US2721949A - Betatron - Google Patents

Description

Oct. 25, 1955 K. GUND ET AL 2,721,949

BETATRON Filed oct. 28, 195o 2 sheets-sneer 1 Zi-067g 0215 ,Kzraaf @zaza/( 07ans erger Oct. 25, 1955 K. GUND ET AL 2,721,949

BETATRON Filed oet. 2s, 195o 2 sheets-sneu 2 United States Patent BETATRON Konrad Gund and Hans Berger, Erlangen, Germany Application October 28, 1950, Serial No. 192,731

Claims priority, application Germany October 31, 1949 6 Claims. (Cl. 313-62) This invention is concerned with a betatron in which the electrons are injected as a narrow beam about azi muthally into the stabilizing guiding field region close to its inner border.

The term betatron refers to a device in which electrons are moved along approximately circular paths in a r magnetic guiding field and accelerated by means of a magnetic flux of increasing strength passing through said circular paths. The device may therefore also be referred to as a magnetic induction accelerator.

In a betatron or magnetic induction accelerator of this type there is available for the injector only a very limited space at the inner border of the guiding field, For this reason it has not been possible heretofore to employ large pre-acceleration voltages which require relatively large dimensions of the injector. Also with regard to accessibility, heat removal and voltage supply leads, it is a disadvantage to dispose the injector at the inner border of the guiding field, especially in the case of sealed vacuum vessels.

These difliculties are avoided by the invention, by disposing the injector at the outer border of the stabilizing guiding field region and selecting its pre-acceleration voltage so that the electrons enter from the injector into the outer stabilizing guiding field region with a velocity which corresponds, at the guiding field intensity operating during the injection, to the gyratory velocity of the electrons on alpath close to the inner guiding field border, and by providing in the vicinity of this path a system for the deflecttion of the electrons, about azimuthally in relation to the guiding field. The electrons which leave the acceleration system of the injector in the form of a narrow beam travel first transversely across the guiding field and, as far as they enter the deflection system, they are ejected therefrom, like from an inner injector, into the inner stabilizing guiding field region.

The injector may be spaced from the outer border of the guiding field so that it does not obstruct expansion of the equilibrium orbit after completion of the `acceleration for the purpose of casting the electrons into the outer space. The injector may therefore be made of any desired and suitable size. There is sufficient space available for accommodating the means for supplying higher pre* acceleration voltages. Nevertheless, the electrons are caught in the inner stabilizing guiding field region, which is advantageous because in such case, by maintaining the guiding field constant during the period of securing the electrons, the acceleration vessel can be uniformly filled with electrons, making it possible to obtain an increase of the space charge.

It is advisable to maintain the guiding eld during the injection constant, at a field intensity which directs the entire electron beam emanating fromthe outer injector into the deflection system. This may be accomplished as disclosed in co-pending application Ser. No. 192,832 filed October 28, 1950, e. g., by means ot' passing through a coil surrounding only the accelerating field, immediately before the beginning of the injection and for the duration thereof, a current which is variable as a function of time and of such intensity, character, and direction that the entire magnetic field within such coil changes as a function of time so that, as a reaction, the guiding eld remains constant, such reaction being based on the fact that the entire field of the betatron or accelerator, being fed from an electric oscillating circuit, must have a sinusoidal character. It is also possible to employ for the same purpose a core made of a magnetic material with substantially rectangular magnetization curve, said core being disposed axially of the betatron and bridging the air gap between the accelerating poles. This core is magnetically saturated in every half-wave 0f the current source, in one or another direction, due to strong magnetization caused by the current in the exciter winding of the betatron and in its state of saturation does not disturb the relation of the betatron fields. At each passage of the exciter current 4through zero, however, a reversal of field direction in the core takes place after a relatively low current intensity has been reached. The rapid change of flux during the reversal of the magnetic field direction causes in the exciter winding the induction of an electromotive force which obstructs the increase of the exciter current and the result is that this current increases again only after the magnetic saturation in the core has again been restored. However, it is also possible to introduce a large portion of the electrons into the deflection system without maintaining the guiding field absolutely constant, provided care is taken that the electrons leave the injector somewhat divergently, with the result that, while the guiding field is changed by a few percent, a large number of electrons will always reach the deflector at the proper angle. lt is true, that the electron current emanating from the electron source must in such a case be of greater intensity than if the guiding field is maintained constant. It is, however, easy to meet such requirement because the electron current is of a magnitude on the order of a few milliamperes.

The deflection system of course must not obstruct the electrons which already have been caught and revolve near the inner border of the guiding field. The system therefore provides a generally S-shaped deflection condenser built so that at least its ejection opening projects into the inner stabilizing guiding eld region and its entrance opening is preferably placed radially more toward the inside than the ejection opening.

The above-noted and additional objects and features of the invention will be presently explained with reference to the accompanying drawings. These drawings show, by way of example, an embodiment of the invention comprising a vacuum vessel of a betatron for a final velocity of 6 in. e. v. with an outside injector the injection voltage of which is 60 lcv.

