US3456207A

US3456207A - Integral cavity multicavity linear beam amplifier having means for applying a d.c. voltage across the interaction gaps

Info

Publication number: US3456207A
Application number: US585439A
Authority: US
Inventors: George M W Badger
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1966-10-10
Filing date: 1966-10-10
Publication date: 1969-07-15
Anticipated expiration: 1986-07-15

Description

July 15, 1969 G. M. w. BADGER 3,456,207

INTEGRAL CAVITY MUL'I'ICAVITY LINEAR BEAM AMlblFllflR HAVING MEANS FOR APPLYING A D.C. VOLTAGE ACROSS THE INTERACTION GAPS Filed Oct. 10, 1966 FIG. I PRIOR ART FIG. 3

25 l FIG. 2

INVENTOR.

GEORGE M.w. BADGER ATTORNEY United States Patent US. Cl. 33045 4 Claims ABSTRACT OF THE DISCLOSURE A multicavity amplifier tube is disclosed. The tube is of the integral cavity type wherein the internal walls of the cavity define the vacuum envelope of the tube. A plurality of the cavity resonators, which are located downstream of the input resonator, include R.F. bypass structures surrounding the drift tube members such that independent operating D.C. potentials may be applied to one of the interaction gap defining members within each of the cavities relative to the opposed gap defining memher. In this manner, accelerating D.C. voltages may be applied to the beam along with the R.F. voltages developed between the gap defining members by the circulating currents to improve the efficiency and stability of the tube. The R.F. bypass structure is disposed inside the vacuum envelope and comprises a section of transmission line including a radial transmission line portion with a vacuum tight insulator disposed surrounding the periphery of the radial transmission line portion. The bypass transmission line sections are dimensioned to produce an effective R.F. short at the gap defining member within the resonator such that the bypass structure does not interfere with the circulating R.F. currents within the cavity resonators.

Heretofore, D.C. accelerating voltages have been applied across the reentrant interaction gaps of external cavity multicavity klystron amplifiers for increasing their overall R.F. efiiciency. In an external cavity klystron, the cavity resonators are each separated into two portions, an evacuated portion containing the interaction gaps, and a non-evacuated portion. Typically, the non-evacuated portion surrounds the evacuated portion and is separated therefrom by a cylindrical R.F. window member sealed at its ends between the axially spaced end walls of the cavity and surrounded by the side walls of the cavity.

In the prior external cavity, having the accelerating voltage applied across the interaction gaps, R.F. by-pass capacitors were built into the end walls of the external cavity portions such that the opposed reentrant gap defining members, which are afiixed to the opposed end walls of the evacuated portions of the cavity, could be operated at different potentials. Such by-pass capacitors were relatively simple structures comprising only a thin slab of dielectric material sandwiched between radially overlapping end wall portions of the external cavity portion. Such capacitors did not need to be gas tight, as they were disposed in the non-evacuated portion of the external cavity.

In the present invention, an internal multicavity klystron includes an R.F. by-pass means, such as a choke or by-pass capacitor, in its evacuated integral cavity structure for separating the cavity wall structure to permit application of different D.C. potentials to the opposed reentrant gap defining member of one or more cavities.

In addition, a gas tight insulator assembly is connected between the by-passed structure to complete the vacuum 3,456,207 Patented July 15, 1969 envelope and to hold off the applied DC. potential between the separated portions of the cavity. The integral cavity linear beam amplifier of the present invention has substantially increased overall R.F. efliciency as compared to other integral cavity linear beam amplifiers.

The principal object of the present invention is the provision of an improved integral cavity multicavity linear beam amplifier.

One feature of the present invention is the provision of an R.F. by-pass and a gas tight insulator assembly in an integral evacuated cavity structure of a linear beam amplifier for electrically isolating the gap defining members of the cavity as regards D.C. potentials while not interrupting the R.F. circuit of the cavity, thereby permitting an accelerating potential to be applied across the interaction gap of the tube for increasing the overall R.F. efiiciency of the tube.

