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Патент USA US3050662

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Aug. 21, 1962
Filed Aug. 12, 1960
5, 1
n., ,.
United States atet
Patented Aug. 21, 1952
even check all emission of ions and electrons if they are
George C. Baldwin, Schenectady, N.Y., assignor to Gen
eral Electric Company, a corporation ‘of New York
ejected from separated sources.
For most e?icient operation of an ion propulsion sys
tem, the above described neutralization process must,
5 therefore, insure that the total ion and electron currents
from the vehicle be exactly equal in order that thrust
Filed Aug. 12, 1960, Ser. No. 49,366
7 Claims. (Cl. 313—63)
be sustained. Also, neutralization must be achieved in
a manner so that the initial spatial separation of electron
and ion streams ejected by the vehicle is reduced to_a
My invention relates to force developing systems and,
in particular, to methods and apparatus employing a beam 10 minimum, producing an electrically neutral beam, 1n
order to avoid wasteful power drain. This requires that
of high speed ions to develop a force for propulsion
the discharge velocity of the electrons match that of the
Ion propulsion is a means for generation of reaction
It is an object of my invention to provide new and im
thrust in which electrostatic ?elds impart momentum to
ions of an expellant substance. The expellant must be 15 proved apparatus and methods employing a beam of high
speed ions to develop a force for propulsion purposes m
ionized, its ions and electrons separately accelerated to
which the total electrical charge of the beam at its exit
the appropriate velocities by independent sets of electro
point is substantially zero.
static ?elds ?xed in the vehicle, remixed and exhausted
It is another object of my invention to provide new
from the vehicle in more or less well collimated fashion.
The vector sum of the momenta of all particles per 20 and improved methods and apparatus for neutralizing the
discharge of an ion propulsion system.
manently leaving the vehicle per unit time is the average
It is still another object of my invention to improve
thrust produced in that time interval.
the ef?ciency of operation of an ion propulsion system.
The propulsion of a vehicle in space utilizes the re
One of the features of my invention consists in pass
action force mv on the vehicle due to ejection of a mass
m of expellant at velocity v and mass ?ow rate m. In 25 ing a beam of high speed ions utilized for force develop
ing purposes through a drift space in which the ions en
order to accomplish a given mission, a de?nite increment
counter a cloud of electrons and thoroughly mingle there
of momentum mv must thereby be imparted to the ex
with before emerging from the space and limiting the
pellant. In order to conserve expellant mass, it is de
number of electrons emerging from the space to that re
sirable to make the exhaust velocity v as high as possible.
Since v is limited for chemical propellants, electrical 30 quired to neutralize the electrical charge of the ions.
In one of its forms, my ion beam force developing
propulsion schemes are of considerable current interest.
system employs two successive electrodes at substantially
equal potentials, through which the ion beam is passed,
These involve the ionization of the expellant and its
acceleration by electric ?elds. It is necessary, however,
that the vehicle maintain electrical neutrality. This re
quires that the ejected ion beam be neutralized by the
and an arrangement to introduce electrons between the
electrodes. The electrons so introduced are trapped in a
potential well created by positive ion space charge be
tween the electrodes in which they oscillate, but do not
necessarily leave the vehicle. By neutralizing space
mixture of an equal current of electrons. Attempts to
introduce electrons into accelerated ion beams have
proved di?icult because of the high electric ?elds created
by the ion space charge. These ?elds cause the electron
to execute intense oscillations which preclude the match
ing of electron and ion velocity and constitute a severe
energy drain upon the vehicle power supply.
charge in the region ‘between electrodes, they prevent
the formation of strong electric ?elds in this region,
allowing other electrons emitted in the space to be drawn
from the vehicle by the already emitted ion beam at low
uniform potentials, thereby avoiding excessive drain of
When an ion beam is used as the force developing sys
tem for propelling a vehicle, as a consequence of the elec
The invention, both as to its organization and opera
trical isolation and the low intrinsic capacitance of the
tion, together with further objects and advantages there
of, may best be understood by reference to the following
description taken in connection with the accompanying
drawings, in which:
system, the potential of the system will change rapidly
if any net current is drawn causing it to interact with
the beam expelled. This potential change may rapidly
become su?icient to stop all net charge ?owing from the
system during a time interval corresponding to a very 50
short motion of the expellant particles. Described in
FIG. 1 is a schematic cross section of a portion of an
ion propulsion system embodying my invention, and
FIGS. 2—6 are curves illustrating certain operational
another way, for each ion ejected, an electron remains
characteristics of the system of FIG. 1.
