Energy Efficiency
Ion thrusters are frequently quoted with an efficiency metric. This efficiency is the kinetic energy of the exhaust jet emitted per second divided by the electrical power into the device.
The actual overall system energy efficiency in use is determined by the propulsive efficiency, which depends on vehicle speed and exhaust speed. Some thrusters can vary exhaust speed in operation, but all can be designed with different exhaust speeds. At the lower end of Isps the overall efficiency drops, because the ionization takes up a larger percentage energy, and at the high end propulsive efficiency is reduced.
Optimal efficiencies and exhaust velocities can thus be calculated for any given mission to give minimum overall cost.
Read more about this topic: Ion Thruster
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—Samuel Goldwyn (1882–1974)