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• W2890945441 abstract "Ion thrusters are Electric Propulsion systems used for satellites and space missions. Withinthis work, the High Efficient Multistage Plasma Thruster (HEMP-T), patented by theTHALES group, is investigated. It relies on plasma production by magnetised electrons.Since the confined plasma in the thruster channel is non-Maxwellian, the near-field plumeplasma is as well. Therefore, the Particle-In-Cell method combined with a Monte-CarloCollision model (PIC-MCC) is used to model both regions. In order to increase the sim-ulated near-field plume region, a non-equidistant grid is utilised, motivated by the lowerplasma density in the plume. To minimise artificial self-forces at grid points bordered bycells of different size a modified method for the electric field calculation was developed inthis thesis. In order to investigate the outer plume region, where electric field and collisionsare negligible, a ray-tracing Monte-Carlo model is used. With these simulation methods,two main questions are addressed in this work.What are the basic mechanisms for plasma confinement, plasma-wall-interactionand thrust generation?For the HEMP-T the plasma is confined by magnetic fields in the thruster channel, generatedby cylindrical permanent magnets with opposite polarity. Due to different Hall parameters,electrons are magnetised, while ions are not. Therefore, the dominating electron transportis parallel to the magnetic field lines. In the narrow cusp regions, the magnetic mirror effectreduces the electron flux towards the wall and confines the electrons like in a magneticbottle. At the anode, propellant gas streams into the thruster channel, which gets ionisedby the electrons creating the plasma. As a result of the electron oscillation between the twocusp regions, ionisation of the propellant gas is efficient.The magnetic field configuration of the HEMP-T also influences the plasma potential insidethe thruster channel. Close to the symmetry axis, the mainly axial magnetic field results ina flat potential. At the inner wall, the field configuration reduces the plasma wall interactionto only the narrow cusp regions. Here, the floating potential of the dielectric channel walland its plasma sheath result in a rather low radial potential drop compared to the appliedanode potential. As a result, the electric potential is rather flat and impinging ions at thethruster channel wall have energies below the sputter threshold energy of the wall material.Therefore, no sputtering appears at the dielectric wall. At the thruster exit the confinementby the magnetic field is weakened and the potential drops with nearly the full anode voltage.The resulting electric field accelerates the generated ions into the plume and generate thethrust, but they are also able to sputter surfaces. During terrestrial testing, sputteringat vacuum vessel walls leads to the production of impurities. The amount of back-fluxtowards the channel exit is determined by the sputter yield of the vacuum chamber wall. Alarge distance between thruster exit and vessel wall reduces the back-flux and smooths thepattern of deposition inside the thruster channel. Dependent on their material, the evolvingdeposited layers can get conductive, modify by this the potential distribution and reducethe thrust.For the HEMP-T, ions are mainly generated at high potential close to the applied anodepotential. Therefore, the accelerated ions producing the thrust gain the maximum energyas observed in experiment. Ions emitted from the thruster into different angles in theplume contribute mainly to the ion current at angles between 30 ◦ and 90 ◦ . They mainlyoriginate from ionisation at the thruster exit. The resulting angular distribution of theejected ion current is close to the one of the experiment, slightly shifted by about tendegrees to higher emission angles. In front of the thruster exit, electrons are trapped byelectrostatics forces. This enhanced density allows ionisation and an additional electrondensity structure establishes.What are possible physics based ideas for optimisation of an ion thruster?An optimised thruster should have a high ionisation rate inside the thruster channel, lowerosion and an ion angular distribution with small contributions at high angles for min-imised thruster satellite interactions. In experiments, the HEMP-T satisfies already quitenicely these requests. In the simulations, low erosion inside the thruster channel and angularion distributions close to the experimental data are demonstrated. However, the ionisationefficiency is lower and radial ion losses are larger than in experiment. A possible explanationof these differences is an underestimated transport perpendicular to the magnetic field lines,well known for magnetised plasmas.A successful example for an optimisation using numerical simulations is the reduction ofback-flux of sputtered impurities during terrestrial experiments by an improved set-up ofthe vacuum vessel. The implementation of baffles reduces the back-flux towards the thrusterexit and therefore deposition inside the channel. These improvements were successfully im-plemented in the experiment and showed a reduction of artefacts during long time measure-ments. This leads to a stable performance, as it is expected in space." @default.
• W2890945441 created "2018-09-27" @default.
• W2890945441 creator A5005844980 @default.
• W2890945441 date "2018-09-05" @default.
• W2890945441 modified "2023-09-27" @default.
• W2890945441 title "Kinetic Simulation of Ion Propulsion Systems" @default.
• W2890945441 hasPublicationYear "2018" @default.
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