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Electric Propulsion Systems overview

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Whats PLASMA

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Plasma = Partially Ionized Gas (Ions, Electrons, Neutrals) with Equal Charges (Quasineutrality)

 

Minimum energy required is the ionization energy of the gas to remove an electron from the atom. There are many different processes (electron collision, etc.) with different probabilities (collision cross section).

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What kind of Ionization Mechanisms exist

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What happens to a conductor immeresed into a plasma

 

1. Electrons have a higher velocity than ions and stream first to the conductor

2. Conductor becomes negatively charged (with respect to the plasma)

3. It becomes so negative that the electrons don‘t flow to the conductor any more (equilibrium)

4. The conductor is now on the „Floating Potential“ Φfwith respect to the „Plasma Potential“ Φpin the main plasma body

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What is schielding of a potential in a plasma

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Overview Resistojet

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Propellant: can be everything

Isp 250 - 840s

Thrust 0.1 - 6N

Power 0.5 -30kW

 

Temperature limited by heat exchanger (Tungsten) – max. 3000 K

Used for attitude control, competes against monopropellant (Isp=150-250) and cold gas (Isp=80 s) thrusters

•Also waste water can be used (space station)

•Efficiency: 80%

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Important Influence on Efficiency for Electrothermal Thrusters

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Under high temperature, propellant (e.g. H2) dissociates (H-H) and even ionizes⇒kinetic energyis reduced (and temperature lost)

•Because gas moves very quickly through chamber-nozzle, there is no time for atoms to re-combine to re-gain this energy

•Influenced by temperature (Isp) and pressure (high pressure – smaller losses)

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Arcjet overview

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Propellant swirled into chamber (increases time for heating)

•Either low voltage (100 V) and high current (hundreds of A) discharge or high-frequency high voltage discharge

•Very high local heating along center line (fully ionization)

•Strong temperature gradient towards walls•Cathode erosion limits lifetime to typically 1,500 hours

•Also microwave and AC discharges