Physics of the high specific impulse alternative low power hybrid ion engine (alphie): Direct thrust measurements and plasma plume kinetics

Abstract

The Alternative Low Power Ion Engine (alphie) is a high specific impulse plasma thruster different from conventional gridded ion engines (GIEs). It uses only one external cathode and ions and electrons flow through the open spaces of its two grids, whereas only ions are transported through the GIE ion optics. Ionizing electrons from the cathode move inward to the alphie ionization chamber and ions, which are neutralized by electrons from the same cathode, exit along the opposite direction. These currents together with the voltages applied to the grids produce a self-consistent electric field that accelerates the charges. The one-dimensional ion velocity distribution and the electron energy spectra in the collisionless alphie plasma plume are studied along its axial axis of symmetry. The thruster produces a mesothermal plasma flow with a non-monotone plasma potential profile along the axial direction. The ion populations observed are of those accelerated by the self-consistent electric field and a low velocity group that results from the charge exchange collisions in the thruster. Both populations remain essentially unaltered in the plasma flow. Conversely, the two electron groups observed merge along the axial direction of the plume following the changes in the plasma potential. The temperatures of ion populations are high by the neutral gas heating inside the thruster by high-energy ionizing electrons. The direct measurement of thrusts of 0.8–3.5 mN for argon gives 13 900–20 000 s specific impulses. These high values might be explained by the additional contribution to the thrust by the remaining non-ionized hot neutral gas effusion through the apertures of grids.

Kinetic simulation of ion thruster plume neutralization in a vacuum chamber

“…In other words, there are two types of electrons in the steady-state ion thruster plume: thermal electrons trapped around the plume core and electrons flowing downstream driven by the potential gradient. These two distributions were also seen in the experiment of Conde et al [11], who identified low-energy and highenergy groups.…”

“…In reality, the apertures cause variations in ion density, and electrons present to some extent inside the aperture [14, 42]. Unlike a hollow cathode, electrons are emitted from the neutralizer wall surface similar to a filament neutralizer [10,11], a direct emission neutralizer [43], or a diode mode neutralizer [44]. Collisions between Xe and e − , Coulomb collisions between Xe + and e − , the presence of multiply-charged ions (i.e.…”

“…Since the 2B-BP case, which includes both the grounded chamber wall and background pressure, is the closest condition to actual ground testing among the cases that we consider, it is useful to compare the predicted maximum plume potential and electron temperature with experimental studies [8][9][10][11][12]. Our maximum plume potential of 110 V is higher than that of Foster et al [8], Foster et al [9,12], Polansky et al [10], and Conde et al [11] who measured 4-10, 3.5, 20-70, and 27, respectively. Our maximum electron temperature of 15 eV can similarly be compared with measured values of 2-2.7, 2-20, 2-7, and 2, respectively.…”

“…In terms of neutralizer coupling, electron temperature was measured in the vicinity of the neutralizer for the NEXT thruster [9]. In addition to current and temperature, Polansky et al [10] studied ion and electron number densities downstream in the radial direction, and Conde et al [11] investigated the energy distribution of ions and electrons in a GIT plume. However, to date, few studies have experimentally investigated the detailed electron motion.…”

“…This study specifically seeks to understand the effect of the vacuum chamber BC and the finite background neutral density on the GIT plume, including beam ions and neutralizer electrons. In ground tests, the thruster system and neutralizer may be floated to simulate a space environment [2,7] or simply connected to the ground [10,11,35]. In the former case, the relative potential of the thruster to the chamber is determined so that the same amount of ions and electrons are emitted.…”