The problem
An ion thruster expels positively charged particles, so a separate electron source is needed to keep the outgoing beam and spacecraft from charging up. A hollow cathode supplies electrons from a hot insert inside a tube. Can it sustain the current needed by a higher-power iodine thruster while hot iodine leaves the emitter intact?[1]
An ion thruster’s positive-ion beam needs an electron source, called a neutralizer, to avoid spacecraft charging. Can an iodine-fed hollow cathode sustain the needed ampere-range current without unstable operation or progressive damage to its insert?
The hollow tube contains a heated insert. Electron release from a hot material is thermionic emission; a plasma discharge carries those electrons outside. The emitter must remain functional in a hot, chemically reactive environment. April 2026 C12A7 electride tests did not sustain stable iodine discharge. August LaB6 tests achieved 8.5 cumulative hours of iodine operation before a pumping-system defect stopped the experiment. Changes found afterwards in the emitter’s surface and geometry constrain compatibility; they do not turn the test’s stopping time into a measured failure lifetime.[1] [2] [3]
Scope
Iodine-fed thermionic plasma-bridge hollow cathodes providing sustained ampere-range neutralization with stable discharge and retained emitter material.
What remains open
Why it matters
The thruster and its electron source could share iodine, reducing the need for a second propellant system.[1]
Solid iodine can be stored in an unpressurised tank. A compatible high-current cathode could let ion production and electron neutralisation share that supply, potentially removing a separate noble-gas tank and feed system. The simplified propulsion system would still have to maintain the electron current needed to balance its ion beam.[1]
Definitions
2 terms
Thermionic emission
Electron release from a sufficiently hot emitting material. A hollow cathode combines a heated insert with a discharge that carries electrons into the surrounding plasma.[1]
Neutralizer
An electron source that balances a thruster’s positive-ion beam, helping prevent spacecraft charging. In a single-propellant iodine system, it would operate with the same propellant as the thruster.[1]
Sources
- Review on the current state of iodine compatible neutralizers
Notes on Review on the current state of iodine compatible neutralizers
Review explains electron neutralisation and the distinction between established low-current iodine devices and a durable hot plasma-bridge cathode at higher current.
- On the compatibility of C12A7 electride and iodine in planar and hollow cathodes
Notes on On the compatibility of C12A7 electride and iodine in planar and hollow cathodes
Tested C12A7 electride cathodes did not sustain stable iodine discharge and showed material changes. The proposed degradation mechanisms remain qualified.
- On the compatibility of LaB6 and iodine in planar and hollow cathodes
Notes on On the compatibility of LaB6 and iodine in planar and hollow cathodes
LaB6 hollow-cathode testing accumulated 8.5 hours before a pumping-system defect ended the run. Post-test emitter changes leave prolonged compatibility unestablished.