Hall-Effect Thruster

A magnetic trap holds electrons prisoner while xenon ions — too heavy to care — blast past them at 15 kilometres per second.

A Hall-effect thruster converts electrical power (from solar panels or a reactor) into kinetic energy of a propellant gas. Xenon atoms are ionised inside an annular ceramic channel, then flung out the back by a strong electric field. The reaction pushes the spacecraft in the opposite direction. The thrust is tiny — tens to a few hundred millinewtons — but the propellant is used so sparingly that a spacecraft can accelerate for months or years on what a chemical rocket would burn in seconds. That efficiency (measured as specific impulse, Isp) is what makes Hall thrusters the workhorse of modern satellite station-keeping, orbit-raising, and increasingly, deep-space primary propulsion.

At the open end of the discharge channel, a hollow cathode injects electrons. A set of electromagnets creates a radial magnetic field of roughly 100–300 gauss across the channel. Electrons are so light they get trapped by this field and spiral in closed azimuthal loops — the 'Hall current' the device is named after. The trapped electrons slow down enough that an axial electric field (typically 200–500 V, set between the anode deep inside the channel and the cathode outside) can stand up without being short-circuited. Heavy xenon atoms, injected at the anode, don't care about the magnetic field — they waltz straight through, get struck by the spiralling electrons, lose an electron themselves, and become ions. Those ions see the electric field and are accelerated to exhaust speeds of 10–30 km/s. A neutraliser cathode then adds electrons back into the plume so the spacecraft doesn't build up charge. The only moving parts are electrons and ions.

In plain terms

Imagine you have a garden hose, but instead of spraying water you're spraying really fast-moving invisible atoms. The atoms push backward so hard that the nozzle — and the rocket attached to it — gets pushed forward. Hall thrusters do exactly that, except the atoms are xenon gas, they go so fast they glow blue, and the whole engine fits in a lunchbox.

Also called: HET, stationary plasma thruster, SPT, Hall thruster, ion thruster, electric propulsion, EP thruster.

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A short history

The underlying physics — electron drift in crossed electric and magnetic fields — was identified in the mid-nineteenth century by Edwin Hall (1879), but the idea of using it for propulsion emerged independently in the Soviet Union in the late 1950s. Askold Zharinov at the Kurchatov Institute of Atomic Energy demonstrated electrodeless ion acceleration in crossed E×B fields between 1958 and 1961. Alexey Morozov led the parallel development of the Stationary Plasma Thruster (SPT) concept from 1962, focusing on creating a sustained electric field inside the bulk plasma using a transverse magnetic field. By the mid-1960s, the first SPT laboratory models were operating with gaseous propellants. The first flight hardware — a pair of SPT thrusters — flew on the Soviet Meteor meteorological satellite in 1971, and Soviet spacecraft quietly accumulated over 140 operational Hall thrusters through the Cold War. Western engineers only obtained detailed technical data after the fall of the Soviet Union; the first US spacecraft to carry a (Russian-built) Hall thruster was the STEX satellite in 1998. ESA flew the PPS-1350-G on SMART-1 in 2003 for the first fully Hall-thruster-propelled interplanetary mission, and commercial adoption accelerated rapidly thereafter.

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