In his classic text, Physics of Electric Propulsion ,
Prof. Robert Jahn of Princeton University defined electric propulsion as:
Electric Propulsion (EP): The acceleration of gases for the
purpose of producing propulsive thrust by electric heating, electric body forces, and/or
electric and magnetic body forces.
Unlike chemical propulsion, which relies on the stored
internal energy in the molecular bonds of its propellant, in electric propulsion
the energy is obtained from an external power source. This is the main advantage
of electric over chemical propulsion, since the amount of energy that can be
externally applied is limited only by available technology. In chemical
propulsion, the dependence on internal energy limits the maximum specific
impulse to about 450 s, whereas in electric propulsion specific impulses of over
17,000 s have been obtained in the laboratory [2, 3]. Current flight hardware
operates around 330 s for hydrazine chemical rockets, 500 s for arcjets, 1600 s
for Hall thrusters, and 3000 s for gridded ion thrusters .
Since external power sources are required, the high-specific
impulse capabilities of electric propulsion come at the expense of power and
mass. This limited the application of electric propulsion on-board spacecraft
until the 1990s as spacecraft power finally began to increase to meet the
growing needs of communication satellites . The so-called "power supply
penalty"  places a premium on maximizing the specific power (in W/kg) of
power supplies used for electric propulsion systems. Other design constraints,
such as launch vehicle selection, mission time, and payload further complicate
the situation. As can be shown, the end result is that the highest attainable
specific impulse is not always the optimum choice for a given mission profile.
The different electric propulsion technologies were first
categorized by Stuhlinger  and Jahn . Adopting the definitions of Jahn
, the thruster categories are:
Electrothermal propulsion: acceleration of a propellant gas by
electrical heat addition and expansion through a convergent/divergent
nozzle. Examples include resistojets and arcjets.
Electrostatic propulsion: acceleration of an ionized propellant
gas by the application of electric fields. Examples include gridded ion
thrusters, colloid thrusters, and field emission electric propulsion (FEEP).
Electromagnetic propulsion: acceleration of an ionized propellant
gas by the application of both electric and magnetic fields. Examples
include Hall thrusters, pulsed plasma thrusters (PPT), pulsed inductive thrusters (PIT), and
magnetoplasmadynamic thrusters (MPDT).
Descriptions of the thruster types, flight programs, and development trends
can be found in Ref. [1, 2, 7-9] and the September-October 1998 issue of the AIAA
Journal of Propulsion and Power (Vol. 14, No. 6).
Goebel and Katz at the NASA Jet Propulsion Laboratory have published a textbook in 2008 that "that delves into the basics of two of the more modern electric engines that are finding increasingly more applications, specifically ion and Hall thrusters, in an attempt to provide a better understanding of their principles."  Soft copies are available here. Hard copies are available through Amazon.com.
 Jahn, R. G., Physics of electric propulsion, 1st ed., New York,
 Curran, F. M., Sovey, J. S., and Myers, R. M., "Electric propulsion: An
evolutionary technology," IAF-91-241, 42nd Congress of the International
Astronautical Federation, Montreal, CA, Oct. 5-11, 1991.
 Byers, D. C., "An experimental investigation of a high-voltage
electron-bombardment ion thruster," Journal of the Electrochemical
Society, Vol. 116, No. 1, pp. 9-17, 1969.
 Sutton, G. P. and Biblarz, O., Rocket propulsion elements, 7th ed.,
New York, John Wiley & Sons, 2001.
 Oleson, S. R., Myers, R. M., Kluever, C. A., Riehl, J. P., et al.,
"Advanced propulsion for geostationary orbit insertion and north-south
station keeping," Journal of Spacecraft and Rockets, Vol. 34, No. 1,
pp. 22-28, 1997.
 Stuhlinger, E., Ion propulsion for space flight, 1st ed., New York,
 Jahn, R. G. and Choueiri, E. Y., "Electric
propulsion," in Encyclopedia of Physical Science and Technology,
Vol. 5, 3rd ed., Academic Press, 2002, pp. 125-141.
 Martinez-Sanchez, M. and Pollard, J. E., "Spacecraft electric
propulsion - an overview," Journal of Propulsion and Power, Vol. 14,
No. 5, pp. 688-693, 1998.
 Pollard, J. E., Jackson, D. E., Marvin, D. C., Jenkin, A. B., et al.,
"Electric propulsion flight experience and technology readiness,"
AIAA Paper 93-2221, 1993.
 Goebel, D. M., Katz, I., "Fundamentals of Electric Propulsion: Ion and Hall Thrusters," John Wiley & Sons, 2008.
EP Research & Development
EP research and development is a worldwide activity, with major efforts in the United States, Europe, Japan, & Russia.