US2524252A

US2524252A - Electron accelerator of the microwave type

Info

Publication number: US2524252A
Application number: US6416A
Authority: US
Inventors: William C Brown
Original assignee: Raytheon Manufacturing Co
Current assignee: Raytheon Co
Priority date: 1948-02-05
Filing date: 1948-02-05
Publication date: 1950-10-03
Anticipated expiration: 1967-10-03

Description

Oct. 3, 1950 w, c, BROWN 2,524,252

ELECTRON ACCELERATOR 0F THE MICROWAVE TYPE Filed Feb. 5,4 1948 Patented oct. 3, 1950 ELECTRON ACCELERATOR OF THE MICROWAVE TYPE William C. Brown, Lincoln,` Mass., assignor to Raytheon Manufacturing Campana, Newton, Mass., a corporation of Delaware Application February 5, 1948, Serial No. 6,416

8 Claims. l

This invention relates to electron accelerators, and more particularly to microwave devices for so accelerating electrons as to cause the same to attain velocities representing energies measuring in the millions of electron-volts.

Microwave devices for this general purpose have heretofore not heen very satisfactory due to the difficulty of maintaining the electrons in step with the microwave energy utilized for the acceleration.

The phase velocity of microwave energy the usual microwave device is in excess of the speed of light, and inasmuch as it is not possible to move charges at such speeds, the electrons soon fall out of step with the accelerating energy and only very limited electron velocities result.

It is, therefore, among the objects of the present invention to provide a microwave device incorporating means for reducing the phase velocity of the energy propagated therethrough to less than the speed of light, whereby electrons `projected therein are enabledto keep in step with any given phase of such energy and, so, become continually accelerated to the desired velocities.

It is another object of the present invention to provide a microwave device in which the electrons traveling therethrough are subjected to the ac celerating influence of microwave energy, the

phase velocity of which changes along the length of the device, whereby as the electrons acquire greater and greater velocity, they are affected by energy of continually increasing phase velocity.

These, and other Objects of the present invention, which will become more apparent as the den tailed description thereof progresses, are attained, briefly, in the following manner:

The phase velocity op of microwave energy propagated down a wave guide may be expressed by the equation:

U11- where w is the angular frequency of the energy, and o is the phase constant thereof;

Now, inasmuch as, for any particular' angular frequency, the phase constant is determined by the physical parameters of the wave guide, the

phase constant of the energy of said particular angular frequency may be considered as the phase `constant of said wave guide.

If, in Equation l, the phase constant is given the phase velocity of the wave guide under consideration As stated in earlier portions of this specification, it is desired, iirst, to reduce the phase velocity to something lessV than the speed of light and, then, to increase said phase velocity along the length of the guideto bring about an irl-phase relationship between the velocity of the accelerating electrons and any given phase of the microwave energy. In other words, it is desired that:

where c is the velocity of light, and a: is a nite value which decreases along the length of the guide in a direction receding from the pointof coupling between the microwave generator and said guide.

Transposing the terms of Equation 2, the phase constant may be expressed:

Means are provided, adjacent the point of coupling between the energy generator and the wave guide, for introducing electrons into the latter, the energy propagated through the Wave guide continually accelerating said electrons to give the saine velocities corresponding to millions of electron-volts.` A target at the far end of the wave guide may be bombarded by the accelerated electrons,possibly, to emit X-rays, and the like, or, possibly for useinerely as a source of high voltage.

Inthe accompanying specication there shall be described, andin the annexed drawing shown, an illustrative embodiment of the electron accelerator of the present invention. It is, however, to` be clearly understood that the present invention is not to belirnited to the details `herein `shown and described forpurposes of illustration only, inasmuch as changes therein may be made without the exercise of invention, and within the `true spirit and scope of the claims hereto 'appended.

In said drawing, the single` gure is a vertical sectional view taken susbtantially through the center of a microwave device made in accordance with the principles of the present invention.

