JPS6397150A - Magnetic coil assembly for adjusting and deflecting incident electron beam in electron beam system like tomographic scanner using computer - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is suitable for producing x-rays in a tomographic x-ray transmission system of the type disclosed in U.S. Pat. U.S. Patent Application No. 43, filed October 14, 1982 by Rand
This invention relates to an improvement in an electron beam control assembly for a scanning system of the type disclosed in US Pat. No. 4,252. The contents of the US patent of Boyd et al. and the US patent application of Rand are quoted here. The present invention also relates to a deflection magnet assembly and related functions for sweeping an electron beam across an x-ray target. In particular, the rotating magnetic field of the deflection magnet approximates a pure dipole field that is always constant in magnitude and direction.

FIG. 1 is a schematic illustration of a computer-based tomographic x-ray transmission scanning system 10 of the type covered in the Boyd et al. patent, and only a brief description of what is necessary is provided here. System 10 is divided into three main operational elements. That is, the electron beam generation and control assembly 12
, detector array 14, and data acquisition and processing elements (not shown) not related to the present invention. See Figure 2. The present invention relates primarily to the apparatus and operation of electron beam generation and control assembly 12. The housing 26 of the assembly defines an elongated vacuum-tight chamber 28 extending between the rear end 16 and front end 20 of the system. This housing is divided into three coaxial sections.

namely, the rearmost chamber section 34, the intermediate control section 36, and the forwardmost section 38. A device such as a vacuum pump, generally designated 40, evacuates all chambers of their internal gases.

An electron gun 42 is positioned near the rear end 16 of chamber section 34 to create a continuously expanding electron beam 44 and direct the beam through chamber section 34 to control chamber 36 . An intermediate control chamber section 36 sweeps and bends the electron beam 44 through the front section 38 of the assembly and focuses it on a target 50 to generate x-rays. Specifically, the control chamber upper comb 36 includes a 42-bundle coil 46 and a deflection coil 48 that bend the incoming beam from section 34 into the forwardmost chamber section 38 . At the same time, the coil provides a beam spot which is located at the forward end 20 of the chamber section 38.
is received by the X-ray target 50. The generated X-rays are detected by detector array 14, and the output data is sent to a computer processing section, indicated by arrow 22 in FIG. 1, where it is processed and recorded. The computer includes means for controlling the electron beam generation and control assembly 12 as indicated by arrow 24 in FIG.

Field of use of the electron beam and system 10 - clinical diagnosis - and
Medical data obtained from X-rays is highly influenced by aberrations of electron beam optics. It is therefore highly desirable to eliminate any deviations from ideal electron beam optics.

Since the system beam optics are nearly perfect, the deflection coil magnetic field must approximate as closely as possible a pure dipole field.)

In view of this need, it is an object of the present invention to provide a deflection magnet in which non-uniformities in magnetic field magnitude and direction are acceptably small, resulting in a very high quality scanning beam spot. There is a particular thing.

A parallel object is to provide a deflection magnet assembly for sweeping an incident electron beam, the deflection magnet assembly being capable of sweeping an incident electron beam to permit rotation of a deflection surface about a magnet axis to control the electron beam.

Yet another object of the invention is that the magnetic field is uniform across the plane perpendicular to the axis, and the direction of the magnetic field is in this plane and is constant in space and in time around the axis. The object of the present invention is to provide a deflection magnet assembly of the type described above, which can be rotated as the rotation speed increases.

These and other objects are accomplished by a deflection magnet, which is a magnetic coil assembly shaped to produce a magnetic field magnitude and direction that approximates that of a pure dipole field.

That is, the number of turns of the coil used in the assembly, the coil radius, and the angular position of the turns and coil end connections are designed to approximate the magnitude of the magnetic field expressed by the following equation.

1Bl=f■]7〒15''=Bo (1) Regardless of the direction of the magnetic field and the coordinates r, θ, and φ, Bo is constant everywhere within the cylinder containing the beam.

Bx/By=-tan e (2)-
tanφ is unrelated to r and θ. (Coordinate system and r
, 0 and φ are shown in Figure 3.4).

In the embodiment of the deflection magnet, the total number of coil turns and the spacing between each turn are determined so as to satisfy the tolerances determined by the error formulas for the constancy of the non-rotating magnetic field of each coil and for the constancy of the rotating magnetic field in the deflection plane. is selected.

