US2922921A - Compact linear accelerator - Google Patents

Description

Jan. 26, 1960 J. c. NYGARD COMPACT LINEAR ACCELERATOR 2 Sheets-Sheet 1 Filed Oct. 28, 1954 Jan. 26, 1960 J. c. NYGARD 2,922,921

COMPACT LINEAR ACCELERATOR Filed Oct. 28, 1954 2 Sheets-Sheet 2 FIG. 3

United States Patent 2,922,921 COMPACT LINEAR ACCELERATOR John C. Nygard, Waltham, Mass., assignor to High Voltage Engineering Corporation, Cambridge, Mass., a corporation of Massachusetts Application October 28, 1954, Serial No. 465,327 3 Claims. (Cl. 315--'5.42)

This invention relates to microwave linear accelerators for the acceleration of electrons to high energy, and in particular to a microwave linear accelerator wherein an electron discharge device which is used to supply highfrequency power to the acceleration tube or waveguide of the linear accelerator is simultaneously used to inject electrons into the waveguide, thereby eliminating the nec'essity for a separate electron injector.

In a microwave linear accelerator electrons are injected into one end of a Waveguide in which a traveling electromagnetic wave is produced by means of highfrequency power fed into the waveguide from a highfrequency oscillator such as a magnetron or klystron. The apparatus is designed so that the electromagnetic wave has an axial electric field component, and the Waveguide is iris-loaded so that the phase velocity of the traveling wave is very nearly equal to the velocity of light in vacuo, or 3 10 cm./sec., The electrons are injected into the, waveguide at a velocity which is almost equal to the velocity of light, so that the electrons start traveling down the waveguide with almost the same velocity as the phase velocity ofthe wav Some of the electrons will find themselves in anaccelerating electric field which raises their velocity to a value very nearly equal to the velocity of light. These electrons can then gain only a negligible amount of additional velocity, and so they remain in the accelerating field throughout the length of the waveguide, continually gaining energy from the wave in the form'of an increase in the electrons mass. If this accelerating field is E volts/cm. and if the waveguide is L cm. long, the electrons in the field will issue from the waveguide with an energy (E+ EL) electron volts, where E is the energy ofthe] electronsat injection into the Waveguide. j

As stated, the traveling wave is produced by means of a high-frequencyoscillator, such as a magnetron or a klystron, whose high-frequency power output is delivered to the waveguide in such a way as to produce the desired wave. In some cases where added power is desired, the output from the oscillator may be amplified by one or more high-frequency amplifiers, such as klystrons. i

- My invention relates to microwave linear accelerators wherein the high-frequency power source for the- Waveguide includes "at least one electron discharge device which produces a'n electron beam, and my'invention comprehends the simultaneous use of such an electron discharge device as the electron injector for the linearaccelerator.

In the preferred embodiment of my invention, a klystron which forms a part of the high-frequency power supply is simultaneously used as the electron injector. By using a klystron as the'dual-purpose electron discharge device, the electron beam may be injected into the waveguide in such away that a maxi-mum number of electrons receive optimum acceleration in the waveguide. However, my invention also' comprehends the use of other high-frequency power units as the electron injector, such as a traveling-wave amplifier tube. High-frequency 2 power units, such as magnetrons, which do not produce an electron beam, are not suited'to use as the dual purpose electron discharge device of my invention.

In the drawings:

Fig. 1 is a diagrammatic view in cross-section of a microwave linear accelerator in which, in accordance with my invention, a klystron oscillator simultaneously serves as the high-frequency power source and as the electron injector; t

'Fig. 2 is a diagram illustrating the general configuration of the lines of force of the electric field within the waveguide of the linear accelerator of Fig. 1;

Fig. 3 is a view similar to that of Fig. 1, showing a microwave linear accelerator in which, in accordance with my invention, a klystron amplifier simultaneously serves as a part of the high-frequency power source and as the electron injector; and

Fig. 4 is a view similar to that of Fig. 1, showing a,

microwave linear accelerator in which a traveling-wave amplifier simultaneously serves as a part of the highfrequency power source and as the electron injector.

