Plasma Thrusters

Abstract

AbstractPlasma thrusters are a class of electric propulsion in which the working medium has the form of plasma in the acceleration zone. The presence of plasma, both positive ions and negatively charged electrons, in the acceleration gap distinguishes plasma thrusters from ion thrusters where the acceleration gaps contain only positively charged ions. In ion propulsion, the space charge field restricts the emissions of ions from the emitter (ion generator), and thus ion thrusters have a relatively low thrust density and require a high acceleration voltage. As a result, they can be efficient only with an acceleration voltage >1 kV and exhaust velocities of ≥30 km/s.Because the acceleration gaps in a plasma thruster contain both positive ions and electrons, no space charge is needed. For this reason, there are no limitations in theory on the thrust density in plasma thrusters, and the exhaust velocity may range from a few km/s to hundreds or more km/s. Of course, different plasma thruster designs are optimal for each range of exhaust velocities and power levels.Plasma thrusters, like other electric propulsion thrusters, are of interest for space technology primarily because of their capacity to reach high exhaust velocities, greater than those attained by thermochemical (liquid or solid) fuel rocket engines. In a number of instances, other features of such thrusters may also be of interest.The chemical energy contained in the working medium is not sufficient to attain high velocities. For this reason, plasma thrusters, like other rocket engines, require special sources of energy. Today, these sources may be either the Sun or nuclear processes of one kind or another (fission, radioactive decay). For each specific flight, optimal velocity parameters exist, defined with respect to minimum total mass of propellant and power source. The optimal exhaust velocities for the majority of purposes are currently within the range of approximately 15–30 km/s.These thrusters are classified primarily on the basis of the mechanism for accelerating the plasma. Three basic types of plasma thrusters are identified: Thermal plasma thrusters; electromagnetic (Ampere's force) plasma thrusters; and the third class encompasses plasma thrusters in which acceleration is achieved through both kinetic gas pressure and Ampere's force.The division of plasma thrusters into three groups on the basis of the acceleration mechanism is very general. However, detailed classification of all possible plasma thrusters on the basis of their physical or design features is virtually impossible, This article describes only representatives of the three types of plasma thrusters that have already functioned in space.