Another feature of the present invention is the same as the preceding feature, wherein the R.F. by-pass is a capacitor or a choke.

Another feature of the present invention is the same as any one or more of the preceding features wherein the R.F. by-pass is evacuated and vacuum sealed at its outer periphery by a gas tight cylindrical insulator structure.

Another feature of the present invention is the same as any one or more of the preceding features wherein the by-pass structure includes a pair of concentrically disposed axially coextensive tubular members extending axially into the cavity resonator and forming one of the reentrant interaction gap defining structures.

Another feature of the present invention is the same as any one or more of the preceding wherein the by-pass structure includes a pair of axially spaced radially coextensive members defining a section of radial line.

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:

FIG. 1 is a longitudinal cross sectional view of a cavity resonator portion of a prior art external cavity klystron,

FIG. 2. is a longitudinal sectional view, partly schematic, of an integral cavity multicavity klystron amplifier embodying features of the present invention, and,

FIG. 3 is an enlarged view of an alternative embodiment to that portion of the structure of FIG. 2 delineated by line 3-3.

Referring now to FIG. 1, there is shown a prior art external cavity resonator 1 of a klystron amplifier. The cavity resonator 1 includes an evacuated portion 2 and an external non-evacuated portion 3, which surrounds the evacuated portion 2. The evacuated portion 2 is defined by a pair of axially spaced centrally apertured, diskshaped end walls 4, as of copper. A pair of conductive drift tube members 5 are axially aligned with the central apertures in the end walls and project from the end walls 4 toward the center of the cavity 1. The mutually opposed inner ends of the reentrant drift tube members are spaced apart to define an electronic interaction gap 6 therebetween. A beam of electrons is projected axially through the drift tube segments 5 for interaction with the electric fields of the cavity 1 in the gap 6. A cylindrical gas tight R.F. window 7 is sealed at its ends to .the end walls 4 of the cavity 1 and coaxially surrounds .the interaction gap 6.

The external portion 3 of the cavity 1 includes a rectangular waveguide section 8 closed on its ends by end Walls 9 and disposed with its broad walls 11 axially spaced apart in the direction of the beam path. The broad walls 11 are centrally apertured to receive the central evacuated cavity portion 2. Conductive spring fingers 12 ring the apertures in the broad walls 11 for completing electrical contact between the composite cavity

end wall portions

11 and 4. A pair of rectangular contacting tuning plungers 13, only one of which is shown, are movable toward and away from the beam axis for tuning the composite cavity 1.

An R.F. by-pass capacitor 14 forms a portion of the upper broad wall 11 of the external waveguide cavity portion 3. The capacitor 14 comprises a thin annular slab of dielectric 15, such as Mylar or Teflon, sandwiched between an annular overlapping portion 16 of the broad Wall 11 and an underlying portion 17 of the upper broad wall 11. This capacitor 14 forms an R.F. by-pass for the circulating cavity currents, while permitting a DC potential to be applied across the interaction gap 6 of the cavity 1.

When an accelerating potential is applied across certain of the interaction gaps of a multicavity klystron amplifier the overall efiiciency of the tube is increased. When acceleration is combined with depressed collector operation, 70 to 80% overall R.F. efiiciency is obtainable.

The prior art R.F. by-pass structure is very simple because the capacitor 14 may be disposed in a nonevacuated portion of the cavity 1. This is not true for an integral cavity klystron amplifier.

Referring now to FIG. 2, there is shown a longitudinal sectional view, partly schematic, of a 4 cavity klystron amplifier 18 incorporating features of the present invention. The tube 18 includes an electron gun 19 for forming and projecting a beam of electrons 21 over an elongated linear beam path to a collector electrode 22 for collecting and dissipating the energy of the beam 21.

