Note: Descriptions are shown in the official language in which they were submitted.
The present invention pertains to ion sources.
More particularly, it relates to ion sources capable of
producing high-current, low-energy ion beams.
Earlier work led to the development of
electrically-energized ion beam sources for use in
connection with vehicles moving in outer space. A plasma
was produced and yielded ions which were extracted and
accelerated in order to provide a thrusting force. That
technology eventually led to designs for the use o~ ion
sources in a wide range of industrial appllcations as
referenced in AIAA Journal Vol. 20, No. 6, June 1982,
beginning at page 745. As there particularly discussed,
ions were selected by a screen grid and withdrawn by an
accelerator grid. While prior gridded ion sources were
useful improvements in such applications, they led to
complexity of construction and alignment together with a
need to use care in handling in order not to affect such
alignment. Yet, they have proved to be of value in
themselves and the observation of their operation has
contributed to advancement.
A wide variety of ion source shapes and
arrangements have been sugyested, including both angular
and annular. Representative is United States Patent
4,361,472 - Morrison. Particular approaches utili~ing
what may be called other varieties of differently-shaped
sources, including annular, are discussed and shown in
United States Patent 4,277,304 - Horiike et al. Still
other plasma-using ion sources were set forth in an
article entitled "Plasma Physics of Electric Rockets" by
George R. Seikel et al., which appeared in Plasmas and
Magnetic Fields in Propulsion and Power Research, NASA,
SP-226, 1969. While numerous ion thrusters are described,
particular attention is direc~ed to pages 14-16 and
Figures I-16 and I-17 and the teachings with regard to the
magnetoplasmadynamic arc thrusters. In addition, this
article contains an extensive bibliography.
Most prior ion sources have used electromagnets
for the purpose of producing the magnetic field which
contains the electrons in a plasma. Again somewhat
representative is the electron-bombardment engine shown
and discussed at page 179 of the Proceedings of the NASA-
University Conference on Science on Technology of Space
Exploration, Vol. 2, NASA, SP-ll, November 1-3, 1962.
Moreover, a permanent-magnet ion engine (source) also was
discussed and shown in that publication on page 180.
To offset the limitations upon gridded ion
sources, others have developed what may be termed gridless
ion sources. In those, the accelerating potential
difference for the ions is generated using a magnetic
field in conjunction with an electric current. The ion
current densities possible with this acceleration process
are typically much greater than those possible with the
gridded sources, particularly at low ion energy.
Moreover, the hardware associated with the gridless
acceleration process tends to be simpler and more rugged.
One known gridless ion source is of the end-Hall
type as disclosed by A.I. Morosov in Physical Principles
of Cosmic Electro-Jet Engines, Vol. 1, Atomizdat, Moscow,
1978, pp. 13-15. Also known is a closed-drift ion source
in which the opening for ion acceleration is annular
rather than circular. This was described by H~ ~ Kaufman
in "Technology of Closed-Drift Thrusters", AI~A Journal,
Vol. 23, pp. 78-87, January 1985. The closed-drift type
of ion source is typically more efficient for use in its
original purpose of electric space propulsion. However,
the extended-acceleration version of such a closed-drift
ion source is sensitive to contamination from the
surrounding environment, and the previously-disclosed
anode-layer version of the closed-drift ion source is
relatively inflexible in operation.
Additional background with respect to gridless
ion sources will be found in III All-Union 15 Conference
on Plasma Accelerators, Minsk, 1976; and IV All-Union
Conference on Plasma Accelerators and Ion Injectors,
Moscow, 1978.
6~
A siynificant effort also has been made in the
use of plasmas for the achievement of a fusion reaction.
A mirror effect has been employed in the field of fusion
machines in order to enhance ion containment. In that
case, however, the magnetic field has been strong enough
to directly affect the ion motion~
Of course, there are many other prior
publications which mention the "Hall effect". As that
effect may be observed to occur in earlier literature, it
can be misleading. This application primarily pertains to
the end-Hall configuration which, in itself, has already
been documented as above discussed.
In light of all of the foregoing, it is an
overall general object of the present invention to provide
a new and improved high-current, low-energy ion-beam
source.
