Coupled Cavity Slow Wave Circuit And Tube Using Same

Abstract

A coupled cavity slow wave circuit, such as a cloverleaf circuit, and a microwave tube using same are disclosed. The slow wave circuit includes an array of cavity resonators arranged successively along the beam path with adjacent ones of the cavities having a common end wall structure. A plurality of generally radially directed coupling slots are cut through the common wall between adjacent resonators to form a plurality of axially aligned arrays of coupling slots angularly displaced around the beam path. Each array of slots is angularly displaced about the beam path from the adjacent array by (360/N) degrees where N is the number of axially aligned arrays of slots. Each axially aligned array of coupling slots includes means for interrupting the flow of electrons through the slots such as a blocking member or the slots are staggered or offset in radial or angular position such as to block off a line-of-sight path parallel to the beam through at least a portion of each array of coupling slots to inhibit cumulative electromagnetic interaction between undesired beamlets in the arrays of slots and the fields of the slow wave circuit, whereby the efficiency and stability of the tube are increased.

Description

DESCRIPTION OF THE PRIOR ART

Heretofore, cloverleaf slow wave circuits for microwave tubes have included eight arrays of axially aligned coupling slots disposed at (360/8) degrees or 45 degree intervals about the beam path. The slots provide wave energy communication through an array of cavity resonators sequentially arranged along the beam path to form a slow wave circuit. Such a slow wave circuit is disclosed and claimed in U.S. Pat. No. 3,233,139 issued Feb. 1, 1966 and assigned to the same assignee as the present invention.

One problem with this prior art slow wave circuit is that at relatively high power levels, as of 1 megawatt or above, and with relatively long pulse widths, as of pulse widths greater than 10 microseconds, it is found that the electric fields within the successive cavity resonators are such as to accelerate electrons down the length of the slow wave circuit along paths generally parallel to the beam path and through the axially aligned coupling slots. As a result, eight beamlets are formed surrounding the main beam. With these relatively high power levels, the electrons within the beamlets are accelerated to relatively high energies and they bombard both ends of the circuit to produce melting and arcing within the tube. At all power levels, the energy extracted from the fields of the circuit to accelerate the undesired beamlets substantially reduces the efficiency of the tube.

SUMMARY OF THE PRESENT INVENTION

The principal object of the present invention is the provision of an improved coupled cavity slow wave circuit and microwave tubes using same.

One feature of the present invention is the provision, in a coupled cavity slow wave circuit of a microwave tube having N number of angularly displaced arrays of coupling slots equally spaced around the beam path, of means disposed intermediate the ends of the slow wave circuit for interrupting the flow of electrons along a secondary beam path parallel to the primary beam path and through at least the inner radial portion of each of the arrays of coupling slots, whereby undesired beamlets through the coupling slots are blocked to inhibit cumulative electromagnetic interaction between the beamlets and the fields of the slow wave circuit.

Another feature of the present invention is the same as the preceding feature wherein the means for interrupting the flow of electrons along the secondary beam paths comprises means for blocking a line-of-sight path parallel to the primary beam path through each of the inner radial portions of each of the arrays of coupling slots.

Another feature of the present invention is the same as the next preceding feature wherein the means for blocking the line-of-sight path comprises a radial displacement of the geometric centers of the coupling slots of each array along the path through each array of coupling slots.

Another feature of the present invention is the same as the second feature wherein the means for blocking the beamlets comprises an angular displacement of the geometric centers of the coupling slots of each array along the path through each array of slots for blocking a line-of-sight path parallel to the beam path.