Fig. 1 shows a sectional view taken approximately along the dot-dash line 1-1 of Fig. 2;

Fig. 2 is a sectional plan view of the vacuum vessel shown in Fig. .1, illustrating the path of travel of the electrons.

.Figs 3 toV 6 indicate in diagrammatic manner sectional views of means for controlling the flow of electrons.

The generally toroidal ceramic vacuum Vessel 1 with the sealing stub 2 and the electron exit stub 3 closed by a thin metal window is provided with the tubular extension@ closed by the ceramic plate 5, containing the injector which isthus disposed completely outside of the outer boundary of the stabilizing zone 6 of the guiding field. The injector comprises a filament 7, the electrode S surrounding it, and the electrodes 9 and 10 forming a sofcalled electrostatic immersion lens. The-action of lthe electrodes 9 and 1f) produces at a point spaced from the filament 7 an enlarged image of the filament, and the electron beam leaving the filament 7 will therefore be of a divergent form. Accordingly, the electrons emerge from the injector in the form of a slightly divergent beam which enters into the outer stabilizing region of the guiding field. The electrodes 9 and 10 are connected relative to the filament 7 at voltages of about +2.5 kv. and +7 kv. The electrode 8 is connected to about the same potential as the filament only twice during every cycle of the betatron field and only for a short time, namely, about 10-4 seconds, whereas during the remaining time it is connected to a negative locking potential. Ahead of the electrode 10 is disposed an acceleration electrode 11, to which is connected, e. g., a voltage of +60 kv. Ahead of the ejection opening of this injector with a pre-acceleration system is disposed a deflection condenser consisting of the two plates 12 and 13, with the aid of which the direction of the electron beam can be easily adjusted. The electrodes 9, 10 and 11, and also the plates 12, 13 are suitably made of soft iron, so that they magnetically shield the electron beam from the leakage field of the betatron, at least while it is still weak. On the plates 12, 13 of the deiiection condenser voltages of a few hundred volts are sufficient to adjust the electron beam in the desired direction. The electrode 13 of the deliection condenser is connected with the acceleration electrode 11, and lies therewith by way of the Contact spring 14 at the grounded inner coating 15.

It shall now be assumed that the electrons in the injector, by applying the proper voltage at its acceleration electrodes, are given a velocity which, at the guiding field intensity operating during the injection, corresponds to the gyratory velocity of the electrons along the path 16 close to the inner boundary circle 17, and that the guiding field is maintained constant during the injection. If such conditions prevail, the electrons traverse the guiding field approximately in the manner shown in Fig. 2, and enter the entrance opening of the defiection condenser 1.8 shortly before reaching the inner boundary circle 17.

The deflection condenser 18 comprises the grounded, generally S-shaped outer condenser plate 20 facing the equilibrium orbit 19, and the two inner arcuately shaped counterplates 21 and 22 connected against ground to voltages of about -10 kv. and +10 kv., the average width of the condenser being about l mm. At its entrace is provided a small electrode 23, the purpose of which is to adjust, if necessary, the direction in which the electron beam enters the deflector. The voltages at the electrodes 21-23 are suitably chosen so that the electron beam, while the guiding field is being maintained constant, travels through the deflector without great losses at the electrodes, and leaves it azimuthally.

There are provided, near the entrance of the deflection condenser 18, pocket-shaped trapping cages 24 and 25 for the purpose of avoiding disturbances of the secured electrons by secondary electrons which are released by the electron beam at the electrode 20 or at the inner wall of the vacuum vessel during the process of securing, or shortly thereafter.

The injection voltage and the guiding field intensity, which is maintained constant, are suitably brought into such a reciprocal relationship that the electrons leaving the deflector undergo relatively slight radial oscillations in order to uniformly fill the guiding field with electrons. Moreover, it is advantageous to give the guiding field in the area of the first revolutions or oscillations such a radial dependency that the electrons make about two revolutions for every radial oscillation, thereby creating especially favorable conditions which are operative to avoid hitting of the defiector by the electrons.

The further acceleration of the electrons and also the ejection of the rapid electrons, e. g., with the aid of a so-called disrupting coil, is not hindered by the injector because the latter is positioned outside of the boundary circle 6, in the vicinity of which the electron paths become unstable. It is also easily possible to provide a deflection condenser 26, which defiects the rapid electrons outwardly in the form of a narrow beam passing through the exit stub 3.

The filament 7 and the electrodes 8 and 9 are adjustably supported by the molybdenum pins 27 which are sealed in the ceramic end plate 5 and serve at the same time as voltage supply terminals. Two brackets 2B and 29 of ceramic material, which are screwed to the end plate 5, serve to support the electrodes 10 and 11 and the defiector 18. The acceleration system and the detiector thus form a structural unit which can be tested, before being introduced into the vacuum vessel, in an auxiliary vessel which must be placed in a stationary magnetic field analogous to the guiding field of the betatron. After the testing, the system is inserted into the vacuum vessel 1 and connected therewith by means of a vacuum-tight seal 30. The vessel is thereupon evacuated by way of the pump stub 2, then it is heated and the metal part of the injector is annealed by high frequency.

