US2876373A

US2876373A - Magnet system for the focusing of electron beams

Info

Publication number: US2876373A
Application number: US630781A
Authority: US
Inventors: Veith Werner Adam; Meyerer Paul
Original assignee: Siemens AG
Current assignee: Siemens and Halske AG; Siemens AG
Priority date: 1956-03-01
Filing date: 1956-12-27
Publication date: 1959-03-03
Anticipated expiration: 1976-03-03
Also published as: NL99653C; FR1164780A; GB853061A; DE1076280B; CH355530A; NL214979A

Description

March 3, 1959 w. A. vElTH ETAL 2,876,373

MAGNET- SYSTEM FOR THE FocusrNG oF ELEcTRoN BEAMS Filed Deo. 27, 1956 6 Sheets-Sheet 1 March 3, 1959 w. A. vl-:ITH ET AL 2,876,373

MAGNET SYSTEM FOR THE FOCUSING OF ELECTRON BEAMS Filed Dec. v27, 1955 s sheets-sheet 2 Fig. 3

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MAGNET SYSTEM FOR THB FOCUSING OF ELECTRON BEAMS y Filed Deo. 27, 1956 6 Sheets-Sheet 3 35 N 33 25 S 2o 27 `22 2s S N l \N s 32 36 2,9 28

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EI.. TRON BEAMS FiledDec. 27. 1956 MAGNET SYSTEM FOR THE FOCUSING 0F ELECTRON BEM/ls Filed Dec. 27, 195e March 3, 1959 w. A. vElTH ET AL 6 Sheets-Sheet 6 Z Z www m V o 9 2 \1. 7 5 V IIIII/ 1 I I I I I I I l s I Il III J1 n K /l N S KS N U 9 A 4 1I I U N ,s RI|IVAOM ,l y M 3 5 5IN 2 [INH l .R/u s Nm@ ZN f s 1. 5 a b 9 9. lll m.. H F F .jadezf/or. Menger J//m C@ United States PatCfO MAGNET SYSTEM Fon THE FocUsING oF ELEcrRoN BEAMs Werner Adam Veith and Paul Meyer-er, Munich, Germany, assignors to Siemens & Halske Aktiengesellschaft, Berlin and Munich, a corporation of Germany The present invention relates to a system of magnets for focusing at least one electron beam, particularly for traveling wave tubes, in which the polarity of the mag- 1 netic poles or pole shoes of the focusing magnet, which poles or shoes are arranged one behind the other in the direction of the electro-n beam, is periodically alternated so as to produce an alternating magnetic eld having a distribution of the magnetic field intensity which extends sinusoidally in the direction of the electron beam.

Upon amplifying or generating very 'short waves in traveling wave tubes, the difficulty occurs, as is known, of conducting the electron beam elo-se by the delay line in order to obtain a good coupling of the electromagnetic wave with .the electrons kand nevertheless prevent an electron impingement on the delay line. For this purpose it is necessary that the electron beam be conducted in focused form along its direction of discharge.

There are already known magnetic kfocusing arrangev ments in connection with which either 'a lhomogeneous or an alternating magnetic field is ,produ-ced along 'the direction of discharge within the discharge vessel. In order to produce a homogeneous magnetic field along the discharge direction, magnets are arranged in star formation, either parallel to the direction of discharge or at the ends of the delay line at right angles to the direction of discharge. By means of these arrangements, a homogeneous magnetic field is produced along the discharge path. In traveling wave tubes, this discharge path is relatively long so that the production of a homogeneous magnetic field requires a large magnetic energy ycontent of the magnets. In order to make this field uniform, themagnet poles adjacent the direction of discharge are connected with soft iron cylinders. This .magnetic shunt requires a further energy content of the magnets so'that the magnets of this arrangement must be 'of very large size. As a result of this the focusing device also becomes bulky and very heavy.

