Method and apparatus for electron beam focusing adjustment by electrostatic control of the distribution of beam-generated positive ions in a scanning electron beam computed tomography scanner

In a scanning electron beam CT system, the electron beam is focused by controlling the distribution of beam-generated ions electrostatically. The upstream (self-expanding, de-focusing) beam region and downstream (converging, self-focusing) beam region are distinguished by the absence or presence of beam-generated positive ions. The relative lengths of these two beam regions are electrostatically controlled such that beam de-focusing in the upstream region compensates for beam self-focusing in the downstream region. In this fashion, essentially zero external focusing strength is required, and the magnetic focus coil used in the prior art is eliminated. Located downstream from the electron gun, a positive ion electrode ("PIE") determines the position of the boundary between the two regions, and thus the relative length of each region. The PIE is a disk-like electrode, mounted coaxially to the beam optic axis within the drift tube, and coupled to a large positive potential. Varying the PIE potential varies the inter-region boundary position, and thus the relative magnitudes of the beam de-focusing and self-focusing effects. A PIE focus potential is determined by varying the potential while examining the output of electron beam monitors with an oscilloscope. Further, by dynamically varying the PIE potential, the present invention adjusts electron beam focusing, even during a scan. Positive ions are removed from the upstream region by a periodic ion clearing electrode ("PICE") whose high rate of change of axial potential creates alternating axial fields that rapidly sweep away ions.

FIELD OF THE INVENTION 
The present invention relates generally to scanning electron beam computed 
tomographic X-ray systems, and more particularly to focusing and adjusting 
the focus of the electron beam in such systems without using magnetic or 
electrostatic lenses. 
BACKGROUND OF THE INVENTION 
Scanning electron beam computed tomography ("CT") systems are described 
generally in U.S. Pat. Nos. 4,352,021 to Boyd, et al. (Sep. 28, 1982), and 
4,521,900 (Jun. 4, 1985), 4,521,901 (Jun. 4, 1985), 4,625,150 (Nov. 25, 
1986), 4,644,168 (Feb. 17, 1987), and 5,193,105 (Mar. 9, 1993), all to 
Rand, et. al. Applicant refers to and incorporates herein by reference 
each above listed patent to Rand, et al. 
As described in the above-referenced patents, an electron beam is produced 
by an electron gun at the upstream end of an evacuated generally conical 
shaped housing chamber. A large electron gun potential (e.g., 130 kV) 
accelerates the electron beam downstream along the chamber axis, and 
further downstream, a beam optical system focuses and deflects the beam to 
scan along an X-ray producing target. It is understood that the final beam 
spot on the target is much smaller than the original beam size upon 
exiting the electron gun. 
The beam optical system includes a magnetic focus coil, quadrupole coils, 
and deflection coils. The X-rays penetrate an object (e.g., a patient) and 
are detected by a detector array 22. The detector array 22 and targets 14 
are coaxial with, and define planes orthogonal to, the system axis of 
symmetry 28. The output from the detector array is digitized, stored, and 
computer processed to produce a reconstructed X-ray video image of a 
portion of the object. 
In the chamber region upstream of the beam optical system, a diverging beam 
is desired. In the upstream region, the electron beam can advantageously 
self-expand due to its own space-charge. The self-expansion depends on the 
force created by the electron space-charge. By contrast, downstream from 
the beam optical system, a converging, self-focusing, beam is desired. 
The vacuum chamber contains residual or introduced gas that ionizes in the 
presence of the electron beam, producing positive ions. While these 
positive ions are useful in the downstream chamber region where a 
converging beam is desired, in the upstream region they can detrimentally 
counteract the desired beam expansion. Unless removed by an external 
electrostatic field upstream, the positive ions become trapped in the 
negative electron beam, neutralizing the space-charge needed for the 
desired beam self-expansion. In fact, neutralization can destabilize and 
even collapse the beam. 
The usual arrangement in prior art scanning electron beam scanners is to 
remove such positive ions by passing the electron beam axially through at 
least one ion clearing electrode ("ICE") located in the upstream region. 