Referring to the drawings and first to Fig. 1' thereof, .therein is diagrammatically shown a waveguide 1 of a conventional microwave linear accelerator. High-frequency power, for the production of the'desired traveling wave in the waveguide 1, is fed to the waveguide 1 from a multiple-resonator klystron oscillator 2 by a suitable transmission line 3. Except as noted hereinafter, the

klystron 2 may be of a conventional type. Electrons emitted from a cathode 4, to which-a negative potential is applied by voltage source 5, are accelerated toward a pair of grids 6 which constitutte a part of the bounding surface of an input cavity resonator 7. The accelerated electrons are maintained as an electron beam 8 by means of focusing magnets 9 provided at intervals along the length of the klystron 2.

' The resonator 7 is tuned to the desired frequency f, and is so designed that electromagnetic oscillations therein create a high-frequency electric field between the grids 6 and perpendicular thereto. A high-frequency signal,

having the frequency f, is fed into the input resonator 7 by a transmission line 10, and the resultant oscillations in the resonator 7 create an electric field between the grids 6 which alternately accelerates and decelerates the electrons traveling therebetween. The input resonator 7 thus velocity modulates the electron beam 8.

After emerging from the grids 6, the electrons travel, through' a first drift-space 11, in which the accelerated electrons catch up to the unaccelerated electrons preced-,

ing them, and the decelerated electrons drop back to the unaccelerated electrons following them, so that the elec-. tron beam 8 becomes bunched. The bunched beam then travels through a second pair of grids 12, which constitute a part of the bounding surface of a second cavity resonator 13. The second resonator 13 is substantially identical to the input resonator 7, and is tuned to the same frequency f. The passage of the electron beam 8 in bunched form through the second resonator 13 induces oscillations in the second resonator 13, and the resultant high-frequency electric field between the grids 12 velocity-modulates the electron beam 8 still more.

, After emerging from the grids 12, the electrons travel through a second drift space 14, in which still more bunching of the electron beam 8 occurs.- The highly bunched beam then travels through a third pair of grids. 15 which constitute a part of the bounding surface of an output cavity resonator 16. The output resonator 16 is substantially identical to the other two resonators 7, 13, and is tuned to the same frequency f. The passage of the highly bunched electron beam 8 through the output resonator 16 induces oscillations in the output 3 resonator 16 which are of much greater amplitude than the oscillations in the input resonator 7.

In this manner, the klystron 2 has amplified the signal which was fed into the input resonator 7. In the apparatus of Fig. l, the klystron 2 is to operate as an oscillator, and so a portion of the high frequency output from th'e'out'put resonator 16 is fed back to the input resonator 7 through the transmission line 10. Since a klystron is a relatively broad-band device, a high-Q cavity resonator 17 should be included in the feed back transmission line 10 for proper frequency control. The rest of the highfrequency output from the output resonator 16 is fed to the waveguide 1 through the transmission line 3.

In a microwave linear accelerator, the frequency of the traveling wave is preferably as high as possible. A frequency as high as 10,000 megacycles would be desirable, but the power tubes currently available limit the frequency attainable. At the present time, a frequency of 3,000 megacycles may be taken as representative, which corresponds to a wavelength of 10 cm. Maximum highfrequency power is desired, and so the klystron 2 should be designed so as to give maximum power. At the present time, klystrons delivering power of up to on the order of 10 megawatts can be constructed. Using such a high-power klystron in the apparatus of Fig. l, the oathode 4 will deliver about 100 amps. and the voltage source 5 will deliver 100 kv. Since klystrons are about 30 percent efiicient at the present time, this input of megawatts will result in an output of about 3 megawatts.

Owing to the high power involved, the klystron 2 is pulsed, rather than being operated continuously. Hence, the voltage source 5 comprises a modulator giving an output of square-wave, 100-kv. pulses about 2 microsecs. in duration, vw'th a pulse repetition rate of about 35 to 350 p.p.s. The pulse length is seen to be very long compared with the period of the high-frequency oscillations. In describing the operation of microwave linear accelerators, reference is made throughout the description herein to conditions during the pulse. Thus, for example, the 3- megawatt output of the klystron hereinbefore mentioned refers to the power output during the pulse and not to the average power output.

Referring again to Fig. l, the electron beam 8 cmerges from the grids into a third drift space 18. The passage through the output resonator 16 results in additional velocity modulation of the electron beam 8, so that it arrives at an anode 19 as a very highly bunched beam. The function of the anode 19 is merely to collect the electrons in the electron beam 8, and in a conventional klystron, the anode 19 is a solid conductive block having cooling means for dissipating the heat generated by the bombardment thereof by the electron beam 8.

