Survey of electromagnetic accelerators for space propulsion

132 Pages·2002·6.35 MB·English

https://ntrs.nasa.gov/search.jsp?R=19660010232 2019-02-09T08:37:57+00:00Z
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SURVEY OF ELECTROMAGNETIC
ACCELERATORS FOR SPACE PROPULSION
by Stanley Domitz, H. G. Kosmuhl, Peter Rumins,
and N. John Stevens i
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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON,'D; C. MARCH' 1966
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TECH LIBRARY KAFB, NM
0130b35
NASA TN D-3332
SURVEY OF ELECTROMAGNETIC ACCELERATORS
FOR SPACE PROPULSION
By Stanley Domitz, H. G. Kosmahl, Peter Ramins,
and N. John Stevens
Lewis Research Center
Cleveland, Ohio
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
For sole by the Clearinghouse for Federal Scientific and Technical Information
Springfield, Virginia 22151 - Price $1 .60

CONTENTS
Chapter Page
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I INTRODUCTION 1
. . . . . . . . . . . . . .
I1 PERFORMANCE EVALUATION AND COMPARISON 5
. . . . . . . . . . . . . . . . . . . . .
I11 DIRECT-CURRENT ACCELERATORS 11
. . . . . . . . . . . .
IV ALTERNATING-CURRENT PLASMA ACCELERATORS 61
. . . . . . . . . . . . . . . . . . . . . .
V PULSED PLASMA ACCELERATORS 87
. . . . . . . . . . . . . . . . . . . . . .
VI GENERAL CONCLUDING REMARKS 123
iii