Viewed from a further point of view, a deflection magnet includes a magnetic shield that increases and confines the magnetic field produced by the windings, shields the region inside the magnet from external magnetic fields, and
The effective radii of the coil winding and the 7349 winding are made equal. This equalizing action of the shield compensates for the difference between the radius ax of the X coil winding and the radius ay of the 7349 winding,
This simplifies the coil structure by allowing different coil radii to be employed. The present invention will be described in detail below with reference to the accompanying drawings.

A schematic illustration of an embodiment of a coil magnet assembly 49 embodying features of the invention is shown in plan view in FIG. 7 and in cross-section in FIG. Although the invention is described as being used in a computer-based tomographic scanner, it is generally applicable to deflection magnets and to sweeping charged particle beams. Depending on the configuration of the magnet assembly 49,
That is, the angular distribution θt of the conductor windings t; the total number of turns t; the shape of the end connection between the axially extending portions of each conductor winding, e.g.
The end connections between the axial coil sections of t and π-Ot and O
an end connection between the axial coil portions of t and π-θt, and -
By the shape of the end connection between Ot and -(π-0t) (see FIG. 4) and by equalizing the effective radii of the different coils, the desired magnetic field magnitude and directional characteristics can be determined. Furthermore, a highly permeable shield 59 shields the internal region of the magnet from external magnetic fields and confines the magnetic field generated by the windings.

The design and construction of the magnetic shield allows the radius of the coil to be as uniform as desired.

Before considering the construction details of magnet assembly 49, it is useful to understand the theory embodied in the coil configuration in order to fully understand its design.

The theory of the entire system 10 is detailed in the aforementioned US patent application and need not be repeated here. The theory discussed here is relevant to the design of deflection magnets.

See Figure 3. A magnetic coil assembly, such as assembly 49, is shown in a highly simplified manner useful for considering the characteristics of a pure dipole field. Cartesian coordinate axes x, y
, z are appropriately determined, and the magnetic coil assembly is defined with reference to this. For simplicity, one coil turn is shown from one of the continuous X coils 51 and one coil turn from each of the two continuous Y coils 53-53. These coil turns have an axial section extending parallel to the Z-axis and an end connection interconnecting the two axial sections of each coil turn.

The cross section of each coil lies in a plane parallel to the xy plane, and the radial position therein is represented by polar coordinates (r, θ). The deflection plane of the magnetic field is a plane perpendicular to the direction of the magnetic field. The plane of deflection is represented by φ and two axes (cylindrical axis 55). Here, φ is the angle between the X axis and the deflection plane. (For example, the fourth
In the figure, the X-axis lies within the deflection plane. φ=0) The currents X and Iy of each coil create magnetic fields Bx and By, respectively.

If each φ and the two axes determine the deflection plane, the current Ix=
Io sin (lφ is the magnetic field Bx==-Bo sin
φ, and the current Iy=Io cosφ creates a magnetic field By=Bo cosφ. Ideally, the resultant magnetic field, B = B x + B y, creates a pure dipole magnetic field inside the finite cylindrical magnet, and the magnetic field is zero outside. That is, the magnitude of the magnetic field is r, θ, 2, φ
It is constant regardless of , and is given by the following equation.

1Bl= v”F”; i lower + By”=Bo (
1) The magnetic field direction is independent of r, θ, 2 (strictly, the confined space does not need to be cylindrical) Bx/By=-tan φ (2) The beam is initially centered on the Z axis, and then deflected Due to the proximity to the plane, symmetry considerations suffice to ensure that BO is independent of r, θ, and φ to keep the field strength as desired. That is, it is not important that BO changes with 2. In practice, a 0.2% BO variation across the beam path in the cross-section of the magnet is acceptable. To approximate a pure dipole field in the magnetic coil assembly 49, equations (1) and (
2) requires that the conditions associated with and be satisfied for each cross-section and with an accuracy of 0.2%.

As described below, in one feature of the present invention, formula (1)
The angular position θt of the coil portion in the axial direction is selected so that the magnetic coil assembly approximates the ideal expressed in (2) to the required accuracy, that is, to the extent that it satisfies a tolerance of 0.2%.

The following effective error (R
MS error) and the following effective errors associated with the rotating magnetic field in the deflection plane do not exceed 0.2% of the value of the magnetic field on the magnetic axes 55 (FIG. 3), the required accuracy of 0.2% is obtained.