In accordance with my invention, and as shown in Fig. l, I provide an aperture 20 in the anode 19 in order to permit a portion 8' of the electron beam 8 to travel through the aperture 20 and into the waveguide 1, in which some of the electrons in the electron beam 8 become accelerated to high energy by the traveling wave existing in the waveguide 1. Since only a portion 8' of the electron beam passes through the aperture 20, the heat generated by the impinging of the rest of the electrons in the beam 8 upon the anode 19 must be dissipated, and for that purpose a coil 21 of copper tubing through which water is circulated is wound about the anode 19.

Consider first the properties that We wish the electron beam 8 to have as it is injected into the waveguide 1. Referring now to Fig. 2, therein is shown the general configuration of the lines of force of the electric field in the waveguide 1, with the arrows pointing in the conventional sense, so that an electron will be accelerated against the arrows. The wave, and hence the field pattern'shown in Fig. 2, is moving from left to right with a phase velocity c=3 l0 cm./sec. In the following discussion, it will be assumed that we are riding with the wave, so that the waveguide 1 is moving from right to left with the velocity c and the field pattern shown in Fig. 2 is stationary.

Consider now an electron which enters the wave at the point A, where the electric field is zero. Initially the electron is neither accelerated nor decelerated, and so if it enters the wave at the velocity 0, it will travel the length of the waveguide 1 without gain or loss in energy. In fact, the electron enters the wave at a velocity less than c and so it tends to drop back to the point B. At B, however, the electron is in an accelerating field, and thus is speeded up by the wave. If the initial velocity of the electron is sufficiently. great, the accelerating field will speed up the electron to very nearly the velocity c, so that upon reaching the point B it travels in phase with "the wave. Since the electron cannot travel faster than c, it will remain at B and continue to gain energy from the wave in the form of an increase in the electrons mass.

If the initial velocity of the electrons is insufficiently great, it will drop back towards the point C. If the electron drops back beyond the point C without gaining enough velocity from the wave to keep in phase with the wave, it will then enter a decelerating field. The electron will therefore continue to lose velocity until it drops back beyond the point D. The slower the initial velocity of the electron, the greater the danger that it will continue to drop back without ever gaining an appreciable amount of energy from the wave.

The foregoing discussion is greatly oversimplified, but it shows that the electrons should be injected into the waveguide 1 with as high a velocity as possible.

Referring again to Fig. 2, electrons between A and C are in an accelerating field, while electrons between C and D are in a decelerating field. Since the wave travels with the velocity 0, all electrons which move with respect to the wave will move from right to left. An electron having a velocity less than the wave will require more time to drop back from A to C than will be required for it to drop back from C to D, since the accelerating field impedes its dropping back while the decelerating field assists it. This means that the electrons will tend to cluster about the points A and D, with a relative scarcity of electrons about the point C. More precisely, since electrons at A and D are necessarily traveling more slowly than the wave, the electron beam will become bunched slightly to the left of the points A and D.

Again, the foregoing discussion is greatly oversimplified, but it shows that the electron beam in the waveguide 1 is a bunched beam.

It is now clear that the electron beam 8' of Fig. 1 is well-suited for injection into the waveguide 1. Many of the electrons in the beam 8' will have energies on the order of twice the voltage of the voltage source 5. Thus, if the voltage source 5 has a -kv. output, many of the electrons in the beam 8' will have energies of about 200 kev., the additional energy having been gained from the resonators 7, 13, 16.

As the electron beam 8 enters the output resonator 16, it will be highly bunched. The largest bunches will arrive, with a frequency f, so that they encounter a decelcrating electric field as they pass between the grids 15. They therefore lose energy which is imparted to the high-frequency field in the output resonator 16. However, between the largest bunches, there will arrive other bunches, with a frequency of f or some multiple thereof, and most of these bunches will encounter an accelerating field as they pass through the resonator 16. Some of the electrons in these bunches will have already experienced a net energy gain in traveling through the first two resonators 7, 13 and thus some of the electrons in the beam 8 as it emerges from the output resonator 16 have energies as high as twice the klystron-injector energy.