The ICE is coupled to an electrode potential of about 1 kV, and creates a 
transverse electric field. The transverse field sweeps away the slowly 
moving positive ions, without disturbing the considerably faster moving 
electrons, which have been accelerated by some 130 kV. 
In this manner, ICE's remove positive ions only from the upstream region, 
permitting positive ions to accumulate downstream from the beam optics 
system. Downstream, positive ions beneficially neutralize beam 
space-charge, which permits the beam's attractive magnetic field to 
converge and self-focus the beam. Thus, downstream, convergence depends on 
the magnetic field created by the electrons in the electron beam. 
The result is a self-repulsive, de-focusing beam in the upstream region, 
and a self-focusing beam in the downstream region. Elements of the beam 
optical system then focus and fine tune the converged beam as it scans 
along the X-ray target, to produce a sharp reconstructed X-ray image. 
The upstream and downstream chamber regions are segregated by a 
washer-shaped positive ion electrode ("PIE"), coupled to a high positive 
potential, e.g., 2 kV. The PIE creates a large axial field that prevents 
positive ions (formed downstream) from migrating upstream, where their 
presence would be detrimental. Near the intersection of the upstream and 
downstream regions, there is usually placed a magnetic solenoid focus coil 
that provides and fine tunes the beam focus in response to a varying coil 
current. 
While magnetic solenoid focus coils are used in the prior art, it is 
advantageous to reduce the number of components, including such coils, in 
a scanning electron beam CT system. 
In a scanning electron beam CT system, there is a need for a mechanism that 
can focus the electron beam, and for a means for adjusting such mechanism, 
that obviates the need for a magnetic solenoid focus coil. Further, there 
is a need for an electron beam focus mechanism that permits dynamic focus 
fine tuning during a scan. 
The present invention discloses such a mechanism. 
SUMMARY OF THE INVENTION 
The present invention focuses an electron beam in a scanning electron beam 
CT system by electrostatically controlling the distribution of 
beam-generated ions. The relative lengths of the upstream (self-expanding, 
defocusing) and downstream (converging, self-focusing) beam regions are 
electrostatically controlled such that beam de-focusing in the upstream 
region compensates for beam self-focusing in the downstream region. If the 
relative lengths of the two regions are properly chosen, essentially zero 
external focusing strength is required, and the magnetic focus coil used 
in the prior art is eliminated. 
Located downstream from the electron gun, a positive ion electrode ("PIE") 
determines the position of the boundary between the two regions, and thus 
the relative length of each region. The PIE is a disk-like electrode, 
mounted coaxially to the beam optic axis within the drift tube portion of 
housing 10, and coupled to a large positive potential. The resultant axial 
field prevents positive ions from migrating past the PIE, upstream toward 
the electron gun. Varying the PIE potential varies the inter-region 
boundary position, and thus the relative magnitudes of the beam 
de-focusing and self-focusing effects. The required PIE potential is 
determined by varying the potential while examining the output of 
appropriate electron beam monitors. 
Further, by dynamically varying the PIE potential, the present invention 
adjusts electron beam focusing, even during a scan. 
In the preferred configuration, the drift tube region separating the 
electron gun from the PIE is too narrow to accommodate a conventional ICE 
without modifying the drift tube. Therefore, positive ions are removed 
from this upstream region by a periodic ion clearing electrode ("PICE"). A 
PICE comprises several spaced-apart washer-like electrodes coaxial to the 
beam optic axis, with alternate PICE electrodes coupled, respectively, to 
large and small potentials. The resultant high rate of change of axial 
potential creates alternating axial fields that rapidly sweep away ions. 
Alternative designs with conventional, e.g., transverse field, ICE's are 
also possible. 
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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1 and FIG. 2, a generalized scanning electron beam CT 
X-ray system 8 includes a vacuum chamber housing 10 wherein an electron 
beam 12 is generated by an electron gun 32 located in upstream region 34, 
in response to high excitation (e.g., 130 kV). The electron beam is then 
caused by optical system 38 to scan at least one circular target 14 
located within front lower portion 16 of chamber 12. 
