US3448325A - Linear beam tube having a beam collector cooled by radiation through an infrared window - Google Patents

Description

June 3, 1969 HEN ET AL 3,448,325

LINEAR BEAM TUBE HAVING A BEAM COLLECTOR COOLED BY RADIATION THROUGH AN INFRARED WINDOW Filed Sept. 6, 1966 Fi i? :2 o

Q T n a INVENTORS ERSING L. LIEN ILLAM J. EDIGH BY 04 RNEY United States Patent 3,448,325 LINEAR BEAM TUBE HAVING A BEAM COLLEC- TOR COOLEI) BY RADIATION THROUGH AN INFRARED WINDOW Erling L. Lien, Los Altos, and William J. Leidigh, Belmont, Calif., assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed Sept. 6, 1966, Ser. No. 577,440 Int. Cl. H01j 25/34 US. Cl. 315-3.5 9 Claims The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457) The present invention relates in general to linear beam tubes and, more particularly, to such tubes employing a beam collector electrode which is cooled by radiation through a gas tight infrared wave permeable window. Such tubes are especially useful for, but not limited in use to, portable microwave velocity modulation power tubes as may be employed, for example, in satellite communication systems or for transmitting information from deep space probes.

Heretofore, radiation cooling of refractory beam collector electrodes through a glass vacuum envelope has been employed in power grid tubes. In such tubes, the beam collector electrode surrounds the grid and cathode electrodes and could not operate above a relatively low temperature, as for example, 1000 C. since it acts like an oven and would otherwise overheat the grid and cathode emitter in use. The radition efliciency of such a radiator is limited by the operating temperature of the radiating element.

Radiation cooling of the beam collector electrodes in linear beam velocity modulation tubes has been employed. However, in these prior tubes the radiating beam collector element formed the vacuum envelope of the tube and was typically made of a good thermal conductor such as copper. The problem with this type of collector is that it must operate at a relatively low temperature as of 500 C. to prevent melting thereof and to maintain sufiicient strength to hold out the atmospheric pressure. As a consequence, the radiation cooling efiiciency is relatively low and the collector becomes excessively large and heavy for many portable applications.

In the present invention, the beam of a linear beam tube is collected in a refractory metal collector electrode operating at a relatively high temperature, as of greater than 1000 C. The collector electrode is cooled by radiation through an infrared Wave permeable vacuum tight window portion of the tubes vacuum envelope. In a preferred embodiment of the present invention, the radiating collector electrode operates between 1300 C. and 1500" C. and is positioned adjacent a thermally shielded infrared reflector which reflects back directed infrared energy through the infrared window and away from the tube. In addition, a suppressor electrode, operating at a negative potential relative to the beam collector, is positioned adjacent the beam entrance thereof to drain positive ions from the collector to prevent ion focusing inside the collector. When the collector is operated as a depressed collector for increased overall radio frequency conversion efiiciency, the negative suppressor electrode also prevents thermionic electrons generated inside the collector from flowing back to the R.F. circuit or down the beam path toward the cathode electrode. Collectors of the present invention have achieved 75% radiation efficiency at 300 watts C.W. input with 20,000 hours expected operating life.

The principal object of the present invention is the ice provision of an improved linear beam tube having a radiation cooled beam collector.

One feature of the present invention is the provision of a heat radiating refractory metallic beam collector at the terminal end of a linear beam tube with the collector radiating its infrared energy away from the tube through a separate infrared wave permeable vacuum tight window, whereby the beam collector may be operated at temperatures in excess of 1000 C. to provide substantial radiating eificiency for cooling the collector of the tube.

Another feature of the present invention is the same as the preceding feature including a thermally shielded infrared reflector positioned at the end of the tube between the collector and the main body of the tube for reflecting infrared energy emitted from the collector out the end of the tube through the infrared Window, whereby the cooling efficiency of the collector is enhanced.