A plurality of reentrant integral evacuated cavity resonators 23 are disposed along the beam 21 intermediate the gun 19 and the collector 22 for cumulative electromagnetic interaction with the beam 21. Each of the cavities 23 is similar in that it includes a hollow cylindrical conductive chamber, as of copper, coaxially aligned with the beam 21. A pair of tubular drift tube segments 24, as of copper, project axially into the cavity 23 from opposite end walls 25 of the cavity 23. The mutually opposed inner ends of the drift tubes 24 define an electronic interaction gap 26 of the cavity.

In the first cavity resonator, both drift tubes are carried from the end walls 25. In the other cavities, one of the drift tube segments 24 is conductively connected at its root portion to the end wall 25 of the cavity 23. The other drift tube segment 24' passes coaxially through a conductive tubular sleeve member 27 in electrically noncontacting relation therewith. The sleeve 27 is connected at its root portion to the end wall 25 of the cavity 23 and is preferably terminated short of the inner end of the drift tube segment 24' such as not to shield the drift tube segment from the R.F. electric fields across the gap 26 of the cavity 23.

The axially coextensive regions of the sleeve 27 and the drift tube segment 24' define a coaxial line segment portion of an R.F. by-pass structure of the cavity 23. The R.F. by-pass drift tube segment 24' is carried, intermediate its length, from the end wall 25 of the adjacent cavity resonator 23. The radial gap between the mutually opposed end walls 25 of adjacent cavity resonators 23 defines a section of radial transmission line, forming a second portion of the R.F. by-pass structure. The axial spacing between the adjacent end walls 25 is made close over a radial distance sufiicient to define a total transmission path length from the inner end of tube 27 outwardly of the resonator 23 of approximately a quarter Wavelength. The axial spacing at that point 30 is greatly increased to provide a substantial increase in characteristic impedance of the section of radial transmission line, thereby reflecting a very low (short circuit) R.F. impedance to circulating R.F. cavity currents at the inner tip of sleeve 27.

A cylindrical insulator 28, as of ceramic, is sealed in a gas tight manner at its ends between opposed end walls 25 of adjacent cavities 23. The insulator 28 is located at the periphery of the radial section of the R.F. by-pass structure and completes the vacuum envelope forthe R.F. by pass portion of the cavity 23.

The last or output cavity 23' is similar to the previously described cavities except that the output drift tube segment 24' is frustoconical to accommodate an expanding beam 21 as it enters the collector 22. Also the sleeve 27 has a frustoconical shape to conform to the shape of the drift tube 24'. The output drift tube 24' is carried at its root from a transverse annular header 31, as of copper. A cylindrical vacuum tight insulator 28 is sealed between the header 31 and the outer wall 25 of the last cavity 23'. The collector 22 is carried from the header 31 via a gas tight cylindrical insulator 32. A centrally apertured cup member 33, as of copper, is sealed to the header 31 and conical output drift tube 24 to define the radial transmission line section of the R.F. by-pass structure for the output cavity 23'. In addition, the cup member 33 defines an annular chamber 34 for applying a liquid coolant to the output drift tube 24". The drift tube coolant chamber 34 is connected in series via insulative tube 35 with a liquid coolant pipe 36 which is wound around the collector 22 for cooling thereof.

A tapped power supply 37 applies an anode voltage between a cathode electrode 38 of the gun 19 and the first and second cavity resonator, which are operated at ground potential, for accelerating the beam to an initial beam velocity. Additional D.C. accelerating voltages are applied from the power supply 37 across each of the interaction gaps 26 except for the first interaction gap 26, which has no DC. potential applied thereacross. The accelerating potential applied across the interaction gaps in the second and third cavity substantially increases the overall R.F. eficiency, whereas the potential applied across the output gap substantially increases the power output. A 20% depression of the collector electrode potential relative to the beam potential, as it emerges from the output cavity 23, also substantially increases overall R.F. efiiciency.

