US3428848A

US3428848A - Synchronous wave linear accelerator wherein the slow wave circuit couples only to the positive synchronous wave

Info

Publication number: US3428848A
Application number: US578440A
Authority: US
Inventors: Bruno W Zotter
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1966-09-08
Filing date: 1966-09-08
Publication date: 1969-02-18
Anticipated expiration: 1986-02-18

Description

Feb. 18, 1969 B w. ZOTTER 3,428,848

SYNCHRONOUS WAVE LINEAR ACCELERATOR WHEREIN THE SLOW WAVE CIRCUIT COUPLES ONLY TO THE POSITIVE SYNCHRONOUS WAVE Filed Sept. 8, 1966 22 & I5 B I FIG,2

INVENTOR, ammo w. ZOTTER 79 ATTORNEYS United States Patent O 3,428,848 SYNCHRONOUS WAVE LINEAR ACCELERATOR WHEREIN THE SLOW WAVE CIRCUIT COUPLES ONLY TO THE POSITIVE SYN CHRON OUS WAVE Bruno W. Zotter, Belmar, N.J., assignor to the United States of America as represented by the Secretary of the Army Filed Sept. 8, 1966, Ser. No. 578,440 US. Cl. 315-35 Int. Cl. H01j 25/36 This invention relates to linear particle accelerators and more particularly to a means for accelerating electrons by microwave energy propagating in a transverse mode on the electron beam.

The acceleration of charged particles in a travelling wave linear accelerator is usually accomplished by coupling to the fast space charge or longitudinal wave. One of the limitations of such a system is that due to the increase of wavelength with increasing beam velocities, the length of the drift regions separating the accelerating regions becomes increasingly longer and the length of each section must be calculated separately. Also, the electron beam is bunched in tighter and tighter bunches so that the output from such devices is in the form of bursts of high velocity particles rather than being continuous.

It is an object of the present invention to provide a linear electron accelerator wherein the above noted limitations are eliminated.

It is another object of the present invention to provide a linear electron accelerator wherein RF energy is converted into electron energy with relatively high efliciency.

Theoretically, it is known that when a transverse-field slow wave circuit interacts with an axial electron beam, the beam is found to propagate four waves which are known as transverse electron beam waves or normal modes. The electron beam is assumed to drift with some constant velocity ,u in a homogeneous magnetic field B directed along the Z axis which coincides with the beam equilibrium position. Two of these waves or modes are known as synchronous waves, one of which is positively polarized. This positive synchronous wave involves only transverse displacement and carries positive RF power. The longitudinal velocity of particles carrying such a positive synchronous wave is greater than the modulated beam velocity. When the velocity increase becomes larger compared to the initial beam velocity, the axial or longitudinal beam velocity will be proportional to the transverse displacement of the positive synchronous wave from the axis or beam equilibrium position. The present invention is based on this linear relationship.

In brief, the present invention provides a unique means for accelerating electrons by microwave energy propagation in a transverse mode on an electron beam. It includes a travelling wave tube having means for projecting an axial electron beam and a transverse-field slow wave circuit in coupling relationship to the electron beam. Also included are means for propagating a microwave signal of relatively high power along the transverse slow wave circuit whereby the signal interacts with the beam to excite a positive synchronous wave on the beam. The slow wave circuit couples only to the positive synchronous waves whereby the electron beam energy is continuously accelerated as it interacts with the coupling slow wave circuit.

For a better understanding of the invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawing in which:

FIG. 1 schematically illustrates a travelling wave particle accelerator embodying the present invention; and

4 Claims "ice FIG. 2 illustrates a typical transverse-field slow .wave circuit for use in the travelling wave particle accelerator of FIG. 1.

