US2667597A - Velocity modulated electron discharge device - Google Patents

Description

R. S. BAILEY Jan. 25, 1954 ECTROZJ DISCHARGE DEVICE Filed Jun 14, 1948 2 Sheets-Sheet l 1N VEN TOR. ROBERT 5. BAILEY ATTORNEY Jan. 26, 1954 R. s. BAILEY VELOCITY MODULATED ELECTRON D ISCHARGE DEVICE 2 Sheets$heet 2 Filed June 14, 1948 INVENTOR. $055197 5. BA/Lf) ATTORNEY Patented Jan. 26, 1954 VELOCITY MODULATED ELECTRON DISCHARGE DEVICE Robert S. Bailey, New York, N.

International Stand 1 Claim.

My invention relates to electron discharge de vices and is particularly directed to improvements in construction and methods of operation of devices of the velocity-modulated types.

In the conventional velocity-modulated microwave generator, a beam of electrons is projected through aligned windows in opposite sides of a cavity resonator, the time phase of the electrons in the windows with respect to the electric fields at the windows being adjusted so that the electrons are bunched. Undulating energy may then be extracted from the electron bunches when passing through the Windows of a second and similar cavity resonator or first cavity by a negatively charged reflector or so-called reflex electrode. In such generators the principal mode of operation contemplated consists of straight longitudinal movement of the electrons through a cylindrical cavit resonator with the walls of the cavity resonator through which the beam passes always 180 degrees out of phase, the single frequency of operation being fixed primarily by the dimensions of the cavity. The wave length, operating in the conventional longitudinal mode, appears to be a function primarily of the radius, r, for a cylindrical cavity resonator and is approximately equal to 2.617.

The object of my invention is improved'apparatus and methods for generating and/or amplifying microwaves.

A more specific obj means for increasing a velocity-modulated ect of my invention is the operating frequency of electron discharge device with a cavity resonator of a given physical size.

A still more sp cific object of my invention is a velocity-modulated electron discharge device employing cylindrical cavity resonators the operat ing wave length of which is less than 2.611, where r is the radius of the cavity resonator of the device.

These and other objects are achieved in a structure of my invention comprising, essentially, a cylindrical wave guide of such length and diameter that there may be excited therein oscillations of a higher order and frequency than that of the longitudinal mode of the conventional velocity-modulating cavity resonator. f, for example, oscillations of the transverse electric mode, designated as the TEo,1,1 mode, are established Y., assignor to an! Electric Corporation, New York, N. Y., a corporation of Delaware Application June 14, 1948, Serial No. 32,945

in my Wave guide, a resonant frequency corresponding to a wave length of approximately one half the wave length in the longitudinal mode may be obtained. In the 'IEo,1,1 mode of the cavity resonator or the corresponding mode TE'o,1 of the wave guide, the electric field appears as circular lines of force in planes transverse to the axis of the wave guide and substantially concentric therewith so that electrons entering one end of the wave guide are propelled by an alternating electric field in a spiral path to the opposite end. The potentials within the wave guide will bunch the electrons and thereafter energy at the mode frequency may be extracted from the bunches in a catcher or debuncher cavity.

The above-mentioned and other objects and features of my invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by ref erence to the following description of preferred embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 shows a longitudinal one-half section of one of said embodiments, and

Fig. 2 shows a longitudinal one-half section of another embodiment of my invention, and

Fig. 3 shows still another embodiment of my invention involving a reflex principle.

In Fig. 1, a velocity-modulated electron discharge device is shown, the device being adapted for either generation or amplification of oscillations in the low centimeter range. At l is a cylindrical wave guide, the radius of the wave guide being chosen large enough to accommodate the lowest frequency, or longest wave length, expected to be amplified or generated in the device.

alignment with the annual window 3 in the end plate of the guide. Extending from the other end of the Wave guide is the drift tube 4, the aligned catcher or debuncher cavity structure 5, and the reflector electrode 6. The Walls of the wave guide, drift tube, cavity structure and the glass bulbs 1 and 8 are placed end to end and may be hermetically joined so that the enclosed space may be evacuated.

The particular cathode shown comprises an annular sleeve, is indirectly heated, and is placed concentric with the center line of the wave guide structures. The side of the cathode annulus facing the wave guide is flattened and coated with electron emissive material. A non-emitting plate 8 is positioned in the center of the cathode annulus. The window 3 in the wave guide, also annular in shape, is approximately coextensive with the emitting portion of the cathode. Control grid in and accelerating and focusing grid H, diagrammatically shown in Fig. l, are placed in front of the cathode and acce erate and direct a tubular stream of electrons through the window 3. Magnetic focusing may be used, if desired.

An antenna I2 to excite oscillations in the wave guide is positioned at or near the cathode end and may comprise a loop, or length, depending upon the position of the antenna and the mode of oscillations to be estab-. lished in the wave guide. The antenna shown is a probe and is energized through thecoa xial cable is which may be connected to an independent source of oscillations to be amplified or to the catcher cavity for feed back energy and selfsustained oscillations. It is important that the excitation of the cavity be accomplished. in such a way, well known to the art, that circular electric field lines be obtained in a plane transverse to the axis of the cavity.

