Single particle simulations of electron transport in the near-field of Hall thrusters

Abstract

The results of 3D, single particle electron trajectory calculations are presented for the near-field of a laboratory E × B Hall plasma thruster. For a prescribed static magnetic and electric field distribution, single electrons are launched and tracked from a simulated cathode. Collisions with external thruster surfaces are accounted for; however, field fluctuations are disregarded. Bulk statistics including the channel to beam electron current ratio, electron lifetimes and spatial distributions of the number density, mean energy, energy distributions, velocity distributions and velocity component ratios are catalogued. For conditions typical of a moderate power Hall thruster, the mean lifetime of electrons in the domain of axial scale length, L = 0.3 m, is approximately 120 ns. Electrons which eventually enter the channel are found to strike the thruster ∼10 3 times as frequently as electrons which exit the domain in the plume. For the static E and B field distributions used in this study, the channel to beam current ratio is found to be on the order of 0.1 and the velocity ratio, V E /V E×B , over the channel has a mean of ∼0.5, with higher values driven largely by collisions with the thruster indicating the importance of such events in driving transport into the channel.

Finite Pressure Effects in Magnetically Shielded Hall Thrusters

“…The other two locations were outside the magnetic circuit of the thruster, referred to as externally mounted configurations, at either the near location of R/R m = 2.64 or the far location at R/R m = 4.90. With both external mounts being outside the separatrix [38,39] of the magnetic circuit where the field is weak, the electrons are not magnetized and their coupling with the discharge is impeded [16,38,39,[58][59][60][61][62][63]. …”

“…Indeed, studies of Hall thrusters with internally mounted cathodes have shown this is the most energetically favorable configuration and provides for a high-level of performance primarily through decreased plume divergence and improved cathode coupling [16,38,39,[58][59][60][61][62][63]. Visual observations, shown in Figure 3, also reveal that thrusters with external cathodes are typically surrounded by a "halo" as the electrons collide with background gases causing neutral emission [58].…”

Measurement of Cross-Field ElectronCurrent in a Hall Thruster Due to Rotating Spoke Instabilities

“…[14][15][16] More recently direct electron trajectory modeling has suggested that surface collisions in the near-field with the thruster pole pieces also play a role. 17 Various theories in support of the second mechanism have been proposed, including resistive instabilities due to electron collisions 18 , shear flow instabilities 19 , electrothermal instabilities 13,20 , critical ionization velocity phenomena 4,21 and many others, at frequencies ranging from the low kilohertz up into the megahertz. The proposed mechanisms number far greater than the quantitative assessments of transport due to any particular mechanism, and there is no clear picture of which mechanisms are dominant under which operating conditions.…”

On the role of fluctuations, cathode placement, and collisions on the transport of electrons in the near-field of Hall thrusters

“…Recent kinetic studies indicate that electron transport to the channel from the cathode is strongly dependent on collisions with the front-face of the thruster. 11 Given the geometry of the magnetic field lines in the vicinity of the cathode ͑see Fig. 2͒, it is clear that altering the position of the cathode will change the initial magnetic field lines electrons travel on and modify the locations of wall-collision events.…”

“…A uniform sheath of thickness 1 mm ͑approximately the Debye length assuming n e ϳ 10 9 cm −3 and T e ϳ 5 eV immediately adjacent to the thruster face͒ and potential drop 15 V was applied on all thruster surfaces as done in the previous work. 11 This value is an estimate motivated by the simplified analysis presented by Bittencourt, 20 which gives the wall potential relative to the bulk plasma ͑the sheath potential drop͒ as ⌬ =−kT ln͑m i / m e ͒ / ͑4e͒ where k is Boltzmann's constant, T is the effective temperature of the plasma ͑assuming the electrons and ion are in thermodynamic equilibrium at the same temperature, T͒, e is the fundamental charge, and m i and m e are the ion and electron masses, respectively. For a xenon plasma, ln͑m i / m e ͒ϳ12.4.…”