Referring now to FIG. 3, there is shown an alternative cavity embodiment of the present invention. In this embodiment, the cavity resonators 23 are essentially the same as in FIG. 2 except that the sleeve member 27, forming the coaxial line segment of the R.F. by-pass, is replaced by a radial conductive disk member 41 carried from the drift tube member 24'. The disk member 41 forms, with the outside surface of the adjacent cavity end Wall 25, a radially slotted conductive structure. The end wall 25 of the by-passed cavity 23 is radially interdigitated with the radial slot to form a radially meandering R.F. transmission line. The length of this radial line from the cavity 23 to the abrupt high impedance transition at shoulder 42 is preferably an odd number of quarter wavelength to reflect an effective R.F. short circuit at the entrance to the R.F. by-pass structure inside the cavity 23.

In operation (see FIG. 2) an input microwave signal to be amplified is applied to the input cavity 23 via a coaxial input coupler 43 and coupling loop 44. The input signal excites the input cavity to velocity modulate the beam 21 at the gap 26. The velocity modulation is transformed into current density modulation of the beam 21 in the field free region inside the succeeding drift tube 24. This current modulation excites the second cavity which excitation further velocity modulates the beam. In addition, an accelerating potential is applied to the beam in the second cavity. The beam acceleration increases the R.F. beam current and causes an increase in the overall R.F. elficiency. Likewise, accelerating potentials are applied to the beam in the third and output cavities. The third cavity further increases the R.F. efficiency, whereas the output cavity extracts the output signal from the beam 21 and the accelerating potential increases the power output of the tube. The depressed collector 22 also increases the overall R.F. efliciency of the tube. With a near optimum design of the tube parameters, the tube 18 should yield about 80% overall R.F. efliciency.

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 drawings shall be interpreted as illustrative and not in a limiting sense. i

What is claimed is:

1. An integral cavity multicavity linear beam amplifier apparatus including; means forming an electron gun for projecting a beam of electrons over an elongated linear beam path; means forming a collector electrode disposed at the terminal end of the beam path for collecting and dissipating the energy of the beam; means forming a plurality of evacuated integral cavity resonators axially spaced along the beam path for successive electromagnetic interaction with the beam to produce current density modulation of the beam with signal energy; said cavity resonators having a pair of axially spaced interaction gap defining members; means for extracting output signal R.F. energy from the modulated beam for application to a load; at least one of said evacuated integral cavity resonators including, means forming an evacuated R.F. bypass structure for DC. isolating one of said gap defining members from the other While not isolating said gap defining members for RF. energy, said R.F. bypass structure including a pair of closely spaced conductive members, one of said members being connected to one of said gap defining members and defining with the other closely spaced conductive member a section of RP. transmission line, said transmission line including an outermost section defined by the space between a pair of axially spaced radially coextensive conductive member portion defining a section of radial R.F. transmission line therebetween, and means forming a hollow cylindrical insulator body sealed in a gas-tight manner across said section of radial transmission line at the outer periphery of said radial section, said insulator completing the vacuum Wall of said bypass portion of said cavity resonator and serving to hold off a DC. potential applied across said gap defining members.

2. The apparatus of claim 1 wherein said R.F. by-pass structure includes a pair of spaced apart concentrically disposed axially coextensive tubular conductors projecting axially into said cavity resonator for forming one of said interaction gap defining members.

3. The apparatus of claim 2 wherein the inner one of said concentric tubular members projects further into said cavity resonator than said outer tubular member.

4. The apparatus of claim 1 wherein said R.F. by-pass structure includes a conductive member interdigitated with a slotted portion of said conductive member which is connected to said gap defining member to define a meandering R.F. transmissionpath therebetween.

References Cited UNITED STATES PATENTS 2,958,804 11/1960 Badger et al 3l55.52 X 2,970,242 1/1961 Jepser 3155.39 3,274,430 9/1966 El-Hefni 3l5--5.52 X

NATHAN KAUFMAN, Primary Examiner US. Cl. X.R. 3155.39