Another feature of the present invention is the same as any one or more of the preceding features wherein the radiating collector electrode has its radiating surface roughened and/or coated with a black coating to increase the radiating efllciency of the collector.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of a suppressor electrode positioned near the beam entrance opening to the beam collector and operated at a negative potential relative to the beam collector for draining positive ions from the collector and for preventing thermionic emission from the collector from flowing back down the beam path to the R.F. circuit and toward the cathode electrode.

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 schematic diagram of a linear beam tube incorporating features of the present invention.

FIG. 2 is an enlarged sectional view of the structure of FIG. 1 delineated by line 22, and

FIG. 3 is a sectional view of the structure of FIG. 2 taken along the line 33 in the direction of the arrows.

Referring now to FIG. 1 there is shown a linear beam tube 1 incorporating the radiation cooled beam collector structure 2 of the present invention. More particularly, the tube 1 is an electrostatically focused extended interaction klystron. The klystron tube 1 has a cathode electrode 3 at one end operating in conjunction with a focus electrode 4 and anode electrode 5 to form and project a beam of electrons over an elongated linear beam path 6 to a hollow beam collector electrode 7 at the terminal end of the tube 1.

An electromagnetic interaction circuit 8 is disposed intermediate the cathode 3 and beam collector 7 for cummulative interaction with the beam to produce amplification of microwave signal wave energy applied to the input end of the circuit 8 via an input R.F. coupler 9. The amplified signal wave energy is extracted from the interaction circuit 8 via output R.F. coupler 11 and fed to a suitable utilization device or load, such as an antenna, not shown.

The interaction circuit 8 comprises a plurality of extended interaction helix resonators 12 successively disposed along the beam path 6 and separated by microwave field free drift regions 13. The helix resonators 12 each comprise a length of metallic tape conductor, as of molybdenum, wound in the shape of a helix 14 and connected at opposite ends to the side walls of a conductive metallic chamber 15, as of molybdenum. The input resonator 12 has its resonant helix coupled to the end of the center conductor of the coaxial R.F. input coupler 9. The end walls of the cavities 15 are centrally apertured for passage of the beam therethrough.

In helical resonators 12 the molybdenum tape helix 14 is supported by one beryllia and two quartz rods, not shown. The beryllia rod serves to conduct the heat from the helix 14 to the molybdenum shell 15. The helix assembly 14 is simply clamped in place by the resonator shell 15. The helical resonator 12 operates at its lowest resonant frequency and is electrically half a wavelength long. Due to end effects, in particular the presence of the beam hole, the variation of the axial component of the electric field is approximately sinusoidal. The helix 14 is, therefore, as far as beam interaction is concerned, effectively a full wavelength long. In order to minimize the variation of the beam coupling coeflicient over the operating voltages, the helix length is adjusted to a peak-to-peak separation in the electric field corresponding to an electronic phase shift fl =2.5 radians at a beam voltage of 3 kv. At the same voltage the normalized inner radius of the helix 14 is 1.13 radians. The value of the interaction impedance (R /Q) is 230 ohms and the square of the beam coupling coefficient M is 0.58.

To obtain 30 MHz for the tube 1 bandwidth at 2.3 gigahertz, the buncher cavities 12 are stagger-tuned. Tuning of the helical resonator 12 is accomplished by perturbing the radial component of the electrical field. Using a radially oriented conductive cylindrical tuning plunger, not shown, located half way along the length of the resonator 12 where the radial field is strongest, up to 100 MHz. tuning is easily realized with only a 5 percent decrease in R Q. The buncher cavities 12 serve to velocity modulate the beam with the signal energy and to successively increase the current density modulation of the beam bunches in successive drift regions 13.

The output circuit comprises a double gap 7r mode cavity resonator 16 having conically shaped resonator end walls. The distance between the penultimate resonator 12 and the center of the output resonator 16 is selected for optimum bunching at the output interaction gaps. The collector electrode entrance 17 is located as close to the output interaction gaps as possible because of the rapid spread of the beam under R.F. condition. The resulting interaction impedance (R /Q) of the output resonator 16 is 285 ohms.