Another feature of the present invention is the same as the second feature wherein the means for blocking the beamlets comprises an electron barrier structure disposed in a plurality of the cavity resonators for blocking the secondary line-of-sight path for the electrons parallel to the beam path.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view, partly in schematic form, of a prior art microwave tube,

FIG. 2 is an enlarged sectional view of that portion of the structure of FIG. 1 taken along line 2--2 in the direction of the arrows,

FIG. 3 is a plot of power output versus time depicting the waveform for an output pulse derived from the output of the tube of FIG. 1,

FIG. 4 is an exploded side elevational view of a portion of the structure of FIG. 1 delineated by line 4--4 and modified to incorporate features of the present invention,

FIGS. 5A and 5B are sectional views of the structure of FIG. 4 taken along section lines 5A and 5B, respectively,

FIGS. 6A and 6B are sectional views of the structure of FIG. 4 taken along section lines 6A and 6B, respectively, and depicting an alternative embodiment of the present invention,

FIG. 7 is an enlarged sectional view of the structure of FIG. 4 taken along line 7--7 in the direction of the arrows and depicting an alternative embodiment of the present invention, and

FIG. 8 is a sectional view of the structure of FIG. 7 taken along line 8--8 in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a prior art microwave slow wave tube 1 to be modified to incorporate features of the present invention. More particularly, the microwave tube 1 includes an electron gun assembly 2 for forming and projecting a beam of electrons 3 over an elongated beam path to a beam collector structure 4. A coupled cavity cloverleaf slow wave circuit 5 is disposed along the beam path intermediate the gun 2 and collector 4 for electromagnetic interaction with the beam to produce an amplifier output signal. A beam focus solenoid 6 is coaxially disposed of the slow wave circuit 5 for producing an axially directed magnetic field in the beam path 3 for focusing the electron beam through the slow wave circuit 5 to the collector 4.

The slow wave circuit 5 includes a plurality of axially aligned cloverleaf coupled cavity resonators 7 successively arranged along the beam path for cumulative electromagnetic interaction with the beam 3. Each cloverleaf cavity 7 includes a pair of axially spaced end walls 8 with an inwardly protruding scalloped side wall to provide four conductive nose portions 9 inwardly projecting at 45 degree intervals around the beam path toward the beam axis (see FIG. 2). Adjacent cavity resonators 7 share a common end wall structure 8. The inwardly projecting nose portions 9 in adjacent cavity resonators 7, are angularly displaced about the beam path by 45 degrees.

A pair of radially directed elongated inductive coupling slots 11 are disposed on opposite sides of the nose portions 9 (see FIG. 5) to provide negative mutual inductive coupling between adjacent cavity resonators 7. The coupling slots 11 are radially elongated for increasing the inductive coupling between adjacent cavities. The inductive coupling slots 11, in the prior art, were axially aligned along a path parallel to the beam path and the eight arrays of coupling slots were located at 45 degree intervals about the beam path 3.

Wave energy to be amplified is applied to the upstream cavity 7 via the intermediary of an input waveguide 12 having a wave permeable vacuum-tight window 13 sealed thereacross for maintaining a vacuum within the evacuated microwave circuit 5. Midway along the coupled cavity slow wave circuit 5 the circuit includes a circuit sever 14 which comprises a solid centrally apertured conductive disk without coupling slots to prevent wave energy communication between the upstream slow wave circuit portion 15 and a downstream slow wave circuit portion 16. The disk 14 is centrally apertured to permit passage of the electron beam 3 therethrough. Wave attenuative members 19 and 20 are coupled to the downstream cavity 7 of the upstream circuit portion and the upstream cavity of the downstream portion 16 for absorbing wave energy coupled into the cavities on opposite sides of the sever 14.

In the upstream slow wave circuit portion 15, the microwave energy applied to the upstream cavity establishes a wave on the circuit which cumulatively interacts with the beam to produce bunching thereof. The bunched beam passes from the upstream circuit portion 15 into the downstream circuit portion 16 for exciting a growing wave in the downstream circuit portion 16. The wave in the downstream portion 16 cumulatively interacts with the bunched beam to produce a growing wave on the downstream circuit portion 16. Output wave energy is extracted from the downstream end of the downstream circuit portion 16 via an output waveguide 17 which is sealed by a wave permeable gas tight window assembly 18. The r.f. output energy extracted from the waveguide 17 is fed to a suitable load, not shown.

