Apollo 17 astronaut Dr. Harrison "Jack" Schmitt and UW grad student Craig Schuff. (Sept. 2011)

NEEP533 Course Notes (Spring 1999)
Resources from Space

Lecture #31: Charge!

Title: Plasma and Electric Propulsion

NEEP 533/Geology 533/Astronomy 533/EMA 601: Resources from Space

April 7, 1999

Next: Lecture 32: Fusion propulsion
Previous: Lecture 30: Recent probes for the exploration of Mars
Up: Resources from Space syllabus

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 on Earth
1960	NASA Lewis; JPL	NASA establishes an electric propulsion research program
1964	USSR	Operate first plasma thruster in space (Zond-2)
1998	US	XIPS xenon ion electrostatic thruster used in space

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 in the figure below, taken from J.D. Huba, NRL Plasma Formulary (Naval Research Laboratory NRL/PU/6790-94-265, 1994).

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.

Maxwell's equations for the microscopic electric (E) and magnetic (B) fields

Lorentz force on a particle of charge q

The result of this equation is that charged particles spiral along lines of magnetic force with the gyrofrequency (also called the cyclotron frequency), at a distance called the gyroradius (or Larmor radius), , where is the average thermal velocity of a particle. Because the electron's mass is much smaller than any ion's mass, the electron gyrofrequency is much faster than the ion gyrofrequency and the electron gyroradius is much smaller.

The Lorentz force leads to several charged-particle drifts, even in static electric and magnetic fields. These include drifts due to ExB motion, magnetic-field gradients, and magnetic-field curvature.

Plasmas Will Try to Reach Thermodynamic Equilibrium

Maxwell--Boltzmann

Maxwellian

n

₀

k

^-23

^-19

Plasmas are Dynamic Entities

Debye Shielding

here

Plasma Parameter

Debye sphere

Electrostatic potential sheaths

sheaths

double layers

Quantum mechanics and atomic physics

line radiation

Local thermodynamic equilibrium (LTE)

High density plasma, so collisional effects dominate radiative ones.
Characterized by the electron temperature, because electrons dominate the collisional processes.

Coronal equilibrium

Optically thin plasma
Collisional ionization, charge exchange, and radiative recombination dominate.

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

Resistojet
Arcjet
RF-heated

Electrostatic

Electrodynamic

Magnetoplasmadynamic (MPD)
Hall-effect
Pulsed-plasma
Helicon

Photon

Solar sail
Laser

Advanced

Fusion
Gas-core fission
Matter-antimatter annihilation
Tether
Magnetic sail

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).

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 Glenn 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 resulting Lorentz force has two components:

Pumping: a radially inward force that constricts the flow.
Blowing: a force along the axis that produces the directed thrust.

The figure at left is from the University of Stuttgart's Electric Space Propulsion web site.

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

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

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

Texts

General plasma physics

Francis F. Chen, Introduction to Plasma Physics and Controlled Fusion (Plenum, New York, 1983).
N.A. Krall and A.W. Trivelpiece, Principles of Plasma Physics (McGraw-Hill, New York, 1973).
D. Nicholson, Introduction to Plasma Theory (Wiley, New York, 1983).

Plasma and electric thrusters

Note: These texts are still useful, despite having been written some years ago.

Robert G. Jahn, Physics of Electric Propulsion (McGraw-Hill, New York, 1968).
Ernst Stuhlinger, Ion Propulsion for Space Flight (McGraw-Hill, New York, 1964).

Journals and conferences

Journal of Propulsion and Power, American Institute of Aeronautics and Astronautics (AIAA).
Journal of Spacecraft and Rockets, American Institute of Aeronautics and Astronautics (AIAA).
IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers.
AIAA/SAE/ASME/ASEE Joint Propulsion Conference

International Electric Propulsion Conference
NASA workshops on specific types of thrusters are held with fair regularity, and advanced-propulsion systems are often discussed in conferences devoted to long-range missions.

Worldwide Web

Selected sites related to plasma and electric thruster research.

Air Force Office of Scientific Research, Electric Propulsion group
Centrospazio (Italy) Space Technology Laboratory
Cornell University Research in Nonequilibrium Gases and Plasmas
Edwards Air Force Base (Phillips Laboratory) Propulsion Directorate
Hughes Space and Communications Xenon Ion Propulsion web page
ISAS Institute of Space and Astronautical Science (in Japanese) Electric Propulsion Department
NASA's Glenn Research Center at Lewis Field Electric Propulsion group
University of Giessen (in German), I. Physikalisches Institut: Abt. Plasma- und Atomstoßphysik - Ionenquellen
University of Illinois Space Propulsion Laboratory
University of Michigan, Plasmadynamics and Electric Propulsion Laboratory
Princeton University, Electric Propulsion and Plasma Dynamics Laboratory
Stanford, Plasma Dynamics Laboratory
University of Stuttgart (Germany) Electric Space Propulsion group
University of Tokyo Electric Propulsion Department

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

Bookmarks

Questions

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