US3673528A

US3673528A - Wide frequency response line scan magnetic deflector

Info

Publication number: US3673528A
Application number: US119584A
Authority: US
Inventors: William C Hughes
Original assignee: General Electric Co
Current assignee: General Electric Co; INDIANA NATIONAL BANK
Priority date: 1971-03-01
Filing date: 1971-03-01
Publication date: 1972-06-27
Anticipated expiration: 1989-06-27

Abstract

A magnetic deflector for an electron beam is disclosed. The deflector is characterized by a frequency response of several megahertz and a linear deflection. The wide frequency response and linearity are achieved by utilizing particularly shaped pole pieces within the electron beam gun. In additional embodiments, combinations of deflecting elements are disclosed.

Description

United States Patent Hughes [4 1 June 27, 1972 [54] WIDE FREQUENCY RESPONSE LINE 2,909,688 10/1959 Archard .335/210 x SCAN MAGNETIC DEFLECTOR 3,197,678 7/1965 Primas.... ..335/209 3,482,136 12/1969 Herrera ..250/49.5 C X [72] inventor: William C. Hughes, Scotia, NY.

73 Ass nee: General Electric Com Examiner ceorge Hams 1 AttmeyRichard R. Brajnard, Paul A. Frank, Charles T. Filed: March 1, 1971 Watts, Frank L. Neuhauser, Oscar Bl Waddelland Joseph B. 21 Appl. 190.; 119,584 Fmma [57] ABSTRACT US. Cl. ..335/210, 313/79, A magnetic deflector for an electron beam is disclosed The [58] Fieid Search /49 C deflector is characterized by a frequency response of several "250/49 13/73 76 megahertz and a linear deflection. The wide frequency response and linearity are achieved by utilizing particularly shaped pole pieces within the electron beam gun. in additional [56] References cued embodiments, combinations of deflecting elements are dis- UNITED STATES PATENTS Closed- 2,777,958 1/1957 Poole ..250/49.5 D Chins, 8 Drawing Figures PATENTED'JUNZ'I'ISR SHEET'10F3 FIG H/S ATTORNEY WIDE FREQUENCY RESPONSE LINE SCAN MAGNETIC DEFLECTOR THE DISCLOSURE This invention relates to magnetic deflection devices, and, in particular, to magnetic deflection devices for use in electron beam recording guns.

In electron beam devices, magnetic deflection is preferred to electrostatic deflection for several reasons. Magnetic deflection allows wider deflection angles with less spot degradation (spread or deformation). Since magnetic deflection is current dependent, it ismore compatible with semiconductor circuitry. Further, since the magnetic field required for a given deflection does not increase as rapidly with beam voltage as the electrostatic field required, it is more suited to high voltage guns.

A swept electron beam has been used to record on a variety of media, such as thermoplastic, photographic and magnetic films. In this application, the beam is used to scan a moving tape, generally along only one axis. It is generally required that the beam produce a very small spot (e. g. less than 6 microns in diameter) and have a high current density. Also, it is advantageous to place the final lens of the gun as closely as possi ble to the recording media, which allows minimal distance for deflection. In addition, it is desirable to have a high repetition rate.

These combined specifications require an electron gun to have wide deflection angles, wide frequency response, high voltage, linear sweep and low spot degradation.

While these requirements would seemingly be fulfilled by a magnetic deflection system, such systems have been little used in this type of application because of the non-linearity in the deflection vs. current characteristic of the magnetic deflection system. Also, it is difiicult to obtain a wide frequency response with conventional magnetic deflection systems. Further, there is often a problem in fitting a conventional deflection yoke into the mechanical design of the gun.

Thus, until now, one had the choice of accepting the performance available with electrostatic deflection or utilizing electronic correction in the deflection amplifier to correct for the non-linear deflection characteristic of a magnetic deflector.

The foregoing difficulties are compounded when one desires to use multiple, cascaded deflection systems. This would occur when it is desired that the beam strike the target at a particular angle across the entire trace; for example, normal to the plane, of a flat target. One deflector deflects the beam to the desired location, a second deflector straightens" the beam to the desired angle. As another example, when the deflector is to be used with an electron lens and the lens must be close to the target, it has been found that redirecting the beam through the nodal point of the lens enables wide deflection angles, despite the close proximity of the lens to the target.

In view of the foregoing, it is therefore an object of the present invention to provide a magnetic deflection system having wide frequency response.

It is a further object of the present invention to provide a magnetic deflection system for a frequency response extending to at least 9 Mhz.

It is another object of the present invention to provide a magnetic deflection system having a wide deflection angle and a linear sweep.