In operation as an amplifier, the desired waves are applied to the antenna. Assume that the applied waves have a length less than 2.611, where r is the wave guide radius, and that the antenna is a probe at the center of the end plate of he wave guide as shown, then, oscillation in the transverse electric mode, TE-0,1, will travel lengthwise of the wave guide. prise circumferential electric lines in planes transverse to the wave guide traveling lengthwise of the wave guide at the group velocity Vg. Then, electrons roiected into such an oscillatory field at a velocity near that of Vg will spiral helically about the center line of the wave guide and will travel along the wave guide, through the annular window is and into the drift tube The alternating field potential within the wave guide will velocity modulate the electrons and bunch'them along the spiral path and electrons passing from the second annular window it continue bunching in the drift tube.

Accordingly, the final result of my invention is a helical stream of bunched electrons with a I buch frequency corresponding to wave lengths much shorter than the wave length possible in cavity resonators of conventional velocity modulators. Calculations show that in the TE mode, the waves that can be amplified may be of any length shorter than the cut-off wave length of 1.64? and that with a wave guide having a radius of l centimeter, the wave length may be 1.6% centimeters or less. Such frequencies are not possible with conventional velocity-modulated tubes of similar dimensions.

To prevent reflections and standing waves in the wave guide, the output end may be coated with a layer of resistance material, such as carbon, of such depth and composition as to have a resistance equal to the characteristic impedance of the wave guide.

For extracting the radio frequency energy from the bunched electrons a wave guide 5 similar in size and shape to the buncher l ,is preferred, so that the boundary condition may be similarand the broad-band characteristics of the buncher wave guide preserved.

Electronsemerging. from. the outer end of the debuncher wave guide 5 are absorbed and reprobe of the proper.

Such oscillations comamplitude and/or frequency modulate the high collecting electrode '0, and oscillatory energy is picked up by the probe H5 at the output end of the debuncher cavity and conducted away through the coaxial cable l'l. A portion of this energy may be fed regeneratively to the buncher wave guide for sustained ocillations or for increased amplifier gain. If the feedback circuit is frequency selective, the resonant frequency of the feedback circuit may determine the operating frequency of the device.

In Fig. 2 is shown another velocity modulated electron discharge device of my invention. Where wave guides several wave lengths long are not desired and where narrow band or fixed frequency operation is. desired, yet where the high frequency of wave guide modes are necessary, the device of Fig. 2 may be used. Here the wave guide 18 is but a half guide wave length long and is operable, in its fundamental mode, with a transverse electric field, designated TEo,1,1. The loop is in the side of the wave guide will excite such a mode. The resonant wave length, A res. of the shortened cavity is 2L Y res: v 1 (its) where L is the length and r is the radius of the resonant wave guide or cavity resonator. The electric field here also appears as circular lines of force in planes perpendicular to and concenmoved by the tric with the axis of the cavity. Electrons from.

cathode 2 projected into the cavity at about the group velocity will travel helically and bunch circumferentially. Drift space 4 allows continued bunching, and cavity zll-dimensioned for TE0,1,1 oscillations, is phased to receive and ex tract high frequency energy from the bunches. The output circuit may comprise a loop 22 in the side of the cavity on the end of a coaxial cable 22. Electrode 6 collects the spent electrons.

Alternatively, the high frequency energy from the bunched electrons may be derived by returning the bunches to and into the opening in the buncher cavity. As shown in Fig. 3 a. reflector electrode 23 with the proper negative potential is placed across the electron trajectory for reversing the direction of electron flow and returning the electron bunches to the cavity E8 in phase with the oscillations in the cavity.

In each of the embodiments disclosed, a longitudinal magnetostatic field may be employed to keep the electrons in their proper orbits. One specific suggestion for application of magnetic focusing to my cavity resonator or wave guide oscillators is shown in Fig. l where a direct current coil 24 is telescoped over the wave guides or resonators to establish magnetic lines substantlally parallel to the axis thereof. Signals imposed on the magnetic field would, conveniently,

frequencies of the device.

'While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

I claim:

an electron discharge device comprising a wave guide, said wave guide being a plurality of guide wave lengths long, an antenna in said wave guide for establishing electromagnetic oscillations in sald wave guide of a frequency above the cut-oil" frequency of said waveguide, windows in opposite ends of said wave-guide offset laterally with respect to the longitudinal axis of said guide, an electron source at one end of said wave guide to release a stream of electrons into one window and hence to and through the other window, a drift tube and a catcher cavity aligned with said other window. 7

ROBERT S. BAILEY.

References Cited in the file of this patent UNITED STATES PATENTS N me Date Number a Potter July 5, 1938 Number 6 Name Date Blewett et a1 May 13, 1941 Hansen et a1 Oct. 21, 1941 4 Tonks Mar. 17, 1942 Lindenblad Oct. 27, 1942 Llewellyn Jan. 16, 1945 Mason July 3, 1945 Hansen et a1 Aug. 27, 1946 Hartley et a1 Sept. 10, 1946 Samuel Oct. 15, 1946 Okress Oct. 22, 1946 Derby Dec. 25, 1951

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