When scanned by the focused electron beam, the target 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 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, producing 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. 
As described earlier, gases in housing 10 produce positive ions in the 
presence of the electron beam 12. While positive ions are beneficial in 
the downstream, self-focusing region 36, they must be removed (or at least 
be suitably controlled) in the upstream, self-expanding defocusing region 
34. 
According to the present invention, beam optical system 38 includes a PIE 
48, a PICE 52, deflecting coils and quadrupole coils, collectively coils 
42. Coils 42 contribute a focusing effect, which is used to help shape the 
beam spot as it scans one of the targets 14. It is to be noted that beam 
optical system 38, in contrast to the prior art, does not include a 
magnetic focus coil. 
As shown in FIGS. 2 and 3, PIE 48 and PICE 52 are mounted within housing 10 
between the electron gun 32 and coils 42 such that the electron beam 12 
passes axially through the PICE and PIE coaxially along the system axis of 
symmetry 28. 
With reference to FIG. 3, PIE 48 is preferably a planar washer whose center 
opening is at least as large as the electron beam diameter at that region, 
typically about 2 cm. PIE 48 is preferably coupled to a large positive 
potential (e.g., +2 kV) V.sub.48. The PIE is made from a relatively inert 
conducting material that does not outgas within chamber 10, e.g., 
stainless steel or copper. The PIE is mounted within chamber 10 using one 
or more posts 54 made from an insulating material such as ceramic. 
The PIE produces an axial field that prevents positive ions from migrating 
upstream toward PICE 52 (see FIG. 4C). Such upstream migration would be 
detrimental and would interfere with the production of a sharply 
self-focused beam spot at the X-ray target. According to the present 
invention, PIE 48 further serves to sharply define the interface between 
the upstream region and the downstream region. 
In prior art scanning electron beam CT system such as that disclosed in 
U.S. Pat. No. 5,193,105, the drift tube was sufficiently wide to permit 
using at least one ICE upstream of a PIE, to remove positive ions in the 
upstream, beam expanding region. However, in the present invention, the 
drift tube is narrowed in diameter to about 3.8 cm, too small a dimension 
to accommodate an ICE. 
Therefore, in place of an ICE, the present invention removes positive ions 
from the upstream region 34 using a PICE 52, disposed adjacent electron 
gun 32 and upstream from PIE 48. PICE 52 preferably comprises a plurality 
of disk-like elements 70, 72 spaced apart coaxially along the axis 28. The 
PICE elements preferably stainless steel or a similar relatively inert 
conducting material that does not outgas within chamber 10. Insulating 
posts 54 are used to mount PICE 52 within the drift chamber 10. 
Alternate PICE electrodes, e.g., 70, 70A, 70B, 70C are together coupled to 
a first potential source V.sub.70, and the intermediate electrodes, e.g., 
72, 72A, 72B are together coupled to a second potential source V.sub.72. 
In the preferred embodiment of FIG. 3, seven PICE disks are used, V.sub.70 
.apprxeq.-2 kV and V.sub.72 .apprxeq.0 V (e.g., ground), although other 
potentials could be used, including possibly +2 kV and ground. A design 
consideration for the PICE is that within a relatively short lateral 
distance, e.g., about 5 cm, a sufficiently high rate of change of axial 
potential must be created to rapidly remove ions. The V.sub.70, V.sub.72 
potentials are sufficient in magnitude to create the desired field but, in 
comparison with the 130 kV electron gun potential, are not sufficient to 
disturb the electron beam flow. 
Further, PICE 52 advantageously subjects the electron beam 12 to an 
electric field notwithstanding discontinuities in housing 10 that create 
gaps, such as 37, over which a conventional ICE could not be used 
(assuming space permitted such use). 
With reference to FIG. 3, it is understood that upstream from PIE 48 (e.g., 
to the left of PIE 48), positive electrons are removed by PICE 52, and the 
electron beam 12 expands, or de-focuses, due to space-charge of the 
electrons within the beam. The magnitude of the defocusing force at 
various points along axis 28 will vary with the beam diameter and 
space-charge density. 