It is a further object of the present invention to provide a magnetic deflection system for use with electron beams of high current density.

It is a further object of the present invention to provide a magnetic deflection system having a low spot degradation.

It is another object of the present invention to provide a magnetic deflection system comprising cascaded deflection elements providing wide deflection angles and a linear sweep.

The foregoing objects are achieved in the present invention pole pieces in close proximity to the electron beam. The shaping of the pole pieces is a function of the positioning of the deflection system relative to the target. This function produces a family of curves or shapes for the pole pieces depending upon the distance to the target. In an alternative embodiment of the present invention, electrostatic deflecting plates are positioned between the shaped, magnetic pole pieces and utilized to extend the high frequency response'of the deflection system.

A more complete understanding of the present invention may be obtained by a consideration of the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a conventional magnetic deflection system and the parameters utilized in determining the shaping of the pole pieces.

FIG. 2 is a diagram useful in explaining the present invention.

FIG. 3 illustrates a graphical representation of the shape of different pole pieces depending upon the distance between the deflection system and the target.

FIG. 4 illustrates a crossvsectional view of a magnetic deflection system in accordance with the present invention.

FIG. 5 illustrates an end view of a magnetic deflection system in accordance with the present invention.

FIG. 6 illustrates a graphical representation of the shaping of the pole pieces in accordance with the distance of the magnetic deflection system from the target.

FIG. 7 illustrates a graphical representation of a pole piece for the magnetic deflection system when cascaded with other deflecting elements.

FIG. 8 illustrates the combination of various shaped pole pieces used to deflect the electron beam to the nodal point of an electron lens.

Referring to FIG. 1, there is illustrated the basic elements utilized in a magnetic deflection system. A magnetic deflection system generally comprises a magnetic core 10 having a plurality of turns of wire 1 I wound thereon and magnetic pole pieces or faces 12 for establishing a uniform magnetic field across the gap 3 defined by pole pieces 12. The amount of deflection is determined by the magnetic field, which in turn is dependent upon the current through coil 11 and the number of turns of coil 11. The amount of deflection is also dependent upon the length L of pole pieces 12.

As an electron beam enters the field established between pole pieces 12, the electrons of the beam follow a curved path such as path 14. With no applied magnetic field, the electrons are not deflected and pass directly through the gap along path 15. The amount of deflection d of the electron beam upon intercepting target 13 is dependent upon the distance S between the deflecting elements and target 13 and the length of the plates L as well as the strength of the magnetic field. Specifically, the deflection sensitivity (in meters per ampere) is given by the following relationship:

Deflection sensitivity (e t/mv) [(S0.5L) LN/g] 1) A more complete understanding of the nonlinearities involved in a magnetic deflection system may best be understood by considering FIG. 2 in which there is illustrated a simplified cross-sectional diagram of the magnetic deflecting system of FIG. 1. The amount of deflection of the electron beam is determined by the length of radius R which in turn is determined by the strength of the magnetic field. The angle through which radius R rotates is determined by the length L of pole pieces 12.

In the basic deflection system illustrated in FIG. 2 are a number of areas which serve to make the deflection vs. input current characteristic of the deflection system nonlinear. A first of these resides in the nature of the beam path between pole pieces 12. For zero deflection, the length of the beam path, and hence the length of time a given electron is traveling between pole pieces 12, is at a minimum since the length of the beam path between pole pieces 12 is equal to L. Since the wherein there is provided a magnetic deflector having shaped pole pieces are rectangular and the beam path is the arc of a circle of radius R, the length of the beam path between pole pieces 12 is greater during deflection than it was before, thereby increasing the angle through which the beam is deflected. Thus, for small radii the beam is deflected relatively more than it should be.

In addition, the amount of deflection of the electron beam is determined in part by the angle of deflection and distance S-L. A portion of the deflection, Y, is determined by the displacement of the beam as it is deflected between pole pieces 12. The amount of displacement (d-Y) is a tangential function of angle 6. Further, displacement Y is a non-linear function of 6.

For zero deflection, an electron traveling through pole pieces 12 will pass through origin 0 along beam path 15 and strike target 13 at point 40. For a given amount of deflection, 0, the electron beam is curved while passing between pole pieces 12 and exits at an angle 0 from the normal to the target, follows path 14 and strikes target 13 at point 41.

In addition to correcting the above enumerated nonlinearities, the deflecting system of the present invention further enables the high repetition rates necessary in a line scanning system. In general, a wide frequency response is obtained by a proper balance between the inductance exhibited by the deflection system with the capacitance of the windings.

