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Claims  |
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What is claimed is:
1. An electrode assembly for correcting space-charge density non-uniformity
in an electron beam generated in a vacuum housing chamber containing a low
pressure gas from which positive ions may be created, the electron beam
traveling in a downstream direction defining a Z-axis, the assembly being
disposed substantially coaxially with the electron beam along the Z-axis
and comprising:
first and second members, spaced-apart diametrically relative to the
Z-axis, defining a first electrode pair;
third and fourth members, spaced-apart diametrically relative to the
Z-axis, defining a second electrode pair;
means for coupling said first, second, third and fourth members
respectively to first, second, third and fourth potential sources;
said potential sources creating a potential difference between said members
comprising each said electrode pair causing each said electrode pair to
create an electric field;
the assembly producing a resultant electric field equal to the vector sum
of each said electric field created by each said electrode pair;
wherein said resultant electric field is controllably rotated, by varying
chosen ones of said potential sources, to an orientation controllably
removing sufficient positive ions to compensate for space-charge density
non-uniformity in said beam.
2. The assembly of claim 1, wherein:
said first and second potentials are substantially equal and opposite; and
said third and fourth potentials are substantially equal and opposite.
3. The assembly of claim 1, wherein relative to said Z-axis each said
member defines a radius R, and said beam defines a radius r.sub.o, where a
ratio defined by R/r.sub.o is substantially constant;
said R/r.sub.o ratio substantially eliminating a voltage gradient along
said Z-axis, minimizing positive ion migration and attendant non-uniform
space-charge distribution in said beam.
4. The assembly of claim 1, wherein said members comprising each said
electrode pair are substantially cylindrically symmetrical about said
Z-axis to each other.
5. The assembly of claim 1, further including:
at least two additional members comprising at least a third electrode pair;
means for coupling a source of potential to each said additional member;
the potential V.sub.i applied to each member in the assembly being:
V.sub.i =V.sub.x .times.cos .theta..sub.i +V.sub.y .times.sin .theta..sub.i
where .theta..sub.i is the average angle of member i, and where V.sub.x and
V.sub.y respectively represent potential applied at an extreme X-axis and
Y-axis member position, said X-axis and Y-axis being mutually orthogonal
and defining a plane normal to said Z-axis.
6. The assembly of claim 1, wherein at least said first and second members
are planar.
7. The assembly of claim 1, further including:
a planar disk element defining a central opening sized to permit passage of
said beam therethrough, disposed coaxial with said Z-axis downstream from
said assembly; and
means for coupling said planar disk to a source of positive potential
sufficient to create an axial field blocking upstream migration of
positive ions toward said assembly.
8. The assembly of claim 1, further including:
a plurality of planar disk elements, each defining a central opening sized
to permit passage of said beam therethrough, spaced-apart and disposed
coaxial with said Z-axis upstream from said assembly;
means for coupling alternate ones of said planar disk elements to a first
source of disk potential; and
means for coupling intermediate ones of said planar disks to a second
source of disk potential;
wherein a potential difference between said first and second sources of
disk potential creates an alternating axial field between adjacent ones of
said planar disks such that substantially all positive ions created within
or near said disks are swept away.
9. The assembly of claim 1, further including an ion clearing electrode
comprising:
first, second, third and fourth electrodes forming a constant radius
cylinder, symmetrically disposed coaxially with said Z-axis;
means for coupling said first, second, third and fourth electrodes
respectively to first, second, third and fourth electrode potential
sources;
said second and third electrode potential sources each being approximately
half of said first electrode potential source, and said fourth electrode
potential source being substantially zero;
wherein said electrodes establish a substantially uniform electric field
while sweeping away positive ions created therein or nearby.
