US2819393A - Magnetic induction type electron accelerator - Google Patents

Description

Jan. 7, 1958 R; WIDERUE 2,819,393

MAGNETIC mnuc'rrou TYPE ELECTRON ACCELERATOR Filed April 1, 1953 2 Sheets-Sheet 1 l f i a A INVENTOR BY M JWNXQPJQW ATTORNEYS Jan. 7, 1958 R. WIDERGE 2,819,393

' MAGNETIC moucnou TYPE ELECTRON ACCELERATOR Filed April 1. 195a 2 Sheets-Sheet 2 V INVENTOR BY JJI MM MQM W ATTORNEYS United States Patent "ice MAGNETIC INDUCTION TYPE ELECTRON ACCELERATOR Rolf Wideriie, Ennetbaden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint stock company Application April 1, 1953, Serial No. 346,123

Claims priority, application Switzerland April 3, 1952 Claims. (Cl. 250-27) In an induction accelerator, the electrons moving in the evacuated acceleration tube are led away from the circular path traveled during the acceleration by an enlargement or reduction of the equilibrium circle (expansion or contraction) always at the end of the acceleration period. By this means the electrons can be directed for example toward an anticathode disposed in the interior of the acceleration tube near the circular path, where they produce X-rays, or they can be led through the tube wall to the outside and leave the accelerator for other uses. Expansion and contraction are usually effected by variation of the ratio between the flux accelerating the electrons and the control flux maintaining the electrons in their orbit and occurring between the circular so-called control poles. These two fluxes can be produced by a single exciter winding fed from the alternating current network. To vary the ratio, use can be made of saturation phenomena in the iron parts of the accelerator; or special coils may be provided which when they carry current vary at least one of said fluxes. For example, a connection for the generation of expansion impulses is known from Swiss Patent No. 260,195 which contains a coil that embraces the acceleration flux and in which, through the discharge of a condenser, current impulses are produced which effect an enlargement of the electron path. The recharging of the condenser occurs by the same coil as a result of the voltage induced in it by the acceleration flux. This connection is suitable also for accelerators in which electrons are accelerated alternately in opposite directions in consecutive half-periods of the alternating current supply network (duo-radiation accelerator), because the direction of the current impulses alternates also.

It may now be desirable to employ for the utilization of the energy of the accelerated electrons selectively an expansion or a contraction, either so as to produce selectively electron rays or X-rays, or in order selectively to bombard one of two anticathodes, one of which being arranged within and the other without the acceleration circle inside the acceleration tube.

The present invention relates to a device for the selective use of expansion or contraction, requiring a minimum of means. The device of the invention is characterized by a coil which embraces the acceleration flux, two thyratrons connected in parallel with opposite polarity, means for the alternate supply of positive voltage impulses to the control grid of the thyratrons each time the network voltage passes through zero, a condenser connected in series with the two thyratrons, and a reversing switch which connects the assembly formed by the condenser and the thyratrons, in one position of the switch, with the coil, and in the other position, with the series connection of the coil and exciter winding, this series connection being poled in such a way that the voltages connected at the coil and exciter winding are opposed.

The embodiment of the invention to be described is illustrated in the accompanying drawings in which Fig. 1 is an electrical schematic diagram showing the essential components and their circuit relationship. Figs. 2-4 are graphs picturizing operation of the circuit in a sequential manner.

With reference now to the drawings the exciter winding 1 for the accelerator is fed from the alternating current network indicated by symbol with the voltage U connected in parallel thereto is a condenser 5 for the improvement of the output factor. The coil 2 serves for the variation of the ratio between acceleration flux and control flux. It embraces the acceleration flux alone, and hence is disposed spatially within the control poles of the accelerator. In this coil an alternating voltage U; is induced by the acceleration flux. A condenser 4 is connected in series with two thyratrons 6 and 7 connected in parallel with opposite polarity i. e. in front-to-back relation. The arrangement 8 includes means which alternately supply to the control grids of the two thyratrons positive impulses, and impulses which are in locked phase relation with the network voltage. These impulses begin approximately with each passage of the network voltage through zero and last somewhat longer than one quarterperiod. In its one position (position e), the reversing switch 9 connects the series connection of the condenser 4 and the thyratrons with the ends of coil 2, in its other position (position k) with the ends of the series connection of coil 2 and exciter winding 1. The latter parts are connected in opposing directions, that is, the voltages U and U connected to them have opposite directions. For this purpose, the end a of coil 2 is connected with the co-phased end I: of the exciter winding 1. Simultaneously with the reversal effected by the reversing switch 9, the phases of the positive voltage impulses supplied to the control grids of the thyratrons are changed by 180 in relation to the network voltage by the reversing switch 10. The mode of operation of the device of the invention will be described with reference to Figs. 24, which show operation of the circuit in chronological sequence.