The correction for nonlinearities is obtained by the shaping of the pole pieces in accordance with the deflection distance S, as will be more fully enumerated hereinafter.

In order to achieve wide frequency response, it is desirable to make the pole pieces as small and closely spaced as possible, e.g. less than beam diameters apart. This reduction in size will result in smaller shunt capacitance across the coil which will further increase the self-resonant frequency of the deflector. The reduction in size produces the advantage of enabling the deflector to be placed inside the vacuum envelope containing the electron beam generating means. Presently available ferrite materials for the core are compatible with the vacuum environment and permit a frequency response of several Mhz.

FIG. 3 illustrates a family of curves for different values of S illustrating the corrective shape for pole pieces 12. For the deflection system illustrated in FIG. 1, that is, with rectangular deflection plates, the amount of deflection is given by the following equation:

As discussed above, for greater angles of deflection, that is, smaller R, the deflection is greater than it should be for a given radius. To linearize the deflector, a way must be found to decrease the deflection sensitivity at larger deflection values. This can be done by shaping the pole pieces so that at wider deflection angles the beam leaves the deflection field sooner. In other words, the pole pieces are shaped as a function of the deflection so that at smaller radii less deflection is produced.

When fringe fields are neglected, it can be shown that the desired shape for the pole pieces is given by the following equation:

[(S 4S+ 2) X +(2S S) X+S*S+l/4]% (4) This equation has been normalized by dividing by L, the length of the plate on axis; i.e. at S l the pole piece is just touching or is very close to the target. The equation holds for 0 g X 1. FIG. 3 illustrates a family of curves for S l and S w and for three values in between. The above equation does not become undefined as S as may be seen by inspection of the equation using the binomial theorm to expand the square root term.

FIG. 4 illustrates a cross-sectional view of a magnetic deflector in accordance with the present invention. The curves of FIG.- 3 represent only the upper half of pole piece 20 in FIG. 4. The lower half of pole piece 20 is the mirror image of the upper half. Since relatively large deflection angles (greater than 100) are not being utilized, the entire curve from FIG. 3 need not be used. Thus, the pole pieces in FIG. 4 include straight edge portions 25. The straight edge portions approximate the exact pole shape and do not affect the performance of the deflector since large deflection angles are not being utilized.

If it is desired to raise the upper frequency limit of the deflection system, beyond the relatively high upper limit already obtained by the present invention, electrostatic deflection plates 21 can be inserted between magnetic pole pieces 20. In operation, the electrostatic deflection plates would be activated only at the very high repetition rate frequencies. At lower repetition rates they would not participate in the deflection.

The response curves for the electrostatic and the magnetic deflection systems would be arranged so as to complement one another. That is, as the frequency response of the magnetic deflection system falls off, the response of the electrostatic deflection system would be increased. This can be readily accomplished by suitable frequency filtering in the amplifier driving the electrostatic deflectors.

In order to minimize any disturbance caused in the magnetic field by the introduction of the electrostatic deflection plates, it is preferred that deflection plates 21 be formed as a thin layer or film of electrically conductive material on nonmagnetic substrate 22. It is understood, of course, that other electrostatic deflection plates may be utilized provided they do not distort the magnetic deflection field.

FIG. 5 illustrates an end view of the combined electrostatic and magnetic deflection systems illustrated in FIG. 4 and shows the relative positioning of the magnetic and

electrostatic deflection elements

20 and 21 respectively.

When it is desired to have the beam intercept the target at a particular angle, a pair of magnetic deflection plates can be utilized having shaped pole pieces as illustrated in FIG. 6. For the first pole piece, whose shape is similar to that illustrated in FIG. 3, the left hand leading edge 61, nearest the source of the electron beam, is perpendicular to the axis of the deflection system. The trailing edge of the first magnetic deflection system has a shaped configuration described by the following equation:

A comparison of this equation with equation (4) will reveal a slight difference to allow for the second deflection system. The net effect of the first deflection system is the same as that for FIG. 3, however, the electron beam exits the pole pieces sooner at greater deflection angles than it would if the pole pieces were simply rectangular. However, in this embodiment, the first deflection system alone is not fully corrected.

The second deflection system has its leading edge represented by a family of curves for various values of S, the distance in which deflection takes place. The trailing edge of the second magnetic deflection pole piece 62 is perpendicular to the axis of the deflection system.

