US2465786A - Accelerating apparatus for charged particles - Google Patents

Description

March 29, 1949. J, BLEWETT 2,465,786

ACCELERATING APPARATUS FOR CHARGED PARTICLES Filed Jan. 24, 1947 :5 SheefS-Sheet 1 Fig. I.

F 0 F ll I m i o E1 0 I E V T/ I \Fl 1 )3] G 53 E]\ F// 02 2 62 I \\F I? Q w 1 -rt Inventor:

John F? Blewe'bb, b 7f'M 6. J

H is Attorney.

March 29,1949. I J, P, BLEWETT 2,465,786

ACCELERATING APPARATUS FOR CHARGED PARTICLES Filed Jan. 24, 1947 3 Sheets-Sheet 2 Fig.3.

Inventor:

John F? Blewett,

H is Attorney.

March 29, 1949. J. P. BLEWETT ACCELERATING APPARATUS FOR CHARGED PARTICLES s Sheets-Shet 3 Filed Jan. 24, 1947 SO RESONATOI? DEV/ CE DEV/CE DELAY DEV/CE DELAY DELAY Inventor": John F? BIewe'tt by 7 (A) w M is Attorney.

Patented Mar. 29, 1949 ACCELERATING APPARATUS FOR CHARGED PARTICLES John P. Blewctt, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application January 24, 1947, Serial No. 724,066

v 7 Claims. 1

The present invention comprises an improved apparatus for accelerating charged particles and in particular to electrons.

Kerst Patent 2,297,305 issued September 29, 1942, and other patents of later dated, describe magnetic induction accelerating apparatus whereby charged particles are accelerated in an annular chamber by a magnetic field, the apparatus including a magnetic core of iron which is interlinked with the acceleration orbit in the annular chamber.

The iron core which forms partof conventional accelerating apparatus presents some disadvantages. Such an iron core is costly. Because of its great weight, it is difiicult to fabricate and handle. results in field distortions which may produce disturbances in the operation of the device. Field distortions may be particularly troublesome at the moment of electron injection when the timevarying field has just passed through a zero value. An apparatus containing an iron core is limited to field values of a maximum of approximately 15,000 to 20,000 gauss.

tion of the iron core is likely to result in wave form distortion and attendant circuit difficulties.

In accordance with my present invention I have provided an apparatus which comprises a modified accelerating structure having an air core, that is charged particles are caused to be accelerate-d therein without the emotion of iron core. It is desirable in such air core apparatus that the means whereby the magnetic accelerating field is generated should couple as closely as possible with the electron beam so that energy will not be stored in a greater volume of air than necessary. It is desirable that the magnetic field which is generated by this means should be relatively small at the electron orbit and that its variation with the radius should-be so small that it will not interfere with the guide field. These are contradictory conditions to be met by a single magnetic winding. An elongated coil will have a small external field but the coupling coefiicient will be low. A short coil will result in good coupling but the accelerating field at the orbit will be highly dependent on the radius.

In an apparatus embodying my invention a magnetic guide field and a magnetic time-varying field are generated by separate means. One field-generating means generates a guide field which varies in a predetermined-relation with respect to the radius of the orbit of acceleration and another magnetic meansis provided to generate time-varying magnetic fields which so neutralize one another that they do not appreciably modify the magnetic guide field at the orbit of the accelerated electrons; the coupling of such In the accompanying drawing Fig. 1 is a diagram showing spatial relationships of magnetic fields; Fig. 2 is a graph of field distribution and magnitude; Fig. 3 shows in vertical section an accelerator having a plurality of correlated magnetic windings for generating magnetic fields for guiding and accelerating electrons; Fig. 4 shows a portion of the acceleration chamber which contains an electron gun for introducing electrons to be accelerated; Fig. 5 is a circuit connection diagram; Fig. 6 is a sectional plan view of an accelerating chamber which is equipped with means for supplemental high frequency acceleration; Fig. 7 is a graph showing diagrammatically the relation of cooperating accelerating fields in the latter device; Fig. 8 is a diagram of coil connections and Fig. 9 is an alternative circuit con- A coil arrangement suitably applying the invention is shown diagrammatically in Fig. 1. In this figure the point 0 represents the location of the orbit of electrons whose motion is to be controlled by the coil arrangement. It will be shown that by proper distribution and energization of the coils they can be caused both to accelerate and to guide electrons in the orbit O in a manner which fully realizes the objects previously set forth.

