US2520447A - Device for accelerating electrically charged particles, such as electrons and ions - Google Patents

Description

Aug. 29, 1950 R. WIDEROE 2,520,447

DEVICE FOR ACCELERATING ELECTRICALLY CHARGED PARTICLES, sucH AS ELECTRONS AND IONS Filed June 15, 1948 2 Sheets-Sheet 1 3- ,usec.

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DEVICE FOR ACCELERATING ELECTRICALLY CHARGED PARTICLES, sucu AS ELECTRONS AND IONS Filed June 15, 1948 2 Sheets-Sheet 2 0 0 0 N A I 1 3mm 22%; W

13W 43W 29 PWIAW Patented Aug. 29, 1950 DEVICE FOR ACCELERATING ELECTRI- CALLY CHARGED PARTICLES, SUCH AS ELECTRONS AND IONS Rolf Wideriie, Zurich, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Application une 15, 1948, Serial No. 33,154

In Switzerland June 16, 1947 For the acceleration of electrons and ions with the aid of high frequency electric potential fields,

it has been proposed already to mak use of a' device in which the electrons or ions (called broadly particles in the following) pass through one or more fields of potential one or more times. The particles can attain a very high kinetic energy in this way. The electrodes for producing the accelerating fields of potential enter thereby as parts of one or both conductors in the high frequency energy line, and the relative spacing of electrodes corresponding to each other amounts to a fourth or a multiple of the latter of th wave length of the standing waves that are formed between the two conductors of the power line, for each two successive acceleration positions.

A circular arrangement of the electrodes has been suggested as especially satisfactory, the particles to be accelerated being forced to move on a circular path by means of a magnetic control field directed perpendicular to the plane of the path.

Particles with great inertia (protons, deuterons) exhibit a speed greatly dependent on the energy of the particle, the result of this being that either the acceleration frequency or the path radius of the particles or both, with changing particle energy, must be changed so that the acceleration frequency shall always agree with the frequency of passage of the particles, or a whole multiple of the same, which is necessary if the particles are to be accelerated a number of times in the acceleration stretches between each two successive electrodes.

A somewhat more detailed description of a device of this general nature can be found in my co-pending United States application for patent Serial No. 766,698, filed August 6, 1947.

Great increases in energy result in such great increase in speed that it is hardly possible in practice to change the acceleration frequency or the path radius in the same proportion.

According to earlier proposals, the acceleration frequency can be made a multiple of the passage frequency and then this multiple can be diminished in stages with increasing speed of the particles.

By these step-like changes, the particles are brought in each case from a subsynchronous condition into an asynchronous condition, in order then to be collected again in the new subsynchronous condition. This collection, however, presents difliculties, since the damping of the resulting phase oscillations is small, and thus a considerable number of particles are lost during 12 Claims. (01. 250-27) each change. In this way, only a small output of accelerated particles is obtained, this being a drawback especially with a large number 01' such changes.

The purpose of the present invention is to lessen or to prevent as far as possible the loss in particles by facilitating the collection of the particles during the change from one synchronous state into the next one. According to the invention, the time curve of the frequency (in) of the acceleration energy during each synchronism change is so fitted to the time curve of the passage frequency (hr) of the accelerated particles that at the moment of the agreeing of th frequency (In) of the accelerationenergy with a multiple (M) (selected at the time) of the passage frequency UT) of the particles, the derivative, with time, of the frequency of the acceleration energy corresponds to the derivative of the passage frequency of the particles, with time,

multipled by the same multiple (M) If both the two frequencies and also their derivatives with time to the factor M are equally large at the same moment, this means that the relative speed of the particles to the acceleration field to be considered as revolving rotary field becomes zero or approximately zero at the moment when the particles pass by the stability limit of the particles phase (relative to the accelerating rotary field). In this case, even the smallest amount of damping is suflicient for collecting the particles in a synchronous manner. Since a certain damping is always provided by the rising particle energy, practically all the particles of the acceleration voltage can be collected synchronously with the device according to the invention.

The invention will now be explained with the aid of Figs. 1 to 3. in connection with. practical examples. Figure 1 shows a mod of operating an ion accelerator with a single external generator. Figure 2 shows a mode of operation with two external generators, and Figure 3 shows a constructional example of the device according to the invention with the circuit.

