NEEP602 Course Notes (Fall 1997)
Resources from Space

Lecture #31: Charge!

Title: Plasma and Electric Propulsion

November 12, 1997

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Selected events in the history of plasma and electric propulsion

Year People Event 
1906 Robert H. Goddard Brief notebook entry on possibility of electric propulsion 
1929 Hermann Oberth Wege zur Raumschiffahrt chapter devoted to electric propulsion 
1950 Forbes and Lawden First papers on low-thrust trajectories 
1952 Lyman Spitzer, Jr. Important ion-engine plasma physics papers 
1953 E. Saenger Zur Theorie der Photonrakete published 
1954 Ernst Stuhlinger Important analysis. Introduces specific power 
1958 Rocketdyne Corp. First ion-engine model operates 
1960 NASA Lewis; JPL NASA establishes an electric propulsion research program 
1964 USSR Operate first plasma thruster in space (Zond-2) 
2002 US Possible solar-electric propulsion 

Plasma physics overview

A useful working definition of a plasma, taken from F.F. Chen, Introduction to Plasma Physics, is 

``A plasma is a quasineutral gas of charged and neutral particles which exhibits collective behavior.'' 

  • Quasineutral means that the number of positive and negative charges are very nearly equal. 
  • Collective behavior means that the state of the plasma in regions somewhat distant from the point of interest may affect the behavior. 
Plasmas exist at widely varying densities and temperatures, as shown if you click on the figure at right, taken from J.D. Huba, NRL Plasma Formulary (Naval Research Laboratory NRL/PU/6790-94-265, 1994).
Plasma regimes in density and temperature 

Key equations and physical effects governing plasma behavior

Numeric formulas for many of the quantities discussed below are given in the Plasma Formulary in cgs units. SI units are used here.

High-Exhaust-Velocity Thrusters

Plasma and electric thrusters generally give a higher exhaust velocity but lower thrust than chemical rockets. They can be classified roughly into five groups, the first three of which are relevant to the present topic and will be discussed in turn.

Electrothermal thrusters

This class of thrusters (resistojet, arcjet, RF-heated thruster) does not achieve particularly high exhaust velocities. The resistojet essentially uses a filament to heat a propellant gas (not plasma), while the arcjet passes propellant through a current arc. In both cases material characteristics limit performance to values similar to chemical rocket values. The RF-heated thruster uses radio-frequency waves to heat a plasma in a chamber and potentially could reach somewhat higher exhaust velocities.

Electrostatic thrusters (ion thrusters)

This class has a single member, the ion thruster. Its key principle is that a voltage difference between two conductors sets up an electrostatic potential difference that can accelerate ions to produce thrust. The ions must, of course, be neutralized--often by electrons emitted from a hot filament. The three main stages of an ion-thruster design are ion production, acceleration, and neutralization. They are illustrated in the figure below. The basic geometry of an actual ion thruster appears at right on the cover from a recent Mechanical Engineering (from PEPL home page).  Mechanical Engineering cover 
Ion thruster stages

 Two of artist Pat Rawling's conceptions of spacecraft using ion thrusters appear below. Fission reactors are located at the ends of the long booms in these nuclear-electric propulsion (NEP) systems. These and other designs are from NASA Lewis Research Center's now-defunct Advanced Space Analysis Office's (ASAO) Web page.


NEP Mars approach  Hydra multiple-reactor NEP vehicle 

Electrodynamic thrusters

Magnetoplasmadynamic (MPD) thruster
In MPD thrusters, a current along a conducting bar creates an azimuthal magnetic field that interacts with the current of an arc that runs from the point of the bar to a conducting wall. The reulting Lorentz force has two components: MPD thruster plasma flow simulation The basic geometry is shown at right in a frame from a computer simulation done by Princeton University's Electric Propulsion and Plasma Dynamics Lab. A movie of an MPD thruster, from the PEPL, can be accessed by clicking here. Erosion at the point of contact between the current and the electrodes generally is a critical issue for MPD thruster design.
Hall-effect thruster
SPT thruster In Hall-effect thrusters, perpendicular electric and magnetic fields lead to an ExB drift. For a suitably chosen magnetic field magnitude and chamber dimensions, the ion gyroradius is so large that ions hit the wall while electrons are contained. The resulting current, interacting with the magnetic field, leads to a JxB Lorentz force, which causes a plasma flow and produces thrust. The Russian SPT thruster, shown at right, is presently the most common example of a Hall-effect thruster.

Image source: University of Michigan Plasmadynamics and Electric Propulsion Laboratory

Pulsed-plasma thruster
In a pulsed-plasma accelerator, a circuit is completed through an arc whose interaction with the magnetic field of the rest of the circuit causes a JxB force that moves the arc along a conductor.
Helicon thruster
Helicon device The principle of the helicon thruster is similar to the pulsed-plasma thruster: a traveling electromagnetic wave interacts with a current sheet to maintain a high JxB force on a plasma moving along an axis. This circumvents the pulsed-plasma thruster's problem of the force falling off as the current loop gets larger. The traveling wave can be created in a variety of ways, and a helical coil is often used. The plasma and coil of a helicon device appear at right. Source: UW Plasma Aided Manufacturing Center

Useful references


General plasma physics
Plasma and electric thrusters
Note: These texts are still useful, despite having been written some years ago.

Journals and conferences

Worldwide Web

Selected sites related to plasma and electric thrusters

Presently, these do not contain a great deal of information, but they will undoubtedly develop and are worth keeping an eye on.

Space-related, government, and other potentially useful Web sites


  1. Categorize and describe the three main types of plasma and electric thrusters.
  2. Describe the three main stages of an ion thruster.
  3. What two major forces are at work in the plasma plume of a magnetoplasmadynamic (MPD) thruster?

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Next: Lecture 32: Fusion propulsion
Previous: Lecture 30: Chemical rockets
Up: Resources from Space syllabus 
Dr. John F Santarius

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