The RMS error in the non-rotating case is: (a) Separately for each pair of coils over all azimuthal angles (0),
and the RMS from the on-axis value of the magnetic field in the cross section, averaged over all radii (r') up to the specified radius.
It is a deviation.

The RMS error in the case of rotation is (b) over all values of the angle φ that defines the plane of deflection for two pairs of coils acting together, and for a particular radius (r
) was averaged over all radii (r') up to It is the RMS deviation of the magnetic field in any cross section from its on-axis value.

Of course, the 0.2% tolerance and the particular maximum radius r may be different for other applications.

To begin with, the requirements for magnetic field uniformity are most stringent where the electron beam exits the magnetic field of the deflection magnet assembly 49. This state is represented in FIG. 4 with φ=0. A quantitative measure of the non-uniformity of the non-rotating magnetic field in the plane of FIG. 4 is given by the RMS error in item (a) above.

where a is the radius of the coil; B is the value of the magnetic field at r', O; Bo is the value of the magnetic field at the origin; and r is the maximum polar radius at which the beam spreads.

It is sufficient to keep this error to 0.2% or less.

The average variation of the magnetic field over the beam path is on the order of 0.1%, and since most of the beam deflection occurs at small radii, the deflection of individual electrons varies by less than 0.1%.

As the angle φ rotates, the quantification of the variation of the magnetic field in the plane of deflection is
The amount given by the RMS error in item (b) above is 0.2%.
Must be less than or equal to

It is convenient to calculate the quantities expressed by error formulas (3) and (4) without considering end effects, that is, by using approximate calculations for an infinitely long deflection magnet.

Considering the error equation (3) for the homogeneity of a non-rotating magnetic field (φ=O), it is convenient to describe the magnetic field as a multipole or multipole expansion.

The 2n-pole component of the magnetic field is given by the following equation.

t=1 η=1/4πε C is the free space impedance (even multipoles are zero due to symmetry).

Ideally, the contribution of the dipole (n=1) is finite (S□≠0) and all other multipoles are zero. In this case B y = B o == S□ = constant.

Using this multipolar expansion of the magnetic field, we get:

-16= Regarding the constancy of the rotating magnetic field in the deflection plane, the following error formula (4)
Considering that the multipole equation corresponding to equation (3') is: For a selected value of r/a determined by the beam shape, and for a selected value of T the problem now becomes the equation (
3') and (4') to determine the method of calculating the value of θt that gives acceptable values for (4'). There are six different ways to solve this problem, which are described below.

(a) Zero multiple solution method (ZMS) Selecting the conductor angle so that 5n=O for S1≠0 and n=3.5.7−...(2T+1) (for example, superconducting (for particle beam transport magnets) is common.

Alternatively, if the value of r/a) is small and r/a) can be ignored for the first nonzero multi-2T+2 pole term, this solution is sufficient.

In scanner system 10 (FIG. 1), it is desirable to have the coil radius as small as possible for a given coil length and beam path so that the aperture can be used as large as possible. In short, (r/a) is 1
(In fact, the chosen value is r/a=0.867). For this value of (r/a), the 2MS solution was insufficient for T≦4. T>
This solution could not be applied to 4. T=
4, the value of Ot is as follows.

Other solutions include:

2T (c) Mildew vrm + a low red paste - Italian ESSS ↓ upper t
1 2T t 2 2T (e) Minimum non-rotational error solution (MNRES) In this solution, all values of Ot may be changed independently,
The value of Ot was then selected by a computer program to minimize the magnitude of equation (3').

(e) Small difference: MRES This solution method is similar to the MNRES solution method in (e), but the value of Ot was selected to minimize the magnitude of equation (4'). The values of error formulas (3') and (4') are T=4
Table 1 shows the solution (a)-(e) as T=18, and the solution (b)-(e) as T=18. r/a
A simple calculation of the magnitude of error 1 for =0.867 is:
T must be at least 1 to obtain the required magnetic field homogeneity.
It showed that it must be 6. T described later
Calculations with T=18 showed that T=18 gives the required field homogeneity.

(Of course, there is a continuum of solutions that includes (b) and (c), and there is another continuum of solutions that includes (e) and (e).) Ot
The values of are shown in Table 2 for each solution with 18 turns.