10. An electrode assembly for correcting space-charge density
non-uniformity in an electron beam generated in a vacuum housing chamber
containing a low pressure gas from which positive ions may be created, the
beam traveling in a downstream direction defining a Z-axis, the assembly
being disposed substantially coaxially with the electron beam along the
Z-axis and comprising:
at least two pairs of electrodes, each electrode pair comprising two
members that are spaced-apart diametrically relative to the Z-axis and are
substantially cylindrically symmetrical to each other about said Z-axis;
means for coupling said members to potential sources such that the
potential V.sub.i applied to each member in the assembly is:
V.sub.i =V.sub.x .times.cos .theta..sub.i +V.sub.y .times.sin .theta..sub.i
where .theta..sub.i is the average angle of member i, and where V.sub.x and
V.sub.y respectively represent potential applied at an extreme X-axis and
Y-axis member position, said X-axis and Y-axis being mutually orthogonal
and defining a plane normal to said Z-axis;
wherein relative to said Z-axis, the electron beam defines a radius r.sub.o
and each electrode pair defines a radial distance R, such that a ratio
defined by R/r.sub.o is substantially constant;
said R/r.sub.o ratio substantially eliminating any voltage gradient along
said Z-axis with the result that positive ions within said electrode
assembly will not migrate along said Z-axis;
the assembly producing a resultant electric field equal to the vector sum
of fields produced between each electrode pair;
wherein varying chosen ones of said V.sub.i potential permits controllably
rotating said resultant electric field to an orientation causing removal
of sufficient positive ions to compensate for space-charge density
non-uniformity in said beam and permits said electron beam to focus
sharply upon a desired target.
11. The assembly of claim 10, wherein at least one of said at least two
pairs of electrodes is planar.
12. The assembly of claim 10, further including:
a planar disk element defining a central opening sized to permit passage of
said beam therethrough, disposed coaxial with said Z-axis downstream from
said assembly; and
means for coupling said planar disk to a source of positive potential
sufficient to create an axial field blocking upstream migration of
positive ions toward said assembly.
13. The assembly of claim 10, further including:
a plurality of planar disk elements, each defining a central opening sized
to permit passage of said beam therethrough, spaced-apart and disposed
coaxial with said Z-axis upstream from said assembly;
means for coupling alternate ones of said planar disk elements to a first
source of disk potential; and
means for coupling intermediate ones of said planar disks to a second
source of disk potential;
wherein a potential difference between said first and second sources of
disk potential creates an alternating axial field between adjacent ones of
said planar disks such that substantially all positive ions created within
or near said disks are swept away.
14. The assembly of claim 10, further including an ion clearing electrode
comprising:
first, second, third and fourth ion clearing electrodes forming symmetrical
segments of a constant radius cylinder, said electrodes being
symmetrically disposed coaxially with said Z-axis such that, relative to
said Z-axis, said first and fourth electrodes are diametrically opposed
and said second and third electrodes are diametrically opposed;
means for coupling said first, second, third and fourth electrodes
respectively to first, second, third and fourth electrode potential
sources;
said second and third electrode potential sources each being approximately
half of said first electrode potential source, and said fourth electrode
potential source being substantially zero;
wherein said electrodes establish a substantially uniform electric field
while sweeping away positive ions attempting to pass longitudinally
therethrough.
15. In a computed tomography X-ray scanning system, an electron beam
production and control system for producing X-rays, said system
comprising:
an evacuated housing chamber having an upstream end, a downstream end, and
defining a Z-axis extending therebetween, and further containing a low
pressure gas from which positive ions may be created;
means, disposed within said upstream end of said chamber, for producing an
electron beam and directing said beam in a downstream direction at least
initially along said Z-axis;
means for correcting space-charge density non-uniformity of said electron
beam by subjecting at least a portion of said electron beam to a rotatable
electric field that controllably removes positive ions, said means being
disposed substantially coaxially with said electron beam along said
Z-axis;
a stationary target, disposed within the downstream end of said chamber for
emitting X-rays upon impingement by said electron beam;
means for deflecting and focusing said electron beam upon said target;
wherein said means for correcting promotes production of a more sharply
focused electron beam upon said target than if said means for correction
were not used.
16. The system of claim 15, wherein said upstream end of said chamber and
said means for deflecting and focusing are separated; by less than about
50 cm.