In accordance with the present invention there
is provided an ion source comprising means for introducing
a gas, ionizable to produce a plasma, into a region within
the source, an anode disposed within the source near one
end of the region, a cathode disposed within the region
and spaced from the anode, means for impressing a
potential between the anode and the cathode to produce
electrons flowing generally in a direction from the
cathode toward the anode in bombardment of the gas to
create the plasma, and means included within the source
for establishing within the region a magnetic field the
strength of which decreases in the direction from the
anode to the cathode and the direction of which field is
generally from the anode to the cathode.
Leading aspects of the approach taken are that
the electrons may be produced independently of any
bombardment of the cathode, the magnet means may be
located outside the region on the other side of the anode
and the gas may be introduced and distributed uniEormly
transverse to that direction.
The features of the present invention which are
believed to be patentable are set forth with particularity
6~
in the appended claims. ~he organization and manner of
operation of the invention, together with further objects
and advantages thereof, may be understood by reference to
the following description Qf an embodiment thereof taken
in connection with the accompanying drawings, in the
6~
several figures of which like refexence numerals identi~y
like elements ana in which:
Figure 1 is an isometric view, partially broken
away into cross-section, illustrating an end-Hall ion
source constructed in accordance with one speci~ic
embodiment of the present invention;
Figure 2 is a schematic diagram of energization
and control circuitry;
Figure 3 is a cross-sectional view of an upper
portion of that shown in Figure 1 with additional
schematic and pictorial representation; and
Figures 4-7 are graphical representations
depicting operational characteristics of the device of
Figure 1.
An end-Hall ion source 20 includes a cathode 22
beyond which is spaced an anode 24. On the side of anode
24 remote from cathode 22 is an electromagnet winding 26
disposed around an inner magnetically permeable pole piece
28. As shown, the different parts of the anode and
magnetic assemblies are of generally cylindrical
configuration which leads not only to symmetry in the
ultimate ion beam but also facilitates assembly as by
stacking the different components one on top of the next.
Magnet 26 is confined between lower and upper
plates 30 and 32. Plate 30 is of magnetically permeable
material, and plate 32 is of non-magnetic material.
Surrounding anode 24 and magnet winding 26 is a
cylindrical wall 34 of magnetic material atop which is
secured an outer pole piece 36 again of magnetically
permeable material. Anode 24 is of a non-magnetic
material which has high electrical conductivity, such as
carbon or a metal, and it is held in place by rings 38 and
40 also of non-magnetic material.
Held in a spaced position between plate 32 and
ring 38 is a distributor 42. Circumferentially-spaced
around its peripheral portion are apertures 44 located
beneath anode 24 and outwardly of opening 45 into the
- ~ ~ 6 ~
bottom of anode 24 and from which its interior wall 48
tapers upwardly and outwardly to its upper surface 50. As
will be observed in Figure 1, the interior edge of pole
piece 36 is disposed outside a projection of interior wall
48.
Disposed centrally within inner pole piece 28 is
a bore 52 which leads into a manifold or plenum 54 located
beneath apertures 44 through which the gas to be ionized
is fed uniformly into the discharge region at opening 46.
Cathode 22 is secured between bushings 56 and 58
electrically separated from but mechanically mounted from
outer pole piece 36. Bushings 56 and 58 are electrically
connected through straps 60 and 62 to terminals 64 and 660
From those terminals, insulated electrical leads continue
through the interior of source 20 to suitable connectors
(not shown) at the outer end of the unit.
The entire assembly of the different plates and
other components is held together by means of elongated
bolts 68 fastened by nuts 70. This approach to assembly
is convenient and simple, as well as being rugged and
eliminating critical alignment o the different
components. The approach also facilitates easy
disassembly for cleaning of parts from time to time, an
expected necessity in view of ultimate contaminatlon such
as from loose flakes of deposited material. When
necessary, heat shields may be included between different
parts of the assembly such as internally around anode 2
and at the back of the assembly below plate 30.
In the above discussion, use has been made of
the words "above" and "below". That use is solely in
accordance with the manner of the orientation shown in
Figure 1. In practice, ion source 20 may have any
orientation relative to the surroundings. Moreover r wall
34 may be secured within a standard kind of flange shaped
to fit within a conventional port as used in vacuum
chambers.
Figure 2 depicts the overall system as utilized
in operation. Alternating current supply 80 ener~izes
:`
` ~LZ~88~
5a
cathode 22 with a current Ic at a voltage Vc. A center
tap of the supply is returned to system ground as shown
through a meter Ie which measures the electron emission
-
from the cathode. Anode 24 is connected to the positive
potential of a discharge supply 82 returned to system
ground and delivers a current id at a voltage Vd. Magnet
26 is energized by a direct current from a magnet supply
84 which delivers a current Im at a voltage Vm. The
magnetically permeable structure, such as wall 34, also is
connected to system ground.