When the combined magnetic deflection systems illustrated in FIGS. 6 are utilized, the deflection distance S is then the length from the leading edge 61 of the first magnetic deflection system to trailing edge 62 of the second magnetic deflection system, since it is only over this interval that the electron beam is deflected. The distance S as shown in FIG. 6 should not be construed as indicating that trailing edge 62 is neces sarily immediately adjacent the target. The shape of the leading edge of the second magnetic deflection system is obtained in accordance with the following equation:

It is important to note with respect to the second magnetic deflection system how the value X is measured, that is from right to left rather than from left to right.

In operation, a magnetic deflection system in accordance with FIG. 6 will displace an electron beam a predetermined amount from the zero deflection path followed by the beam and will cause the beam to strike a target at a predetermined angle, for example, normal to the plane of the target, assuming the target is perpendicular to the axis of the deflection system. Deflection is obtained by varying the current through windings l l and hence varying the radius R through which the electron beam is deflected in the first deflector. The second deflector also has a variable current flowing in the windings thereof so as to deflect the beam through an appropriate radius curve to bring the beam back to a direction that is perpendicular to the plane of the target and parallel to the undeflected beam path. As with the shaped pole piece illustrated in FIG. 3, the deflection d produced by the deflection systems of FIG. 6 is linearly proportional to the input current.

FIG. 7 illustrates a shaped pole piece for bringing parallel but separated electron beams together at a point on the extended axis of the deflection system. As before, the amount of deflection is linearly proportional to the input current. The proper shape for these deflection plates is obtained from the following equations:

In FIG. 7, the plate shape for S=3 and D=2 is illustrated. Utilizing a pole piece as illustrated in FIG. 7 in a magnetic deflection system, a plurality of parallel electron beams perpendicular to and incident upon leading edge 71 will be deflected by pole piece 70 and exit past trailing edge 72 at such an angle that all of the beams will intercept the axis of the deflection system, if extended, at a point a distance S 3 from the leading edge. Such a deflector would find utility, for example, in conjunction with an electron lens in which it is desirable that the electron beam pass through the nodal point of the lens. By utilizing a magnetic deflector as illustrated in FIG. 7, the nodal point of the electron lens would be placed at S 3 on the axis of the magnetic deflection system.

FIG. 8 illustrates one embodiment of the present invention in which all three types of magnetic deflectors are utilized. The system of FIG. 8 utilizing magnetic deflectors in conjunction with an electron lens enables the electron lens to be placed very close to the target and provide wide deflection angles in the very short distance available.

An electron .beam entering the deflection systems from the left will be deflected by first magnetic deflector 81 through an angle such that, in conjunction with second magnetic deflector 82, the electron beam is displaced a distance d that is linearly proportional to the input current. Second magnetic deflector 82 realigns the electron beam to a direction that is parallel to the axis of the deflection system. Third magnetic deflector 83 then redirects the beam to the nodal point of electron lens 84. Note in this combination that trailing edge 62 of the second magnetic deflector is coincident with leading edge 71 of the third magnetic deflector. This need not be the case, however. Second and third magnetic deflectors can be separated provided that there are no stray fields to divert the electron beam from its course parallel to the axis of the deflection system.

There has thus been described a magnetic deflection system having a linear response and a wide frequency range. The first magnetic deflector alone can provide a deflection linearly proportional to input current or, with a slightly different shape, in combination with a second magnetic deflector provide a deflection that is linearly proportional to input current and, in addition, reorients the electron beam to a predetermined direction independent of the amount of deflection. In addition, a third shaped pole piece has been described that can be utilized to linearly redirecting parallel electron beams to a predetermined point. In addition, it has been shown how the already high frequency response of a magnetic deflector in accordance with the present invention can be extended through the use of an electrostatic deflection means that is activated in a complementary fashion to the frequency response of the magnetic deflector. In a magnetic deflector built in accordance with the present invention, specifically in accordance with FIG. 3 and in which S 1.31, the measured inductance of the deflector with 1 inch leads was 4 rnicrohenries and the self-resonant frequency was about Mhz. Measurements of deflection linearity indicate that the deflection is a linear function of input current to better than 0.6 percent. For

a magnetic deflection system in accordance with FIG. 1, that is, with rectangular pole pieces, the non-linearity would have been at least 2.4 percent.

Having thus described the invention it will be apparent to those skilled in the art that various modifications can be made within the spirit and scope of the present invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A magnetic deflection system for deflecting an electron beam comprising:

a magnetic core, first and second pole pieces magnetically coupled to said core and a coil of wire wound on said core;

said first and second pole pieces being positioned on opposite sides of said electron beam and in close proximity thereto for controlling the deflection of said electron beam by a magnetic field induced therebetween by current through said coil;

said first and second pole pieces being shaped as a function of the distance from the deflection system to a target to linearize the deflection of said electron beam as a function of current through said coil.