17. The system of claim 16, wherein said means for correcting includes:
at least two pair of electrodes, each electrode pair comprising two members
that are spaced-apart diametrically relative to the Z-axis and are
substantially cylindrically symmetrical to each other about said Z-axis;
means for coupling said members to potential sources such that the
potential V.sub.i applied to each member in the assembly is:
V.sub.i =V.sub.x .times.cos .theta..sub.i +V.sub.y .times.sin .theta..sub.i
where .theta..sub.i is the average angle of member i, and V.sub.x and
V.sub.y respectively represent potentials applied at an extreme X-axis and
Y-axis member position, said X-axis and Y-axis being mutually orthogonal
and defining a plane normal to said Z-axis;
wherein relative to said Z-axis, said electron beam defines a radius
r.sub.o and each said electrode pair defines a radial distance R, such
that a ratio defined by R/r.sub.o is substantially constant;
said R/r.sub.o ratio substantially eliminating any voltage gradient along
said Z-axis with the result that positive ions within said electrode
assembly will not migrate along said Z-axis;
the assembly producing a resultant electric field equal to the vector sum
of fields produced between each electrode pair;
wherein varying chosen ones of said V.sub.i potential permits controllably
rotating said resultant electric field to an orientation causing removal
of sufficient positive ions to compensate for space-charge density
non-uniformity in said beam and permits said electron beam to focus
sharply upon a desired target.
18. The system of claim 15, further including:
a planar disk element defining a central opening sized to permit passage of
the electron beam therethrough, disposed coaxial with said Z-axis
downstream from said means for correcting; and
means for coupling said planar disk to a source of positive potential
sufficient to create an axial field blocking upstream migration of
positive ions toward said means for correcting.
19. The system of claim 15, further including:
alternating axial field means for sweeping away positive ions in a region
of said housing chamber that includes a discontinuity.
20. The system of claim 19, wherein said alternating axial field means
includes:
a plurality of planar disk elements, each defining a central opening sized
to permit passage of said beam therethrough, spaced-apart and disposed
coaxial with said Z-axis upstream from said means for correcting;
means for coupling alternate ones of said planar disk elements to a first
source of disk potential; and
means for coupling intermediate ones of said planar disks to a second
source of disk potential;
wherein a potential difference between said first and second sources of
disk potential creates an alternating axial field between adjacent ones of
said planar disks such that substantially all positive ions created within
or near said disks are swept away.
21. The system of claim 15, further including an ion clearing electrode
disposed within a region of said housing chamber, said ion clearing
electrode comprising:
first, second, third and fourth ion clearing electrodes forming symmetrical
segments of a constant radius cylinder, said electrodes being
symmetrically disposed coaxially with said Z-axis such that, relative to
said Z-axis, said first and fourth electrodes are diametrically opposed
and said second and third electrodes are diametrically opposed;
means for coupling said first, second, third and fourth electrodes
respectively to first, second, third and fourth electrode potential
sources;
said second and third electrode potential sources each being approximately
half of said first electrode potential source, and said fourth electrode
potential source being substantially zero;
wherein said electrodes establish a substantially uniform electric field
while sweeping away positive ions attempting to pass longitudinally
therethrough or created therein. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates generally to scanning electron beam systems
for X-ray production in a computed tomography X-ray transmission system,
and more particularly to controlling the uniformity of the beam
space-charge density, especially by means of positive ions.
BACKGROUND OF THE INVENTION
Scanning electron beam computed tomography systems are described generally
in U.S. Pat. No. 4,352,021 (Boyd, et al.) issued Sep. 28, 1982. The theory
and implementation of devices to help control the electron beam in such
systems is described in detail in U.S. Pat. No. 4,521,900 (Rand, et al.),
issued Jun. 4, 1985; U.S. Pat. No. 4,521,901 (Rand, et al.) issued Jun. 4,
1985; U.S. Pat. No. 4,625,150 (Rand, et al.) issued Nov. 25, 1986; and
U.S. Pat. No. 4,644,168 (Rand, et al.) issued Feb. 17, 1987. Applicants
refer to and incorporate herein by reference each above listed patent to
Rand, et al.
As described in detail in U.S. Pat. No. 4,521,900 to Rand, et al.
(hereafter "Rand, et al. '900"), an electron beam is produced by an
electron gun at the upstream end of an evacuated generally elongated and
conical shaped housing chamber (or "drift tube"). A large electron gun
potential (e.g., 130 kV) accelerates the electron beam downstream along a
first straight line path defining the chamber Z-axis. Further downstream a
beam optical system including focus and deflection coils deflects the beam
into a scanning path. The deflected beam exits the beam optical system and
impinges a suitable target for producing X-rays. The X-rays penetrate an
object (e.g., a person) and are then detected and computer processed to
produce an X-ray image of a portion of the object. Prior art electron beam
systems such as described in the above-referenced patents
characteristically had relatively long conical shaped housing chambers,
e.g., 3.8 meters.