A gas flow controller 88 operates an adjustable
valve 86 in the conduit which feeds the ionizable gas into
bore 52. Cathode supply 80 establishes the emission oE
electrons from cathode 22. Anode potential is controlled
by all of: the anode current, the strength oE the magnetic
field and the gas flow.
While an electromagnet version has been shown, a
permanent-magnet version also has been tested. A
permanent-magnet was installed in place of winding 26 of
the illustrated electromagnet and as part oE inner pole
piece 28. In that case, gas flow may be brought through
the ion source to plenum 54 by a separate tube. Using the
permanent-magnet, the number of electrical power supplies
was reduced, because magnet supply ~4 no longer was
necessary. Use of the permanent-magnet had no adverse
affect on the performance to be described.
For a generalized description of operation,
reference should be made to Figure 3. Neutral atoms or
molecules are indicated by the letter "0". Electrons are
depicted by the negative symbol "-" and ions are indicated
by the plus sign "+".
The neutral atoms or molecules o~ the working
gas are introduced to the ion source through ports or
apertures 44. Energetic electrons from the cathode
approximately follow magnetic field lines 90 back to the
discharge region enclosed by anode 24, in order to strike
atoms or molecules within that region. Some of those
collisions produce ions. The mixture of electrons and
ions in that discharge region forms a conductive gas or
plasma. Because the density of the neutral atoms or
molecules falls off. rapidly in the direction from the
.~ 8~i~
anode toward the cathode, most of the ionizing collisions
with neutrals occur in the region laterally enclosed by
anode 24.
The conductivity parallel to the magnetic Eield
is much higher than the conductivity across that field.
Magnetic field lines 90 thus approximate equipotential
contours in the discharge plasma, with the magnetic field
lines close to the axis being near cathode potential and
those near anode 24 being closer to anode potential. Such
a radial variation in potential was found to exist by the
use of Langmuir probe surveys of the discharge. It was
also found that there is a variation oE potential along
the magnetic field lines, tending to accelerate ions Erom
the anode to the cathode. The cause oE this variation
along magnetic field lines is discussed later. The ions
that are formed, therefore, tend to be initially
accelerated both toward the cathode and toward the axis of
symmetry. ~aving momentum, those ions do not stop at the
axis of the ion source but continue on, often to be
reflected by the positive potentials on the opposite side
of the axis. Depending upon where an ion is formed, it
may cross the axis more than once before leaving the ion
source.
Because of the variety of the trajectories
followed, the ions that leave the source and travel on
outwardly beyond cathode 22 tend to form a broad beam.
The positive space charge and current of the ions of that
broad beam are neutralized by some of the electrons which
leave cathode 22. Most of the electrons from cathode 22
flow back toward anode 24 and both generate ions and
establish the potential diEference to accelerate the ions
outwardly past cathode 22n Because of the shape of the
magnetic field and the potential gradient between the
anode and cathode, most of the ions that are generated
leave in the downstream direction.
The current to the anode is almost entirely
composed of electrons, including both the original
electrons from cathode 22 and the secondary electrons that
8~i~
result from the ionization of neutrals. ~ecause the
secondary electron current to anode 24 equals the total
ion production, the excess electron emission from cathode
22 is sufficient to current-neutralize the ion beam when
the electron emission from cathode 22 equals the anode
current.
The cathode emission Ie can be considered as
being made up of a discharge current Id that flows back
toward the anode and a neutralizing current In that flows
out with the ion beam:
Ie = Id + In (1)
Because the ions that are formed are directed by the
radial and axial electric fields to flow almost entirely
into the ion beam, the current Ia to the anode is
primarily due to electrons. This electron current is made
up of the discharge current Id from the cathode plu5 the
secondary electron current Is from the ionization process,
or:
Ia = Id + Is (2)
Equating Ie and Ia then gives:
In = Is- (3)
From conservation of charge, the ion-beam current Ib
equals the current Is of secondary electrons, so that:
In ~ Ib~ (4)
For the condition of equal electron emission and anode
current, then, the electron current available for
neutralizing the ion beam equals the ion-beam current.
Apart from the foregoing general description of
the ion production process, it is instructive to consider
that which occurs in more detail. There are two major
~ ~.2~
mechanism by which the potential difference which
accelerates the ions is generated by a magnetic field
generally of the diverging shape as shown in Figure 3.