The brackets 28 and 29 are metallized within the guiding field at the sides facing the electron beam, and the corresponding metal coatings are connected with the metallic inside lining 15 of the vacuum vessel. The brackets carry on their outside narrow silver strips forming the leads for the electrodes 21-23 of the deficctor. The leads are insulated by means of mica against the metallic inside lining of the vacuum vessel and are metallically connected with the bushings 31-33.

It is possible, by suitably dimensioning the defiector 18, to obtain not only the defiection of the electrons which have traversed the guiding field, but also a constriction of the electron beam and therewith greater security against subsequent collision of the electrons with the defiector after one or more revolutions. For, if the width of the deflector is small in relation to its length, all electrons of the beam gather in its first section in a focal point after the beam has traveled in this section along a circular arc across an angle a1 of 63.7. By proper selection of the voltage at the electrode 21, it is possible to place this point in the middle between the plates 20 and 21 of the first section. If the first section of the defiector is of such length that the focal point, as can be seen in Fig. 3, occurs at the end of this section, and if the second deflector section-which must be curved more than the first section in order to deflect the electrons azimuthally in relation to the guiding fieldis formed so that it has the same proportions as the first section, in other words, if the second section has also a length extending across an angle a2 of 63.7, then the electrons form in the second section a beam which represents a reduced centro-symmetric replica of the beam in the first section.

The above-noted values for the angle a (63.7) are true provided that no magnetic field exists in the defiector. However, the magnetic field of the betatron is also effective in the deflector. Therefore, the deliection voltage in the first section of the condenser must be made larger, and in the second condenser section it must be made smaller than if the arrangement were free of a magnetic field, so as to eliminate the effect upon the arcuate path resulting from the magnetic field. Assuming a homogeneous magnetic field intensity in the defiector, there will result, depending on the field intensity of the guiding field, a certain reduction of the angle a1 and an increase of the angle az, these values increasing in accordance with the intensity of the magnetic field. The electron beam does not leave the defiector any longer in parallel to the entering beam, but at an angle =a2-oci, as shown in Fig. 4. The angle is the larger, the greater the intiuence of the magnetic field compared to that of the electric fields of the defiector.

However, it is also possible to proceed as shown in Fig. 5, by making the angles ai and az of the same magnitude and placing the focal point not at the juncture of the two condenser parts, but in the first condenser part. Of course, in such a case there will not be obtained a reduction ratio for the width of the beam which corresponds to the ratio of the radii of the respective condenser parts.

It is advantageous for the insulation of the leads to build the deflector so that its positive and its negative deection voltages are identical. In order to obtain in such a case, under the conditions prevailing in a betatron, a beam of parallel and strongly concentrated electronic rays in azimuthal direction from the deilector, it is possible to proceed, according to Fig. 6, so that the first condenser part of the length ai and the bending radius r1 is extended beyond the focal point of the electrons with a restricted cross-section, and more heavily bent in the restricted portion, as indicated in Fig. 6 by the bending radius r2 and the angle u2.

The accompanying claims defined what is considered new and desired to have protected by Letters Patent of the United States.

We claim:

1. A betatron having a generally circular evacuated vessel and apparatus for accelerating and stabilizing electrons describing within said vessel an equilibrium orbital path, said apparatus comprising a device for producing electrons and introducing said electrons into said vessel, said device comprising an electron gun disposed outside said equilibrium orbital path and forming an electron source and accelerator, and an electron deilector within said equilibrium orbital path forming with said gun an electron path intersecting said orbital path, said deilector comprising opposed electrodes, one of said electrodes having a surface opposed to the perimeter of said vessel.

2. A betatron according to claim l, wherein said electrode having a surface opposed to the perimeter of said vessel is generally S-shaped providing a concave portion proximate to said accelerator and a convex portion remote therefrom as viewed in the direction of electron flow, said other opposed electrode having concave and convex portions opposed to those of said S-shaped electrode and also being generally S-shaped and comprising two electrically separated arcuate portions.

3. A betatron according to claim 2, wherein said concave portions are of greater length than said convex portions, and the spacing between said concave portions exceeds that of said convex portions.

4. A betatron according to claim 2, comprising an auxiliary electrode adjacent to but electrically separated from the portion of said electrode disposed nearest said accelerator.

5. A betatron according to claim 2, comprising an electron trap disposed at that end of each of said S- shaped electrodes proximate to said accelerator.

6. A betatron according to claim 1, comprising common supporting means mounting said electron gun, ac# celerator and deflector as a unit, said supporting means having opposed metallized surfaces, the inner walls of said vessel having a metallized coating, and means electrically connecting said metallized surfaces and coating.

References Eited inthe le of this patent UNlTED STATES PATENTS 2,193,602 Penney Mar. 12, 1940 2,528,541 Pajes et al Nov. 7, 1950 2,533,859 Wideroe Dec. 12, 1950

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