For an understanding of the invention reference should first be had to the coils C1, C2 and D1, D2 which have the primary function of providing an accelerating flux within the electron orbit. As previously noted, it is desirable that the coils which perform this function shall have relatively little effect upon the field at the electron orbit. That a coil arrangement such as is represented by coils C and D can achieve this result may be understood by reference to Fig. 2.

Referring first to the coils C1 and C2, the field in their plane of symmetry can be calculated as :a function of radial distance from the axis of the coils and proves to have the form indicated by the graph R of Fig. 2. If taken with respect to the plane of symmetry, the field thus plotted will have an axial but no radial component. For

10 strength with radius.

Also, the remanence in such iron core At higher values saturanection diagram.

purposes of the present invention the radius of the coils is so chosen that the minimum of the field pattern indicated at A lies at the radius of the electron orbit. There exists also a location at which a second pair of coils D1 and D2 can be located such that the maximum field due to the coils D1, D2 will lie at over the minimum due to the coils C1, C2. As a matter of fact, referring to Fig. 1, there exist loci OF along Which this condition can be satisfied. If now the ampere turns in the coils D1, D2 bear the right relation to the ampere turns in the coils C1, C2, the fields at 0 due to the two pairs of coils will cancel because of the establishment by the coils D1, D2 of the field having the distribution and magnitude indicated by the curve S of Fig. 2. Since 0 is an extremum for both pairs of coils, the first derivative of the field with radius also disappears at 0. If now the correct location on OF is chosen for the coils D1, D2, the second derivatives of the field may also be made to cancel. Under these conditions the field due to the four coils will be efiectively zero over a considerable region around the orbit. In practice, however, it may prove that exact cancellation of the second derivatives is not as desirable as an approximate matching of field characteristics which may give a field which is not zero but is small over a larger region. It is worthy of note that Maxwells equations insure that zero field over a range of radius in the plane of symmetry also results in low fields as one leaves the plane of symmetry in the axial direction.

While the fields attributable to the coils C1, C2 and D1, D2 cancel in the region of the orbit 0, it will be understood from a consideration of Fig. 2 that they are additive in the region within the orbit and are, therefore, effective if varied conjointly by application of properly varying potential to produce an accelerating flux which links the orbit. That such a flux may cause continuous acceleration of electrons disposed at the orbit is a fact which is well known in connection with the operation of betatrons.

In order that the gyration of electrons accelerated in the manner indicated in the foregoing may be confined to a single orbital path, it is necessary as has been established in connection With betatron theory to provide at the electron orbit a guide field which falls off as rin the region of the orbit, 11. having a value between 0 and 1. A practical coil configuration for realizing this object in an air core system is represented by the coils G1, G2 and H in Fig. 1. It has been found empirically that in such a coil configuration, a field whose variation is of the P variety is realized at the orbit O and that the index n is approximately proportional to the distance d between the coils G1, G2.

In operating a coil system such as that described for the purpose of accelerating electrons to relatively high energy levels, it is assumed that the various coils are energized in synchronism and that the relative directions of their energization is as indicated by the current direction signs applied in Fig. 1. More specifically, the coils D1, D2; C1, C2 and G1, G2 are excited in a common direction while the solenoidal coil H is differentially excited with respect to the other coils.

The application of a coil system such as that which has thus been described to a practical apparatus is illustrated in Fig. 3. The apparatus shown in Fig, 3 comprises a frame 1 consisting of any suitable insulating material, such as wood,

or a molded plastic, on which are supported sev-: eral sets of windings to be described presently and an annular evacuated accelerating chamber 2. Communicating with chamber 2 is a side chamber 3 into which are sealed conductors 4 (see Fig. 4) leading to an electron gun 5 whereby electrons to be accelerated are introduced into the annular chamber 2. The chamber 2 is highly evacuated.