The first of those mechanisms is the reduced plasma
conductivity across magnetic field llnes 90. ~he strong-
field approximation is appropriate for the typical field
strength of several hundred Gauss (several times 10-2
Tesla) used in the disclosed end-Hall source. The ratio
of conductivity parallel to the magnetic field to that
transverse thereto is, thus, expressed:
a,~ /v)2~ (5)
where w is the electron cyclotron frequency and v is the
electron collision frequency. The electron collision
frequency is usually determined by the plasma fluctuations
of anomalous diffusion when conduction is across a strong
magnetic field. Using Bohm diffusion to estimate that
frequency, it can be shown that:
~ , = 256. (6)
Because sohm diffusion is typically accurate only within a
factor oE several, the ratio expressed in equation t6)
should be treated as correct only within an order of
magnitude. Even so, it is expected that:
~ >~ ~1 t7)
From this difference in conductivity parallel
and normal to the magnetic field, it should be expected
that the magnetic field lines as shown in Figure 3 would
approximate equipotential contours in the plasma.
Further, the field lines closer to the anode would be more
positive in potential. Radial surveys of plasma potential
have been made using a Langmuir probe. Those surveys
showed some potential increase in moving off the
longitudinal axis deflned by the concentricity of anode 2~
6~
to a magnetic field lying close to anode 2~. However, the
increase was found to be only a fraction of the total
anode-cathode potential difference. The bulk of the
latter potential difference appeared in the axial
direction. That is, a major portion of the difference
appeared to be parallel to the magnetic field where, from
equation t7), the potential difference might otherwise be
expected to be small.
The time-averaged force of a non-uniform
magnetic field on an electron moving in a circular orbit
within source 20 is of interest. For a variation of field
strength in only the direction of the magnetic field, that
force is parallel to the magnetic field and in the
direction of decreasing field strength. Assuming an
isotropic distribution of electron velocity, two-thirds of
the electron energy is associated with motion normal to
the magnetic field, so as to interact with that field.
With the assumption of a uniform plasma density, the
potential difference in the plasma is calculable by
integrating the electric field required to balance the
magnetic-field forces on the electron, yielding:
~Vp = (kTe/e) ln (B/Bo), (8)
where k is the Boltzman constant, Te is the electron
temperature in K, e is the electron charge and B and Bo
are the magnetic field strengths in two locations. The
grouping, kTe/e is the electron temperature in electron-
Volts. Assuming B > Bo, the plasma potential at B is
greater than that at Bo.
~ xial surveys of plasma potential in the
described end-Hall source are found to be in approximate
agreement with equation (8). It is noted that there is an
additional effect of plasma density on potential, and a
more complete description of the variation of plasma
potential with magnetic field strength would also have to
include that effect.
Variation of plasma potential as given by
equation (8) is significant in that it enables control of
the acceleration of the ions by a variation in the plasma
potential parallel to the magnetic field, which is caused
by the interaction of electrons with the magnetic field.
This is different from high-energy applications as in
fusion, where the magnetic field is strong enough to act
directly on the ions. The latter is called the "mirror
effect" and is described by a different equation.
The ions are at least primarily generated in the
discharge plasma within anode 24 and accelerated into the
resultant ion beam. The potential of the discharge plasma
extends over a substantial range. As a result, the ions
have an equivalent range of kinetic energy after being
accelerated into the beam. The distribution of ion energy
on the axis of the ion beam has been measured with a
retarding potential probe. With the assumption of singly-
charged ions, the retarding potential, in Volts, can be
transla~ed into ion kinetic energy as expressed in
electron-Volts. Kinetic energy distribution obtained in
this matter have been characterized in terms of mean
energy and the rms derivations from mean energy and are
depicted in Figures 4 and 5 for a wide range of operating
conditions. It is found that the mean energy (in
electron-Volts) typically corresponds to about sixty
percent of the anode potential (in Volts), while the rms
deviation from the mean energy corresponds to about
thirty-percent in the apparatus of the specific
embodiment.