Moreover, further bunching of the electron beam 8 takes place in the third drift space 18, so that the electron beam 8 arrives at the anode 19 as a series of bunches, the series being repeated with a frequency f, and some of the bunches being composed of electrons having energies of twice the klystron-injector energy.

' Referring now to Fig. 2, it will be recalled that the major portion of the useful electron beam in the waveguide 1 is concentrated in bunches just to the left of the points A, D. These bunches are spaced a wavelength apart. Now the electronbeam 8' (Fig. 1) which is injected into the waveguide 1 includes bunches of electrons having energies of twice the klystron-injector energy, and also spaced a wavelength apart. By proper adjustment of a phase control device 22, which forms a part of the transmission line 3, these bunches may be injected into the waveguide 1 so as to arrive in phase with those regions of the wave where bunching naturally occurs.

Thus high-energy, high-current portions of the electron beam 8' are injected into that portion of the traveling wave which provides maximum acceleration. By my in vention, not only'is the necessity for a separate electron injector eliminated, but the output of the linear accelerator is maximized through the injection of high energy electron bunches into the waveguide of the linear accelerator in phase with the accelerating portions of the traveling wave.

-It will be recalled that, although some of the bunches in the electron beam 8 are composed of high-energy electrons, the bunches which contain the highest-energy electrons are not necessarily the bunches which contain the highest current. As hereinbefore pointed out, the largest bunch (and hence the bunch containing the highest current) loses much of its energy in passing through the output resonator 16. However, the electron beam 8 has an average current of 100 amperes, so that even though the bunches containing the highest energy electrons contain a current which is low compared with 100 amperes, these bunches will nevertheless have a current well in excess of the current required to be injected into the waveguide 1, which is about 1 ampere. In fact, the current in the electron beam 8 is too large for injection into the waveguide 1, and must be reduced by making the area of the aperture 20 smaller than that of the electron beam 8, so that some of the electrons in the beam 8 are collected on the anode 19 and their energy dissipated in the form of heat. The size of the aperture 20 is thus selected such that the desired beam current is transmitted therethrough. Thus, for example, if the klystron beam 8 carries 100 amperes and has a diameter of /2-inch, an aperture 20 having a diameter of or less would admit an electron beam 8 of about 1 ampere.

It has just been stated that no more than about 1 ampere need be injected into the waveguide 1. The reason for this is the fact that the beam current of a microwave linear accelerator can be increased for a given high-frequency power input only at the expense of the beam energy. Thus, for example, a linear accelerator that can accelerate electrons to a maximum energy of about mev. with negligible beam current can accelerate electrons to a maximum energy of only about 6 mev. if the beam current is about ampere.

As hereinbefore stated, the klystron which is used as the electron injector need not be the high-frequency oscillator, but may also be a high-frequency amplifier of the high-frequency power supply. Fig. 3 shows a klystron amplifier 2' which injects electrons into the waveguide l in the same manner as the klystron oscillator 2 of Fig. 1. The high-frequency output of a magnetron oscillator 23 is amplified by the klystron 2' and then fed into the waveguide 1. v

In the apparatus of Fig. 4, the high-frequency output of the magnetron oscillator 23 is amplified by a traveling wave tube 24, and then fed into the waveguide 1. A portion 25' of the electron beam 25 of the traveling wave tube 24 is directed into the waveguide 1 through an aperture 26 in an anode 27 so that the traveling-wave tube 24 serves both as high-frequency amplifier and electron in-' jector.

The traveling wave in a traveling wave tube is similar to that within the waveguide of a microwave linear accelerator, and so the diagram of Fig. 2 also serves to illustrate the electric field in the traveling wave tube. However, in the case of the traveling wave tube, the field pattern shown in Fig. 2 moves from left to right with a phase velocity much less than the velocity of light, and

the electrons in the electron beam of the traveling wave tube move from left to right with a velocity greater than the phase velocity of the wave. As an electron moves from D to C, it is decelerated, and hence some of its kinetic energy is converted into the electromagnetic energy of the wave. As the electron moves on from C to A, it is accelerated, and hence gains energy from the wave. However, the electron spends more time in traveling from D to C than in traveling from C to A, so that the net effect is a transfer of energy to the wave. Moreover, the electrons tend to form bunches in the regions just to the right of points A and D.

