical plasma-wall sheath develops. The ion-wall contact is thus limited to the cusp re-
gion. The small radial drop of the potential towards the wall gives rather low energies of
ions impinging at the wall and minimizes erosion in the HEMP-T.
In the near-field plume, which extends from the thruster exit plane to some centimeters
downstream, the ion emission characteristics is defined. The ratio of radial and axial elec-
tric field components in this region determines the ion emission angle which should be
minimized for maximum thruster efficiency. The plasma discharge in the channel produces
high plasma densities and the subsequent drop from plasma to vacuum potential occurs
further downstream for higher densities. This increases the ratio of radial and axial electric
field components because the plasma expands radially outside of the confinement from the
dielectric discharge channel walls. The potential structure in the near-field plume impacts
also the supply of electrons for the channel discharge because the electrons enter the channel
from the plume. An effect which arises from this coupling is the breathing mode oscilla-
tion. It is an oscillation which is observed in all plasma quantities and is located near the
thruster exit. The oscillation frequency measured in the simulation is in good agreement
with a predator-prey estimate which validates this ansatz. However, the electron tempera-
ture, assumed constant in the predator-prey model, correlates inversely with the oscillation,
i.e. it is minimal at the current maximum and vice versa, which contributes to the observed
oscillations. Because of the oscillation of the plasma number density, the potential drop also
oscillates in the exit region and thus the ratio of radial to axial electric field components,
which results in the oscillation of the mean ion emission angle.
Regarding suitable models for a combined simulation of channel and plume plasmas, the
PIC model for channel and near-field plume is explicitly coupled to a hybrid fluid-PIC
model for the plume. The latter treats the electrons as a fluid, hence increasing the effective
spatial and temporal resolutions which can be applied in the plume simulations at the cost
of reduced accuracy of the electron model. Plasma densities decrease by two orders of
magnitude two meters downstream from the channel exit. The explicitly coupled kinetic
and hybrid PIC models are well suited for the computation of a HEMP-T and its plume
expansion, but they disregard the coupling of channel and plume plasmas for which other
methods are necessary. For this purpose a new approach is presented with a proof-of-
principle validation. The limited spatial resolution in the plume can be overcome with the
mesh-coarsening method, which increases the resolution in regions of low plasma density
without numerical artifacts. Sub-cycling for the electrons in the plume can then be used
to increase the temporal resolution in the plume. The combination of both methods, called
the sub-cycling mesh-coarsening (SMC) algorithm in the scope of this work, promises high
savings in computational cost which can make a combined simulation of plume and channel
plasmas feasible.