Fig. 1 shows a practical example for the case where the particles are accelerated with the aid of a single acceleration system. In order for the radius of the particle paths to remain as exactly constant as possible, it is necessary that the acceleration frequency (In) which has a serrated course (solid-line curve in Fig. 1) should run proportional to the frequency (11') of the particles moving along subsynchronously, in its periodically ascending sections, 1. e. the

B represents the control field for radius R at moment t R represents the mean radius of the paths of the particles 0 represents the speed of light 1 e represents kinetic energy at rest of the particles m represents the inertia of the particles e represents the charge of the particles The frequency of the circling particles (IT) is shown by the dot and dash curve sections, where, however, it is always the passage frequencies multiplied by a definite factor (M) that are entered. In the interval, for instance, be-

tween A and A, i. e. during a change in the acceleration frequency between twenty-eight and thirty-five megacycles, legended MHz on the drawing, the particles can be accelerated from 1.4 to 1.75 megacycles, after they have been accelerated already to 1.4 megacycles before the moment A by the use of a multiplication factor M=25. In further travel (after A), the particles can be accelerated from 1.75 to 2.19 megacycles in using a multiplication factor of M=16, etc. with step-like reduction of factor M.

In order to assure a sequence, without gaps, of the individual acceleration stages (differing in the point of time) with different multiplication factors, the quotient of the greatest and smallest acceleration frequency is preferably fitted at least approximately to the quotient of two successive multiplication factors, in connection with which it should be noted that the multiplication factors show the ratio of the acceleration frequency to the passage frequency of the particles.

Before the moment A, the particles run subsynchronous to the acceleration frequency (multi'plication factor M=25) and are accelerated thereby in such a manner that the path radius remains constantly equal to R0, as indicated by the curve p. In the time interval A-S, the acceleration frequency is diminished from the value C to D. In the short time interval AS (about -20 a sec.) the particles are brought out of their synchronism with the acceleration frequency and consequently are no longer, accelerated in their passage through the potential field, as it would be necessary because of the continuous increase of the magnetic control field B in this temporal interval A-S But since on the other hand control field B decreases in a radial direction according to the equation B0,t=th8 control field for radius R0 at time t k=the exponent of the radial fleld decrease At=the time interval A-S This radial reduction AR of the path of the particles is indicated for the interval AS by section a of curve 10. Caused by the reduction AR of the path radius R, the passage frequency (j'r) of the particles increases according to the following rule:

as is shown by the straight line b for a multiplication factor M= in Fig. 1, where it is the frequency of the particles corresponding to the control field Bo.

so that its curve joins the straight line 1; tangentially in the neighborhood of point D, and specifically for a period of time that corresponds approximately to a, tenth of the whole period of 20 increase SA'. At this moment D, the parterval A--S are collected so that their frequency follows the acceleration frequency in a subsynchronous manner again in making use of a multiplication factor of M=20, and the path radius increases as shown by curve After about a fifth of the period of increase SA, i. e. at the moment E, the frequency curve in of the acceleration energy reaches the straight line F-C', that corresponds to the frequency curve of the particles for R=Ro, whereby the change in synchronism is completed in the time lapse A-E.

In the time interval E--A, the particles again run subsynchronously with the acceleration frequency as before moment A, but the ratio of the frequency of the particles to the acceleration frequency now amounts no longer to 1:25 (M=) but to 1:20 (M=20). At the moment A, a new change in synchronism begins again, the frequency multiplication factor M falling to 16, but theoperation corresponding in all details to that described for the interval A--A', which is expressed in Fig. 1 by the same reference characters marked with a prime.

The time interval A-S, during which the acceleration frequency drops, is with advantage not greater than about a fifth of the whole period of increase SA, so that the particles can remain in the asynchronous condition only for a short time in relation to the time of running subsynchronously. In this time interval A-S, it is advisable to 'reducethe acceleration voltage at least to half its maximum value, in order to avoid influencing the particles running asynchronously, 65 which could give rise to harmful oscillations.

Fig. 2 shows as another practical example, how the device according to the invention operates with two acceleration systems energized separately. In this figure, the straight -line sections in dot and dash lines again represent the passage frequencies UT) of the accelerated particles multiplied by different multiplication factors M.