The output resonator 16 is coupled via loop 18 to the coaxial output coupler 11. Tuning of the output resonator 16 is by means of an inductive plungernot shown. The 21r-mode resonant frequency for cavity 16 is 3550 MHz. The output loop 18 satisfactorily loads both the 1r and 211'- modes of resonance so that the 21r-mode loaded Q is about 220. The degree of 21r-mode unloading required to start oscillation is approximately a factor two.

The output resonator shell and walls are fabricated from copper-clad molybdenum. The drift tube 19 of the output cavity 16 and its single support .arm 21 are made from molybdenum. Copper plating is used to reduce the RF. conduction losses, resulting in a circuit efficiency of 96.7 percent.

The tube 1 is electrostatically focused by means of a plurality of ring electrodes 22 operated at cathode poten tial and successively axially spaced apart along the beam path 6. Potential supply leads 23 for the ring electrodes pass through insulator assemblies 24 in the walls of a vacuum envelope 25 of the tube 1. The vacuum envelope structure 25 is evacuated to a low pressure as of torr, and encloses the cathode 3, interaction circuit 8 and beam collector electrode 7. The output terminal end of the vacuum envelope is sealed by a disk-shaped infrared wave permeable window member 26 as of sappbire or quartz approximately 3 inches in diameter and 0.125" thick.

An outwardly flared, generally parobolic shaped, infrared reflector structure 27 is disposed between the beam collector electrode 7 and the interaction circuit 8 for refleeting infrared wave energy emitted from the collector electrode 7 through the infrared window 26 and away from the tube 1 for cooling thereof. The beam collector electrode 7 and reflector 27, in a preferred embodiment, operate at a potential negative with respect to the interaction circuit 8 to form a depressed collector, thereby enhancing the overall R.F. efficiency of the tube 1.

A cathode potwer supply 28 supplies the cathode-toanode operating potential as of 3 -kv. relative to the grounded anode 5 and interaction circuit 8. An anode-tocathode cylindrical insulator 29 forms a portion of the vacuum envelope structure 25 and holds off the anode-tocathode potential. A separate beam collector power supply 31 supplies the cathode to beam collector operating potential, as of +2 kv. Thus, the beam collector 7 and reflector 27 are operated at a negative potential of 1 kv. relative to the grounded interaction circuit 8 and anode portion of the envelope structure 25, thereby obtaining the increased R.F. efficiency attendant the depressed collector mode of operation.

In operation, the input signal velocity modulates the beam. The beam is further velocity modulated and current density bunched by the stagger tuned buncher cavities 12. The bunched beam excites the output cavity 16. The amplified output signal is extracted from the output cavity 16. The spent beam is collected in the collector electrode 7, thereby heating the collector to a temperature falling within the range of 1300" C. to 1500 C. At these temepratures the collector is an efiicient thermal radiator, radiating the major part of its energy in the form of infrared ray-s out the end of the tube through the sapphire window 26.

In a typical example of an s band tube 1, the tube :provided RF. power output of watts with 35 MHz bandwidth, with 42 db. of gain at 36%-overall conversion efliciency. For this case, the radiation cooled collector must dissipate 200 watts at 75% radiation efficiency. The other 25% of the energy is removed from the collector region by thermal conduction and radiation back down the beam path 6.

Referring now to FIGS. 2 and 3 the beam collector feature of the present invention will .be described in greater detail. The collector electrode 7 is formed by a hollow generally egg-shaped member having an inverted frustoconical beam entrance portion 35 closed by a domeshaped potrion 36 which receives the beam incident thereon. This shape produces even beam interception over its interior beam collecting surfaces as the beam expands due to space charge repulsion. The electrode 7 is preferably made of chemically vapor deposited tungsten having a wall thickness of about 0.010". The vapor deposition leaves the external surface rough due to the tungsten crystallite orientation which is normal to the surface. This rough exterior surface increases the radiation efliciency of the collector 7. In addition, the radiation efiiciency may be further increased by coating the exterior surface of the electrode 7 with a black coating of zirconium carbide.