For this reason, use has been made of another focusing 'pole shoes are provided which are yso developed and arranged that a sinusoidal distribution of the field intensity is produced along the axis of the electron beam. This `coaxial arrangement of the magnets with respect to the electron beam has the disadvantage that themagnets cannot be made as sho-rt as desired since in such case the amplitudes of the sinusoidal distribution of the eldjintensity along the axis of the electron bea'm would bejtoossmall, thus limiting 'the intensity ofthe yelectron Patented Mar. 3,

current ofthe electron beam to be conducted in focused form. As a result of this, the wavelength of the electromagnetic wave which is to be amplified or produced has a lower limit in the case of traveling wave tubes or similar veryhigh frequency tubes while there is an upper limit for the amplification o-r the output power, as will be presently explained more in detail.

Therefore, the problem of the invention was to provide a focusing device for traveling wave tubes or the like which assures a good focusing of the electron vbeam even in case of very high electron current densities of the electron beam.

The essential feature of the invention is that the direc,- tions of magnetizationin the magnet ends of the focusing magnets extend substantially at 'right angles through the electron beam or perpendicularly to the direction of the electron beam.

The invention makes it possible, in a particularly advantageous manner, to select the focusing magnets as long as desired in their magnetizing direction by arranging bar magnets in a plurality of planes at right angle to the axis of the discharge system and outside the discharge vessel in such a manner that two magnets are opposite each other in a plane symmetric to the electron beam. ln order to avoid as a result of stray fields unnecessary increase of the flux lrequired to produce the sinusoidal field along the direction of discharge, the magnets which are directed with the south pole facing 'the electron beam are displaced by 90 with respect `to the magnets which have their north pole directed toward the electron beam, in the plane perpendicular to the electron beam. In order to obtain a sinusoidal field distribution along the electron beam when the magnets are arranged at right angles thereto, the poles of the magnets are so connectedwith the pole sho-es that those poles of the magnets which are adjacent to the electron beam are provided with pole shoes shaped in such -a manner that in planes at right angles to the axis of the discharge system similar poles which are adjacent to the electron beam are connected and that the pole shoes have a polarity which alternates along the direction of discharge.

A further feature of the invention is that a plurality of focusingv magnets, `the magnetizing directions of which extend in the magnet poles at right angles to the electron beam 'are so developed and arranged that the lmagnet poles extend parallel 'to each other and parallel to the electron beam. The magnets are in such arrangement of very large width as compared with the yknown bar magnets, this width extending over the entire discharge path, and the height and thickness of the magnets are 'dimensioned so as to obtain the magnetic llux and required field intensity for'the sinusoidal eld necessary in the direction of discharge. If the field intensity requirement of such an arrangement is veryhigh, as is true in particular at Very high frequencies, itis advantageous -to arrange two toroid-like magnets symmetrically to the electron beam in such a manner that two similar magnet poles arealways opposite each other.

In accordance with Na further object and feature, the invention provides an arrangement wherein every two 'magnets are similarly spaced from the electron beam and so located ina plane formed by the longitudinal axis land by any arbitrary straight line perpeudicularto the longitudinal axis that their magnet poles adjacent the electron 'beam have the same polarity. For this purpose, it is advisable from a construction standpoint that in 'two planes-Which are at right angles to each other and the line of intersection of which constitutes the longitudinal axis of the electron beam, thereare always arranged two magnets in such a vmanner that the .similar poles of the anagnetplane of one plane lying .in the .vicinity y,ofthe electron beam are south poles and those of the other plane are north poles.

The arrangement of the magnets parallel to the electron beam makes it in a simple manner possible to connect the south poles adjacent to the electron beam magnetically with each other and the north poles adjacent to the electron beam magnetically with cach other by a plurality of pole shoes, such that a pole shoe which connects the south poles is followed alternately in the direction of discharge by a pole shoe which connects the north poles. The poles of the magnets which are located remote from the electron beam are suitably magnetically connected with thick soft iron plates so that the magnetic impedance in these plates is negligibly small.

A further feature of the invention is that the focusing magnets form an even-numbered polygon around the electron beam in a plane perpendicular thereto and that similar poles adjoin each other at the corners. One advantageous structural development of this arrangement is that the magnets form a square polygon extending in' a plane perpendicular to the electron beam, the center of which forms the axis of the electron beam. In order to produce the sinusoidal field distribution along the electron beam, the similar poles at the corners are connected by pole shoes, the longitudinal axis of the pole shoes extending at right angles through the electron beam.