According to the present invention, PIE 48 separates the upstream region 
(e.g., the beam expanding or de-focusing region) from the downstream 
region (e.g., the beam converging or self-focusing region). Because 
positive ions exist downstream from PIE 48 (e.g., to the right in FIG. 3), 
the electron space-charge is neutralized and the beam will converge or 
self-focus toward axis 28 due to the beam's self-magnetic field. The 
magnitude of the self-focusing force will vary along axis 28 as a function 
of the beam diameter and current density, which produces the self-magnetic 
field. 
According to the present invention, between the upstream and downstream 
regions, there will be an axial location for which the beam expanding 
effect in the entire upstream region compensates the beam converging 
effect in the entire downstream region. Then essentially zero external 
focusing force will be required to focus the scanning electron beam on to 
a target 14. 
By varying the PIE potential V.sub.48, the boundary between the ion-free 
region and the neutralized region can be moved. In this manner, the 
relative length of the two regions can be varied until an upstream region 
length and a downstream region length result, for which the beam 
converging and diverging effects compensate one another. 
Preferably the magnitude of PIE V.sub.48 required for compensation is 
determined by varying V.sub.48 while observing the output from beam 
monitoring devices 56 (see FIG. 2) on an oscilloscope. The optimum value 
of V.sub.48 will be used. The use of "W"-wire devices 56 for monitoring 
beam quality in a scanning electron beam CT system is described in U.S. 
Pat. No. 4,631,741 (1986) to Rand, et al. Because the use of beam 
monitoring devices is known in the art, further description is not 
presented here. 
Within system 8, stray electric and magnetic fields, and inexact dimensions 
of chamber 10 can vary ion distribution within the beam, and require fine 
tuning of the electron beam focus. This fine tuning is also provided by 
varying PIE potential V.sub.48, which potential may be varied to control 
the focus dynamically during a scan. For example, an appropriate V.sub.48 
may be a DC voltage level about which a varying AC voltage is 
superimposed. If desired, the shape of the AC voltage can be suitably 
keyed by computer mechanism 24 to compensate for stray fields and 
inexactness in chamber 10 dimensions, as a function of beam scan position 
along an X-ray target 16. 
It will be appreciated that although the PIE-PICE combination may 
constitute an electrostatic lens, this lens is weak and negative, 
contributing little to the overall focusing effect. The focusing effect of 
such a lens is significantly less than that of the beam self-forces, which 
are controlled by the beam ion distribution, which in turn is controlled 
by the PIE potential, according to the present invention. 
The present embodiment varies both the strength of the effective 
electrostatic lens and the beam ion distribution as a function of the PIE 
potential. However, since the electrostatic lens strength and ion 
distribution are actually separate effects, a suitable electrode 
arrangement would permit focusing by maintaining the electrostatic focus 
effect constant or zero, while varying the beam magnetic self-forces by 
means of the ion distribution. 
FIG. 4A depicts the PICE-PIE assembly 52-48 in longitudinal cross-section, 
showing the role of PIE 48 in demarking the interface between the upstream 
space-charge limited, de-focusing beam region, and the downstream 
neutralized, self-focusing beam region. FIG. 4B depicts the electrostatic 
potentials measurable at various locations along axis 28. FIG. 4C depicts 
the electric fields along Z-axis 28, resulting from the PICE and PIE 
units, and also shows the motions of the ions. 
In summary, in contrast to the use of a magnetic focus coil in the prior 
art, the present invention configures a PIE unit to provide the necessary 
focusing and dynamic focusing. Further, the PICE-PIE configuration used in 
the present invention requires less drift tube space, consumes less power, 
and operates more reliably than a prior art magnetic focus coil. 
Modifications and variations may be made to the disclosed embodiments 
without departing from the subject and spirit of the invention as defined 
by the following claims. For example, although a focus mechanism has been 
described for use in a scanning electron beam CT system, such mechanism 
could be used in other applications as well, e.g., high current electron 
accelerator injectors, and possibly electron beam welders.