The 4-turn results (Table 1) show that the zero multipole solution is clearly inferior to all other solutions, and therefore this solution was not carried out further in a row. The results for 18 turns show that in all cases the uniformity of the non-rotating magnetic field is most critical.

In this application example, 18 turns were selected. The RMS error is acceptable for all solutions and 2X10−” (0,
2%) or less.

Of course, turns larger than 18 turns are desirable from the standpoint of further increasing the uniformity of the magnetic field, but the angular separation of the X and 7 turns was next considered. Preferred dimensions (radius a
2 inches) and construction method, the minimum allowable separation of a pair of X and y turns is 0.62°. (This is a purely mechanical constraint. To machine the shape of the coil,
This is because we require a minimum separation of 0.020 inches between the closest portions of the book's X and Y conductors.

It may be possible to change this constraint by adopting a different manufacturing method. ) with 18 turns for each solution, the minimum angular separation is shown in Table 2. In this requirement, the solution ESSS
and ME S S S 2 are allowed.

Kaya LL Magnetic field non-uniformity (RMS error) equation (3') obtained for a long cylindrical magnet using various solution methods for the conductor spacing
Formula (4') Raw ticket method (f) MRES 2. I XIO""
5,19X10-” conductor position (Ot value (degrees)) for the form of main table 1-”18
Winding 2 4.780 4.665 4.746
4.768 4.8483 7.984 7.8
68 7.949. 7.932 7.9204
1.1.212 11.095 11.177 1
1.143 10,9335 14.478 1
4.359 14.442 14.436 14.49
36 17.792 17.671 17.75
5 17.630 18.0237 21.16
g 21.046 21.132 2+,, 100
20.994g 24.624 24.498
24.587 24.495 24.2509
28.179 28.049 28.140 27.
900 28.06310 31.855 31.
721 31.815 31.893 31.7341
1 35.685 35.544 35.643
35.172 35.54112 39.709
39.560 39.664 39.767 39.5
4613 43.983 43.824 43.9
35 43.382 43.6] 114 48.5
90 48.417 48.538 4L5]7 48
．． 23015 53.664 53.47],
53.606 52.977 53.40516
59.442 59.217 59.37459.21
,0 59.13317 66.444 66.1
58 66.358 65.391 65,36818
76.464 75.983 76.3+8 7
5.927 76.268 Above 18 turns for a given radius, no solution is acceptable for this application. For practical reasons, i.e. to optimize the end connections, ESS
I actually chose S.

In short, in the -set of possible solutions, the angular position θt of the axial portion 60 of each coil is selected according to T. The parameter δ is (1) (r/a = 0 in the illustrated embodiment of the coil assembly 49
, 867, T = 18) as the value that minimizes the aforementioned RMS error in the non-rotating case; (2) (
Again, in the illustrated example, r/a=0.867°T=18
) as the value that minimizes the aforementioned RMS error in the case of rotation; or (3) as δ=O. Of these, the last two are acceptable. This last value, which is the most compact solution of this type, is detailed below. All three formulations are both RM
The limit that the S error is equal to or less than 0.2% is satisfied. Of course, it depends on the coil parameters (e.g., small wire diameter or large coil diameter), but when choosing the value of Ot, E
One or more of the other solutions may be applied in addition to or in addition to the SSS solution. A set of these solutions includes the angular position Ot of the axial portion 60 of each coil that minimizes the stated RMS error for a non-rotating magnetic field or the stated RMS error for a rotating magnetic field. It can be treated as an independent parameter T that can be changed as follows.

Please refer to the plan view shown in FIG. 7 and the sectional view of the portion shown in FIG. The magnetic coil assembly 49 of the present invention implements the theoretical R85S solution described above.One coil assembly includes a cylindrical coil form 57 (FIG. 8) of dielectric material, such as resin-impregnated fiberglass, and an overlying magnetic shield 59. (Fig. 7). Coil form 57 is 3
Six slots 61-61 are each cut or otherwise created in its outer surface as indicated by Xl-X36 to receive the like-identified axial portions of the X-coil 51. (Only half of the coil form is shown.) Similarly, the form 57 has 36 slots 63-63, respectively labeled Y19-YB2, each receiving an axial portion of a particular X coil. I'll put it in. The coil assembly consists of two 18-turn X coils 51 and 18
X coils 53 of turns, each spanning an arc of about 180' according to the configuration and position of FIG. 3.4 and solution T=18 for a given radius. The X coils are adjacent to each other and span an arc of approximately 360°. This also applies to the X coil. The X-coil and the X-coil are rotated 90 degrees with respect to each other, and the X-coil has a somewhat larger radius than the X-coil, as explained below.