Because the electrons are negatively charged, the resultant space-charge
causes the electron beam to diverge or expand in the upstream chamber
region between the electron gun and the focus and deflection coils. This
expansion is beneficial because the beam diameter at the target varies
approximately inversely with the beam diameter at the focus and deflection
coils. In the chamber region downstream from the focus and deflection
coils, a converging electron beam is desired. In that downstream region,
the beam preferably is neutralized by positive ions produced by the
electrons from residual gas in the chamber, or from a gas purposely
introduced into the chamber. This neutralization causes the beam to
self-focus sharply upon the target to produce a sharp X-ray image. In the
ideal case, the electron beam is perfectly uniform in current density,
diverging upstream and converging to sharply self-focus downstream.
Although a diverging beam is desired in the upstream chamber region,
positive ions can counteract divergence. Positive ions are present because
the electron beam interacts with residual gases that inevitably remain
after evacuation, or with gases purposely introduced into the chamber. In
the upstream chamber region, positive ions are detrimental because they
tend to neutralize the space-charge, preventing electron beam divergence.
This in turn increases the beam width at the target, resulting in a
defocused X-ray image. Neutralization also can result in the beam becoming
unstable and collapsing completely.
By contrast, positive ion neutralization can be beneficial in the chamber
region downstream from the focus-deflection coils. Here neutralization
eliminates the electron self-repulsion, while the beam's attractive
magnetic field converges and self-focuses the beam. Elements of the beam
optical system are then used to fine tune the converged beam to produce a
sharp X-ray image.
Thus, while positive ions can be beneficial downstream from the
focus-deflection coils, they are detrimental in the upstream region. In
prior art tomography systems such as described in the Rand, et al. U.S.
Pat. No. 4,521,900 patent, positive ions were removed by causing the
electron beam to pass axially through an electrically biased ion clearing
electrode (or "ICE") mounted in the upstream chamber region. The ICE
created a relatively large transverse electric field that swept away the
slow moving positive ions, without disturbing the considerably faster
moving electrons. Such ICEs required large electrode potentials (e.g.,
about 1 kV) to produce the large electric field needed to remove ions on
an "all or nothing" basis.
Ideally the electron beam should be homogenous, i.e., with a uniform
electron distribution, so the beam acts as its own perfect lens:
self-diverging in the upstream chamber region and self-converging in the
downstream chamber region to focus sharply on the target. A uniform
space-charge density is desired because any optical aberrations due to the
electron beam self-forces would then be eliminated. In addition to
degradation from ions, the electron beam space-charge density may not be
perfectly uniform due to imperfections in the electron gun and in the beam
optics system.
It is believed that the relatively long length of prior art housing
chambers contributed to beam space-charge homogenization by smoothing or
evening out the electron distribution. In essence, the distance between
the electron gun and beam optics was sufficiently long to allow the
electron beam to expand and become more uniform without requiring special
mechanisms to compensate for beam non-uniformity.
For reasons of economy, maintenance and ease of installation in hospitals,
it is advantageous to construct a scanning electron beam system using a
housing chamber shorter than used in prior art systems. Unfortunately,
however, the resultant shorter distance between the electron gun and beam
optics prevents the beam from expanding sufficiently to become
homogeneous. Further, the construction of shorter housing chambers may
create discontinuities, typically near vacuum valve couplings and flanges.
These discontinuities create gaps in the electric field generated by ion
controlling devices, thus allowing some ions to remain in the upstream
region where they further degrade beam expansion.
In summary, in an electron beam scanner system employing a relatively short
length housing chamber, there is a need for a method and apparatus for
removing positive ions, and for controlling the positive ion distribution.
Such method and apparatus should compensate for beam space-charge density
non-uniformity, thereby eliminating any aberrations due to the beam
self-forces. Unfortunately, prior art ICEs with their "all or nothing"
characteristic simply do not provide any mechanism for controlling
space-charge uniformity of the electron beam, and do not remove all ions
when operating over discontinuities. The present invention discloses an
ion controlling electrode assembly and a method to fulfill these needs.