The solid-line curve I shows the serrated frequency 'course (modulation) of one accelerating system, and the solid-line curve II, the frequency course (modulation) of the other accelerating system, a phase difference of approximately 180 existing between the two modulations. Up to moment A, the particles are accelerated by acceleration system I, the relation of the passage frequency of the particles to the acceleration frequency I being equal to 1:25 (M=25). At this moment, the system II starts, its derivative of the frequency with time corresponding at point D, ac-

The acceleration frequency is to be influenced cording to the invention, to the derivative of the frequency with time of point C of system I. By this, it is achieved that the particle is accelerated further from the point D on through the system II, the ratio of the passage frequency of the particle to the acceleration frequency 11 being equal to 1:20 (M=20) It is advantageous in this connection to maintain on the one hand the action of system I beyond point about up to' point H, and on the other hand to permit the action oi system II to start already at point E.

In the time interval F-G, therefore, the two systerns act at the same time on the particles, 50 that the range or the asynchronous particle movement will in practice be infinitely small, so that the change in synchroni'sm is accomplished with a minimum of disturbances. The path radius of the particles consequently remains constanwRo. so that the magnetic field. in the radial direction, can be kept about three to four times narrower than in former acceleration systems.

This overlapping of the effect in point of time of the two accelerating systems can be achieved by selecting the time duration of the frequency rise for the two systems to be at least a fifth longer than the duration of the frequency drop.

The acceleration frequencies are so to be selected in relation to each other that in the interval F-G, the passage frequency of the particles runs subsynchronously to the two acceleration frequencies as exactly as possible. The greater the deviation from this requirement, the greater will he the number of the particles no longer capable of being collected during the change in synchronism.

In the time interval A-A'. the particles are accelerated by the system II, whereupon, in the range F'--G', a change of the acceleration system is made again, that is accomplished just as the change in range FG, that is indicated by the same reference characters, only provided with a prime.

In the range to the right of A, system I again taltes over the acceleration, etc. In order that the particles in the interval G-F' are not aflected by system I, or in the next succeeding like interval by system II, it is advisable in this period of frequency diminishing to reduce the acceleration voltage at least to half its maximum value, it not switching it out entirely.

Switching-in again must take place so rapidly in the ascending part that, before completed synchronism change at D or D, the voltage has attained its full value.

The voltage course in the field (F-G) of synchronism change is preferably chosen so that in the period (F-G) in which the tangents on the curves of the two acceleration frequencies (I and H) are approximately parallel, the voltage at the one high frequency generator drops to zero and that or the other high frequency generator rises from zero to its maximum value.

other taking place exactly as was explained in connection with Fig. 2. By the use of more than two acceleration-systems, the individual transmitters acting on these systems are without load during long intervals of time, which has very favorable eifect with high transmitter power.

It is of course possible also to permit the frequency curves to depart from the straight line Ihe time interval (F-G) of the synchronism in amplitude and in frequency, which, however,

does not present any diiiiculties technically.

Instead of two acceleration systems, three or more could be used, in which case these systems in turn progressively take over the acceleration of the particles, the transfer from one system to the course shown in Figs. 1 and 2, this causing a change in the path radii of the accelerated particles. A path radii change of this kind can be of importance at the beginning and end of the acceleration operation, if it is desired to introduce the particles into the acceleration tube or take them out of the same.

The change in acceleration frequency in the sense of Figs. 1 and 2 can be carried on without difficulty with means generally known, such for instance as rotating condensers, mechanically variable inductances and also by a variable premagnetization of magnetic materials, the magnetization curve of which does not ascend linearly.

Fig. 3 shows a constructional example of the device according to the invention with the circult.

In an exhausted annular glass tube 30 are located, for instance, four acceleration systems for producing the accelerating electrical field at places 3I--34. For this purpose an annular metal tube 35 with the electrodes in the form of perforated plates 36-39 is provided, opposite each of which a hollow cylinder ill-43 is placed. These hollow cylinders are connected at their ends M-tl with the outer grounded metal tube 35 and are connected at places 48-54 to the appertaining high frequency transmitters. The

distance between the end 44 of cylindrical electrode 40 and electrode plate 36 as measured in a' clockwise direction corresponds to a quarter of the wave length of the standing wave forming in tube 35 or can be an integer multiple of the same. The electrode ends 43-41 and the appertaining plate electrodes 3'l-39 are given the same spacing as that existing between end 44 and plate 38. The distance between electrode end M and electrode end 45 as measured in a clockwise direction however amounts to the whole wave length or the standing were formed in the tube or an integer multiple thereof, which multiple corresponds to the factor M indicated in Figs. 1 and 2, and the same applies to the distance between electrode ends 45 and 46, I8 and 41,

, etc.