Referring now more in detail to the aforesaid illustrative embodiment of the present invention, with particular reference to the drawing illustrating the same, the numeral Il] generally designates an electron accelerator [comprising a cathode structure I I, an anode structure I2 spaced from and surrounding said cathode structure, means I3 for establishing a magnetic field in the space between said cathode and anode structures in a direction transverse to the electron path therebetween, the structures I I, I2 and I3 constituting a microwave generator of the magnetron type, and means I4, receptive of the energy generated by said microwave generator, for accelerating electrons projected `therein as will hereinafter be more fully described.

The cathode structure II includes a sleeve I5 made, preferably, of nickel, and having a portion thereof coated, as at I6, with electron-emissive material, for example, of the alkaline earth metal oxide type, the coated portion being bounded by anges I'I constituting shields for preventing passage of the emitted electrons longitudinally of the sleeve I5.

The upper end of the sleeve I5 is closed by a disk I8 to the inner surface of which one end of a cathode heater I9 is xed, said cathode heater passing downwardly through the sleeve I5 and being connected at its lower end, as at 2t, to a lead-in conductor 2l, the latter, in turn, passing downwardly and out of the sleeve I5 by way of a glass seal 22. Heating current may be supplied to the heater I9 by means of a voltage source 23 connected between the sleeve I5 and the conductor 2l.

The anode structure I2 includes a cylindrical member 211 made, preferably, of copper, and provided with a plurality of inwardly-directed, radial vanes 25, the inner ends of said TJanes being spaced from the cathode sleeve I5 and providing electron-receiving faces 25 substantially coextensive with the coated portion I6 of said cathode sleeve. with that portion of the cylindrical member 24 lying therebetween, constitute a cavity resonator adapted, when suitably excited, to generate electrical oscillations of a frequency which is a function of the geometry of the elements making up the same. Preferably, vthe device of the present invention is so dimensioned as to generate energy in the microwave region of the spectrum, for example, of a wave length of 10 centimeters or less.

In order to suppress spurious oscillations or provide adequate mode separation and, thus, increase the efficiency of the device, alternate vanes 25 are electrically interconnected, at the upper edges thereof, by a low impedance conductor 21 and, at the lower edges thereof, by a similar conductor 28. Intervening vanes 25 are electrically interconnected, at the upper edges thereof, by a low impedance, dish-shaped member 29 having a central opening, 3! and, at the lower edges thereof, by a low impedance conductor 3|.

The lower end of the cylindrical member 24 is closed by an end plate 32 and the upper end thereof is closed by a glass dome 32.

In order to bring the cathode structure II to the outside of the anode structure I2, the' end plate 32 is provided with a central aperture in which a sleeve is xed, the lower end of said sleeve being fastened to a glass sleeve 34 which, in turn, is fastened to a ferrule 35 carried by the cathode sleeve I5. l

Each pair of adjacent vanes 25, together The magnetic field establishing means I3 includes dish-

like pole pieces

36 and 37 spaced from the Vanes 25 at the upper and lower edges thereof, said pole pieces having

flange portions

38 and 39 extending through the cylindrical member 24 of the anode structure I2 and engaging, for example, a horseshoe magnet 50, whereby a magnetic field is set up in the interelectrode region of the device, in a direction perpendicular to the electron path between the cathode and anode structures.

When a device, such as has thus far been described, is supplied with heating current, as from the voltage source 23, and anode voltage, as from a voltage source 4I, one terminal of which is connected to the cathode sleeve I5 and the other terminal of which is grounded, as is the anode structure I2 itself, electrical oscillations are generated in the cavity resonators thereof in a manner well known to those skilled in the art.