SUMMARY OF THE INVENTION
The present invention is a relatively short length ion controlling
electrode assembly for use in a short length housing chamber in a computed
tomography X-ray transmission scanning system. Because the chamber is
short, the electron beam cannot adequately expand between the electron gun
and beam optics. The present invention compensates for this by
controllably removing some (but not necessarily all) ions, thereby
adjusting the electron beam's space-charge density distribution. As a
result, beam spot resolution and thus image sharpness is improved.
The electrode assembly is disposed within the vacuum housing chamber
between the electron gun and the focus-deflection coils such that the
electron beam passes axially through the electrode assembly along the
Z-axis. The electrode assembly includes a rotatable field ion controlling
electrode ("RICE") and, downstream therefrom, a positive ion electrode
("PIE"). Alternative embodiments further include an optional periodic
axial field ion controlling electrode ("PICE"), located at the most
upstream region of the electrode assembly, and one or more optional ion
clearing electrodes ("ICEs"), located on either side of the RICE.
The RICE improves image sharpness by homogenizing the electron beam
space-charge density, thereby linearizing the beam optics and eliminating
aberrations. Some but not necessarily all positive ions are controllably
removed by subjecting the electron beam to a small, rotatable transverse
electric field generated by the RICE. The field is preferably on the same
order of magnitude as the field created by the electron beam, and is
rotated by varying the electrical potential coupled to the elements
comprising the RICE. The electric field is adjusted until the electron
beam space-charge density is homogenized, and so the scanner system's
X-ray image exhibits maximum resolution or sharpness.
The RICE includes at least two pair of spaced-apart elements, an equal and
opposite electrical potential preferably being coupled to each element in
an element pair. The elements comprising each element pair are preferably
cylindrically symmetrical to each other about the Z-axis. The RICE
preferably is shaped like a cone that expands downstream such that the
distance from the Z-axis to each RICE element is approximately
proportional to the electron beam radius at each point. This geometry
tends to make the potential along the beam's Z-axis constant. As a result,
positive ions trapped within the electron beam tend not to drift axially
along the Z-axis, which drift could produce severe beam optical
aberrations.
The potential applied to a RICE element may be AC, DC or a combination
thereof. Because a potential difference exists between the elements
comprising an element pair, a transverse electric field exists
therebetween. The transverse electric field generated by the RICE as a
whole is the vector sum of the electric fields generated by the element
pairs comprising the RICE. If the RICE electrode potentials are DC, the
resultant RICE transverse field will be static, but if an AC electrode
potential is present, a dynamic or rotating field mode occurs. By varying
the electrode potentials, the RICE field can be rotated to control how
many positive ions, if any, are allowed to remain within the RICE to
modify and improve the beam space-charge uniformity. The resultant more
homogenized space-charge density reduces (or eliminates) aberrations due
to the electron beam, allowing for easier focusing by the scanning
system's beam optical components.
In a preferred embodiment, the electrode assembly further includes a
positive ion electrode ("PIE") disposed coaxially downstream from the
RICE. The PIE is biased to create a large axial field that prevents
upstream migration of positive ions, which migration could interfere with
the production of a sharply self-focused uniform beam at the X-ray target.
In another aspect, the present invention further includes a short-length
periodic axial field ion controlling electrode ("PICE"). The PICE is
located adjacent the electron gun, in the upstream region of the housing
chamber where small size and discontinuities preclude the effective use of
a conventional ICE. The PICE comprises several spaced-apart washer-like
electrodes coaxial to the Z-axis, with alternate electrodes coupled to a
relatively large potential (e.g., -2 kV) relative to intermediate
electrodes (e.g., 0 V). The PICE's small size allows it to operate in the
upstream housing chamber region across discontinuities and thus remove
positive ions from this region.
In yet another aspect, the present invention further includes first and
second conventional ion clearing electrodes (ICEs), preferably coaxially
disposed immediately upstream and downstream from the RICE. These ICEs are
coupled to a large negative potential (e.g., -1.5 kV) to sweep away all
positive ions on either side of the RICE. Essentially these ICEs may be
used to control the electric field in any region of the housing chamber
between the electron gun and beam optics that is not controlled by the
RICE, PIE and, if present, PICE.
In summary, the present invention controls the ion distribution density
within an electron beam by rotating the vector direction and strength of a
relatively low magnitude transverse field created by a RICE. A PIE
prevents any upstream migration of positive ions into the RICE where they
could interfere with RICE operation. A PICE removes ions from the upstream
region of the chamber where discontinuities are present, and conventional
ICEs remove ions from regions of the chamber not controlled by the RICE,
PIE or PICE. The resultant electron beam produces a controllably sharper,
higher resolution X-ray image.