lowering the vehicle potential and increasing the ?eld
In the structure of FIG. 1, ions are emitted normally
about the vehicle which retards the ions. It is, therefore,
imperative that electrons be emitted along with the ions, 55 from the surface of a curved emitter l and pass through
the aperture 2 in an accelerating electrode 3 into a ?eld
free region or drift space 4 de?ned by a neutralizing elec
trode 5 and a drift electrode 6. Electrodes 5 and 6 are
maintained at substantially the same potential, which is
guarantees that steady emission is at least on the average
60 intermediate the potential of emitter 1 and accelerating
electrode 3. For example, electrodes 5 and 6 may be
In an ion propulsion system, therefore, it is also neces
at zero potential, while emitter 1 is at a potential which
sary that the total electron current exactly match the
2500 volts positive and electrode 3 at a potential 2500
total ion current if the vehicle is not to acquire a charge.
volts negative with respect to electrodes 5 and 6. The
So far as the vehicle potential is concerned, electrons
ion beam would then receive a ?nal velocity equal to that
could be emitted at some part of the vehicle other than 65 received in a direct potential fall of 2500 volts. Ob
the point of emission of the ions and need not match the
viously, other values of either emitter or accelerating
and it has been recognized that it is desirable to mix such
electrons with the ion beam in the interest of ef?ciency.
The equality of ions and electrons in the current then
ion density or velocity. This, however, not only is a
di?‘icult feat to perform, but moreover the capacitively
stored energy of the ion beam constitutes an unwanted
drain on the power source.
More importantly, the strong
local space charge ?elds inevitably present oppose and
electrode potentials may be employed as dictated by the
desired ?nal ion energy.
Emitter 1 may be of the perfusing type and may com
prise a louvered, sintered, or woven structure of a suit
able metal, such as tungsten or tantalum, having a high
work function. Vapors of an ionizable gas, such as
cesium, are supplied to one surface of emitter 1 from a
thermally insulated reservoir 7 containing metallic cesium
intermediate that of the accelerator electrode 3 and ion
emitter It and, moreover, is located at sui?cient distance
from accelerator electrode 3. The neutralizer electrode
through a duct 8. The ionizable vapor Within reservoir
7 is maintained at su?'icient pressure to produce a desired
5 is then properly positioned at the point where the
potential again reaches that of electrode 6.
rate of flow by a heater of a suitable material, illustrated
In accordance with my invention, I provide beam neu
as a resistance 9, to which heater current is supplied
tralization by establishing a stationary cloud of electrons
ovver leads 10. The emitter 1 is also maintained at high
in the region 4 between the neutralizing electrode 5 and
temperature by a similar heater, not shown. A cesium
the drift electrode 6. The electrons emitted from the
atom, upon striking the hot emitter material during its 10 cathode 11 are retained by positive ion space charge
passage ‘through the porous structure of emitter 1, be
in the drift space 4. These electrons, when introduced
comes ionized so that a supply of cesium ions is provided
into this region, exchange suf?cient momentum in scatter
from the porous emitter surface 1. The ions formed from
ing collisions with the ions to insure trapping and, there
cesium which has passed through the porous emitter 1
are accelerated by the difference in potential between the
emitter electrode and the accelerating electrode 3 to form
a beam of high speed ions. The shapes of emitter elec
fore, accumulate until they have completely neutralized
the region. This region constitutes an electron reservoir
from which current required to maintain the system
neutrality is supplied. By regulating the electron emis
trode 1 and accelerating electrode 3 are such that ions,
sion from cathode H, such that it can replenish the elec
which leave the emitter surface normally, move radially
trons drawn from the drift region at a rate which equals
to pass through the aperture in accelerating electrode 3 20 the rate at which they leave the system, this region func
into the ?eld free region 4 where they ?ow in parallel
tions as a plane emission limited electron source and a
paths as a collimated beam until they exit through an
steady state condition is achieved. In this steady state
aperture in electrode 6. Any suitable structure (not
condition, the potential of the ?lled electron trap region
shown) may be employed to support the electrodes in
remains slightly negative and electrons ?ow smoothly
?xed spaced relation.
out with the ion beam.