In the present invention, the dish-shaped conductor 29 serves two purposes. It constitutes one of the elements of the mode-suppressing or separating arrangement described in earlier portions of this specification, and it functions as a coupling member to lead the energy generated in the cavity resonators of the device out of the same. As shown herein, said conductor 29 has affixed thereto a wave guide, for example, a spiral rod 42, the turns of which are increasingly spaced from each other as it recedes from its point of contact with the coupling conductor 29. Ordinarily, the phase velocity of an electromagnetic wave traveling down a wave guide is in excess of the speed of light. However, this velocity can be reduced to less than the speed of light, for example, by causing the wave to travel over a path, the length of which is greater than the linear dimension of the guide. In such case, the phase velocity along the extended path will still be greater than the speed of light, but by selecting a proper ratio of distance along the extended path to the distance along the vlinear dimension of the guide, the velocity along said linear dimension may be made less than the speed of light.

Consider the spiral member 42 and neglect, for the moment, the unequal spacing of the turns thereof. When this wave guide is excited, the wave will move along a spiral path with a phase velocity (Equation 1) v and this velocity will exceed the speed of light;

but, the distance covered by any given phase per unit time along the linear dimension represented by the axis of the guide will be much less than the distance covered by said phase in the same unit time along said spiral path, and if the diameter of the member #l2 is suciently great, the linear phase velocity will be less than the speed of light. l

The present invention takes advantages of this phenomenon by providing the outer surface of when a point immediately beyond the ,entrance to the guide is at a positive potential with respect to a point at said entrance become accelerated, and, if the guide is properly constructed to cause the accelerating phase to move along the guide at increasing Avelocities, said electrons remain in step with said accelerating phase and, eventually, attain tremendous velocities.

As indicated above (Equation 2), it is desired that the phase velocity be made equal to something less than the speed of light, in other words,`

to c-.r or that the phase constant of `the-guide be made equal (Equation 3) to `C IIJ uHowever, to satisfy the condition described in the previous paragraph, the phase constant must decrease linearly along the guide. This lis accomplished by increasing the spacing between adjacent turns of the guide herein shown, in other words, decreasing the finite value of x in the expression c-. The linear phase velocitt7 increases because by increasing turn spacing, any

given phase travels a greater linear distance per unit time.

The device of the present invention is completed by supporting a target 44 adjacent the outer end of the wave guide 42, which target may be sealed through the glass dome 32 and groundif ed through a load 45. As the accelerated electrons pile up on the target, the potential of said target increases with respect to ground. If desired, the target 44 may be replaced by one which, when bombarded by high-speed electrons, emits ,QS-rays or the like. H i

Thus, there is provided asirnple electron accelerator or voltage generator utilizing microwave energy for the electron acceleration, said device incorporating a wave guide in which the phase velocity of the energy propagated therethrough is reduced to less than the speed of light, whereby electrons projected therein are enabled to keep in step with any given accelerating phase of the propagated energy and in which the phase veloc- "7 where w is the angular velocity of said energy, c is the velocity of light, and :r is a finite value which decreases along the length of said wave guide in a direction receding from the point of coupling between said energy-generating means and said wave guide; and means, adjacent said point of coupling, for introducing electrons into said wave guide.

2. An electron accelerator comprising: a magnetron oscillator for generating microwave energy of a predetermined wave length; a wave guide, coupled to said oscillator, and having a phase constant, relative to sid energyof predetermined wave length, which is equal to C$ where. e is the angular velocity of said zenergy, c is` the velocity of light, and :r is' aiinitesvalue which decreases along the length of said wave guide in a direction receding from the pointlof coupling` between "said oscillator and said 4wave guide; and a commonmeans, adjacent said point of coupling, forintroducing electrons into said wave guide and into` said magnetron oscillator.

