Hall thrusters

The Hall thruster is the most widely flown electric propulsion device in history. It sits between chemical and gridded ion propulsion in specific impulse, and it beats gridded ion engines decisively on thrust per unit area, which is why it dominates commercial spacecraft.

The crossed-field discharge

A Hall thruster is an annular channel with an anode at the closed end and a cathode outside the exit. A radial magnetic field crosses the channel; an axial electric field runs along it. The physics follows from how differently the two charge species respond to that arrangement.

Electrons emitted from the cathode are drawn toward the anode but encounter the radial magnetic field. Their Larmor radius is small compared with the channel, so they are magnetised: instead of reaching the anode they drift azimuthally around the annulus in an $\mathbf{E}\times\mathbf{B}$ direction. This closed circulating current is the Hall current that names the device.

Trapped electrons make many circuits and collide repeatedly with neutral propellant, ionising it efficiently. The resulting ions are far heavier, so their Larmor radius is much larger than the channel and they are effectively unmagnetised. They ignore the magnetic field and accelerate straight out along the electric field.

The consequence is the device’s central advantage. Because electrons and ions occupy the same volume, the plasma is quasi-neutral, and there is no net space charge to limit the ion current. The Child-Langmuir limit that caps a gridded ion engine’s thrust density simply does not apply.

Against gridded ion engines

The trade is thrust against efficiency. For a mission where transfer time matters, or where available power is modest and the delta-v is a few kilometres per second, the Hall thruster’s thrust density wins. For a deep-space mission accumulating ten or more kilometres per second over years, the gridded engine’s specific impulse wins. See Electric propulsion.

Flight systems

SMART-1 is the illustrative case. It reached lunar orbit from GTO on a single 1.5 kW Hall thruster, taking fourteen months and using about 82 kg of xenon for roughly 3.9 km/s of delta-v. A chemical stage would have done it in days and needed several times the propellant mass.

Erosion and magnetic shielding

For most of the technology’s history, life was limited by a single mechanism. Ions accelerating out of the channel do not all travel axially; some strike the ceramic discharge channel walls and sputter material away. Once erosion exposes the magnetic circuit behind the ceramic, the thruster fails.

This capped qualified life at roughly 7,000 to 10,000 hours, which is adequate for station keeping but not for a multi-year primary propulsion mission. It was the main reason gridded ion engines were preferred for deep space despite their lower thrust density.

Magnetic shielding, developed at JPL and the Air Force Research Laboratory around 2010 to 2014, solved it. The insight was that the magnetic field shape determines the plasma potential distribution near the walls, because electrons move freely along field lines and therefore make field lines close to equipotential. Reshaping the field so that the lines adjacent to the walls connect to high-potential plasma near the anode holds the wall at high potential, so ions are never accelerated into it.

The gain was not incremental. Measured erosion rates fell by two to three orders of magnitude, and qualified life moved from thousands of hours to tens of thousands. Magnetically shielded designs, including HERMeS for the lunar Gateway, are what make Hall thrusters viable as primary propulsion for long missions.

Propellant economics

The Hall thruster is where the electric propulsion propellant question became commercially urgent, because large constellations fly them in thousands.

Xenon

Best performance. An air-separation by-product with small global production, priced in the low thousands of dollars per kilogram, and a single large constellation can move the market.

Krypton

Roughly 10% lower specific impulse and somewhat lower efficiency, at a fraction of the cost. Flown on early Starlink satellites.

Argon

Cheapest and most abundant, with a further performance penalty and a higher ionisation energy that demands design changes. Flown on later Starlink satellites, the first operational use at scale.

Condensable propellants

Iodine and bismuth store far more densely, but plume condensation on spacecraft surfaces and materials compatibility have limited them to small demonstrations.

The move from xenon to krypton to argon is a clear case of a technical specification being driven by supply economics rather than performance. At constellation scale the propellant bill and its supply security outweigh a 10% specific impulse penalty.

References

• Goebel, D. M. and Katz, I. Fundamentals of Electric Propulsion: Ion and Hall Thrusters, JPL Space Science and Technology Series, Wiley, 2008.
• Boeuf, J.-P. “Tutorial: Physics and modeling of Hall thrusters”, Journal of Applied Physics, 121:011101, 2017.
• Mikellides, I. G., Katz, I., Hofer, R. R. and Goebel, D. M. “Magnetic shielding of a laboratory Hall thruster”, Journal of Applied Physics, 115:043303, 2014.
• Hofer, R. R. et al. “Development Approach and Status of the 12.5 kW HERMeS Hall Thruster for the Solar Electric Propulsion Technology Demonstration Mission”, IEPC-2015-186.
• Racca, G. D. et al. “SMART-1 mission description and development status”, Planetary and Space Science, 50:1323, 2002.