As has been pointed out before, in utilizing an ion
The condition described above is illustrated in FIG. 6,
beam as a force developing device, particularly for pro
in which line 1-1 illustrates the position of the emitter
pulsion purposes, it is essential that the beam be electri
surface, line 3—3 the position of the accelerating elec
cally neutral when it emerges from. the system to prevent
trode 3, line 5—5 the position of neutralizer electrode
rapid charging of the system and also to eliminate unde~ 30 5, line 6—-6 the position of the drift electrode 6, and the
sirable space charge ?elds. In order to produce such beam
potential at each point is given by the solid curve. Before
neutralization within the ?eld free region de?ned by
the introduction of electrons, the potential between elec
neutralizing electrode 5 and drift electrode 6, I provide
trodes 5 and 6 has a maximum, as shown in FIG. 4, and
means to introduce or inject into the region a controllable
amount of electrons, illustrated in FIG. 1 as a cathode
is shown as a dashed line in FIG. 6.
In the steady state
neutralized condition, this region is ?eld free because
11, supplied with heating current from a suitable poten
it contains a stationary cloud of electrons.
tial source through a variable resistance 12.
that the drift space may retain electrons without the use
FIGS. 2-5 illustrate possible potential distributions in
the region between accelerator electrode 3 and drift
electrode 6 before electrons are introduced into this
region. A detailed theory of these types of potential dis
tributions has been presented by C. E. Pay, A. L. Samuel
and W. Shockley in “Bell System Technical Journal 17,”
page 49 (1938), for the analogous case of electron beams
in a vacuum tube. Thus, FIG. 2 represents the condi
tion where at point 13 the potential is su?iciently positive
that all ions are re?ected and return to the accelerating
FIG. 3 illustrates a condition where the potential at
point 14 is of such a value that a virtual anode is estab
Some ions will be re?ected and some will pass
beyond this point. Obviously, if electrons can be intro
duced into this region, they will lower the potential to
a value such that all ions will pass beyond the point and
In order
of grids at its entrance and exit, the potential variation
to the left of line 5--5 must be such that a potential bar~
rier is established which extends completely across the
aperture in electrode 3 of FIG. 1, and electrons are not
drawn into the ion acceleration region between electrodes
I and 3 of FIG. 1 since such action would remove elec
trons from the trap region and, bombard the ion emitter.
Similarly, the potential at electrode 6 must be such that
an electron does not pass this point unless it accompanies
an ion which it electrically neutralizes. It can be demon
strated mathematically that the scattering action of the
electric ?eld of each ion on the electrons is such as to
make their motions while in region 4 become entirely
random and insure their retention between electrodes
5 and 6 so that the electrons constitute a cloud of nega
tive charges. It can also be demonstrated mathematicah
ly that the attraction of the positively charged ions be
55 yond electrode 6 will extract electrons from this cloud
FIG. 4 illustrates a potential distribution in which all
at Such a rate that a neutral ion beam is maintained be
none will be re?ected.
ions proceed through the region and electrons can be
retained Within the region.
yond the electrode 6, provided that electrons so drawn
from region 4 are exactly replenished by emission from
From an inspection of FIGS. 3 and 4, it is apparent
cathode ill. Since the region within the electron reser
that the potential distribution curve must pass through
voir, in which the stationary cloud of electrons exists,
a maximum point, 14 or 15, of value not greater than
is substantially a field free region, ions entering the
the emitter potential, within the drift space de?ned by
region retain the velocities with which they enter this
electrodes 5, 6 in order that electrons can be trapped in
neutralized region. When they leave the region to a
this drift space. Furthermore, the electrons emitted
point downstream of the ion beam, each ion is accom
from a source at the potential of electrodes 5, 6 will be 65 panied, on the average, by an electron so that the beam
unable to move through aperture 2, so that they will not
is electrically neutral. In addition, the electron achieves
enter the region of ion acceleration and will not strike
a velocity equal, on the average, to that of the ions, there
the ion emitter 1.
by minimizing the power required for a given thrust to be
FIG. 5 illustrates the ?nal condition, in which the
potential in this region does not possess a maximum, so 70
While in the foregoing a particular embodiment of this
that electrons cannot be retained in the region 4 to
invention has been shown, it will, of course, be under—
neutralize the ion beam, but will, instead, pass through
stood that it is not limited thereto since many other modi?
aperture 2 and strike the ion emitter 1. It can be shown
cations in the circuit arrangement and in the instrumen
that the desired condition for electron retention can
always be achieved if the drift electrode 6 is at a potential 75 talities employed may be made. It is contemplated by the
appended claims to cover any such modi?cations as fall
within the true spirit and scope of this invention.
