A Comparison of Ion Acceleration Characteristics for Krypton and Xenon Propellants within a 600 W Hall Effect Thruster

Abstract

There is growing interest within the electrostatic propulsion community in the use of krypton as a propellant. It is a lower cost replacement for xenon, and is especially of interest for potentially very large solar electric transfer vehicles that may potentially strain xenon production capability. This work compares the internal propellant acceleration of krypton within a laboratory medium power Hall effect thruster to historical xenon data for the same thruster. One case matched in propellant particle flux (matched volumetric flow rates) is presented. The measurements consist of laser-induced fluorescence velocimetry extending approximately the anode to 10 mm outside the thruster into plume along the center of the coaxial acceleration channel. The results show that the acceleration process for krypton is more gradual and produces a lower electric field. As a result, energy conversion is lower than xenon for this flow matched case. In addition, there is clear evidence of ionization throughout the acceleration channel. This may explain a lower performance for krypton as this particular appears to have low propellant utilization. It is not known to what extent the less oscillatory plasma and/or the lower ionization cross-section of the krypton discharge produced this difference relative to xenon.

Laser-induced fluorescence diagnostics of the cross-field discharge of Hall thrusters

“…The laser linewidth is below 1 MHz with a CW source, therefore it can be considered as a Dirac delta function and subsequently neglected. Mechanisms at the origin of the absorption line shape in a HT discharge have been described in detail in many publications, see, e.g., [17,18,28]. We here review and briefly explain absorption line shape broadening effects.…”

“…at 600 V. In both cases, the electric field has been calculated from the Xe II ion mean velocity profile. The properties of the E x field are typical for HTs, see, e.g., [17,18,28,32]. The largest magnitude of the field is around 200-400 V cm −1 .…”

“…The magnitude of the velocity dispersion depends upon the thruster operating conditions, see, e.g., [17]. The velocity dispersion profile along the channel axis is the same with xenon and krypton, see, e.g., [23,28] for measurements with krypton in low-power thrusters. An example of measurement outcomes with Kr as propellant is illustrated in figure 13.…”

Zeeman effect of isotopes of Kr and Xe investigated at the linear plasma device PSI-2

“…We also estimated the influence of the electron-ion broadening in quasi-static approximation using the peak electron density 3 × 10 17 m −3 and temperature 3 eV of a krypton discharge, as plotted in figure 1(b). For that purpose, equation (4) of [35] provides estimates of the thermally averaged Gaunt factors of the Kr I and Xe I levels needed to calculate the width according to equation (24) of [36]. The NIST ASD [26] gives the nearest perturbing levels and the orbital quantum numbers l are 0 for the lower and 1 for the upper levels were used in [36].…”

“…The theoretical description of experimental data for Kr lines suffers even this level of discussion. For instance, only zerofield modeling of the Kr I 760.4 nm or Kr II 728.98 nm transitions is available in the literature [18,24]. We are not aware of studies investigating Kr I lines in the magnetic field of tens of mT with consequent modeling taking the Zeeman effect and the HFS components into account.…”