As indicated above, the mean energies were
obtained on the ion-beam axis. The mean off-axis values
were found to be similar but were often several electron-
Volts lower. Charge-exchange and momentum-exchange
processes with the background gas in the vacuum chamber
result in an excess of low-energy ions at large angles to
the beam axis. These processes are believed to be the
cause of most, or all, of the observed v~riation and mean
energy with off-axis angleO
1~8~
12
Some processes depend on the ion current
density, while some depend more on the kinetic energy of
the ions. The variations of both ion current density and
the current density corrected for kinetic energy are
therefore of interest, and both are depicted in Figure 6
at a typical operating condition. The correction for
energy was obtained by multiplying the measured off-axis
current density by the ratio of off-axis to on-axis mean
energies.
Several ion beam profiles obtained at a distance
of fifteen centimeters from source 20 are presented in
Figure 7. To assure a conservative measure of current
density, those profiles are corrected for energy as
described above. Only half-profiles are shown in Figures
6 and 7, because only minor differences were found as
between the two sides of the axis.
It was noted that the angular spread of the
profiles shown in Figure 7 were generally greater than
that which earlier have been found to exist for gridded
sources. To avoid vignetting of the probe surface by the
electron-control screen in front of the probe at large
angles, the probe was pivoted during these measurements
about the center of the axis plane at a constant
difference from that center. Because ions tend to follow
narrowly straight-line trajectories, the angular variation
is believed to be similar at larger distances, but the
intensity would vary inversely as the square of the
distance.
The ion beam profiles obtained from the end-Hall
source of the present specific embodiment, can be
approximated with
~ i = A cosna, (9)
where A depends on beam intensity, n is a beam-shape
factor~ and is the angle from the beam axis.
For profiles corrected in accordance with off-
axis energy variation, as also indicated in Figure 7,
13
values of n typically range from two to four. The beam
currents as presented in Figures 6 and 7 were obtained by
using the approximation of equation ~9) and integrating
the corrected current density over an angle ~ from zero to
ninety degrees.
Analysis of the discharge process had indicated
that neutralization should be obtained when the cathode
emission is approximately equal to the anode current.
This has been verified with potential measurements using
an electrically isolated probe in the ion beam.
Cathode lifetime tests were conducted with
argon. Using tungsten cathodes with a diameter of 0.50mm
(0.020 inch), lifetimes of twenty to twenty-two hours were
obtained at an anode current of five amperes which
corresponded to an ion beam current of about one ampere.
Lifetime tests were also conducted with oxygen, again
using the same type of tungsten cathode. With oxygen,
lifetimes at an anode current of five amperes range from
nine to fourteen hours.
Tests have also been conducted with use of a
hollow cathode. Using oxygen as a working gas for the ion
source, ion source operation was found to be similar to
that when using a tungsten cathode. Experience with
operation using hollow cathodes in similar vacuum
environments indicates that a lifetime of fifty to one-
hundred hours, or more, might be expected. While the
inert-gas flow to the hollow cathode would, to some
extent, dilute the oxygen or any other reactive gas
employed for plasma production, it is to be noted that the
hollow-cathode gas flow was introduced at a considerable
distance from the main discharge within anode 24.
Accordingly, only a fraction of the inert gas would return
to the discharge region to be ionized.
Another consideration with respect to any ion
source is contamination of the target. To obtain
contamination estimates on the specifically disclosed
device, duration tests were conducted at an anode
potential of 120 V to permit measurements of weight loss
. .
8~
14
or dimension changes. Conservative calculations were used
to translate those measurements into arrival rates at the
target. For example, the cathode weight loss was assumed
to be distributed in a uniform spherical manner, although
the bombardment by beam ions probably results in the
preferential sputtering of material away from the target.
Those arrival rates were then expressed as atom-to-ion
arrival ratios at the target.
The components considered as possibly subject to
erosion are the cathode 22, distributor 42 and anode 24.
Using argon, the impurity ratios for those three
components were, respectively, < 4 x 10-4 with a tungsten
cathode, < 13 x 10-4 for a carbon distributor and~ 0 for a
carbon anode. Using oxygen, the ratios were < 17 x 10-4
for a tungsten cathode < 3 x 10-4 for a stainless steel
distributor and < 2 ~ 10-4 for a stainless steel anode.
It should be noted that the use of a hollow
cathode could eliminate the cathode as a contamination
source. This would leave only the smaller contributions
of the distributor and the anode. OE course, other
materials may be used in the alternative for construction
of either the distributor or the anode. In any event,
contamination is generally low, making the source suited
for many applications.
While the specific approach to construction of
this particular kind of ion source may be varied, there
are several salient features considered to be important.
Therefore, they will now be summarized.