What I claim as new and desire to secure by Letters
5. In an ion propulsion system comprising means for
forming a collimated beam of high speed ions, a pair
of electrodes having apertures therein through which said
Patent of the United States is:
1. In a force developing system, the combination com
prising means providing a beam of high speed ions, means
region through which said beam passes, and means for
establishing a ?eld free region through which said beam
injecting electrons into said ?eld free region substantially
beam passes, said electrodes being maintained at substan
tially the same potential whereby they de?ne a ?eld free ~
is projected, and means including a source of electrons
to neutralize the electrical charge of the beam, the rate
for introducing electrons into said beam as it passes
of injecting electrons into said beam being such that the
through said region at a rate such that the total electrical 10 total electrical charge of the beam is substantially zero
charge of the beam is substantially zero as it emerges
as it emerges from the region.
from said region.
6. In an ion propulsion system employing a collimated
2. In a force developing system, the combination com
beam of high speed ions, the method of neutralizing the
prising means providing a beam of high speed ions, a
electrical charge of the beam which comprises passing the
pair of electrodes de?ning a space through which said
beam through a region free of external ?elds, injecting
beam is projected, means for injecting electrons into said
electrons into the beam as it passes through said region,
space to establish therein a cloud of electrons of a density
establishing a potential barrier across the entrance of the
sufficient to neutralize the charge of the ions and form
region so that electrons cannot freely leave the region,
a ?eld free region, the potential of said electrodes being
the same and of a value relative to the potential of the
beam that ions are decelerated as they enter the space
and substantially all electrons are trapped in said space,
the rate of injection of electrons into said space being
such that the total electrical charge of the beam is sub
stantially zero as it emerges from said space.
3. In a force developing system of the type employing
a collimated beam of high speed ions, the method of neu
tralizing the electrical charge of the beam which com
prises passing the beam through a region free of external
?elds, forming a cloud of electrons in the region through
which the ions pass, injecting electrons into the region
at a rate suf?cient to substantially completely neutralize
the electrical charge of the beam, and permitting only elec
and controlling the rate of injecting electrons into the
region so that only enough electrons enter the region
to substantially completely neutralize the electrical charge
of the beam.
7. In a force developing system, a ?rst electrode, a
second electrode having an aperture therein and arranged
to be maintained at a negative potential with respect to
the ?rst electrode, means supplying an ionizable vapor
to said ?rst electrode whereby a high speed beam of ions
is directed through said aperture, a pair of electrodes
having apertures therein aligned with the aperture in said
second electrode, means for introducing electrons into the
region between said pair of electrodes, ‘the potentials of
said pair of electrodes relative to the potentials of said
4. In an ion propulsion system, the combination com
prising an emitter electrode, an accelerating electrode, a
?rst and second electrodes being such that electrons in
troduced into said region are prevented from reaching
said ?rst and second electrodes and are trapped in said
region to neutralize said beam, said beam as it leaves said
neutralizing electrode, and a drift electrode supported in
?xed spaced relation, said neutralizing electrode and drift
ion beam is maintained beyond said pair of electrodes,
trons utilized for neutralizing the beam to emerge from
the region with the beam.
region attracting electrons at a rate such that a neutral
and means to control the rate of introducing electrons
electrode being maintained at substantially the same po
into said region to equal the rate at which electrons are
tential whereby they de?ne a drift space, means providing
attracted by the beam leaving said region.
ions to said emitter electrode, said accelerating electrode
being at a high negative potential relative to said emitter
References Cited in the ?le of this patent
electrode whereby ions are accelerated to enter into said
drift space, and means for introducing electrons into said 45
drift space to neutralize the electrical charge of the ions
' Langmuir et al. _______ __ Mar. 31, 1959
within the space.
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