Determination of the ionization region in Hall thruster plasmas with low perturbation

Abstract

Information on the ionization of propellant gases is invaluable for Hall thruster plasma studies because the ion beam is the source of thrust generation. This study proposes a new method that identifies the ionization location of Xe+ ions in Hall thruster plasmas by combining the laser-induced fluorescence spectroscopy and floating emissive probe measurements in annular and cylindrical Hall thrusters. In an annular Hall thruster, this method indicated that ionization became the largest at a few millimeters upstream of the maximum radial magnetic field, whose result agreed with a one-dimensional hybrid fluid/particle-in-cell (PIC) simulation result. On the other hand, the ionization region in a cylindrical Hall thruster was located deeper inside the discharge cavity than that of the annular thruster. In addition, a large upstream shift of the ionization region was observed with the decreasing fraction of the radial magnetic field in the cylindrical Hall thruster.

Structure of the ion acceleration region in cylindrical Hall thruster plasmas

“…Therefore, ExB drifting electrons are thought to contribute to inconsistency between the potential contour and the magnetic field line. Figure 3(b), showing the electric potential measured by a floating emissive probe [21], demonstrates that the potential drop is still considerable even beyond z = 60 mm, as in Figure 3(a). In the figure, the white dashed lines represent the observation range to maintain low plasma perturbation (less than 2% discharge current variation).…”

“…The ion trajectories and energy contours exhibit an upstream movement of the acceleration front and a narrowing of the acceleration region as moving away from the thruster axis. These results are considered to be related to the upstream shift of ionization location with the increase in radial component of the magnetic field [21]. It is also noteworthy that the electric potential contour significantly deviates from the magnetic field line in the region where the magnetic field is axially dominant.…”

“…The ion trajectories and energy contours exhibit an upstream movement of the acceleration front and a narrowing of the acceleration region as moving away from the thruster axis. These results are considered to be related to the upstream shift of ionization location with the increase in radial component of the magnetic field [21].…”

“…The magnetic field profile of two thrusters can be found in Ref. [21]. For the annular thruster experiment, IVDFs were measured along the centerline of the discharge channel, i.e., r = 21 mm, and the experimental conditions were kept the same (300 V, 280 W).…”

“…To clarify the ion acceleration in the cylindrical Hall thruster plasma, we compared the ion energy profiles along trajectories 1 and 4 with the conventional annular Hall thruster (figure 4) that had the same outer channel diameter and depth as the cylindrical Hall thruster [11]. The magnetic field profile of two thrusters can be found in [21]. For the annular thruster experiment, IVDFs were measured along the centerline of the discharge channel, i.e.…”