US2880353A - Particle accelerator - Google Patents

Description

3 Sheets-Sheet 1 R. WARNECKE ETAL PARTICLE ACCELERATOR -March 31, 1959 Filed Feb. 23,1954

3 S heets-Sheet I 2.

March 31, 1959 R. WARNECKE' ETAL PARTICLE ACCELERATOR Filed Feb. 2:, 1954 L|E|c. .E .c

March 31,1959 RWAI'QIQECKE ETAL 2,880,353

PARTICLE ACCELERATOR Filed Feb. 25, 1954 I s Sheets-Sheet Q cg 1 I 2 I I E I a I- I (n 5 I Z I I z I Q ,l I 2 I 1 1 I E k l s l "II/III'II.

I 4' I I I: I: I I l 1 I E l I I 7 g l United States Pateflt PARTICLE ACCELERATOR Robert Warnecke and Georges Mourier, Paris, France, assignors to Compagnie Generale de Telegraphie Sans Fil, a corporation of France Application February 23, 1954, Serial No. 411,970

Claims priority, application France February 23, 1953 13 Claims. (Cl. 315-35) The present invention relates to accelerators for electrons and other elemental particles; It has for an object to provide a novel linear accelerator capable of obtaining particularly high accelerations while using voltages of the same order as those presently used. Compared with known linear accelerators, the accelerator according to the invention has moreover the interesting feature of ensuring the displacement of the elemental particles along a perfectly stable trajectory without it being necessary, as is usually the case, to use special devices for this purpose.

The accelerator according to the invention may be used for accelerating electrons, or other elemental particles, such as positive ions, particles, protons and the like.

Briefly, the accelerator comprises an envelope in which prevails a vacuum and are disposed a first electrode and a second electrode parallel to the first electrode by which it is surrounded. Means' are provided for propagating in the space comprised between these two electrodes a beam of elemental particles to be accelerated. Such means may for instance comprise a cathode if the accelerator is an electron accelerator.

A direct current is caused to flow in the second electrode for creating a magnetic field B having circular lines of force coaxial with the electrode. An electric field E having radial lines of force is provided. The above particle beam is propagated normally to both fields.

Delaying elements forming a delay line or several delay line portions, are disposed along that one of the above two electrodes which is of the same polarity as the particles to be accelerated. Ultra-high frequency energy is fed to said delay line. This delay line is of such nature that the phase-velocity of the high-frequency field and that of the beam are substantially the same. Accordingly, the delay ratios for-successive delay line portions decrease in the direction of the beam as the beam velocity increases.

It is known that the-linear velocity of the particles is equal to E/B, where B has an upper critical limit. B being the electrical field and B- the magnetic field prevailing in the space Where the particles concerned are propagated. When this limit is reached, the particles settle on the electrode having a polarity opposed to theirs for instance on the anode in the case ofelectrons.

Furthermore, one of the objects of the invention is to collect these particles without using too high a voltage for generating the field E.

In operation of the accelerator according to the invention the beam of electrons, or other elemental particles to be accelerated having the same mass and the same polarity, while moving at the linear velocity E/B is urged toward the delay line under the action of the ultrahigh frequency field'of which this line is the seat, and the potential energy of the electrons increases at the expense of that furnished to the delay line by a ultrahigh frequency source. As has just been recalled, the ratio E/B is limited and if it exceeds a'certain limit, the

electrons arrive at the anode. But if, at the-moment; when the electrons are very near the delay line, the above ratio is increased, by steps or progressively, the electrons leave the delay line and approach the other electrode, for instance the anode in the case of electrons, and the potential energy of the electrons is able to trans: form into kinetic energy, without the electrons reaching the anodes. The velocity of the electrons, or other particles considered, increases. v

The invention will be described with more detail with the aid of several modes of carrying out the invention, which are given by way of example only. a

In the accompanying drawing,

Fig. 1 is a first embodiment of the accelerator according to the invention, wherein the ratio E/B is varied by steps by modifying the value of B,

Fig. 2 is another embodiment of the accelerator accord-. ing to the invention wherein the ratio E/B is variedby modifying the value of E,

Fig. 3 shows a third embodiment of the accelerator according to the invention wherein the ratio E/B is varied progressively.

In the following the accelerator described will be considered as being an electron accelerator. However, as already mentioned it is obvious that the same could as well be used for accelerating any kind of elemental par ticles provided that the polarity of the delay line is the same as the polarity of the charge of said particles.