Curve U of Fig. 2 represents the course of the feeding network voltage, referred to point b of Fig. 1. Let it be noted that the electrons reach their maximum energy when the acceleration flux is maximum, that is, when the network voltage passes through zero. In that instant the electrons must be removed from the circular path by a current impulse flowing in coil 2. The arrows marked 1 I and 11, respectively, on the time axis of Fig. 2 indicate the temporal position of the acceleration processes in a duo-radiation accelerator; the removal of the electrons from the circular path always occurs at the tips of the arrows.

Curve U of Fig. 2 represents the course of the alternating voltage induced by the acceleration flux in coil 2, referred to point e of Fig. 1. For reasons which will be stated later, the amplitude of this alternating voltage is half the amplitude of the network voltage.

Fig. 3 shows the operations occurring during expansion. It is pre-supposed, therefore, that reversing switch 9 (Fig. 1) is in position e. Curve U again represents the course of the alternating voltage induced in coil 2 and curve U the course of the voltage connected at condenser 4, also referred to point c of Pig. 1. Shortly before the end of the first entered half-period, the condenser 4 is charged on the positive peak of voltage U In that instant of the expansion, that is, near the passage of the network voltage through zero, the discharge of the condenser 4 via coil 2 is made possible again by ignition of the thyratron. Since the circuit formed by these parts causes only small energy losses, discharge occurs in the form of a slightly damped vibration, however, only a half-period is passed through, as the thyratron permits flux only in one direction. But for elucidation, the dotted line in Fig. 3 shows some periods of the slightly damped vibration. (In most cases its frequency is much higher inv comparison to the network frequency than the figure indicates, for reasons of drafting.) Hence, shortly after the passage through zero of the network voltage, the voltage U; at the condenser 4 remains at a negative value, which is almost as great as the productive value prevailing before the expansion. The recharging process of the condenser 4 brings about the occurrence of a current impulse in coil 2 which removes the electrons outward from their circular path. The said negative value of voltage U, is now reached by voltage U shortly before its negative peak. As the thyratron 6 is still unlockedthe times of unlocking of thyratrons 6 and 7 are marked by thick lines on the axes 6 and 7 respectively of Fig. 3condenser 4 is then charged to the negative peak of voltage U Then thyratron 6 is locked again.

During the next passage through zero of the network voltage there follows the expansion for the electrons accelerated in the other direction, in that by ignition of the thyratron 7 the slightly damped vibration circuit consisting of condenser 4 and coil 2 again carries out a free vibration of the duration of a half period.