It is in all arrangements according to the invention advantageous to provide pole shoes of soft iron with a larger cross-section in the vicinity of the magnets than in the vicinity of the electron beam so that the flux requirement necessary for the production of the sinusoidal field distribution can be obtained from the magnets. The pole shoes serving for the magnetic coupling of similar poles are each provided with a bore to receive the discharge vessel, for instance an elongated discharge vessel, in the case of traveling Wave tubes. j

If means must be provided by which the sinusoidal magnetic field is interrupted or extended by homogeneous magnetic steady fields for the coupling or the uncoupling of electromagnetic waves, as in the case of delay lines of traveling wave tubes or in the case of other very high frequency tubes, it is advantageous, for the arrangements in accordance with the invention, for the magnets having the first and last pole shoes respectively to be lengthened at their ends by bar magnets which are disposed at right angles to the longitudinal axis of the discharge vessel and to provide bar magnets also located at right angles to the longitudinal axis of the discharge vessel at a distance away from the ends of the magnet necessary for the coupling and decoupling respectively. The bar magnets may for this purpose be provided near the wall of the vessel with an annular pole shoe for connecting similar poles. The bar magnets are suitably of such dimensions and so connected with soft iron plates with their poles located away from the electron beam that a magnetic steady field is formed between the annular pole shoe and the first and last pole shoes, respectively, of the magneticvsystem in the direction of the longitudinal axis of the discharge system.

The invention will now be explained in further detail with reference to the drawings wherein Figs. 1a and lb show the construction and distribution of the field intensity of the known arrangement; and

Figs. 2 to 9 show in greatly simplified and in part schematic presentation various embodiments in the parts thereof which are essential for the invention.

Identical parts are identically referenced throughout the drawings.

In Fig. la there is shown a part of the known, previously mentioned magnetic focusing device for traveling wave tubes in which the intensity of the field strength in zdirection alternates and is approximately sinusoidal. Coaxial to the z-direction, which at the same time represents the longitudinal axis of the electron beam, annular permananent magnets 1 are serially so arranged that similar poles are adjacent to each other. Between the similar poles, there are provided annular pole shoes 2 having cylindrical rings 3 near the discharge vessel 4. Within the discharge vessel and coaxial to the z-direction there is arranged a helix 5 through which the electron beam is projected. Magnetic lines of flux 6 extend from the cylindrical parts 3 of the pole shoes. The direction of the magnetic lines of flux 6 depends on the polarity of the pole shoes and is shown by arrows.

In Fig. 1b, the field intensity H which occurs along the z-axis of the discharge system of Fig. la is plotted in the direction of the z-axis. The solid sinusoidal curve 7 represents the theoretical field distribution which is necessaryv for the dependable focusing of the electron beam. The dotted line 8 shows approximately the actual field distribution of the arrangement of Fig. 1a produced by the fact that the annular magnets 1 cannot be made magnetically so identical that the amplitudes of the sinusoidal curve of the field intensity are equal.

In order to increase the frequency to be produced or amplified in case of a traveling wave tube shown in Fig. l, it is necessary to reduce the diameter of the helix so that the coupling with the electron beam of the electromagnetic wave conducted over the helix is as great as possible. It is for this purpose for the same initial power or the same amplification necessary to reduce also the cross-section of the electron beam so that the electron current density becomes muc-h greater. An increase in the electron current density, however, causes the repulsion forces of the electrons to become greater so that a greater field-strength amplitude of the sinusoidal field strength distribution in accordance with Fig. 1b is necessary for the focused passage of the electron beam. Since, however, the field strength of a magnet depends on its length, it is not possible in accordance with Fig. 1a to increase the fieldA intensity if it is desired to go over to a higher frequency. Furthermore, with a higher frequency of the wave conducted over the helix, the plasma wavelength becomes smaller. The length L of the sinusoidal wave of the field intensity, however, is in such relationship to the plasma Wavelength that with a decrease in the plasmal wavelength, the length L of the sinusoidal wave of the field intensity must also become smaller.