According to the embodiment shown in Fig. 1, the electron accelerator comprises an envelope 1 wherein a vaccum is caused to prevail, for example, by means of pumping devices diagrammatically shown at 2. Along the axis ZZ of the envelope 1 is disposed a central conductive electrode 3 whose ends are connected respectively to two terminals +A and --A of a source of direct cur,- rent. This current creates a magnetic field B whose lines of force are circles having axis ZZ.

The electrode 3 is in electrical contact with the envelope 1 at the right end, as seen in Fig. 1, but is insulated therefrom at the left end.

A tubular anode 4 is mounted inside the envelope 1 on insulating supports 5. A source of DC. potential 20 is connected to the anode 4 and furthermore to the envelope 1, and maintains the anode 4 at a positive potential E relative to a cathode 6 which is housed in an annular groove, formed in a cylindrical conductive electrode 8 connected to the envelope 1. The cathode is provided with a heater of conventional type and is brought to a potential equal to or slightly different from that of the electrode 3. This cathode is adapted to produce an electron beam 7. An electron optical system, for example of the type described in the U.S.A. application Serial Number 150,357 of March 18, 1950, now US. Patent No. 2,694,783, diagrammatically represented by the electrode 4, makes it possible to bring the electrons to an equipotential corresponding to the potential E/B. The electron beam is tubular and surrounds the electrode formed by the central conductor 3. This last electrode carries delaying elements forming a delay line 9, for example of the vane type. An ultra-high frequency genrator (not shown), feeds ultra-high frequency energy to the delay line through a coaxial line 21, comprising the conductor 3 and a tubular member 10 traversing the wall of envelope 1. In a manner well known to those skilled in the art, the delay line is provided with such a delay ratio that the ultra-high frequency field, propagated along the line, has a velocity equal to that of the beam.

In the embodiment shown in Fig. 1, the accelerator comprises several sections, for example three L, M and N. The delay line portions respectively pertaining to these sections have such decreasing delay ratios inthe direction of the beam that the phase velocity of the ultrahigh frequency field, propagated in the line 9, increases stepwise from section to section. In this way the propagation velocity of this field is always equal to that of the electrons of the beam, this last velocity increasing as will be explained hereinafter. This variation of the delay ratio may be obtained in different well known ways. For example, as shown in the drawing, the depth of the delaying elements, or vanes, forming the delay line 9 may be varied by increasing the diameter of the conductor 3 and/or the spacing between the vanes may be increased.

It will be explained hereinafter what length it is necessary to give to each one of the sections L, M and N for a given ultra-high frequency power applied.

The D.C. field E, or the magnetic field B, are not the same inside the several sections L, M, N. Since the velocity of the electrons moving in the longitudinal direction is E/ B, E may be increased and/ or B may be decreased to increase the electron velocity.

In the example shown in Fig. l the magnetic field B is adjusted. To this end the central conductor 3 comprises two branches 11 which are respectively disposed at the region where one section joins the other. Through these branches the conductor 3 is connected to sources of D.C. voltage (not shown) which together with the source A control the direct current in conductor 3 in each of the sections.

The change in the delay ratio in passing from one section to the other is progressive. To this end the depth of the vanes and/or their spacing varies progressively over a certain length in that region.

The wall of the envelope 1 which faces the outer end of the last section N of the accelerator comprises an annular opening which is closed by a thin metallic diaphragm 12. Outside the envelope is a chamber 13 in which the electrons are used, for example for bombarding a material which it is desired to subject to their action or to any other suitable end.

There will now be given a brief description of the operation of the accelerator according to the invention. This description is, of course, merely illustrative.

When the various sources of voltage mentioned above are applied to the accelerator shown in Fig. 1, an electron beam 7 is emitted by the cathode 6. This beam 7 moves along the delay line 9 at a velocity equal to E/B and passes in succession through the sections L, M, N of the tube. If there were no ultra high frequency field, the beam would move along a straight line and then settle on the anode 4. Owing to the presence of this field the electron beam is urged towards the delay line 9. Theory and practice show that the beam 7 is subject to bunching and that the electron bunches tend to approach the delay line 9.

Some idea of this may be had from the Fig. l where there have been shown several electrons or, e, 'y, 6, e, f. Briefly, it can be seen that the electron e is urged towards the delay line 9 by the ultra-high frequency field existing in this region and the electrons 6 and g tend to join it. Similarly, if the beam is not too near the anode, the electrons a and 7 respectively join the following and preceding electrons. It is clear then that there is a concentration of electrons at the points corresponding to the electrons e of each period of the ultra-high frequency field and deconcentration at the points corresponding to the electrons p.

