C.7.3 Induced plasma characteristics

The plasma produced by electric propulsion thrusters includes the high energy primary ions of the plasma beam (300 eV for SPT, 1 000 eV for ion engines, 8 000 eV for field emission thrusters), and a low energy charge exchanged ions population (up to a few tens of eV) and neutral atoms.

The emission from thrusters can be classified into the following categories (see [16]):

• Primary beam ions
These ions are well focused within the half-angle beam divergence of 10 degrees - 40 degrees. Most thrusters have high ionization efficiencies greater than 95 %. The beam usually consists of singly charged ions, only some percent can be doubly charged (higher percentage in hall effect thrusters). A typical ion beam profile shows a Gaussian distribution, limited by the half-angle divergence.

• Charge-Exchange ions
If high-energy primary ions collide with neutral propellant atoms, a charge-exchange collision can occur. During this collision process, the impulse between each collision partner is exchanged, thus creating fast neutrals and slow ions with thermal velocities:

Due to the Gaussian primary ion beam distribution, the beam potential follows also a similar distribution. Slow ions can be repelled from the positive potential along the thruster centreline and create a substantial charge-exchange ion environment around the thruster. Typical charge-exchange ion energies are several eV up to a few tens of eV. These ions are not limited by the primary ions half-angle divergence and thus can flow towards the spacecraft’s surface, solar arrays or other parts. This backflow current does influence spacecraft charging.

• Neutral atoms
Since propellant utilization efficiency is less than 100 %, also a neutral environment is produced around the thruster. In case of metal propellants, this can cause substantial contamination concerns, but there is no influence on spacecraft charging until ionization occurs.
Normal plasma characteristics on exit plane of usual electric thrusters are summarized in Table C-3 (see [16]).

Table C-4 (see [15] and [16]) lists order of magnitude of the emission versus backflow currents for various electric propulsion thrusters obtained by particle-in-cell (PIC) computer simulations and consistent with ground based measurements.

The current from the primary emitted particle and the charge exchange plasma overall affect the charging state of the spacecraft. Floating potential values of around –10 Volt in GEO have actually been measured in space experiments using ion thrusters on the SCATHA[17] and ATS-6 spacecraft [18]. A complete spacecraft charging analysis cannot be performed without taking these induced plasma interactions into account.