3. An electron accelerator comprising: a cathode; an anode structure, spaced from said cathode, and incorporating a plurality of cavity resonators in which microwave energy of a predetermined wave length is adapted to be generated; a wave guide, coupled to said cavity resonators, and receptive of said energy of predetermined wave length; said wave guide having a `phase constant, relative to said energy of predetermined wave length, which is equal to where w is the angular velocity oi?` said energy, c` is the velocity of light, and x is a finite value which decreases along the length` of said wave guide? in a direction receding fromthe point'of coupling between said cavityresonators and said wave guide; and means, adjacent said pointof coupling, for introducing electrons into said wave guide. f

4. An electron accelerator comprising: a cathode; an anode structure, spaced from said cathode, and provided with a plurality of radiallydisposed arm members; each pair 'of adjacent arm members, together with that portion of said anode structure lying therebetween, constituting a cavity resonator in which microwave energy o1 a predetermined wave length is adapted to be generated; a pair of conductive members electrically connecting, respectively,` alternate and intervening anode arm members;v a wave guide coupled to one of said conductive members; said wave guide having a phase constant. relative to said energy of predetermined wave length, which is equal to where `w is the angular velocity of said energy, c is the velocity of light, and o: is a finite value which decreases along the length of said wave guide in a direction receding from the point of coupling between said one of said conductive where w is the angular velocity of said energy, c is the velocity of light, and is a nite value which decreases along the length of said wave guide in a direction receding from the point of coupling between said oscillator and said Wave guide; and means, adjacent said point of coupling, for projecting electrons into the space bounded by said helix along the axis thereof.

6. An electron accelerator comprising: a cathode; an anode structure, spaced from said cathode,and incorporating a plurality of cavity resonators in which microwave energy of a predetermined wave length is adapted to be generated; an electrically-conductive helix coupled to said cavity resonators, and having successive turns increasingly spaced from each other along the axis thereof; said helix constituting a wave guide having a phase constant, relative to said energy of predetermined wave length, which is equal to where .w is the angular velocity of said energy, c is the velocity of light, and is a nite value which decreases along the length of said wave guide in a direction receding from the point of coupling between said cavity resonators and said wave guide; and means, adjacent said point of coupling, for projecting electrons into the space bounded by said helix along the axis thereof.

7. An electron accelerator comprising: a cathode; an anode structure, spaced from said cathode, and provided with a plurality of radiallydisposed anode arm members; each pair of adjacent arm members, together with that portion of said anode structure lying therebetween, constituting a cavity resonator in which microwave energy of a predetermined wave length is adapted to be generated; a pair of conductive members electrically connecting, respectively, alternate and intervening anode arm members; an electrically-conductivehelix coupled to one of said conductive members, and having successive turns increasingly spaced from each other along the axis thereof; said helix constituting a wave guide having a phase constant, relative to said energy of predetermined wave length, which is equal to where w is the angular velocity of said energy, c is the velocity of light, and a: is a nite value which decreases along the length of said Wave guide in a direction receding from the point of coupling between said one of said conductive members and said wave guide; and means, adjacent said point of coupling, for projecting electrons into the space bounded by said helix along the axis thereof.

8. A high voltage generator comprising: a primary cathode; an anode, spaced from said primary cathode, and incorporating a cavityresonator in which microwave energy of a predetermined wave length is adapted to be generated; a wave guide, coupled to said cavity resonator, and adapted to be excited by said microwave energy; said wave guide having a phase constant, relative to said energy of predetermined wave length, which decreases along the axis thereof in a direction receding from the point of coupling between said cavity resonator and said wave guide; a secondary cathode, disposed adjacent the'relatively high phase constant end of said wave guide, and adapted to introduce electrons therein; a target electrode, disposed adjacent the relatively low phase constant end of said wave guide, and adapted to intercept the electrons emitted by said secondary cathode and accelerated toward said target along said wave guide; and an evacuated envelope enclosing said primary and secondary cathodes, said anode, said wave guide, and said target electrode.

WILLIAM C. BROWN.

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

UNITED STATES PATENTS Number Name Date 2,233,126 Haef Feb. 25, 1941 2,300,052 Lindenblad Oct. 27, 1942 2,414,085 Hartman Jan. 14, 1947 2,434,508 Okress et al. Jan. 13, 1948 2,442,118 Donal, Jr., et al. May 25, 1948