In the prior art tube 1, when the power output was in excess of 1 megawatt peak and the pulses had a duration in excess of 10 microseconds, as shown in FIG. 3, the peak power output suddenly dropped, for the latter half of the pulse width, to about half of the peak power. It was found that this power loss was associated with arcing within the output circuit portion 16. More particularly, it was found that undesired beamlets of electrons were being accelerated through the axially aligned arrays of coupling slots 11 and were bombarding the end walls of the first and last cavity in the downstream circuit portion 16.

When the power output reached a relatively high level, on the order of 1 megawatt and the pulse length was on the order of 10 microseconds or more, it was found that the energy in the beamlets was sufficient to melt portions of the end walls of the end cavities. When the cavity end wall was melted, trapped gas was released producing an arc in the tube which caused the output power to drop to about half power.

It has been found that the arcing can be prevented and the efficiency of the microwave tube substantially increased by interrupting the flow of electrons within the beamlets as by blocking off a line-of-sight path parallel to the beam 3 through each of the arrays of coupling slots, whereby the beamlets do not form to cumulatively interact with the wave on the slow wave circuit 5. One advantageous arrangement of the present invention for blocking the individual beamlets is shown in FIGS. 4 and 5.

Referring now to FIGS. 4 and 5 there is shown an embodiment of the present invention. More particularly, an examination of the melting pattern produced by the individual beamlets, in the prior art tube of FIG. 1, on the end walls of the last and first cavity, as shown in FIG. 2, shows that the melting is produced in registration with the inner radial one-third of the slot length as taken in the radial direction. Therefore, in the embodiments of FIGS. 4 and 5, the radial position of the coupling slots 11 is staggered or varied in the direction taken along the beam path such that a line-of-sight path through each of the arrays of coupling slots is blocked for at least the inner radial one-third region of the slots.

The radial position of the coupling slots 11 may be varied continuously from an inner position as shown in FIG. 5A to an outer position as shown in 5B or the slots may be provided in two or more configurations, such as an inner position configuration as shown in FIG. 5A and an outer slot configuration as shown in 5B. The inner and outer slot configurations are alternated in the direction along the beam path for blocking a line-of-sight path through the coupling slots to interrupt the beamlets. Alternating between the inner and outer slot configurations has the advantage that only two types of common end walls need to be fabricated, whereas in the case where the position of the coupling slots 11 is continuously varied from an inner position to the outermost position, each end wall has a slightly different configuration.

The successive cavity resonators 7 are stacked into the longitudinal array with the nose portions 9, in adjacent resonators, angularly displaced relative to each other about the axis of the beam 3 by 45 degrees. The cavities 7 and end walls 8 are aligned on eight aligning rods passing through axially aligned apertures 23 provided in the outer margin or lip of the cavity resonators 7 and end walls 8. In this manner precise angular alignment of the successive cavities 7 and end walls 8 is obtained.

Referring now to FIGS. 4 and 6, there is shown an alternative embodiment of the present invention. In this embodiment, a line-of-sight path parallel to the beam axis 3 through each of the arrays of coupling slots 11 is blocked by displacing the angular position of the cavities 7 and end walls 8 in successive cavity resonators 7 about the beam axis. The angular position is displaced in successive cavities 7 by a relatively small angle such as 3 degrees, such that as the circuit advances in the direction along the beam path, the arrays of slots transcribe slightly spiraling configurations to block off a line-of-sight path therethrough parallel to the beam path 3, as shown by comparing FIGS. 6A and 6B. Since the electrons are constrained by the beam focusing magnetic field to paths parallel to the beam path the electrons cannot negotiate the spiral trajectory required to remain as a beam within each of the longitudinal arrays of coupling slots 11.

