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Home > Research > Low temperature plasmas > Plasmas for space propulsion

Plasmas for space propulsion

 Towards new thrusters…

To propel a spacecraft, a space vehicle or a rocket works on the same basic principle, namely to create a propulsive force (the trust) by accelerating and excelling mass. Chemical rockets achieve a large thrust by expelling a lot of mass very quickly, and can overcome earth’s gravitation and escape into space. However, this method is very fuel or propellant consuming and therefore very expensive for a long interplanetary mission, or for keeping satellites in a desired orbit.
In this respect electric propulsion is promising and has become more and more popular in recent years. The exhaust velocity of electric propulsion systems can reach up to 102 km/s and the propellant burden is 1/10th of what is used in chemical rockets. This means 1/10th reduction in the propellant burden compared to chemical rockets. The thrust achieved by electric propulsion systems is relatively low (so they can not be used to leave earth) but they provide a large velocity change needed for long interplanetary missions.
The electric propulsion systems already used in space, are gridded thrusters (also called ion engines) and Hall effect thrusters (also called closed drift thrusters). Briefly, the thrust is provided by extracting and accelerating positive ions from a high density plasma. The ion beam is neutralized by electrons from a hollow cathode downstream of the acceleration stage. This neutralisation is needed to avoid a built up of negative charge on the space vessel that will counteract the accelerating field.

 Electric Propulsion at LPP

At LPP, experiments, theory and simulations are carried out in the frame of 3 main projects on electric propulsion

PEGASES (Plasma Propulsion with Electronegative GASES) thruster
The electronegative plasma thruster PEGASES (patented by the Ecole Polytechnique in early 2007) belongs to the electrostatic thruster family. The main innovation is to use both positive and negative ions for thrust. This is done by creating a high density electronegative plasma (a plasma with both positive and negative ions and electrons). A magnetic field is used to filter away the electrons, such that an electron free region is formed at the periphery of the plasma. This electron free region is called an ion-ion plasma where only positive and negative ions are present. It is in this extractor area the positive and negative ions will be accelerated to provide the thrust.

The first prototype has been in operation in a small vacuum chamber since late 2007. Since then, experiments, theory and simulations are carried out to better understand the key processes in ion-ion plasma, filtering, anomalous transport and also to study the use Iodine as future propellant in space propulsion.

 Neptune thruster

Neptune is an original ion thruster where the acceleration grids are biased with RF voltages across a capacitor. Due to the different masses of ions and electrons and due to the different effective surfaces of the plasma grid and the acceleration grid (the first is in contact with the plasma and the second is downstream) the capacitor charges up and rectifies the RF voltage into a DC and RF components. Thus, ions are accelerated continuously while electrons are accelerated in bursts during a small period in the RF cycle when the plasma sheath in front of the grids “collapses”.
The Neptune thruster have several advantages over classical ion thrusters; it is simpler, there is no need for an external neutralizer, the capacitive system ensures that the net current is zero, it can run on a variety of propellant from xenon to iodine and the current drawn across the grids can be two times higher than in a classical system.

 Hall effect thrusters

Studies on Hall effect thrusters at LPP are carried out within the project POSEIDON financially supported by the ANR and the industrial partner Safran Aircraft Engines. In this project, 3D simulations in the real geometries of the thruster will be carried out in collaboration with the CERFACS in Toulouse.

For more information

- >https://www.lpp.polytechnique.fr/Kick-off-of-the-ANR-industrial-chair-on-future-plasma-thrusters-for-low-Earth]

 Publication list on electric propulsion at LPP since 2009

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