Coil form 57 with an outside diameter of 12.375 inches is cut with coil slots 61 to a depth of 0.375 inches and coil slots 63 to a depth of 0°250 inches, with an average radius ax-12,055. inch x coil 5
1 and the axial portion of the X-coil 53 with an average radius ay=12.180 inches. The diameter of the conductor wire is 0.109 inches. The X-coil is essentially the same as the X-coil except that the radius is smaller and the coil forms are offset by 900 degrees.

Xl-X36 (
And X37-X72, Y19-Y54, Y55-Y1
8) Arrange each of the coils in a continuous array of axial sections 60, marked 8).

Ideally, the coil end portions 67 = 67 are designed such that the magnetic field distribution caused by the end connections has the same high frequency spectral description as the magnetic field distribution caused by the axial portion of the winding. If the distribution of windings on the cylindrical surface is ideal (finite dipoles, all other multiballs are zero), a way to achieve this result is by particle accelerators (Parti
cle Acce-1erators), 5,227.
1973 “Magnetic Flux Theory for the Design of Magnet Coil Ends”,
Described by Mills and Morgan. This configuration, applicable to a finite number of turns, is shown schematically in Figure 5.6 for 4 of the 18 coil turns. When applied to the deflection magnet coil assembly 49, when the cylinder is unfolded as shown in FIG. It is symmetrical. Furthermore, each end connection 67 makes an angle α with respect to the axial coil portion 60, which angle is equal to the angle β between the curve 69 and the axial coil portion 60.

Viewed from the side shown in the partial schematic diagram of FIG. 5, the curve 69 and the end connections 67 are straight lines when the coil is placed in the coil form 57, and these are equal for each interconnect 71 and its end connections. We are determining equal angles that take values. For a sufficiently large number of turns, the ideal mathematical properties of this form are approximated with the same precision as the homogeneity of the magnetic field produced by the axial portion of the winding.

The configuration described above is well suited to all equidistant sine solutions (1, 2 and 2) where the axial space available per turn in the 2nd part of the cylinder, ΔZ (FIG. 6), is constant. In fact, we obtain the most compact solution of this type if we make ΔZ the same as the conductor diameter so that all turns touch the ends of the coil. This solution, in which the end connections are exactly inside the ideal magnetic line, is the equally spaced sine solution (b) mentioned above. Therefore, this solution (δ=O) can be adopted using a conductor diameter Δ2 of 0.109 inches and a coil of 18 turns. This solution is shown in Figures 7.9 and 10 with the value of Ot given by the first column of Table 2.

Please refer to FIGS. 9 and 10 and FIG.

In the example of the coil assembly 49, the X coil and the end connection 67 of the
-75 and retaining spacers 77 and gluing them all into slots 79 at the peripheral end of the coil form (which, unlike the central coil form, does not itself contain slots). It is made by. Spacer 75-75 is the coil end 6 6
The above-mentioned predetermined shape of 7 is determined. After attaching the X-coil and its associated spacer to the coil form, a coating 81 (FIG. 10) of liquid dielectric material, such as an epoxy resin, is applied and cured to insulate the X-coil;
and create a smooth base for the X coil at the same height as the X coil slots 63-63. Thus, appropriately sized end spacers 75-75 and retaining spacers 77 are applied, followed by placement of the X-coil,
and another insulating layer 83 such as the epoxy resin material mentioned above.
will be established. The coil assembly includes a magnetic shield 5, which will be described later.
9 is then prepared for application, followed by a final insulation coating (not shown) of fiberglass tape or other suitable medium.

Please refer to FIG. The magnetic shield 59 is a cylinder of high permeability material with an inner radius R, which is outside the coil form and serves three functions.

That is, (1) shielding the inner coil assembly region from external magnetic fields; (2) increasing and confining the magnetic field produced by the windings; and (3) different radii 8x and a.
The effective radius of the X winding and the 7th winding with y are the same.