The coil system provided includes a coil pair l4, 15 which corresponds to the coils G1, G2 of Fig. l, a solenoidal coil l6 which corresponds to the coil H of Fig. 1, and coil pairs I8, l9 and 20, 2| which correspond respectively to coils D1, D2 and C1, C2 of Fig. 1. An energizing circuit for use with the structure of Fig. 3 is shown diagrammatically in Fig. 5. It includes a pair of capacitors, or capacitor banks, 23 and 24 connected to the windings |4l6 and l82| by circuit conductors 25 and 26. These capacitors are adapted to be charged intermittently in parallel by connection to a charging source as indicated schematically at 3|. Resistors 2'! and 28 are respectively interposed between the charging source and the capacitors to limit the rate of charging.

It will be seen that upon closing the circuit 29 by a switch 30 (which in a practical equipment may consist of an electronic switching scheme rather than a mechanical switch as shown), the various coils l4, l5, l6, l8, I9, 20 and 2| will be concurrently energized and that the relative directions of energization will be as indicated in Fig 1. As currents build up simultaneously in the coil pairs 18, I9 and 20, 2| an accelerating flux will be produced in the region enclosed by the annular vessel 2 and at the same time a correspondingly varying guide field will be produced at the orbit of electron gyration within the enclosure 2 by virtue of the simultaneous growth of current in the coils l4, l5 and I6. Because of the pairing of the coils l8, l9 and 20, 2| in accordance with the principles set forth in connection with Fig. 1, the guide field will be essentially independent of the current flow in these coils. It may be desirable in order to minimize field variations due to the coil leads to subdivide the windings, as shown in Fig, 8, into several closely spaced sections connected in parallel, the leads l1, emerging as shown at spaced circumferential points.

In order that a useful result may follow from the coil energization just referred to, it is necessary that electrons be introduced into the annular enclosure 2. This is accomplished by applying to the electron gun 5, Fig, 4, a high voltage pulse at, or very shortly after, the application of potential to the coil system. This may be done in one way by connecting the elements of the gun by conductors 3| to the power source which energizes the coils through a delay device, indicated schematically at 32, and a pulse generator indicated schematically at 33. It is assumed in this connection that the application of potential to the delay device is effective within a very short interval of time to cause a voltage pulse to be generated in the pulse generator 33 and applied by it to the electron gun with the result that a burst of electrons is introduced into the space enclosed by the annular vessel 2.

With the conditions thus postulated, the electrons so provided in the annular enclosure may be accelerated to an energy level on the order of several million electron volts provided proper balance exists between the guide field and accelerating flux as established by the various coils. As is well known from betatron theory, this balance requires that the following relationship "exist:

Average magnetic flux included within the orbit: twice magnetic field at the orbit extended over the entire area enclosed by the orbital path.

This is a relationship which can readily be established by proper proportioning of the ampere turns applied to the various'coils.

The following cycle of operations occurs starting from the moment when the switch 30 of Fig. is closed. In. a few microseconds, when the magnetic field at the orbit has reached a value which will cause electrons of several kilovolts energy to be constrained to travel in the orbit afforded by the annular chamber, the electron gun 5 is automatically energized to inject electrons at such an energy level. The electron gun may be operated for only a brief period or pulse of perhaps a few microseconds length. The electrons which are thus introduced into the evacuated accelerating space are accelerated by the time varying magnetic flux within the orbit while being guided by the field at the orbit to a velocity equivalent to 2 or 3 million electron volts. If the parameters of the various coil circuits were iden- "tical, acceleration of the electrons in a stable orbit would continue indefinitely with continued increases in the coil excitation. However, since it is desirable that after a predetermined acceleration the accelerated electrons be permitted to impinge upon a target such, for example, as the target 22 in Fig. 3 some disturbance of the equilibrium between the accelerating flux and the guide field is indicated. This may be accomplished in one way simply by making the parameters of the fiux producing circuit different from those of the field-producing circuit. If the fluxpro'ducing circuit has a higher natural frequency than the field producing circuit, for example, the guide field will still be increasing as the accelerating flux drops off, and the orbit of the accelerated electrons will be contracted until eventually the electrons strike the target 22. Within the limits of practical construction this may occur, for instance, when the electrons have attained an energy level in the neighborhood of or less than M. E. V.