SUMMARY
A survey of the experimental status of electromagnetic or plasma accelerators is
presented. This survey is limited to the major programs of recent years on plasma ac-
celerators for spacecraft propulsion and does not cover the purely research activities in
plasma physics.
At present plasma accelerators are at the stage of demonstrating feasibility collect-
ing performance data and investigating the problems of operation. A broad approach
covering a wide range of input power requirements and types of accelerators is being
pursued. There is, however, a definite tendency towards devices using coaxial geometry,
minimum electrode areas, and short interaction lengths.
V
CHAPTER I
INTRODUCTION
Accelerators for electric propulsion applications have traditionally been classified as
electrothermal, electrostatic (ion), or electromagnetic (plasma) accelerators. Recent
survey reports have been published for the first two categories (refs. 1-1 and 1-2). Sur-
veys of plasma accelerators have been written by various authors (refs. 1-3 to I-8), but
the surveys that were comprehensive, do not have the latest results. The purpose of
this report is to compile and interpret the latest representative experimental data in the
propulsion-oriented, plasma accelerator program. In general this report represents the
published state of the art as of July 1965. Recent theoretical and purely research-
oriented efforts will be mentioned only when the results have a direct bearing on the ac-
celerator program.
A plasma accelerator is a device in which a partially ionized plasma, a gas com-
posed of free electrons, ions and neutral atoms, is accelerated primarily by means of
the Lorentz (or 7 X g) force. Most accelerators operate in a regime such that the force
acts primarily on the electrons causing them to be accelerated in the thrust direction.
The ions are accelerated by the electric field resulting from the separation of the elec-
trons and ions. Finally, the neutral atoms, if they are to be accelerated, must be moved
along by means of ion-neutral collisions. In this process the plasma remains electrically
neutral on the average, that is, the electron density is approximately equal to the ion den-
sity.
Although the basic force law from which the thrust is derived is common to all
plasma accelerators, there are numerous ways in which the current and magnetic field
can be applied or induced. These can be used to distinguish the various types of acceler-
ators. For example, in a linear cross-field accelerator a steady-state current and mag-
netic field are applied to accelerate the plasma; in Hall devices a steady applied magnetic
field and induced currents are utilized; pulsed accelerators use a pulsed current and in-
duced magnetic field; and traveling wave accelerators use an oscillating magnetic field
and induced currents.
Another characteristic that distinguishes the different types of accelerators is the
regime of operation. If the ratio of the ion mean free path to the accelerator dimensions
is much lower than unity, then the accelerator is said to operate in the collision-
dominated regime. In this regime the plasma is treated as a fluid and the usual
magnetofluid-dynamic equations are used. If this ratio is much higher than unity,
however, then the accelerator is said to operate in the collisionless regime. Here the
particle trajectory equations are used to represent the acceleration process. In the re-
gime between these limits, there is no simplified theoretical approach.
As a result of this situation, it is doubtful if any single, simple theory will describe
plasma phenomena well enough to predict operation over a broad operating range. Al-
though present-day theories are useful for guiding the experimental research, most im-
provements in accelerator design and performance have come about almost entirely from
an empirical approach and it is very likely that plasma accelerators will become opera-
tional before a theory sufficiently refined is available. A phenomenological approach will
be used in this survey to explain the operation of the various accelerators. For those
readers who desire a more complete theoretical background, references 1-9 to 1-14 are
suggested.
The advantage of the plasma accelerator lies in its ability to produce high thrust den-
sities, that is, the devices are not limited by fundamental space-charge laws. Their po-
tential specific impulse ranges from 1000 to 20 000 seconds with no fundamental restric-
tion on the upper limit. Thus with a high thrust density, relatively large thrusts (in the
range of newtons) from small, simple, lightweight accelerators are quite feasible. The
magnetic fields, where applied externally, can generally be obtained by relatively light-
weight, permanent or low-loss magnets. The power-conditioning requirements do not
appear to be severe. The dc devices require a low voltage and high current which can be
obtained from present generators or conceivably from one of the direct energy-conversion
schemes. The radiofrequency power for the induction accelerators can be generated
using rotating machinery. The tubes for microwave-driven accelerators and the capaci-
tor for pulsed accelerators will require a modest developmental effort.
The major experimental effort now is to study and evaluate the operation of the ac-
celerators toward demonstrating feasibility for possible space propulsion applications.
A number of different types of plasma accelerators requiring a wide range of input powers
are being investigated. Development of flyable systems and the technology associated
with these efforts has been logically postponed until acceptable performance of the accel-
erator has been established.
In this survey the division of the plasma accelerators will be by the power-input re-
quirements. Prior to beginning this discussion, a chapter on performance evaluation
and comparison is given to emphasize the fact that care is needed in interpreting and
comparing data. A pr&is of current status containing the recent important results as
well as problems precedes the discussion of each class of accelerator. A general-
considerations section dealing with the particular acceleration process will be given be-
fore the discussion of the experimental results.
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REFERENCES
1-1. Mickelsen, William R. ; and Kaufman, Harold R. : Status of Electrostatic Thrus-
tors for Space Propulsion. NASA TN D-2172, 1964.
1-2. Wallner, Lewis E. ; and Czika, Joseph, Jr. : Arc Jet Thrustor for Space Propul-
sion. NASA TN D-2868, 1965.
1-3. Brewer, G. R. ; Currie, M. R. ; and Knechtli, R. C. : Ionic and Plasma Propul-
sion for Space Vehicles. Proc. IRE, vol. 49, no. 12, Dec. 1961, pp. 1789-1821.
1-4. Seikel, George R. : Generation of Thrust - Electromagnetic Thrustors. Proceed-
ings of the NASA-University Conference on the Science and Technology of Space
Exploration, vol. 2, NASA SP-11, 1962, pp. 171-176. (Also available as NASA
SP-22. )
1-5. Hess, Robert V. : Fundamentals of Plasma Interaction with Electric and Magnetic
Fields. Proceedings of the NASA-University Conference on the Science and Tech-
nology of Space Exploration, vol. 2, NASA SP-11, 1962, pp. 313-336. (Also
available as NASA SP-2 5. )
1-6. Ellis, Macon C. , Jr. : Survey of Plasma Accelerator Research. Proceedings of
the NASA-University Conference on the Science and Technology of Space Explora-
tion, vol. 2, NASA SP-11, 1962, pp. 361-381. (Also available as NASA SP-25.)
1-7. Demetriades, Sterge T. : Plasma Propulsion, pt. II. Astronautics, vol. 7, no. 4,
Apr. 1962, pp. 40-42; 44.
1-8. Dix, D. M. : A Survey of Some Plasma Acceleration Devices for Space Propulsion.
Rept. No. TDR-l69(3713-01)TR-l(SSS-TDR-63-120A),er ospace Corp., May 17,
1963.
1-9. Resler, E. L., Jr. ; and Sears, W. R. : The Prospects for Magneto-Aerodynamics.
J. Aeron. Sci., vol. 25, no. 4, Apr. 1958, pp. 235-245; 258.
1-10. Sears, W. R. : Magnetohydrodynamics in Aeronautics. Preprint No. 709-58, Am.
Rocket SOC., Inc. , 1958.
1-11. Spitzer, L., Jr. : Physics of Fully Ionized Gases. Intersci. Pub. Inc., 1956.
1-12. Chandrasekhar, S. : Plasma Physics. Univ. Chicago Press, 1960.
1-13. Cowling, T. G. : Magnetohydrodynamics. Intersci. Pub. Inc., 1957.
1-14. Demetriades, S. T.; Hamilton, G. L.; Ziemer, R. W.; and Lenn, P. D.: Three-
Fluid Non-Equilibrium Plasma Accelerators. Progress in Astronautics and
Aeronautics. Vol. 9 - Electric Propulsion Development. Academic Press, Inc. ,
1963, pp. 461-511.
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