We believe that the radius equalization in term (3) is novel. Magnetic shielding 59 according to standards to satisfy this new function
, the shielding function, magnetic field increase, and confinement function can be obtained thereby. Previously, magnet designs achieved the required same radius in the windings by spatially mixing different windings. In the present invention, a magnetic shield 59 is placed on the outside of both coils, creating substantially equal effective coil radii, although the actual sizes of the radii are different. Thus, one coil can be wound outside the other, simplifying the construction considerably. This is the basis of the already described use of the coil form 57, with the X coil wrapped around the outside of the X coil and the coil form 57 having a groove for receiving the X coil. The form 57 is amenable to forming grooves at precise locations to establish the angular position of the coil windings. Furthermore, the actual radii of the coils aX and ay are precisely determined and controlled by the coil form thrower 1~, and the precisely determined coil radii are used in conjunction with the following design principles and are the same. Give the effective radius a FET.

Assuming that the condition of infinite magnetic permeability of the shield material is satisfied, if the coil is wound into a cylinder of radius a, and the inner radius of the shield is R (a < R), then the mirror current of the coil current is R ”/a. The dipole magnetic field generated by the coil current and mirror current is [1+(a/R
)” is proportional to ko/a. Difference between the radius of the shield and the coil Δ
a=R-ΔR for R=a-H, and the effective radius is:

a = R[1-1/2 (AR/R
)2 + , , ko (11) For a coil according to the FF 31 set (11), the difference in the effective radius determined from this equation is negligible. That is, it is much smaller than the actual difference in physical radius.

For purposes of explanation, consider an example of the shield 59, X coil, and Y coil of the present invention. In order to satisfy formula (1), the magnetic shielding material is a magnetic material with very high magnetic permeability,
An example is the magnetic material sold under the trade name Molypermalloy by Arenilla Drum Steel Corporation of Pittsburgh, Pennsylvania. The dimensions of the shield are chosen such that its magnetic permeability is close to the maximum value of molypermalloy, 300,000 to 500,000, a value large enough for equation (11) to be valid.

The magnetic material was formed by spirally wrapping 12 layers of tape with a thickness of 0.000 s and a width of 0.25 in. with alternating orientation layer by layer to reduce eddy currents. The inner shield radius R is 12.375 inches, the value of ΔRx for the X coil is 0.320 inches, and the value of ΔRy for the X coil is 0°195 inches. That is, the average coil radius ay is 12.180 inches and ax is 12.055 inches. actual radius ax
The percentage difference between and ay (given by ay-ax) is 1
．． It was 0.3%. In contrast, the difference in the corresponding a EFF is determined by the difference in the parenthetical terms of equation (11), ie, 0.99967-0.99988.

Thus, the effective radius of the X coil and the
It differs by only 0.021%. actually,
This unique magnetic shielding technique allows the required equal effective coil radius to be achieved using a simple configuration that allows the coil and its radius to be determined very precisely.

Those skilled in the art will appreciate that the solution described herein for T=18 was determined for the deflection coil size, approximately 12 inches in diameter. Increasing (or decreasing) the diameter will increase (decrease) the optimal number of turns in the best dipole solution. However, finding the appropriate solution is exactly the same as explained above, and can be found easily. The solution is, of course,
It is not affected by the axial length of the coil assembly 49.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a computer-based tomographic X+ Herans mission scanning system utilizing an assembly for generating and controlling an electron beam within an evacuated beam chamber. FIG. 2 is a cross-sectional view of the system of FIG. FIG. 3 shows the overall shape of the improved magnetic coil of the present invention. FIG. 4 shows a cross section perpendicular to the axis of the magnetic coil of FIG. 3, showing a coil whose deflection plane is φ=O (X coil). FIG. 5 shows the shape and spacing of the coil end connections embodying the present invention in the coiled (cylindrical) state, and FIG. 6 shows the coil in the unfolded state. FIG. 7 is a plan view and FIG. 8 is a cross-sectional view of a portion of a magnetic coil assembly having features of the present invention. FIG. 9 is an enlarged view of a portion of FIG. 7 showing the coil unfolded, showing details of the attachment configuration of the end of the coil. FIG. 10 is an enlarged view of a portion of FIG. 7, showing spacers used in the fractional arrangement of the coils. In the figure: 10... Tomographic X-ray transmission scanning system 12... Electron beam generation and control assembly 14...
Detector array] 6... Rear end of scanning system 20... Front end of scanning system 28... Chamber 34... Rearmost chamber section 36... Forwardmost chamber section 42... Electron gun 44...・Electron beam 46...Focusing coil 48...Deflection coil 49...Deflection magnet coil assembly 50...Target 51...X coil 531...X coil 55...Cylindrical axis (magnetic field axis) 57 ... Cylindrical coil form 59 ... Magnetic shield 60 ... Axial coil portion 61 ... Slot (Xi-X, 36) 63 ... Slot (Y 19- Y 54- ) 67 ... End connection 71...Interconnection 75...End connection spacer 77...Holding spacer 79...Slot 81...Adhesive coating 83...Insulating layer a...Coil radius B... Magnetic field value Bo...Magnetic field value at the origin r...Maximum radius of the beam