If higher energy levels than several million electron volts are desired to be reached, it becomes expedient to make use of a system such as that described in prior application Serial No. 639,462, filed January 5, 1946, by Herbert C. Pollock and Willem F. Westendorp and assigned to the same assignee as the present application wherein use is made of a method for imparting high energy to charged particles by subjecting them to the successive accelerating effects of magnetic and electric fields. The application of this method in connection with my present invention requires that the accelerating chamber 2 be provided with means for producing a high frequency electric field in a localized region within the enclosure. Such means may be of the character illustrated in Fig. 6.

As shown in Fig. 6, the chamber 2 may be provided with. conducting coatings 6, l which are separated by insulating gaps 8, 9 across which high frequency fields may be set up. Lead-in conductors i6, II which are illustrated conventionally are provided to supply high frequency potentials to the conducting coatings 6, 1 which constitute interior electrodes.

In the use of a system incorporating high frequency electrodes as just described, it is expedi- 6 ent to make the parameters of the circuit by which the guide field is produced materially different from that which produces the accelerating flux. Under these circumstances, assuming the difference to be in the right direction, the accelcrating flux may be caused to fall off while the guide field is still increasing so that the function of continuing the acceleration of the electrons may be transferred to the high frequency electrode system. Operation in accordance with this modeof procedure is illustrated in Fig. 7 in which the curve U may be taken to represent the variation in time of the guide field produced by the coils l4, l5 and 1-5. The curve V correspondingly represents the variation in the accelerating flux produced by the accelerating coils l8, l9 and 20, 21, it being 'noted that the resonant frequency of the flux producing circuit is, in this instance, made many times that of the guide field producing circuit. With this arrangement, it is assumed that electrons are introduced into the accelerating orbit at or shortly after the point a at which the accelerating and guide fields are initiated. At a point I), which corresponds to the time at which the accelerating flux wave begins to diverge materially from the guide field wave (which-point may correspond to attainment by the accelerated electrons of an energy level of 2 or 3 M. E. V., a high frequency field indicated schematically by the curve X is abruptly applied to the high frequency electrodes 6 and 7 contained within the accelerating chamber.

This high frequency field may be initiated at a proper time in one way by a circuit arrangement such as that shown in Fig. 5 in which a high frequency source 36 is connected to the energizing circuit of the accelerating and guide field coils through a properly timed delay device 34 which is so adjusted as to assure the abrupt energization of the high frequency source at the time b (Fig. '7). The high frequency source connects with conductors H], H which correspond to the similarly numbered conductors shown in Fig. 6. De-energization of the high frequency source may be accomplished through a second delay device 3! (Fig. 5) which will act at a time c (Fig. 7) which is near the peak of the accelerating flux wave. This will permit the accelerated electrons to diverge from the accelerating orbit and to impinge upon a target placed outside the orbit.

As is explained in the aforementioned application Serial No. 639,462, the frequency of the field applied to the electrodes 6 and i must be correlated to the dimensions of the accelerating chamber 2. For an orbit radius of about 18 inches diameter, a frequency on the order of 200 megacycles is satisfactory.

As shown in Fig. 9 the connections may be modified. The windings l6, l9 and 20 may be connected in series to a capacitor bank 3%. The windings l4, l5 and I6 are connected in series to a capacitor bank 46. The capacitors 36, it are charged in series with a single impedance t l.

The rest of the circuit connections are the same as described in connection with Fig. 5. When the switch 30 is closed the'capacitors 39, t?) discharge through the series-connected coils id, i5, i6 and l8, I9, 20 and'Zl.

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

1. An air core electron accelerating apparatus comprising means providing a space for the gymtion of electrons in a generally circular orbit, means for introducing electrons into said space,

means for generating a time-varying magnetic flux to accelerate said electrons in said orbit, and separate means for generating a timevarying magnetic guide field for maintaining said electrons in said orbit, said guide field generating means comprising coils respectively located inside of and outside of the orbit and connected to cause their action to be cumulative at the orbit so as effectively to restrain said particles to said orbit, and said accelerating flux generating means comprising additional coils inside of and outside of the orbit and connected to cause their magnetic fields to be neutralizing at the orbit.