For purposes of clarity, the required conventional D-C potentials have been omitted from FIG. 1 of the drawing. Referring now to FIG. 1 of the drawing, at 10 there is shown the evacuated envelope of a travelling wave particle accelerator having an electron gun 12 at one end thereof for emitting an axially electron beam towards a target electrode 14 axially spaced therefrom. A longitudinal or axial magnetic field as represented by the arrow B is provided in the usual manner by an energized solenoid 1S and the axial beam is assumed to be initially drifting with some constant velocity #0 in the homogeneous magnetic field B. Intermediate electron gun 12 and target electrode 14 is a transverse synchronous wave coupler 16 which is adapted to excite and couple only to the positive synchronous beam wave or mode hereinabove described. The structural details of a typical transverse synchronous wave coupler 16 known in the art is shown in FIG. 2. As shown, coupler 16 is essentially a finned ridged circular wave guide with the fins 18 twisted at a prescribed axial pitch which is adjusted to give the optimum synchronization with the positive synchronous wave. An RF signal of relatively high power is applied to the coupler 16 at the end thereof proximal electron gun 12 by means of a suitable waveguide coupling structure shown schematically at lead 20. The other end of coupler 16 is terminated by a matched load as shown at 22. The coupler 16 will act as a transverse-field slow wave circuit which interacts with the axially directed electron beam so that it excites and couples only to the positive synchronous wave carried on the beam. Since the positive synchronous wave carries positive RF power, the kinetic energy of the electron beam will be increased as it passes through and interacts with the tranverse-field slow wave circuit provided by coupler 16. Hence, the velocity of the electron beam is accelerated as it passes through the transverse-field slow wave coupler 16 so that its velocity as it passes through the end thereof is much greater than the initial constant drift velocity p0 of the electron beam. As hereinabove noted, this increase in beam velocity is proportional to the transverse displacement of the positive synchronous wave from the longitudinal axis or equilibrium position of the beam. In FIG. 1, the displacement of the positive synchronous wave from the axis as it progresses therealong is represented by the broken sinuous line 24. At the terminus of the coupler 16, the transverse displacement is shown to have thegreatest magnitude, hence the beam velocity will have maximum acceleration as it leaves the coupler 16. The accelerated beam may be refocussed by a suitable electro-optical lens 30 so that the high velocity beam may be directed to the target 14 from which X-rays may be emitted due to the high velocity electron beam striking the target.

An inherent advantage of the velocity accelerator hereinabove described is the high efiiciency with which the RF power can be converted into beam acceleration. Since the coupling impedance of transverse-field structures such as coupler 16 is much higher than that of longitudinal couplers, acceleration can be achieved over a relatively shorter length. Another advantage resides in the =fact that the beam acceleration is continuous.

While there has been described what is at present considered to be the preferred embodiment of this invention,

it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a travelling wave particle accelerator having means for projecting an axial electron beam within an evacuated enclosure at a constant drift velocity, means for accelerating the beam velocity, said accelerating means comprising, a transverse-field slow wave circuit in coupling relationship to said beam, means for propagating a microwave signal of relatively high power along said transverse-field slow wave circuit whereby said signal interacts with said beam to excite a positive synchronous wave thereon, said slow wave circuit coupling only to said positive synchronous wave whereby said electron beam energy is continuously accelerated as it couples with said slow Wave circuit.

2. The accelerator in accordance with claim 1 wherein said transverse-field slow wave circuit comprises a finned ridged circular waveguide, the fins being twisted at a prescribed axial pitch.

3. The accelerator in accordance with claim 1 and fur- References Cited UNITED STATES PATENTS 3,239,712 3/1966 Norris 328233 X 3,315,118 4/19627 Mi'lller 315-3.6 3,332,024 7/1967 Leboutet 328-233 PAUL L. GENSLER, Primary Examiner.

H. K. SAALBACH, Assistant Examiner.

US. Cl. X.R.

Claims

1. IN A TRAVELLING WAVE PARTICLE ACCELERATOR HAVING MEANS FOR PROJECTING AN AXIAL ELECTRON BEAM WITHIN AN EVACUATED ENCLOSURE AT A CONSTANT DRIFT VELOCITY, MEANS FOR ACCELERATING THE BEAM VELOCITY, SAID ACCELERATING MEANS COMPRISING, A TRANSVERSE-FIELD SLOW WAVE CIRCUIT IN COUPLING RELATIONSHIP TO SAID BEAM, MEANS FOR PROGAGATING A MICROWAVE SIGNAL OF RELATIVELY HIGH POWER ALONG SAID TRANSVERSE-FIELD SLOW WAVE CIRCUIT WHEREBY SAID SIGNAL INTERACTS WITH SAID BEAM TO EXCITE A POSITIVE SYNCHRONOUS WAVE THEREON, SAID SLOW WAVE CIRCUIT COUPLING ONLY TO SAID POSITIVE SYNCHRONOUS WAVE WHEREBY SAID ELECTRON BEAM ENERGY IS CONTINUOUSLY ACCELERATED AS IT COUPLES WITH SAID SLOW WAVE CIRCUIT.