The arrangement in Fig. la has as a result of the two reasons explained above a limit wavelength of the electromagnetic wave to be amplified.

It will be seen that the greatest magnetic field occurs between the small spaces d of the cylindrical rings of the pole shoes 2 and that the sinusoidal field required on the z-axis represents a stray field. The size of this stray field depends predominantly on the values l, R and d indicated in Fig. la. l is the distance between the pole shoes 2 which is. determinative for the stray-field shunt outside the discharge vessel; d determines the loss produced by the strong field between the cylindrical rings 3; and R is the radius of the cylindrical ring 3 on which the size of the field-strength distribution on the z-axis depends. It is apparent that with a decrease in l, the field strength on the z-axis also becomes less since the stray-field shunt requires a higher magnetic energy content of the magnet. With a decrease in d, the main field also becomes greater so that still more magnetic energy is withdrawn from the magnetic field on the z-axis. Balancing with R would then only be possible if the radius of the discharge vessel 4 (Fig. la) is made very small. However, this can be done only to the extent permitted by precision mechanics in order that the tolerances, which must be very closely met in the case of traveling wave tubes, do not become too large. These simple considerations show that the possibilities of the known arrangement shown in Fig. 1a are very limited and that this arrangement cannot be used for very short electro-magnetic waves.

Fig. 2 shows schematically an arrangement of bar magnets which makes it possible to produce the sinusoidal kfield along the z-axis of the electron beam, in which connection the length L of one cycle of the sinusoidal magnetic field can be dimensioned in accordance with the above conditions corresponding to the maximum frequencies occurring in traveling wave tubes. The

magnets

9 and 10 are arranged horizontal and symmetric to the electron beam, the north poles adjoining the pole shoe 11. The pole shoe 11 is provided with a borehole 12 to receive the discharge vessel such as 4 in Fig. 1a. At a right angle to the arrangement of the

magnets

9 and 10, the

magnets

13 and 14 are arranged vertically and also symmetrically opposite each other with respect to the axis z of the electron beam. In the case of this pair of

magnets

13, 14, in contradistinction to the first pair of

magnets

9, 10, the south poles are connected Yby the pole shoe 1S so that a magnetic field is produced between the poles shoes 11 and 15. The pole shoe 15 also has a bore-hole .12 to receive the discharge vessel such as the vessel 4 in Fig. la. In the direction of the axis z of the electron beam, there then again follows a magnet arrangement with north poles adjoining the pole shoes.

It will be seen that the system of magnets shown in Fig. 2, in contradistinction to that shown in Fig. l, has a greater degree of freedom with respect to the dimensioning of the magnets. The length of the magnets, which is determinative with respect to the field strength, can be selected as large as desired. Furthermore, the arrangement of the magnets at right angles has the advantage that the. magnetic stray fields outside the field of action of the pole shoes are very small.

Fig. 3 shows the construction of the system of magnets indicated in Fig. 2. The

vertical magnets

13, 14 are followed by the

horizontal magnets

9, 10. They are then followed by a pair of vertical magnets and then again by a pair of horizontal ones, and so forth, until the entire system of magnets extends over the discharge path of the electron beam. The outside poles of the bar magnets are connected

withsoft iron plates

16 and 17. The soft iron plates 16 connect all similar poles to each other, while the soft iron plates 17 serve to connect the dissimilar poles. The soft iron plates are suitably of such size that the magnetic resistance is negligibly small.