If the particles according to the arrangement described in Fig. 2 are to be accelerated, the places 38 and Bil of the hollow cylinders l0 and 42 are connected to the ends of thecowpling coil 52 grounded at the middle. To this coupling coil 52 is coupled a high frequency generator I, this transmitter being indicated only diagrammatically by a, rectangle. The places 4'8 and Slot the hollow cylinders ii and II are to be connected to the ends of the coupling coil 53 that is likewise grounded at the middle. To this coupling coil 53 is coupled a high frequency generator II that is again indicated only by a rectangle. The modulation of the transmitter II isto be displaced in relation to the transmitter I according to Fig, 2 by approximately which can be done easily by relative adjustment of segments 58 and 51.

The frequency modulation of the two transmitters can be accomplished for instance by meansof the two segments 58 and 51, that rotate with relation to the stationary segment plates 58 and 58. 'The amplitude modulation resultant'pf the switching in and out of the accelerating voltages may also be accomplished by contacts or other amplitude varying generally well known means operated by the synchronous motor III. In the illustrated embodiment, a contact mechanism is shown somewhat schematically for efiecting the desired amplitude modulation of each of the high frequency generators. The contact mechanism associated with generator I is seen to be comprised of a circular array of stationary contacts 64 corresponding in number to, and in alignment with; the stationary condenser plates 58 and a rotating wiper contact 65 secured upon the shaft of motor 60, the rotating contact being angularly aligned with the rotating condenser plate 56. All of the stationary contacts '64 are interconnected as shown to a common conductor lead 68, the rotating contact is connected by way of the usual slip ring produced by generator I during the desired frequency decreasing interval in each cycle of frequency modulation.

A similar rotating contactor arrangement is provided for generator II, the array of stationmy contacts being denoted by numeral 68, the rotating contact by numeral 69, and'the conductor leads by and II. The plates 56 and 58 are connected in parallel electrically to the coupling coil 52, and the plates 51 and 59 in parallel to coupling coil 53, and a synchronous motor 60 drives the two movable segments 56 and 51. The motor 60 is driven by the same alternating current network 63 that likewise excites the magnets for producing the magnetic control field that keeps the accelerated particles on the predetermined circular path. Only one exciting coil 6| of these control field magnets is shown diagrammatically, and the magnetic control field B produced by the same and the boundary of which is'indicated in Fig. 3 by the correspondingl legended circle lies penpendicular to the plane of the drawing, passing through the glass tube and the acceleration elements contained in the same. The metal tube is cut apart completely at least at one place 62, in order to prevent shortcircuiting of the magnetic control field.

Insteadof the two high frequency generators, acceleration of the particles can be carried on with a single generator as indicated in Fig. 1. In that case, all the acceleration electrodes in Fig. 3 are connected for instance only to one transmitter I, and the segments of the plates 56 and 58 are to be shaped so that the frequency course shown in Fig. l is obtained. Transmitter II, and the segment plates 51 and 59 are then eliminated.

In conclusion, while a preferred constructional form of the novel device for accelerating charged particles in accordance with the invention has been described and illustrated, it is to be understood that this particular construction is exemplary only, and that various modifications may be made by those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

I claim:

1. Device for the multiple acceleration of electrically charged particles by means of high frequency electric fields of potential comprising an enclosed evacuated chamber providing an orbital path along which said particles are accelerated, a system of electrodes arranged in said chamber along said ath, means for applying a frequency modulated potential to said electrodes to produce corresponding potential fields through which said particles pass, said potential vary cyclically with time between minimum and maximum limits in stepped diminishing multiples of the constantly increasing frequency at which the particles pass through said potential fields, and means establishing a time varied magnetic guiding field through said path normal to the plane thereof distinguished by the feature that the time course of the frequency of said potential fields during each change in synchronism is suited to the time course of the passage frequency of the particles such that at the instant the frequency of said potential fields reaches a match with a given one of the said multiples of the passage frequency of the particles the derivative of the frequency with time of said potential fields corresponds to the derivative of the passage frequency of the particles with time multiplied by said given multiple.

