Krypton Propellant for Hall-effect Thrusters

Overview

Motivation

The typical choice of propellant for Hall Effect Thrusters (HETs) is xenon gas however, several studies have considered the use of krypton as an alternative ([1], [2], [3]). Using krypton can significantly lower the cost of operating an HET: as of May 2017, xenon gas was approximately 45 times more expensive than krypton. Although most simulation work has focused on xenon propellant thrusters, there have been previous attempts to evaluate the use of krypton gas. In 2001 Garrigues [4] used a 1D hybrid model to investigate thruster performance of the SPT-100 with krypton gas, and found that for the same voltage and mass flow rate similar values of thrust may be obtained between xenon and krypton, but a lower experimental efficiency is observed for the krypton gas (30% versus 40% for xenon). In 2008 Yim [5] applied a 2D hydrodynamic model to study krypton operation on the NASA-173Mv1 HET. However the model did not capture the peaks of electric field and electron temperature accurately since the thermal conductivity term in the electron energy conservation equation was neglected. Both previous efforts focused only on the thruster channel. The present work is the first attempt to simulate both thruster and plume with krypton propellant using a 2D axisymmetric model and validate the results with experimental measurements.

Simulation Approach

The simulation tool Hall2De ([6], [7], [8]) which has been developed at the Jet Propulsion Laboratory over the past decade for the purpose of studying HETs is used in the current study. The computational grid is aligned with the magnetic field, and a typical domain includes both the thruster channel and a large plume that extends several channel lengths downstream. The model is axisymmetric, with the radial zero coordinate located at the thruster centerline. Different models are used for each of the plasma species: neutrals, ions and electrons. Under the assumption of free molecular flow, the neutral particle density and velocity distributions are computed with a view-factor algorithm [6]. A hydrodynamic approach is used to model the ions, and they are divided into multiple populations, based on energy. The ions generated upstream of the acceleration region are part of the high energy population, while the ions generated downstream will form the low energy population. Thus, in most cases the HET physics can be modeled with two populations of ions. In addition to solving separate continuity and momentum equations for each population, each charge state is also resolved independently. The different energy populations and charged states interact with each other through ionization, charge exchange and elastic collisions. Finally, the electrons are modeled using a fluid approach. The Generalized Ohm's law in vector form in the directions parallel and perpendicular to the magnetic field lines, coupled with charge conservation and plasma quasi-neutrality yield the plasma potential, while the electron temperature is computed from an energy conservation equation.

In order to use krypton propellant in the simulation, several updates are implemented, related to the properties of the new propellant gas. The ionization cross-sections of krypton affect the ionization rate which results in different numbers of neutral and charged species than in the xenon case. Further, the ionization potential plays a direct role in the electron energy equation, by changing the electron energy losses. Energy loss mechanisms are implemented through the introduction of a collision cross-section corresponding to each process: elastic scattering, ionization and charge-exchange. The krypton implementation is then validated against experimental data for the NASA 300M thruster, and also used to estimate krypton performance for the H6 thruster.

Results

NASA 300M

The starting point for the comparison is a previous xenon simulation, performed for the 400 V, 50 A operating condition. The electron collision frequency profile is informed by experimental measurements of the temperature and thrust. The magnetic field strength was adjusted between different operational conditions in the laboratory, so the collision frequency profile is also expected to change. However, since the magnetic field values were only available for the 400 V and 50 A xenon gas operating condition, the electron collision frequency profile, magnetic field topology and magnitude from this particular test case are used in all simulations of the 300M thruster.

The thrust value from simulation is matched to within 10% of the experiment. The thrust (Fig. 1) is 15% - 25% higher for the xenon propellant than krypton, however, the krypton simulations produced specific impulse (Fig. 2) values between 10% and 16% higher than xenon cases. Additional comparisons to a reduced order model are included in the conference paper.

(a)

(b)

Figure 1: Thrust values from simulation and experiment for (a) xenon and (b) krytpon propellant.