It becomes apparent from equation 8 that the
operation of the present end-~all source benefits greatly
from the fact that the cathode is placed downstream in the
direction of ion flow in a region of low magnetic field.
The inner pole piece 28, or the equivalent permanent-
magnet, increases the magnetic field strength at what
might be called the back of the discharge region within
anode 24. On the other hand, outer pole piece 36, and its
arrangement with respect to the flux path provided,
decreases the field strength near the cathode. Those two
~ 88~
effects, taken together, result in an increased ratio of
field strength in a direction from cathode 22 to the
discharge region.
One result of that increased ratio i5 the
creation of a potential gradient in the plasma which tends
to direct the ions outward from source 20 into a beam.
Through the effect on the potential distribution and,
therefore, on the ions, that effect is used to direct the
ions in the desired direction. This reduces the effect of
erosion which would be caused by ions moving in the
opposite direction and striking interior portions of
source 20.
In the present approach, permeable material is
used to shape and control the magnetic field. That is, it
is a ferromagnetic material that exhibits a relative
permeability ~with reference to a vacuum) that is
substantially greater than unity and preferably at least
one or two orders of magnitude greater.
Distributer 42 is located behind the anode
(opposite the direction of the cathode 22.) Ion source 20
has been operated with that distributor at ground
potential, typically the vacuum chamber potential, and to
which ground the center tap of the cathode is attached.
In normal operation, ground is usually within several
volts of the potential of the ion beam. With that manner
of operation, it was found that the distributor could be
struck by energetic ions in the discharge region, so that
sputtering due to those collisions could become a major
source of sputter contamination from source 20 itself.
Of course, such contamination is undesirable,
because it is included in any material that is deposited
near source 20. In the presently preferred approach, any
such sputtering of distributor 42 is greatly reduced, in
one measured case by a factor of about fifteen, by
electrically isolating distributor 42. When isolated,
distributor 42 electrically floats at a positive
potential. This reduces the energy of the positive ions
.. . .
~2
16
striking it and probably also reduces the number of ions
which may strike it.
In an alternative, others of the conductive
elements within the established magnetic field may be
electrically isolated from the anode and the cathode,
thereby being allowed to float electrically. That also
may include additional field shaping elements located
between the anode and the cathode.
As described, gas distribution is controlled so
that most of the gas flow passes through anode 24L
Because the electrons can cross the magnetic field easier
by going downstream, crossing and then returning to the
anode, increased plasma density downstream of the anode
provides a lower impedance path and reduces the operating
voltage necessary. Plasma density in a region can be
controlled by controlling the gas flow to that region.
Thus, the gas distribution may be used to control the
operating voltage. As may be observed in Figure l, rings
38 and 40 are spaced inwardly from wall 34. This provides
the flow path into the downstream region for enabling such
control of the operating voltage.
That the magnetic field is easier to cross in
the downstream region occurs because the magnetic
integral, ~ x dx, is less between the same field lines in
that region. For example, if the radius of the outer
field line is doubled, the distance between the axis and
that radius is doubled, but the field strength between is
decreased by a factor of four. For further discussion of
the integral of field strength and distance, which in this
case is cut in half, reference is made to the
aforementioned AIAA Journal Volume 20, No. 6 of June 1982,
at page 746.
As specifically illustrated, source 20 and all
essential elements~ except cathode 22, are circular or
annular in shape. Accordingly, the ion beam produced
exhibits a circular cross-section across ics width or
diameter. This ordinarily is suitable for most
bombardment uses.
-
16a
In some applications, however, it may be
preferably to present a beam pattern which is elliptical
or even rectangular. For example, when a strip oE material
38~i~
,
17
is moved through the ion beam, a narrow but wide beam
pattern may be more suitable. That is accomplished by
changing the shape of anode 24 to be elliptical or
rectangular rather than annular as specifically
illustrated in Figure 1.
It will thus be seen that the objectives set
forth in the introduction are achieved. In some cases,
the achievement has been in ~he nature of an improvement
of prior ion sources both of the gridded and the gridless
types. ~t the same time, some salient and unique features
have been described.
While a particular embodiment of the invention
has been shown and described, and alternatives have at
least been mentioned, it will be obvious to those skilled
in the art that changes and modifications may be made
without departing from the invention in its broadest
aspects. Therefore, the aim in the appended claims is to
cover all such changes and modifications as fall within
the true spirit and scope of that which is patentable.