If one assumes that the

vertical magnets

13, 14 of Fig. 3 become progressively thicker in z-direction, the magnets abut against each other without the production of the sinusoidal field distribution on the z-axis being disturbed. If the places of abutment are allowed to pass into one another, there is produced the arrangement of vertical magnets shown in Fig. 4. The

magnets

18 and 19 have a direction of magnetization which is perpendicular to the z-axis. The height of these magnets is small as compared with their width. The south poles of

magnets

18 and 19 are connected by trapezoidal pole shoes 20 and 21 with the rectangular pole shoe 22. The pole shoes 20, 21 and 22 are advantageously made of soft iron so that as high a density of the magnetic lines of force emerging from the pole shoes 22 as desired is possible. The cross-section 23 in the vicinity of the south pole of magnet 1S is selected larger than the cross-section 24 so that the flux requirement determined by the cross-section can be obtained as large as possible from the magnet. The pole shoes 22 are also provided with boreholes 25 to receive the discharge vessel such as 4 shown in Fig. la.

Fig. is a front View of the arrangement of the magnetic system shown in part in Fig. 4 and Fig. 6 is a perspective view, partially in section of the arrangement shown in Fig. 5. The

vertical magnets

18, 19, as already shown in Fig. 4, are connected with

pole shoes

20, 21 and 22 which are arranged one behind the other in the z-direction. Perpendicular to this pair of

magnets

18 and 19 there are arranged parallel to each other two

magnets

26 and 27 which are likewise connected with

pole shoes

28, 29 and 30. The polarity of the' vertical pole shoe 22 must be opposed to the polarity of the horizontal pole shoe 30. The pole shoes 22 and 30 alternate in z-direction so that an alternating magnetic field is produced. In order to obtain a purely sinusoidal distribution of the field strength along the z-axis, the thickness of the pole shoes 22 and 30 and the distance from pole shoe 22 to pole shoe 30 must be in a given relationship to each other. The most favorable relationship can easily be found by experiment. As a result of the

elongated magnets

18, 19, 26, 27 and the alternation in zdirection of

pole shoes

22 and 30, it has become possible to shape the distribution of the field strength on the z-axis sinusoi-dally in the manner shown by curve 7 in Fig. lb. Any possible places of interference in

magnets

18, 19,26 and 27 are counteracted by the elongated form. The outside poles of the

elongated magnets

18, 19, 26 and 27 are magnetically short-circuited by means of the soft iron plates 31 to 38.

In Fig. 7 there is shown a variation of the arrangement of the magnets shown in Fig. 6. This arrangement is particularly advantageous for very high frequency tubes, due to the fact that the field intensity can be greatly increased by means of the toroid-

like magnets

39 and 40. As already stated above, the field intensity of a magnet depends on its length. Due to the toroidal shape, a possibility is afforded of increasing the field strength as much as desired so that this magnetic system can be used for extremely high frequencies. The arrangement of the pole shoes is the same as already described in connection with Fig. 6.

A'possibility of simultaneously varying the field intensity and the iiux requirement and therefore, for all practical purposes, the energy content of the magnets within wide limits, is shown in Fig. 8. The

magnets

45, 46, 47 and 48 are so arranged symmetrical to the z-axis of the electron beam that they form the sides of a square around the electron beam with similar poles always abutting against each other at the corners of the square. The result is that the opposite corners of the square have similar polarity. The corners of similar polarity are connected horizontally with the pole shoes 43 and 44 by way of the central pole shoe 30. In vertical direction, the pole shoes 41 and 42 are connected by way of the central pole shoe 22.

The arrangement of the focusing magnets according to Fig. 8 represents a particularly favorable embodiment with respect to the manner lof manufacture and the dimensioning of the field intensity distribution obtained on the z-axis. Experimental results have given for this embodiment extremely good approximations to the theoretical sinusoidal curve of the field strength distribution on the z-axis.

For the coupling or uncoupling of the electromagnetic wave in the case of very high frequency tubes it is necessary to create a magnetic steady field for the coupling or uncoupling space. For the arrangement in accordance with the invention, this steady field is advantageously produced at the end and at the beginning of the magnet systern. Fig. 9a shows an example of how, in the case of the arrangement shown in Fig. 6, a steady field is produced at the beginning of the magnet system.

