US2992357A - Microwave linear accelerator - Google Patents

Description

July 11, 1961 J. c. NYGARD 2,992,357

MICROWAVE LINEAR AccELl-:Imoa

Filed sept. 29, 195s United States Patent O 2,992,357 MICROWAVE LINEAR ACCEDERATOR John C. Nygard, Lexington, Maw., assigner to High Voltage Engineering Corporation, lBurlington, Mass., a corporation of Maachusetts Filed Sept. 29, 1958, Ser. No. 764,01'4 5 Claims. (Cl. S15-5.42)

This invention relates to microwave linear accelerators and in particular to a microwave linear accelerator with which there is associated a plurality of radio-frequency power sources which are pulsed in sequence in order to provide high average power output. In accordance with one embodiment of the invention the pulsed radio-frequency sources may have their outputs adjusted in such a -manner that it is possible to introduce energy spread -into the radio-frequency spectrum in order to increase dose uniformity. For example, three radio-frequency power sources may be arranged to provide alternately 5 mev., 4 mev., and 3 mev. electron beams.

The invention may best be understood from the following detailed description thereof having reference to the accompanying drawing in which FIG. l is a circuit diagram illustrating one form of the invention; and

FIG. 2 is another circuit diagram illustrating a modification of a portion of the circuit diagram of IFIG. l.

Referring to the drawing and iirst to FIG. 1 thereof, a conventional microwave linear accelerator comprises an accelerating waveguide 1 into which electrons are injected from an electron injector 2. These electrons are then accelerated by means of a traveling electromagnetic wave which is produced in the accelerating waveguide 1 by means of radio-frequency power which is fed into said waveguide 1 from a suitable power source. In accordance with the invention, a plurality of power sources are provided and in FIG. 1 are shown at 3, 4, 5 and 6. These power sources may be of any conventional type such as klystrons, magnetrons or traveling-wave tubes and are each connected to a transmission line 7 by an appropriate phase shifter, indicated at 8, 9, 10 and 11 for the power sources 3, 4, 5 and 6 respectively. A modulator 12 is alternately connected to the power sources 3, 4, 5 and 6 by means of an appropriate distributing device 13, so that the power sources 3, 4, 5 and 6 are alternately energized. By appropriate adjustment of phase Shifters 8, 9, 10 and 11 mismatches are avoided.

Referring now to FIG. 2 of the drawing, the device therein shown is similar to the device o-f FIG. 1. However, between the phase Shifters 8, 9, 10 and 11 and the transmission line 7 there are provided ferrite isolators '14, 15, 16 and =17, respectively. Moreover, each power source 3, 4, 5, 6 has its own modulator 18, 19, 20 and 21, respectively, and sequential operation is obtained by means of an appropriate trigger circuit 22 which sequentialy tires the modulators 18, 19, 20, and 21 by means of the operation of an appropriate distributor device 13.

Each ferrite isolator 14, :16 and 17 acts to reiiect radiation incident from the transmission line 7, but transmits radiation incident thereon from the power sources 3, 4, 5 and 6, respectively. In principle, each ferrite isolator could be replaced by any other device which permits energy incident from the utilization system to be reected but permits radiation incident from the power sources to be transmitted. In short, the function of each ferrite isolator is to act as a switch junction or gating device and might be replaced, lfor example, by a mechanical shutter which permits energy transmission only when the corresponding power source is pulsed.

Patented July i l1, 1961 By adjusting the relative outputs of the radio-frequency power sources so that they are heterogeneous in nature an energy spread is introduced to the radio frequency spectrum, the result of which is to introduce energy spread into the electron stream produced by the linear accelerator, For example, two radio frequency power sources may be arranged to provide in sequence 5 mev. and 1 mev. electron beams. The eifect is the same as having a main generator producing an electron beam of 5 mev. electrons and an auxiliary generator producing an electron beam of l mev. electrons. The effect of such an arrangement on dose distribution in a product being irradiated by these electron beams is described in detail in U.S. Patent No. 2,724,059, issued to A. J. Gale, and need not be described herein in any detail.

Having thus described the principles of the invention together with several illustrative 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. A microwave linear accelerator comprising in comnation a waveguide adapted to support a traveling electromagnetic wave having an axial electric field component ywhose phase velocity is sufficiently slow for electron acceleration, means for injecting electrons into said waveguide .in the proper phase relationship with said wave for acceleration thereof, a plurality of radio frequency power sources, means -for transmitting radio frequency power from each of said radio frequency power sources to said waveguide, whereby each of said radio Ifrequency power sources is adapted to produce a traveling Wave in said waveguide, and means for energizing said power sources in sequence and in proper phase relationship with the injected electrons, whereby successive sequential pulses from said power sources operate on different electrons.

2. Apparatus in accordance with claim I1 wherein each power source comprises a radio-frequency oscillator each of said oscillators being energized by a common modulator to which they are connected by a distributor device.

3. Apparatus in accordance with claim l wherein each power source comprises a radio-frequency oscillator and a modulator, said modulators being each connected to a common trigger circuit by a distributor device.

4. Apparatus in accordance with claim 1 wherein the outputs of said power sources are heterogeneous, whereiby energy spread is introduced into the electron stream produced by said waveguide.

5. A microwave linear accelerator comprising in combination a waveguide adapted to support a traveling electromagnetic wave having an axial electric eld component whose phase velocityis suiiiciently slow for electron acceleration, means for injecting electrons into said waveguide in the proper phase relationship Awith said wave for acceleration thereof, a plurality of radio frequency power sources, means for transmitting radio frequency power from each of said radio 'frequency power sources to said waveguide, whereby each of said radio frequency power sources is adapted to produce a traveling wave in said waveguide, means for energizing said power sources in sequence and in proper phase relationship with the injected electrons, whereby successive sequential pulses from said power sources operate on different electrons, and means for introducing energy spread into the radio frequency spectrum by varying the output characteristics of said radio frequency power sources with respect to each other.

(References on following page) References Cited in ther le of t'his patent UNITED STATES PATENTS Alvarez Mar. 20, 1951 APirce June 12, 1951 5

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