Other features and advantages of the invention will appear from the
following description in which the preferred embodiments have been set
forth in detail in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a generalized scanning computed tomography X-ray
transmission system that uses a relatively short length vacuum housing
chamber to produce and control an electron beam;
FIG. 2 is a longitudinal view of the system shown in FIG. 1;
FIG. 3 is a perspective, expanded view of the ion electrode assembly shown
in FIG. 2, according to the present invention;
FIG. 4A is a longitudinal cross-sectional view of an alternative embodiment
of a RICE comprising four element pairs;
FIG. 4B is a sectional view of the RICE embodiment of FIG. 4A, taken along
the section line B--B';
FIGS. 5A-5D depict correction of beam space-charge density resulting from
an applied external corrective field created by a RICE, according to the
present invention.
FIG. 6A is a longitudinal cross-sectional view of the electrode assembly 44
depicted in FIG. 3;
FIGS. 6B-6D depict electrostatic potential, transverse electric field and
axial electric field at various longitudinal positions along assembly 44.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 and FIG. 2 depict a generalized computed tomography X-ray
transmission scanning system 8 that includes a relatively short length
vacuum housing chamber 10 wherein an electron beam 12 is generated and
caused to scan at least one circular target 14 located within chamber 12's
front lower portion 16. When it strikes the target, the electron beam,
which typically scans 210.degree. or so, emits a moving fan-like beam of
X-rays 18. X-rays 18 then pass through a region of a subject 20 (e.g., a
patient or other object) and register upon a region of a detector array 22
located diametrically opposite. The detector array outputs data to a
computer processing system (indicated by arrows 24) that processes and
records the data to produce an image of a slice of the subject on a video
monitor 26. As indicated by the second arrow 24, the computer processing
system also controls the system 8 and the electron beam production
therein. By repeating the scanning process after the patient has been
moved laterally along the chamber Z-axis 28, a series of X-ray images
representing axial "slices" of the patient's body is produced.
As shown in FIG. 2, an electron gun 32 within the extreme upstream end 34
of chamber 10 produces the electron beam 12 in response to high voltage
excitation (e.g., 130 kV). Although vacuum pumps 36 evacuate chamber 10,
residual gases inevitably remain that produce positive ions in the
presence of the electron beam 12. Gases may also be introduced into the
chamber for the purpose of producing positive ions, since the ions are
beneficial in the downstream chamber region. A beam optical system 38 that
includes a focus coil 40 and a deflection coil 42 is mounted downstream in
chamber 10 to respectively magnetically focus and scan the beam 12
typically about 210.degree. in an arc across an arc-like target 14.
The positive ions that are created can detrimentally neutralize the
electron beam in the upstream region of chamber 10. It is important to
subject the electron beam 12 to an electric field in the upstream region,
but discontinuities such as 37 in the housing 10 create gaps over which
conventional devices for clearing ions cannot be used. In addition,
imperfections in the electron gun cause the electron beam to have a
non-uniform space-charge density in a plane perpendicular to the Z-axis
28. Finally, because the drift distance between the electron gun and the
beam optics 38 is relatively short, e.g., 40 cm or so, the electron beam
12 does not have time for its space-charge density to become sufficiently
homogeneous. The present invention, electrode assembly 44, is designed to
counteract these detriments and to ensure that the X-ray image resulting
from the relatively short length computed tomography system 8 is
sufficiently sharply focused and of high resolution. Assembly 44 must
function within a relatively short chamber length 30, typically about 2.5
meters as contrasted with 3.8 meters for chamber lengths in scanning
systems such as described in the Rand, et al. and Boyd, et al. references.
As shown in FIG. 2, electrode assembly 44 is mounted within housing 10
between the electron gun 32 and the beam optical assembly 38 such that the
electron beam 12 passes axially through assembly 44 along the Z-axis 28.
Ideally the Z-axis 28 is coaxial with the electron beam 12 upstream from
the beam optics assembly 38 within chamber 10, and further represents the
longitudinal axis of chamber 10, and the axis of symmetry for the
electrode assembly 44 and the beam optics assembly 38.