The collector electrode 7 is carried from a surrounding support cylinder 3-7, as of stainless steel with a wall thickness of 0.200", via a pair of orthogonally crossed rods 38. The rods 38, as of 0.040" diameter tungsten rhenium alloy, are anchored at their ends in oval Monel diaphragrns 39 as of 0.005" wall thickness brazed at their margins into oval cutout portions of the support cylinder 37. The lower rod 38' is downwardly bowed while the upper rod 38 is upwardly bowed. The rods 38 are connected to the side Walls of the collector electrode by tantalum eyelets 41, spot welded to the collector 7 and rods 38 and through which the rods 38 pass. The center portions of the rods 38, within the collector electrode, are flattened to a thickness of 0.029" to present a thin beam interception profile to the beam. The oppositely directed bowing of the rods 38 serves to retain the collector electrode 7 in the same position in spite of thermally produced changes in the dimensions of the rods 38, electrode 7 and surrounding support structure 37. For a 300 watt dissipation collector '5 electrode 7, the electrode 7 is about 2 inches in length and 1.25" in diameter at its largest point. The rods 38 are about 2.75" in length.

The generally parabolic shaped infrared reflector 27 is made of 0.005 to 0.010" thick sheet of polished refractory metal having a high infrared reflectivity such as tantalum, molybdeum, or tungsten. The reflector 27 is outwardly flared and positioned facing the infrared permeable window 26 to reflect the infrared radiation, received by the reflector from the collector 7, through the window and away from the tube 1. The reflector 7 has an axial length of about 2.25" and a diameter at its largest point of 2.75". The reflector 27 is supported at its lip 42 from the end of the cylindrical support 37, as by spot Welding.

A pair of thermal shields 43, substantially identical to the reflector 27, are closely and coaxially positioned surrounding the outside of the reflector 27 and spaced therefrom and from each other 'by dimples formed in the mutually opposed surfaces thereof. The reflector 27 and thermal shields 43 are centrally apertured to a diameter of about 0.350" to permit passage of the beam theretlhrough. The thermal shields inhibit back streaming of heat from the reflector 27 to the interaction circuit 8. The reflector 27, heat shields 43, and collector electrode 7 are supported from the output cavity 16 via the intermediary of a cylindrical ceramic insulator 44 which, in addition, permits the separate depressed beam collector potential to be supplied to those elements relative to the anode potential applied to the interaction circuit 8.

The infrared window 26 is brazed at its periphery to a cylindrical copper nickel alloy window frame member 45. The frame 45 is outwardly flanged at 46 for sealing as by Welding to a similarly flanged portion 47 of the cylindrical vacuum envelope 25, as of copper nickel alloy to form a vacuum tight joint therebetween. The window 26 is spaced by approximately 0.125" from the closest point of the collector electrode 7.

The beam collector and radiation structure, with dimensions as aforecited, dissipated 300 watts of beam power with 75% radiation efliciency. The structure will dissipate about 1 kw. beam power with the same efficiency with about a 30 to 50% increase in the diameter of the beam collector and radiation structure. In this case the beam collector would also be coated with a black coating. With further increases in dimensions, a structure of this design is capable of dissipating up to kw. of beam power with the same radiation 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.

What is claimed is:

1. A linear beam tube apparatus including, means forming a cathode electrode for forming and projecting a beam of electrons over an elongated linear beam path, means forming a refractory metallic beam collector at the terminal end of the linear beam path for collecting and dissipating the energy of the beam by infrared radiation, means disposed along the beam path intermediate said cathode electrode and said beam collector for electromagnetic interaction with the beam to produce an output microwave signal, means for extracting the output signal for application to a utilization device, means forming an evacuated envelope structure for enclosing said cathode electrode, said interaction means, and said. beam collector electrode, means forming an infrared wave permeable window structure disposed at the terminal end of the tube apparatus adjacent said beam collector electrode and forming a portion of said envelope structure for passing infrared wave energy emitted from said beam collector through said envelope structure and away from the tube apparatus for cooling thereof, whereby the tube apparatus is cooled by radiation.

