Like a de Laval nozzle, a magnetic nozzle converts the internal energy of the plasma into directed kinetic energy, but the operation is based on the interaction of the applied magnetic field with the electric charges in the plasma, rather than on pressure forces acting on solid walls.
Additional advantages include the capability of modifying the strength and geometry of the applied magnetic field in-flight, allowing the nozzle to adapt to different propulsive requirements and space missions.
The expansion of a plasma in a magnetic nozzle is inherently more complex than the expansion of a gas in a solid nozzle, and is the result of several intertwined phenomena, which ultimately rely on the large mass difference between electrons and ions and the electric and magnetic interactions between them and the applied field.
[2] This magnetic confinement prevents the uncontrolled expansion of the electrons in the radial direction and guides them axially downstream.
The heavier ions are typically unmagnetized or only partially magnetized, but are forced to expand with the electrons thanks to the electric field that is set up in the plasma to maintain quasineutrality.
[3] As a result of the ensuing electric field, the ions are accelerated downstream, while all electrons except the more energetic ones are confined upstream.
This condition prevents the continuous electrical charging of the spacecraft on which the magnetic nozzle is mounted, which would result if the amount of ions and electrons emitted per unit time differ.
The separation of ions due to their inertia leads to the formation of local longitudinal electric currents, that do not violate however the global current-free condition in the jet.
[6] The performance of a magnetic nozzle, in terms of its specific impulse, generated thrust and overall efficiency depends on the plasma thruster to which it is connected.
An efficient magnetic nozzle is sufficiently long to minimize the amount of energy wasted in the radial and azimuthal directions.