Claims (6)

[Claims] (1) In a magnetic coil assembly that adjusts and deflects an incident electron beam in an electron beam system such as a computer-based tomographic scanner, a pair of x and y coils are used.
a coil, a generally cylindrical support form supporting the x and y coils at different radii ax and ay, and a magnetic shield, the x and y coils being positioned at an angular position of π/2 with respect to each other; and is generally cylindrical about its axis, with an angle φ defining a plane of deflection relative to a fixed plane containing the magnetic coil axis, and defined by mutually orthogonal coordinate axes perpendicular to the magnetic coil axis. Polar coordinates (γ, 0) determine the position in the plane of The current Ix proportional to sinφ is −Bo si
A magnetic field Bx equal to nφ is created with a selected precision, and a current Iy proportional to cosφ creates a magnetic field By equal to cosφ with a selected precision, so that the magnitude of the resultant field is Bo=(Bx＾ Approximating to 2+By^2)^1^/^2, it becomes a constant independent of R, θ, and φ, and the magnetic field direction Bx/By is approximated to Bx/By=-tanφ, where r and θ The orientation and configuration of the coil are chosen so that the R for non-rotating
The MS error is: (a) for each pair of coils separately, the square root of the mean of the square of the deflection of the magnetic field in the cross-section from its on-axis value (average over all polar angles θ, and (b) for two pairs of coils acting together, of the magnetic field in the cross section, is the square root of the mean of the square of the deflection from its on-axis value (average over all values of angle φ bounding the plane of deflection and over all radii r′ up to the chosen radius ), the angular position θt of the axial portion of each coil is sinθt=(2t-1)/2t+δ (where t=1, 2, 3...T, and T is the coil winding For a given value of T, the parameter δ is selected according to the root mean square error in the non-rotated case (a) and the square root mean square error in the rotated case (b). or the parameters δ and T are less than or equal to a given percentage of the magnetic field on the axis of the magnet; and said magnetic shield is selected to provide at least one of:
It is made of a high magnetic permeability material with a radius R larger than x and ay,
A magnetic coil assembly characterized in that the image of the coil current is around R^2/a to give the x-coil and the y-coil substantially equal effective radii. (2) A patent claim in which the value of the parameter δ is selected so as to give the error of the root of the mean of both squares a value smaller than or equal to 0.2 percent of the value of the magnetic field on the axis of the magnet. Bending magnet assembly in the first term of the range. (3) For T=18 and r/a=0.867, the value of the parameter δ is selected so as to give the minimum value of the error of the square root of the mean square in the case of non-rotation. Bending magnet coil assembly. (4) Claim 2 in which the value of the parameter δ is selected so as to give the minimum value of the error of the square root of the mean square in the case of rotation for T==18 and r/a=0.867. Bending magnet coil assembly. (5) the angle that each end connection makes with the extension of each axial section is substantially equal to the angle that the curve formed by the intersection of each axial section and a plurality of adjacent end connections makes with the extension of the axial section; 2. A deflection magnet coil assembly according to claim 1, wherein the curve defined by the intersection of said end connection and each axial portion is symmetrical with respect to the respective end connection. (6) A compact configuration is created using δ = 0, where the intersection of the coil end connections and the axial portion of the coil forms a first curve, and a second curve where the individual end connections are symmetrical.
forming a curve and intersecting the corresponding part of the first curve and being inside a third curve symmetrical to and connected to the ends of the first curve. Deflection magnet coil assembly.