The bunch of electrons thus formed approaches the delay line and its potential energy is thereby increased at the expense of the high frequency energy that the delay line loses to it without this causing the kinetic energy of the bunch of electrons to be modified.

If the electrons, whose energy has been thus increased were allowed to attain the delay line this potential energy would be wasted as heat or restored to the delay line. But, if before the bunch of electrons whose potential energy has been thus increased, settles on the delay line, it is moved away from this delay line and is allowed to approach the anode, it transforms its potential energy to kinetic energy. In order to move the electrons away from the delay line it is necessary to increase E and/or decrease B.

In the example shown in Fig. 1 this has been done by decreasing B. Owing to the branches 11 the current which passes through the section N of the accelerator is weaker than that which passes through the section M, etc. The velocity of the electrons therefore increases from one section of the accelerator to another, i.e., from L to M and from M to N. It is in order to retain for the ultra-high frequency field a phase velocity which is substantially equal to that of the electrons that the delay ratio of the delay line is modified as indicated above.

Calculations show that the factors governing the increase in the velocity of the electrons from one section to the other are V V wherein n is the number of sections extending from the cathode. As this velocity approaches the velocity of light this factor decreases.

It will be seen from the foregoing that it is important that each section of the accelerator is terminated, i.e., that the ratio E/B be modified, at the region where the electron bunches are just about to strike the delay line 9. If this ratio is modified earlier, the electrons fail to gain the maximum of the potential energy that the high frequency source is able to yield to them. In the opposite case a part of the beam reaches the delay line.

In order to determine the length of each section L, M, N trial and error method may be used. For example, a certain length is given to the first section L and the current which flows in the conductor 3 is measured. Then the length of the section is varied. At the moment when the optimum length is exceeded, the electrons of the beam 7 commence to settle along the delay line 9 and the measured current increases.

Of course, the passage from one section of the accelerator to the other is effected in a progressive manner as has been indicated above.

Fig. 2, where like reference numbers designate like members shown in Fig. 1, relates to a first modification of the invention. According to this modification the variation of the ratio E/B is obtained by changing the D.C. field E. This is obtained by varying the radius of the anode 4, this radius changing from one section to the other. Between the adjoining sections the anode includes an intermediate portion 14 which is adapted to ensure a progressive change in the velocity of the electrons. Similar intermediate portions 15 are provided in the central electrode 3. This enables any two consecutive sections of the delay line 9 to be so adapted as to prevent reflexions of the wave. The ratio between the successive radii of electron 4 is such that the electric field around the delay line is in the second section V; times greater than in the first section, or more generally times greater in the nth section than in the first.

Fig. 3 shows a further embodiment of the invention in which the variation in the electric field and that in the phase velocity are progressive and continuous. This embodiment is, in fact, the most advantageous. It may he considered that, in this embodiment, each infinitely small portion of the travel of the electrons towards the delay line is accompanied by an increase in the electric field which again causes the electron to rise the same amount towards the anode. The path of the beam 7 remains in this way substantially rectilinear, While the velocity of the electrons increases owing to the transformation of potential energy into kinetic energy as explained above. The anode 4 should in this case have a conical shape, as shown in Fig. 3, and the density and/or the depth of the delaying elements, forming the delay line 9, should decrease from the cathode in the direction of the beam propagation in order to maintain the beam and the high frequency field phase velocity substantially in synchronism. The radius of the negative electrode may be varied as well as that of the anode.

It is merely essential that the distance from the'dela'y line to the anode decrease in suitable" fashion.

It is of interest that the electron beam passes very near the delay line, since it is in fact there that'the U.H.F. fields are the strongest.

According to a convenient arrangement, shown in Fig. 3, the delay line is earthed so that the generators of high frequency energy may also be earthed. The cathode 6 (or any other source of particles) is strongly negative, relative to the delay line and the anode 4 is positive relative to the delay line. The cathode'6 is a part of an electron gun, including a concentrating electrode 16, which is insulated from earth by the element 17, and an accelerating anode 18. The anode 4 would not collect any electrons owing to the fact that the beam of electrons is completely focalized. It is obvious that the more the beam passes near the delay line, the more the voltage necessary on the anode is--low. When the velocity of the electrons becomes near to' the velocity of light, the distance from the delay line to the anode may remain constant, for v (velocity of the electrons) is practically constant, since the equation v=E/B remains valid in relativist mechanics and since B is eonstant,'E

is also constant.

