Gridded ion thruster

A gridded ion thruster is a form of electric propulsion that produces thrust by ionizing a propellant gas and accelerating the resulting ions through a set of electrically biased grids. Compared with chemical rockets it produces very low thrust but uses far less propellant, which makes it suited to long-duration missions.

ION-I’s first thruster is a gridded ion thruster using argon as propellant, rather than the xenon used on most flight thrusters.¹

Principle of operation

Electron source

Ionization starts with electrons. In ION-I’s design these come from a heaterless, insertless hollow cathode, IonSpark, in which a high-voltage tungsten pin emits electrons by field emission.² An electron source is needed both for ionizing propellant in a DC ion thruster and for neutralizing the ion beam in gridded ion and Hall-effect thrusters.

Discharge chamber

Electrons from the cathode are carried into the discharge chamber together with the propellant. When an electron strikes an argon atom with enough energy it can remove one of the atom’s electrons, ionizing it (see Stepwise ionization). Rings of magnets around the chamber confine electrons away from the walls, increasing the number of collisions each electron makes and therefore the plasma density.¹ See Discharge chamber.

Ion optics

The ion optics are a pair of perforated grids at the downstream end of the chamber. The screen grid extracts ions from the plasma and shapes the beam; the potential difference between it and the accelerator grid produces the electric field that accelerates ions out of the thruster. The accelerator grid is biased negatively, which keeps electrons from outside the thruster from flowing back in.³ See Ion optics.

Neutralization

A thruster that expels only positive ions would leave the spacecraft with a growing negative charge, eventually attracting its own exhaust back. Flight thrusters therefore use a second cathode, the neutralizer, to emit electrons into the beam. ION-I’s first thruster does not yet include a separate neutralizer.¹

See also

• IonSpark
• Discharge chamber
• Ion optics
• Stepwise ionization

References

1. ION-I Discharge Chamber subteam, Systems Requirement Review, version 2.3. ↩ ↩² ↩³

2. ION-I Plasma Source subteam, Systems Requirement Review / Systems Definition Review. ↩

3. ION-I Ion Optics subteam, Systems Requirement Overview. ↩