Referring now to Fig. 9a, on the

soft iron plates

34 and 38 of the

magnets

18 and 19 which have the first pole shoe 22, the two

bar magnets

49 and 50 are arranged at right angles` to the electron beam. The direction of magnetization of the

bar magnets

49 and 50 is advantageously selected to be the same as the directions of magnetization of the

magnets

18 and 19, since the length of the steady field is relatively large and the field intensity must be increased. At a distance from the end of the magnet system required for the coupling or uncoupling

bar magnets

51 and 52 are also arranged at a right angle to the electron beam, these magnets being 'magnetically so oriented that the field intensity of the steady field is further jincreased. The bar magnets S0 and 52 are com nected with a soft iron yoke 54. The

bar magnets

49 and 51 are magnetically coupled in exactly the same manner with a soft iron yoke 53. The bar magnets 5l and 52 advantageously have an annular soft iron pole shoe 56 which together with the pole shoe 22 produces a homogeneous steady tield.

In Fig. 9b, the eld intensity is plotted over the z-axis of the electron beam. The sinusoidal curve 57 represents the magnetic field strength in the

magnet system

18, 19. Curve 57 shows the magnetic field intensities of the steady eld in the coupling space. This field intensity of the coupling space is suitably selected in the order of magnitude of the effective value of the sinusoidal wave of the field intensity of the magnet system in order to reduce the initial ripple of the electron beam.

The present invention is not only applicable to traveling-wave tubes or the like, but can also be employed advantageously whenever it is desired to conduct electron beams in focused form over a relatively long path, and the term travelling wave tube as used in the claims therefore is to be interpreted with sensible latitude as including dilerent structures to which the invention may be applied. In addition to the embodiments shown by way of example in the figures, the invention may also be applied to three, four or six-wing magnet systems.

Changes may be made within the scope and spirit of the appended claims.

We claim:

1. A magnet system for focusing at least one electron beam in connection with a travelling wave tube and the like, comprising a plurality of focusing magnets disposed serially in a direction paralleling the direction of propagation of the electron beam and surrounding the electron beam for the extent of the focusing path, the directions of the lines of force extending in said magnets perpendicular to the direction of propagation of said electron beam, pole pieces respectively cooperating with said magnets being similarly serially disposed and alternately magnetically interconnected with identical poles thereof, whereby an alternating magnetic field is produced creating along the beam axis a substantially sinusoidal distribution of the magnetic eld intensity, said focusing magnets torming along planes extending perpendicular to the electron beam structures exhibiting substantially rectangular con; iguration surrounding the electron beam, identical poles of said magnets being interconnected by the respective pole pieces cooperating therewith.

2. A magnet system according to claim l, wherein the respective magnet poles extend parallel to one another and parallel to the electron beam.

3. A magnet system according to claim l, wherein said magnets form a structure exhibiting a square configuration in a plane extending perpendicular to the electron beam, said beam passing centrally through said magnets.

4. A magnet system according to claim 2, wherein said magnets form a structure exhibiting a square configuration in a plane extending perpendicular to the electron beam, said beam passing centrally through said magnets.

5. A magnet system according to claim 2, wherein the longitudinal axes of said pole pieces extend in directions perpendicular to the direction of the electron beam.

6. A magnet system according to claim 3, wherein said pole pieces extend inwardly from the respective corners ot said square structure, the longitudinal axes of said pole pieces extending in directions perpendicular to the direction of the electron beam.

7. A magnet system according to claim 5, wherein said pole pieces are made of soft iron exhibiting a cross-section adjacent the corresponding magnet poles which exceeds the cross-section thereof in the neighborhood of the electron beam.

8. A magnet system according to claim 6, wherein said pole pieces are made of soft iron exhibiting a cross-section adjacent the corresponding magnet poles which exceeds the cross-section thereof in the neighborhood of the electron beam` References Cited in the tile of this patent UNTED STATES PATENTS 2,102,045 Thomas Dec. 14, 1937 2,157,182 Maloi May 9, 1939 2,730,678 Dohler Jan. l0, 1956 2,801,361 Pierce July 30, 1957 2,804,548 Ruska Aug. 27, 1957 2,812,470 Cook NOV. 7, 1957 2,847,607 Pierce Aug. 12, 1958