It is clear that all systems in accordance with the may be reversed. The central electrode may be used as an anode and the delay elements may be located on the outside electrode, provided the magnetic field is correspondingly reversed. Furthermore, if positive particles are to be accelerated, the delay circuit is placed on the anode which may be the central or the outside electrode.

Many modifications and changes may-be made in the various embodiments described hereinbefore.

Thus, for example, instead of obtaining the electrons from a cathode, the electrons or other particles may be introduced in the accelerator according to the invention with a certain initial velocity. They may, for example, be provided by a Van de Graaf machine, or a similar machine.

What we claim is:

1. An accelerator for elemental particles, having a given mass and a given polarity, comprising in a vacuum tight envelope: a first elongated electrode in theform of a bodyof revolution; a second electrode in the-form of a body of revolution coaxial with the first electrode and spaced therefrom; means for creating a radial electric field between said electrodes; means for feeding a direct current to said first electrode for creating a magnetic field having circular lines of force perpendicular to said electric field; means at one end of said electrodes for propagating a beam of said particles in the space comprised between said electrodes normally to both said fields; means for feeding ultra-high frequency energy to that one of said electrodes, whose potential is of the same polarity as said particles; delaying elements along the said one electrode for delaying said ultra-high frequency energy to make its phase velocity substantially equal to the velocity of said particles; means for causing at least one of said fields to change its value at least at spaced points along said electrode for increasing thereby the velocity of said particles; and collecting means for accelerated particles, located at the other end of said electrodes.

6 "2. An accelerator for elemental particles, having a given mass and a given polarity, comprising in a vacuum tight envelope: a first elongated electrode in the form of a body of revolution; a second electrode in the form of a body of revolution coaxial with the first electrode and spaced therefrom; means for creating a radial electric field between said electrodes; means for feeding a direct current to said first electrode for creating a magnetic field'having circular lines of force perpendicular to said electric field; means at one end of said electrodes for propagating a beam of said particles having the samemass and the same polarity in the space comprised between said electrodes, normally to both said fields; means at the other end of said electrodes for collecting said particles; means for feeding ultra-high frequency energy to that one of said electrodes, whose potential is of the same polarity as said particles; delaying elements along the said one electrode for delaying said ultra-high frequency energy to make its phase velocity substantially equal to the velocity of said particles; said elements constituting a plurality of delay line portions, each line portion having a constant delay ratio, the respective delay ratios of these delay line portions decreasing stepwise in the direction of propagation of said beam; and means for causing at least one of said fields to change its value stepwise, at the points where said delay ratios change for increasing thereby the velocity of said particles.

3. An accelerator according to claim 2 wherein the respective lengths of said delay line portions are such that, when the accelerator is in operation, the particles would strike said delay line portions, if at least one of said fields were not changed for decreasing the ratio E/B, E being the electric field and B the magnetic field.

4. An accelerator according to claim 2 comprising energised connections at said first electrode, the portions of said first electrode separating said connections being respectively coextensive with said delay line portions, for

I feeding additional direct currents to said first electrode for stepwise changing the current flowing in said electrode, thereby to decrease said magnetic field for increasing the velocity of said particles.

5. An accelerator according to claim 2 wherein the distance between said two electrodes decreases stepwise, said distance changing at the end of each of said delay line portions.

6. An accelerator according to claim 2 wherein the said phase velocity and the ratios E/B, B being the electric field and B the magnetic field provided between said electrodes, relative respectively to the first and to the nth delay line portion are related as 1 to V2:

7. An accelerator for accelerating elemental particles having the same mass and the same polarity comprising in a vacuum tight envelope: a first elongated electrode in the form of a body of revolution, a second electrode in the form of a body of revolution coaxial with the first electrode andspaced therefrom by a distance which is progressively decreasing from one end of said electrodes to the other; means for creating a radial electric field between said electrodes; means for feeding a direct current to said first electrode for creating a magnetic field having lines of force perpendicular to said electric field; means at said one end for feeding a beam of said particles in the space comprised between said electrodes, and means at said other end for collecting said particles; means for feeding ultra-high frequency energy to that one of said two electrodes whose polarity is the same as the polarity of said particles; delaying elements along the said one electrode, said elements constituting a delay line whose delay ratio decreases progressively from said one end towards said other end of said electrode, for making the phase velocity of said U.H.F. wave substantially equal to the velocity of said particles.