The described processes repeat periodically. The current impulses occurring in coil 2 are entered along the axis i of Fig. 3. Fig. 4 shows the processes occurring during contraction. it is presupposed, therefore, that the reversing switch 9 (Fig. l) is in position k and that at the same time the phase position of the positive grid voltage impulses supplied to the thyratrons 6 and 7 has been changed by 180 by the reversing switch 10. Compare lines 6 and 7 of Fig. 4 with the corresponding lines of Fig. 3. Curve U of Fig. 4 represents the course of the voltage connected to the series connection of the thyratron and condenser 4, again referred to point c of Fig. 1. According to the invention, voltage U is obtained, in that the exciter winding 1 and the coil 2 are connected in series in phase opposition. In order that the voltage U effective on contraction, will be of the ame magnitude as the voltage U effective during expansion, coil 2 should be so dimensioned that the alternating voltage induced in it by the acceleration flux has half the magnitude of the network voltage. If it should be desirable that the impulses for contraction and expansion differ in amplitude, this can be taken into account by a variation of the proportions of coil 2. Curve U of Fig. 4 shows the course of the voltage connected at the condenser 4, likewise referred to point c of Fig. 1. The processes are analogous to those described above in connection with expansion. However, the discharge of condenser 4 upon ignition of a thyratron proceeds not only via coil 2 but via the parallel connection of the exciter winding 1 and condenser with which it is in series. This condenser, however, has such a low impedance for the impulse currents that the mentioned parallel connection has no disadvantageous effect on the contraction process. It is seen by comparison of Figs. 4 and 3 with Fig. 2 that the direction of the current impulses (i flowing in coil 2 is reversed in relation to the direction of movement of the electrons. A simplification of the device, whereby in particular the reversing switch 10 can be locked in, can be achieved in that the feed voltage for the arrangement 8 is taken, not from the network, but for expansion, from coil 2, and for contraction, from the series connection of coil 2 with the exciter winding 1. For this purpose, lines f, g of arrangement 8 should be connected with points c, d of the connection. If necessary, a lowpass filter may be arranged in front of arrangement 8, preventing the impulse-like currents flowing in coil 2 from reaching the entrance of arrangement 8.

Although the device of the invention is suitable particularly for use in dua-radiation accelerators, it is understood that it can be used also in an accelerator in which electrons are periodically accelerated in only one direction of movement.

I claim:

1. The combination in a device for the selective generation of expansion or contraction impulses to effect expansion or contraction of the electron orbit in a magnetic induction type electron accelerator Whose accelerzu tion flux is produced by an exciter winding fed from an alternating supply voltage, of a coil embracing the acceleration flux produced by said exciter winding, :1 first terminal end of said coil being connected to the co phased first terminal end of said exciter winding, two thyratrons connected in parallel in back-to-front relation, means for alternately applying voltage impulses to the control grids of said thyratrons for rendering said thyratrons conductive in alternation each time the supply voltage passes through zero, a condenser connected at one terminal thereof with one common terminal of said thyratrons, a connection between the other terminal of said condenser and the opposite terminal end of said coil, and circuit means including switching means movable to alternative positions for selectively connecting said thyratrons and condenser to said exciter winding and coil, said switching means in one position thereof connecting the other common terminal of said thyratrons to said first terminal end of said coil thereby to connect said thyratrons and condenser only to said coil, and said switching means in the other position thereof connecting the other common terminal of said thyratrons to the other terminal end of said exciter winding thereby connecting said exciter winding and coil in series and wherein the respective voltages on said exciter winding and coil are opposed.

2. A device for selective expansion or contraction of the electron orbit in a magnetic induction accelerator according to claim 1, characterized in that said coil is so dimensioned that the alternating voltage induced in it by the acceleration flux during operation of the acceleration is at least approximately equal to half the terminal voltage of said exciter winding.

3. A device for selective expansion or contraction of the electron orbit in a magnetic induction accelerator as defined in claim 1 wherein said voltage impulses applied to the control grids of said thyratrons are derived from said supply voltage and which further includes second switching means operated simultaneously with said switching means and connected between said supply voltage and said means for applying voltage impulses to the control grids of said thyratrons for changing the phases of said voltage impulses by 4. A device for selective expansion or contraction of the electron orbit in a magnetic induction accelerator as defined in claim 1 wherein said means for applying voltage impulses to said control grids of said thyratrons includes a voltage source established across said series connected thyratrons and condenser.

5. A device for selective expansion. or contraction of the electron orbit in a magnetic induction accelerator as defined in claim 4 and which further includes a low-pass filter connected between the voltage source established across said series connected thyratrons and condenser and said means for applying voltage impulses to said control grids of said thyratrons.

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,217 Snyder Apr. 4, 1950 2,207,392 Zuschlag July 9, 1940 2,480,169 Westendorp Aug. 30, 1949 2,654,838 Wideroe Oct. 6, 1953 2,754,419 Wideroe July 10, 1956

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