Electrostatic Ion Thruster - Method of Operation

Method of Operation

1. Propellant atoms are injected into the discharge chamber and are ionized by electron bombardment, forming a plasma. There are several ways of producing the energetic electrons for the discharge: (1) The electrons are emitted from a hollow cathode and are accelerated on their way to the anode (Kaufman type ion thruster). (2) The electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a self-sustaining discharge and omits any cathode (radiofrequency ion thruster). (3) Microwave heating
2. The positively charged ions move towards the extraction system (2 or 3 multi-aperture grids) of the chamber due to diffusion. Once ions enter the plasma sheath at a grid hole they will be accelerated by the potential difference between the first (screen) and the second (accelerator) grid of the extraction system. The ions are ion-optically focused by the rather large electric field to pass through the extraction holes. The final ion energy is determined by the potential of the plasma (the plasma potential is a few volts larger than the screen grid voltage).
3. The negative voltage of the accelerator grid prevents electrons of the beam plasma outside the thruster from streaming back to the discharge plasma. Electron backstreaming occurs if the potential within the grid is not sufficiently negative, this can mark the end-of-life of the ion thruster. By increasing the negative voltage electron backstreaming can be avoided.
4. The expelled ions propel the spacecraft in the opposite direction according to Newton's 3rd law.
5. Electrons are emitted from a separate cathode placed near the ion beam, called the neutralizer, towards the ion beam to ensure that equal amounts of positive and negative charge are ejected. Neutralizing is needed to prevent the spacecraft from gaining a net negative charge.