(a)

(b)

Figure 2: Specific impulse values from simulation and experiment for (a) xenon and (b) krytpon propellant.

H6

A baseline simulation is prepared for the H6US with xenon propellant, operating at 6 kW and 300 V at a mass flow rate of 19.7 mg/s. Next, a simulation with krypton gas is set up that uses the flow rate, magnetic field topology and magnitude from the xenon case. In addition, since there is no experimental guidance available, the electron anomalous collision frequency profile from the xenon simulation is also used for the krypton propellant.

Table 1 provides a summary of the simulation results for the H6 thruster. The anode current values computed by the code are shown for the three test cases. While the first krypton case yields a power level of 8.64 kW, the second krypton case where the mass flow rate is adjusted to lower the power gives 5.73 kW, showing that the scaling is successful.

The thruster performance parameters are shown in the table: thrust and specific impulse. The thrust value is 14.8% higher than for xenon in the first krypton case, and 18.9% lower in the second. As expected, when operating at similar power levels, the thruster using a propellant that has a smaller atom and a higher ionization potential should produce a lower value of thrust. However, since the mass flow rate is lower and this is inversely proportional to the specific impulse, a higher Isp is observed in both Kr cases. Thus, when using the same flow rate as xenon, a 14.8% higher Isp is computed, while using a reduced mass flow rate produces a value that is 13.3% higher. Further discussion regarding the ion fractions can be found in the 2017 JPC paper.

Table 1: Simulation results.

Species

Xe

Kr

Kr

Flow rate (mg/s)

19.7

19.7

14.1

Anode current (A)

-21.1

-28.8

-19.1

Thrust (mN)

412

473

334

Isp (s)

2200

2530

2420

Conclusions

While the use of krypton propellant is expected to produce lower thrust than xenon, operating the thruster at the same power level will require a significantly lower flow rate, and produce a higher specific impulse.

Future Work

Investigators

Horatiu Dragnea

Acknowledgments

This work has been supported by a NASA Space Technology Research Fellowship. The guidance and direction provided by Alejandro Lopez Ortega and Yiangos Mikellides from JPL are greatly appreciated.

References

  1. Linnell, J. A. and Gallimore, A. D., Internal plasma potential measurements of a Hall thruster using xenon and krypton propellant, Physics of Plasmas, Vol. 3, 2006.

  2. Linnell, J. A. and Gallimore, A. D., Efficiency Analysis of a Hall Thruster Operating with Krypton and Xenon, Journal of Propulsion and Power, Vol. 22, No. 6, 2006

  3. Nakles, M. R., et. al., A Plume Comparison of Xenon and Krypton Propellant on a 600 W Hall Thruster, International Electric Propulsion Conference, IEPC-2009-115, Ann Arbor, MI, September 20-24, 2009.

  4. Garrigues, L., Boyd, I. D., and Boeuf, J. P., Computation of Hall Thruster Performance, Journal of Propulsion and Power, Vol. 17, No. 4, 2001

  5. Yim, J. T., Computational Modeling of Hall Thruster Channel Wall Erosion, Ph.D. Dissertation, Dept. of Aerospace Engineering, University of Michigan, Ann Arbor, MI, 2008.

  6. Katz, I. and Mikellides, I. G., Neutral gas free molecular flow algorithm including ionization and walls for use in plasma simulations, Journal of Computational Physics, Vol. 230, pp. 1454-1464, 2011

  7. Mikellides, I. G. and Katz, I. Numerical simulations of Hall-effect plasma accelerators on a magnetic-field-aligned mesh, Physical Review E, Vol. 86, no. 4, 2012

  8. Lopez Ortega, A. and Mikellides, I. G., A New Cell-Centered Implicit Numerical Scheme for Ions in the 2-D Axisymmetric Code Hall2De, AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA-2014-3621, Cleveland, OH, 28 July - 30 July, 2014.

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