8. An electron accelerator comprising in an evacuated envelope: an elongated electrode in the form of a body of revolution, an anode in the form of a body of revolution coaxial with the first electrode and spaced therefrom; energised connections for negatively biassing said electrode and positively biassing said anode, for providing a radial electric field E between said electrode and said anode; means for feeding direct current to said electrode for providing a magnetic field B having lines of force perpendicular to said electric field; means at one end of said electrode and said anode for propagating an electron beam normally to both said fields in the space between said anode and said electrode; means for feeding U.H.F. energy to said electrode; delaying elements on said elec trode constituting a plurality of delay line sections, each section having a constant delay ratio such that .the phase velocity of said ultra-high frequency energy is equal to the quotient E/B of the electric and magnetic fields prevailing along this delay line section; means for changing said quotient for increasing the velocity of said electrons, at least at certain points of their path,

and means at the other end of said electrodes for collecting accelerated electrons.

9. An accelerator according to claim 8 wherein said means for changing the E/B ratio comprise energised D.C. connections for feeding direct currents to said electrode for stepwise increasing the current flowing in each of said delay line sections from said other end to said one end.

10. An accelerator according to claim 8 wherein the distance between said anode and said electrode is stepwise decreasing from said one end to said other end. this distance remaining unchanged along each one of said delay line sections, for increasing stepwise the value of said electric field.

11. An electron accelerator comprising in a vacuum tight envelope: an elongated electrode in the form of a body of revolution; an anode in the form of a body of revolution coaxial with the first electrode and spaced therefrom; the distance between said anode and said electrode decreasing progressively from one end of said anode and electrode to their other end; energised connections for negatively biassing said electrode and positively biassing said anode for providing a radial electric field between said electrode and said anode; means for feeding direct current to said electrode for providing a magnetic field having lines of force perpendicular to said electric field; means for feeding U.H.F. energy to said electrode; means at one end of said electrode and said anode for propagating an electron beam, normally to both said fields, in the space between said anode and said electrode; a delay line whose delay ratio progressively decreases from said one to said other end for seeping the phase velocity of said ultra-high frequency energy at any point along said delay line substantially equal to the quotient of the electric and magnetic fields preva ing at sa d point, and m a s at d he end of said electrodes for collecting accelerated electrons.

12. An accelerator according to claim 11 wherein said means for propagating electrons in said space comprise: an electron gun having a cathode and an accelerating anode; energised connections for negatively biassing said cathode with respect to said electrode; and energised connections for biassing said accelerating anode to a potential slightly higher than the potential of said electrode.

13. A particle accelerator comprising, within an evacuated cylindrical envelope having a longitudinal axis, an inner electrode extending along said axis, an outer electrode surrounding said inner electrode, coaxial therewith and spaced apart therefrom, one of said electrodes being smooth while the other is shaped as a delay line, whereby a particle and wave interaction space is bounded between said two electrodes, means for injecting into said space an annular beam of electrically charged particles of one polarity, means for establishing a difference of potential between said electrodes to produce in said space electrostatic fields having radial lines of force, wherein said delay line is given essentially the same polarity as said particles with respect to said smooth electrode, means for feeding direct current through said inner electrode to produce in said space a magnetic field having lines of force circular around said inner electrode and perpendicular to said electrostatic lines of force, whereby said particle beam propagates in said space perpendicularly to both said electrostatic and magnetic fields, means for feeding ultra high frequency waves to said delay line to produce interaction between said beam and said waves so that energy is yielded from said waves to said particles owing to said polarity of the delay line, whereby said particles are accelerated, said delay line being arranged to provide therealong wave phase velocities increasing in accordance with the increases of said particle Velocities, means for adjusting the ratio of the electrostatic field strength to the magnetic field strength along said interaction space in accordance with the increases of both said wave phase and said particle velocities, and means for collecting the accelerated particles.

References Cited in the file of this patent UNITED STATES PATENTS 2,233,779 Fritz Mar. 4, 1941 2,439,401 Smith Apr. 13, 1948 2,630,544 Tiley --a Mar. 3, 1953 2,680,823 Dohler et al. June 8, 1954 2,687,777 Warnecke Aug. 31, 1954 2,730,648 Lerbs Jan. 10, 1956 2,774,913 Charles Dec. 18, 1956 FOREIGN PATENTS 984,020 France Feb. 21, 1951

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