2. An air core electron accelerating apparatus comprising means providing a space for the gymtion of electrons in a generally circular orbit, means for introducing electrons into said space, means for generating a time-varying magnetic flux to accelerate said electrons in said orbit, and separate means for generating a time-varying magnetic guide field for maintaining said electrons in said orbit, said guide field generating means comprising coils respectively located inside of and outside of the orbit and connected to cause their action to be cumulative at the orbit so as effectively to restrain said particles to said orbit, and said accelerating flux-generating means comprising a pair of axially spaced coils inside of said orbit and a second pair of axially spaced coils outside of said orbit, the mutual spacing of the members of said coil pairs being effective to cause their magnetic fields at the orbit to be self-neutralizing.

3. Apparatus for accelerating charged parti cles along an orbital path comprising: air-cored guide field windings adjacent said path including first coils inside said path and second coils outside said path, said first and second coils being magnetically coupled to said path to produce conjointly a time-varying magnetic field which constrains said particles to said path; and aircored flux generating windings adjacent said guide field windings including coils inside said first coils and coils outside said second coils, said coils being magnetically coupled to said path to produce conjointly a time-varying magnetic flux which links said path to accelerate said particles but which exerts substantially no deflecting force upon said particles within said path.

4. Apparatus for accelerating charged particles along an orbital path comprising: air-cored guide field windings adjacent said path including first coils inside said path and second coils outside said path, said coils being magnetically coupled to said path to produce conjointly a timevarying magnetic field which constrains said particles to said path; and air-cored fiux generating windings adjacent said guide field windings including coils inside said first coils respectively axially spaced on opposite sides of a plane in which said orbit lies and coils outside said sec. ond coils respectively axially spaced on opposite sides of said plane, said last positively named coils being magnetically coupled to said path to produce conjointly a time-varying magnetic flux which links said path to accelerate said particles but which exerts substantially no deflecting force upon said particles within said path.

5. In apparatus wherein charged particles are accelerated along an orbital path during a predetermined cycle; a source of voltage; air-cored flux generating windings adjacent said path including first coils inside said path respectively axially spaced on opposite sides of a plane in which said orbit lies and second coils outside said path respectively axially spaced on opposite sides of said plane, all said coils being connected to said voltage source to produce conjointly a time-varying magnetic fiux which links said path to accelerate said particles during at least a portion of said predetermined cycle; and air-cored guide field generating windings including coils between said path and said first coils and coils between said path and said second coils, all said last positively named coils being connected to said voltage source to produce conjointly a timevarying magnetic field at said orbit to constrain said particles to said path during said predetermined cycle.

6. In apparatus for accelerating charged particles along an orbital path: flux generating windings adjacent said path including first coils inside said path respectively axially spaced on opposite sides of a plane in which said orbit lies and second coils outside said path respectively axially spaced on opposite sides of said plane, the flux paths within said inner coils and between said inner and outer coils being substantially free of ferromagnetic material and all said coils being connected so that upon excitation of said apparatus current flow in them is in the same direction with respect to their common axis whereby their magnetic fields neutralize one another in the region of said orbital path; and additional windings in close proximity to said orbit for providing upon excitation of the apparatus a radially varying guide field to constrain said particles within said orbit.

7. In apparatus for the acceleration of charged particles along an orbital path wherein a timevarying magnetic flux links said path to accelerate the particles while a time-varying magnetic guide field acts to restrain the particles to the path, the combination which includes a first pair of coils within said orbit axially spaced on opposite sides of a plane in which said orbit lies and enclosing a space substantially free of ferromagnetic material, and a second pair of coils outside said orbit axially spaced on opposite sides of said plane and enclosing a space substantially free of ferromagnetic material, both said pairs of coils being spatially interrelated to produce upon energization of said apparatus a combined magnetic field which is substantially zero in the region of said orbit while producing the timevarying magnetic fiux which links said orbit to accelerate the particles.

JOHN P. BLEWETT.

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

UNITED STATES PATENTS Name Date Steinbeck Dec. 28, 1937 OTHER REFERENCES Number

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