As best seen in FIG. 3, a preferred embodiment of assembly 44 includes a
rotatable field ion clearing electrode 46 ("RICE"), a positive ion
electrode 48 ("PIE"), first and second ion clearing electrodes 50, 50'
("ICEs"), and a periodic axial field ion controlling electrode 52
("PICE"). While FIG. 2 shows the present invention 44 used in conjunction
with a relatively short housing 10, the present invention may also be used
with conventional longer housings. Further, while FIG. 3 depicts assembly
44 as including a RICE, a PIE, two ICEs and a PICE, it is to be understood
that the present invention may be implemented without all of these
elements. For example, while the RICE 46 and PIE 48 normally will be
present, the PICE 52 may be dispensed with where discontinuities are not
present. Further, in systems where it is unnecessary to sweep away further
ions, either or both of ICE 50, 50' may be dispensed with.
The various RICE, PIE, ICE and PICE elements comprising assembly 44
preferably are made from a relatively inert conducting material that does
not outgas within chamber 10, stainless steel for example. Each RICE, PIE,
ICE and PICE preferably is mounted within the chamber 10 using insulated
standoffs 54, e.g., ceramic, as these elements are coupled to potential
sources that create electric fields to which the electron beam 12 is
subjected.
In the relatively short chamber system 8 with which the present invention
may be used, the distance 56 from electron gun 32 to the focus coil 40 is
only about 40 cm, contrasted with 1.5 m in the prior art systems with
which the earlier Boyd, et al., Rand et al. patents were practiced. In
these earlier systems, assembly 44 would have been a single prior art ICE,
extending substantially the entire 1.5 meter distance from electron gun 32
to focus coil 40. In such systems, prior art ICEs would sweep away all
ions, and the relatively large drift length allowed the electron beam
space-charge density to become reasonably homogenous.
However in the present system 8, although approximately 40 cm distance is
available for assembly 44, satisfactory performance is obtained with a
RICE 46 whose length is about 17 cm. Rather than lengthen RICE 46 to the
full electron gun-to-focus coil distance (40 cm) to control positive ions,
positive ions may be essentially totally removed from the otherwise
exposed regions of chamber 10 using ICE elements 50, 50'. If, as depicted
in FIG. 2, one or more discontinuities such as 37 are present, it is
advantageous to include the PICE 52, a relatively short length element
that can operate over such discontinuities.
The elements comprising the assembly 44 depicted in FIG. 3 will now be
described in detail. According to the present invention, RICE 46
preferably creates a rotatable electric field to which the electron beam
12 is subjected, and represents the most important component of the
electrode assembly 44. As shown in FIGS. 3 and 4A, RICE 46 is generally
conical shaped and includes at least two pair of cylindrically symmetrical
elements 58A, 58B and 60A, 60B, each element being spaced apart from its
opposing element symmetrically about the Z-axis 28. Thus in the embodiment
of FIG. 3, two pair of elements are present, with elements 58A and 58B
symmetrically spaced apart from each other about the Z-axis 28, and with
elements 60A and 60B similarly spaced apart from each other.
Other RICE configurations are also possible, FIGS. 4A and 4B depicting, for
example, a generally conical shaped RICE 46' comprising four element pairs
58A and 58B, 60A and 60B, 62A and 62B, and 64A and 64B. In the preferred
embodiment of FIGS. 4A and 4B, the elements in each element pair are
cylindrically symmetrical about the Z-axis 28 to each other. A RICE
comprising more than four element pairs could also be employed. Although
more elements improve the electric field uniformity, they increase the
difficulty in routing the many electrode potentials to the RICE.
Each RICE embodiment 46, 46' is preferably conical shaped such that the
ratio between the electrode radius (R) and the beam radius r.sub.o is
constant. The generally conical geometry is significant because if
R/r.sub.o .apprxeq.constant, then the electrical potential at the electron
beam 12 axis is essentially constant. This means that positive ions
trapped within the beam 12 will not drift axially, downstream or upstream.