The electron beam of the traveling-wave tube 24 of Fig. 4 is created in much the same manner as the electron beam 8 of the klystron 2 of Fig. 1. Thus, electrons emitted from a cathode 4 are given an initial energy of about kev. by a voltage source 5. However, unlike the klystron 2 (Fig. l), the traveling wave tube 24 (Fig. 4) tends to cause the electron beam 25 to form bunches all of which lose energy to the wave. Hence, the traveling-wave tube 24 does not inject electrons into the waveguide 1 with as high an energy as does the klystron 2 (Fig. 1). However, the electrons in the injected beam 25 have energies of about 50 kev., and this will be adequate for proper acceleration in the waveguide 1.

Having thus described the preferred embodiment of my invention, together with several illustrative alternate embodiments thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

I claim:

1. An electron accelerator comprising in combination: an evacuated enclosure adapted to sustain a high-frequency electromagnetic field of such a nature that the electric field component of said electromagnetic field along a rectilinear path through said enclosure accelerates electrons which travel along said rectilinear path in proper phase relationship with said electromagnetic field; means for creating an electron beam; a first space-resonant device supported in the path of travel of said electron beam; means for generating electromagnetic oscillations in said first space-resonant device which are adapted to velocity-modulate said beam so that after leaving said first space-resonant device the electrons of the beam become concentrated in a periodically recurring series of groups, each series including at least one group of relatively high electron density; a second space-resonant device tuned to the frequency of recurrence of said series of groups and supported in the path of travel of said electron beam, whereby said groups of relatively high electron density create electromagnetic oscillations in said second space-resonant device by virtue of their passage through said second space-resonant device, said electromagnetic oscillations therefore accelerating other groups which pass through said second space-resonant device out of phase with said groups of relatively high electron density; means for conveying the energy of the electromagnetic oscillations in said second space-resonant device into said evacuated enclosure, wherein said energy is stored in the form of the electromagnetic field in said enclosure; means for directing at least some of the accelerated groups into said enclosure along said rectilinear path; and a phase shifter coupled between said second space-resonant device and said evacuated enclosure,

whereby the phase relationship between the electromagnetic field 'in' said enclosure and other electron groups than said groups of relatively high electron density. may be adjusted 'so that said other electron groups are accelerated by said electric field component.

2. An electron accelerator comprising in combination: an evacuated enclosure adapted to sustain a high-frequency electromagnetic field of such a nature that the electric field component of said electromagnetic field along a rectilinear path through said enclosure accelerates electrons which travel along said rectilinear path in proper phase relationship with said electromagnetic field; means for creating an electron beam; a first space-resonant device supported in the path of travel of said electron beam; means for generating electromagnetic oscillations in said first space-resonant device which are adapted to velocity-modulate said beam so that after leaving said first space-resonant device the electrons of the beam become concentrated in a periodically recurring series of groups, each series including at least one groupof relatively high electron density; a second space-resonant device tuned to the frequency of recurrence of said series of groups and supported in the path of travel of said electron beam, whereby said groups of relatively high electron density create electromagnetic oscillations in said second space-resonant device by virtue of their passage through said second space-resonant device, means for conveying the energy of'the electromagnetic oscillations' in said second space-resonant device into said evacuated enclosure, wherein said energy is stored in the form of the electromagnetic field in said enclosure; means for directing at least some of the groups of said series of groups into said enclosure along said rectilinear paths; and a phase shifter coupled between said second spaceresonant device and said evacuated enclosure, whereby the phase relationship between the electromagnetic field in said enclosure and said groups directed along said rectilinear path may be adjusted so that said groups directed along said rectilinear path are accelerated by said electric field component.

3. An electron accelerator in accordance with claim 2, wherein said means for generating electromagnetic oscillations in said first space-resonant device includes said second space-resonant device.

References Cited in the file of this patent UNITED STATES PATENTS 2,392,380 Varian Jan. 8, 1946 2,464,349 Samuel Mar. 15, 1949 2,524,252 Brown Oct. 3, 1950 2,556,978 Pierce June 12, 1951 2,582,186 Willshaw Jan. 8, 1952 2,630,544 Tiley Mar. 13, 1953 2,767,259 Peter Oct. 16, 1956

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