2. Device for the multiple acceleration of electrically charged particles by means of high frequency electric fields of potential comprising, an enclosed evacuated chamber providing an orbital path along which said particles are accelerated, a system of electrodes arranged in said chamber along said path, means for applying a frequency modulated potential wave to said electrodes to produce corresponding potential fields through which said particles pass, said potential wave having a sawtooth characteristic varying cyclically with time between minimum and maximum limits in stepped diminishing multiples of the constantly increasing frequency at which said particles pass through said potential fields, the

' cyclically rising sections of said sawtooth poten- 'tial wave running proportionally to the passage frequency of said particles running subharmonically at least in the last three quarters of the whole ascending section of said wave to thereby maintain the radius of the orbital path of said particles substantially constant, and means establishing a time varied magnetic guiding field through said path normal to the plane thereof, distinguished by the feature that at the instant the frequency of said potential fields reaches a match with a given one of the said multiples of the passage frequency of said particles the derivative of the frequency with time of said potential fields corresponds to the derivative of the passage frequency of said particles with time multiplied by said given multiple.

3. Device for accelerating charged particles as defined in claim 2 wherein the quotient of the largest and smallest .values of the frequencies of 7 ing asynchronously during such period and diminishing their radius of path constantly, and, after the particle collection is completed, it coincides, during an additional fifth of the total rise in frequency, with the value proportional to the frequency of the particles rotating in a subharmonic manner.

5. Device for accelerating charged particles as defined in claim 2 characterized by the fact that on a, basis of time the period during which the frequency of said potential field falls from maximum to minimum does not exceed one fifth of the period during which said frequency rises from minimum to maximum value.

6. Device for accelerating charged particles as defined in claim 2 characterized by the fact that during the period in which the frequency of said potential field falls from maximum to minimum values, the strength of such field is reduced to a value not exceeding one half its maximum value.

7. Device for accelerating charged particles as defined in claim 1 characterized by the fact that the least two systems of the high frequency potential fields are utilized in alternation for accelerating the particles, each said system being energized through a separate source of high frequency oscillations.

8. Device for accelerating charged particles as defined in claim 1 characterized by the fact that at least two systems of high frequency potential fields are utilized in alternation for accelerating the particles, and each said system is supplied by its own high frequency generator, the frequency of one system increasing as the other decreases, and vice versa, and the period during which said frequency increases is at least one fifth longer than the period during which it decreases. I

9. Device for accelerating charged particles as defined in claim 1 characterized by the fact that at least two systems of high frequency potential fields are utilized in alternation for accelerating the particles, and each said system is supplied by its own high frequency generator, the frequency of one system decreasing during a part of the time where the frequency of the other system increases, said system being arranged such that during the common time period in each cycle of frequency variation in which the frequencies of both systems exhibit a rising characteristic, the particles run substantially in a. sub-harmonic manner to the frequencies of both said systems.

10. Device for accelerating charged particles as defined in claim 1 characterized by the fact that at least two systems of cyclically varied high frequency potential fields are utilized in alternation for accelerating the particles, and each said system is supplied by its own high frequency generator, the voltage of each system during the portion of the cycle in which the frequency de creases being reduced to a value not exceeding one half of the value which obtains during that portion of the cycle in which the frequency increases, said voltages reatta'ining said latter value before the change in synchronism has been completed.

11. Device for accelerating charged particles as defined in claim 1 characterized by the fact that at least two systems of cyclically varied high frequency potential fields are utilized in alternation for accelerating the particles, and each said system is supplied by its own high frequency generator, said systems being arranged such that during the common time period in each cycle of frequency variation in which the tangents to the two frequency curves are substantially parallel the'voltage of one system falls off to zero and the voltage of the other system rises to its maximum value.

12. Device for accelerating charged particles as defined in claim 1 characterized by the fact that at least two systems of cyclically varied high frequency potential fields are utilized in alternwtion for accelerating the particles, and each said system is supplied by its own high frequency generator, said systems being arranged such that during the common time period in each cycle of frequency variation in which the tangents to the two frequency curves are substantially parallel the voltage of one system falls off to zero and the voltage of the other system rises to its maximum value, said common time period being greater than one third of one oscillation cycle of the phase oscillations of the particles and less than threefold.

ROLF wmERdn REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,242,888 Hollmann May 20, 1941 2,398,162 Sloan Apr. 9, 1946 OTHER REFERENCES Physical Review, 1947, vol. 71, pages 449-450. Physical Review, 1945, vol. 68, pages 143-144.

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