If this drift were not eliminated, positive ions within the beam would
oscillate radially as they drifted, creating a severe non-uniform
space-charge distribution, and beam optical aberrations. Since the
electron beam 12 expands in the region in which the ion electrode assembly
44 or 44' is located, the RICE assembly 46, 46' has a generally cone shape
that expands in the downstream direction. In the embodiment of FIGS. 4A
and 4B, the RICE length is approximately 17 cm, and the slope angle o is
approximately 3.degree.. The upstream RICE diameter is approximately 4 cm,
while the larger downstream RICE diameter is approximately 6 cm. While
FIG. 3 and FIGS. 4A and 4B show conically shaped RICEs comprising curved
elements, some or all of these elements could instead be planar as
indicated by the phantom lines in FIG. 4B. One advantage of planar
elements would be their relative ease of manufacture.
Each element in an element pair comprising the generically conical shaped
RICE 46, 46' is coupled to a source of electrical potential, AC, DC or a
combination thereof, that is preferably equal and opposite to the
potential applied to the other element in the element pair. In FIG. 3, for
example, element 58A is coupled to a potential source V.sub.58A =+V.sub.x,
and element 58B, disposed diametrically opposite from element 58A, is
coupled to a potential source V.sub.58B =-V.sub.x. The potential
differential (i.e., 2 V.sub.x) between these elements 58A, 58B creates a
transverse electric field, and in a similar fashion the potential
difference between each element in each element pair comprising the RICE
creates a transverse electric field. The vector sum of these individual
fields represents the electric field created by the RICE 46, the amplitude
and direction of the resultant field being rotatable by varying one or
more potentials applied to the element pairs. (Of course changing the RICE
geometry mechanically also would affect the resultant electric field, but
modifying the geometry in situ to get a sharply focused X-ray image would
be more difficult.)
Regardless of how many electrode pairs a RICE comprises, several
characteristics will preferably be present. The RICE will be generally
conically shaped such that Z-axis potential differences within the
electron beam are preferably minimized. The RICE will further exhibit
cylindrical symmetry about the Z-axis, the better to present a more
uniform field to the electron beam 12. Preferably equal and opposite
potentials will be coupled to each element comprising a RICE element pair,
to present a more uniform field to the electron beam 12. (A constant bias
potential may also be applied to all electrodes.) Further, the potential
applied to the electrodes is preferably computed geometrically as follows.
If V.sub.x and V.sub.y represent electrode electrical potentials at the
extreme X-axis and Y-axis electrode positions, then the potentials of the
other electrodes take the general form:
V.sub.i =V.sub.x .times.cos .theta..sub.i +V.sub.y .times.sin
.theta..sub.i(1)
where .theta..sub.i is the average angle of electrode i. An electrode
biasing scheme according to equation (1) will produce the most uniform
electric field for a given number of electrodes. For example, in the
embodiment of FIG. 3 where two pair of electrode elements are present, for
elements 60A and 60B, .theta..apprxeq.90.degree. and
.theta..apprxeq.270.degree. respectively, while for elements 58A and 58B,
.theta..crclbar.0.degree. and .theta..apprxeq.180.degree. respectively.
Therefore for a RICE 46 having two element pairs as shown in FIG. 3, the
potentials applied to elements 58A, 58B, 60A and 60B are, per equation
(1), respectively +V.sub.x, -V.sub.x, +V.sub.y and -V.sub.y. By contrast,
for the four element pair RICE 46' embodiment shown in FIGS. 4A and 4B,
equation (1) requires that the potentials applied to elements 58'A, 58'B,
60'A, 60'B, 62'A, 62'B, and 64'A, 64'B (e.g., potentials V.sub.48'A,
V.sub.58'B, etc.) have the values shown in Table 1, below. Electrode
potentials for a RICE comprising a different number of electrode pairs
would be similarly calculated using equation (1), above.
TABLE 1
______________________________________
Element .apprxeq. .THETA.
Potential to Element
______________________________________
58' A 0.degree. +V.sub.x
58' B 180.degree. -V.sub.x
60' A 90.degree. +V.sub.y
60' B 270.degree. -V.sub.y
62' A 135.degree.
##STR1##
62' B 315.degree.
##STR2##
64' A 45.degree.
##STR3##
64' B 225.degree.
##STR4##
______________________________________
The electric field produced by this arrangement is given by:
##EQU1##
where R is the electrode radius measured from the Z-axis. The vector
orientation of the electric field E is given by .phi., where
tan(.phi.)=V.sub.y /V.sub.x (3)
As noted, by driving the RICE electrodes with DC potential, a static field
having a desired magnitude and direction may | | |