2. The apparatus of claim 1 including, means forming an outwardly flared infrared reflector disposed facing said window means adjacent said collector electrode and within said envelope structure between said collector electrode and said electromagnetic interaction means for reflecting infrared energy emitted from said collector electrode through said infrared window means, thereby inhibiting infrared radiation from traveling back toward said interaction means and thus producing enhanced cooling of the tube apparatus.

3. The apparatus of claim 2 including means forming a thermal shield structure disposed between said reflector means and said interaction means, for additionally shielding said interaction means from infrared radiation traveling back toward said interaction means.

4. The apparatus of claim 1 including means forming a suppressor electrode disposed between said beam collector electrode and said interaction means, and operated at a negative potential relative to the operating potential of said beam collector electrode for suppressing thermionic electron emission from said beam collector electrode from traveling back to the RF. circuit and along the beam path in the direction toward said cathode electrode and for draining positive ions from the interior of said beam collecting electrode to prevent ion focusing of the beam therein.

5. The apparatus of claim 1 wherein an outer surface portion of said beam collector electrode is provided with a rough exterior surface to enhance infrared radiation therefrom.

6. The apparatus of claim 5 wherein the rough exterior surface portion of said beam collector includes a black coating.

7. The apparatus of claim 1 wherein said infrared window is formed of a material selected from the class of sapphire and quartz.

8. The apparatus of claim 3 wherein said infrared reflector means and said thermal shield means comprises a plurality of coaxially disposed flared members disposed in spaced apart relationship over substantial mutually opposed surfaces areas thereof.

9. The apparatus of claim 1 including, means forming an electrical insulator disposed intermediate said interaction means and said beam collector means for operating said beam collector means at a potential negative with respect to said interaction means, whereby the beam collector may be operated as a depressed collector for enhancing radio frequency efliciency of the tube apparatus.

References Cited UNITED STATES PATENTS 2,860,277 11/1958 Iversen 313-45 X 2,957,103 10/1960 Birdsall 313-45 X 3,026,435 3/ 1962 McPherson 313-45 X JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner.

US. Cl. X.R.

Claims (1)

1. A LINEAR BEAM TUBE APPARATUS INCLUDING, MEANS FORMING A CATHODE ELECTRODE FOR FORMING AND PROJECTING A BEAM OF ELECTRONS OVER AN ELONGATED LINEAR BEAM PATH, MEANS FORMING A REFRACTORY METALLIC BEAM COLLECTOR AT THE TERMINAL END OF THE LINEAR PATH FOR COLLECTING AND DISSIPATING THE ENERGY OF THE BEAM BY INFRARED RADIATION, MEANS DISPOSED ALONG THE BEAM PATH INTERMEDIATE, SAID CATHODE ELECTRODE WITH THE BEAM COLLECTOR FOR ELECTROMAGNETIC INTERACTION WITH THE BEAM TO PRODUCE AN OUTPUT MICROWAVE SIGNAL, MEANS FOR EXTRACTING THE OUTPUT SIGNAL FOR APPLICATION TO A UTILIZATION DEVICE, MEANS FORMING AN EVACUATED ENVELOPE STRUCTURE FOR ENCLOSING SAID CATHODE ELECTRODE, SAID INTERACTION MEANS, AND SAID BEAM COLLECTOR ELECTRODE, MEANS FORMING AN INFRARED WAVE PERMEABLE WINDOW STRUCTURE DISPOSED AT THE TERMINAL END OF THE TUBE APPARATUS ADJACENT SAID BEAM COLLECTOR ELECTRODE AND FORMING A PORTION OF SAID ENVELOPE STRUCTURE FOR PASSING INFRARED WAVE ENERGY EMITTED FROM SAID BEAM COLLECTOR THROUGH SAID ENVELOPE STRUCTURE AND AWAY FROM THE TUBE APPARATUS FOR COOLING THEREOF, WHEREBY THE TUBE APPARATUS IS COOLED BY RADIATION.

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