2. A magnetic deflection system as set forth in claim 1 wherein said first and second pole pieces are separated by less than 10 times the diameter of said electron beam.

3. A magnetic deflection system as set forth in claim 1 wherein the shaping of said first and second pole pieces is such that the electron beam exits the region between said pole pieces sooner at greater deflection angles than it would if the pole pieces were rectangular.

4. A magnetic deflection system as set forth in claim 1, wherein the self-resonant frequency of said coil and pole pieces is greater than 50 Mhz.

5. A magnetic deflection system as set forth in claim 1 wherein said first and second pole pieces are shaped in accordance with the curve and generated by the equation:

Y =X +SX+S- [(S 4S+2) X"+(2S S)XS+ 1 1/2 wherein X and Yare coordinates of the pole piece and S is the distance in the X direction in which the beam must be deflected.

6. A magnetic deflection system as set forth in claim 1 further comprising:

electrostatic deflection means positioned between said first and second pole pieces to assist in deflecting said electron beam in the same plane as it is deflected by said first and second pole pieces; said assistance complementing the frequency response of said core, coil and pole pieces to extend the frequency response of said deflection system to higher frequencies.

7. A magnetic deflection system as set forth in claim 6 wherein said electrostatic deflection means comprises conductive films on a non-magnetic substrate positioned between said pole pieces.

8. A magnetic deflection system for deflecting an electron beam comprising:

a plurality of magnetic deflection elements arranged in cascading fashion along the path followed by an undeflected electron beam;

each of said plurality of magnetic deflection elements comprising a magnetic core, first and second pole pieces magnetically coupled to said core and a coil of wire wound on said core;

said first and second pole pieces of each magnetic deflection elements being parallel and positioned on opposite sides of said electron beam and in close proximity thereto for controlling the deflection of said electron beam by a magnetic field induced between each of said first and second pole pieces by current through said coil; each of said first and second pole pieces being shaped as a function of the amount of deflection to linearize the deflection of said beam. 9. A magnetic deflection system as set forth in claim 8 comprising first and second magnetic deflection elements; the first element deflecting said electron beam a predetermined amount, the second element aligning the electron beam to a path parallel to and displaced from the electron beam path

Claims

1. A magnetic deflection system for deflecting an electron beam comprising: a magnetic core, first and second pole pieces magnetically coupled to said core and a coil of wire wound on said core; said first and second pole pieces being positioned on opposite sides of said electron beam and in close proximity thereto for controlling the deflection of said electron beam by a magnetic field induced therebetween by current through said coil; said first and second pole pieces being shaped as a function of the distance from the deflection system to a target to linearize the deflection of said electron beam as a function of current throUgh said coil.

5. A magnetic deflection system as set forth in claim 1 wherein said first and second pole pieces are shaped in accordance with the curve and generated by the equation: Y2 -X2 + SX + S - 1/2 - ((S2 - 4S+2) X2 + (2S2 - S)X-S+ 1/4 )1/2 wherein X and Y are coordinates of the pole piece and S is the distance in the X direction in which the beam must be deflected.

6. A magnetic deflection system as set forth in claim 1 further comprising: electrostatic deflection means positioned between said first and second pole pieces to assist in deflecting said electron beam in the same plane as it is deflected by said first and second pole pieces; said assistance complementing the frequency response of said core, coil and pole pieces to extend the frequency response of said deflection system to higher frequencies.

8. A magnetic deflection system for deflecting an electron beam comprising: a plurality of magnetic deflection elements arranged in cascading fashion along the path followed by an undeflected electron beam; each of said plurality of magnetic deflection elements comprising a magnetic core, first and second pole pieces magnetically coupled to said core and a coil of wire wound on said core; said first and second pole pieces of each magnetic deflection elements being parallel and positioned on opposite sides of said electron beam and in close proximity thereto for controlling the deflection of said electron beam by a magnetic field induced between each of said first and second pole pieces by current through said coil; each of said first and second pole pieces being shaped as a function of the amount of deflection to linearize the deflection of said beam.

9. A magnetic deflection system as set forth in claim 8 comprising first and second magnetic deflection elements; the first element deflecting said electron beam a predetermined amount, the second element aligning the electron beam to a path parallel to and displaced from the electron beam path prior to defection.

10. A magnetic deflection system as set forth in claim 8 comprising first and second magnetic deflection elements, the first element deflecting said electron beam a predetermined amount, the second element focusing the deflected beam at a point over the range of deflections of said first deflection element.