Referring now to FIGS. 7 and 8, there is shown an alternative embodiment of the present invention. In this embodiment an electron barrier such as an electrically conductive vane 25 as of copper, is disposed in a plane disposed midway along the cavity 7 and is shaped in plan view to cover one or more of the coupling slots 11. The vane 25 is brazed into a transverse slot in the nose portions 9 of the cloverleaf cavities 7. In successive resonators of the longitudinal array of resonators 7, the angular position of the vane is displaced to cover one or more coupling slots 11. More particularly, when the vane 25 covers two slots the angular position of the vane in adjacent resonators can be offset by 90 degrees such that within four successive resonators all of the eight arrays of coupling slots have been blocked. If the vane 25 covers only one slot, then the angular position of the vane 25 is displaced by 45 degrees in adjacent resonators. A succession of eight resonators 7 will thus produce a blocking of each of the eight arrays of coupling slots 11. In a preferred embodiment, the vane 25 is electrically conductive but this is not a requirement it need only be an electron barrier. For example, the vane 25 could be made of alumina ceramic.

As one moves along the slow wave circuit 5 from the downstream end to the upstream end thereof, the electric fields of the wave decrease in intensity. Thus, the requirement for blocking off the individual arrays of coupling slots decreases. Therefore, the beamlet blocking means may be omitted near the upstream end of the slow wave circuit 5, such as in the upstream circuit portion 15.

Also, in certain hybrid circuit tubes, disclosed and claimed in U.S. Pat. No. 3,289,032 issued Nov. 29, 1966, not shown, the upstream portion 15 of the slow wave circuit 5 is replaced by a succession of klystron type buncher cavities. In such a tube, the downstream circuit portion 16 serves as the output circuit. The beamlet blocking means of the present invention are used to advantage to increase efficiency and to prevent arcing in such a hybrid tube.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

Claims

1. In a tube employing a coupled cavity slow wave circuit, means for projecting a beam of electrons over an elongated beam path, slow wave circuit means disposed along the beam path in electromagnetic energy exchanging relation with the beam, said slow wave circuit means including an array of cavity resonators arranged successively along the beam path with adjacent ones of said cavity resonators having common end wall structures, said common end wall structures having a plurality of generally radially directed elongated open inductive coupling slots therein providing wave energy communication through said common end walls, said coupling slots being arranged in N angularly displaced arrays around the beam path, each array being angularly displaced by (360/N) degrees where N is an integral number greater than 2, THE IMPROVEMENT COMPRISING, means disposed intermediate the ends of said slow wave circuit for interrupting the flow of electrons along a secondary beam path parallel to the primary beam path through at least an inner radial portion of each of the arrays of coupling slots, whereby undesired beam paths through said coupling slots are blocked to inhibit cumulative electromagnetic interaction between undesired beamlets in said array of slots and fields

2. The apparatus of claim 1 wherein said means for interrupting the flow of electrons along a secondary beam path includes means for blocking a line-of-sight path parallel to the primary beam path through at least an

3. The apparatus of claim 2 wherein said beamlet blocking means comprises a successive radial displacement of the geometric centers of the coupling slots of each array along a path through each array of coupling slots for blocking a line-of-sight path parallel to the beam path through at least

4. The apparatus of claim 2 wherein said beamlet blocking means comprises a successive angular displacement of the geometric centers of the coupling slots of each array along the beam path through each array of slots for blocking a line-of-sight path parallel to the beam path through at least

5. The apparatus of claim 2 wherein said beamlet blocking means comprises an electron barrier structure disposed in a plurality of said cavity resonators for blocking a line-of-sight path for the electrons parallel to the beam path through at least an inner radial portion of said slots of

6. The apparatus of claim 5 wherein said electron barrier structure comprises an electrically conductive vane structure dimensioned to block off at least a portion of a line-of-sight path parallel to the beam path through one of said arrays of slots, said vane structure being disposed generally midway along the length of each of said cavity resonators.