US3448313A

US3448313A - Efficient radiation cooled beam collector for linear beam devices

Info

Publication number: US3448313A
Application number: US585459A
Authority: US
Inventors: Dennis I Boilard; Albert Mizuhara
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1966-10-10
Filing date: 1966-10-10
Publication date: 1969-06-03
Anticipated expiration: 1986-06-03

Description

June 1969 0. BOILARD ETAL 3,443,313

EFFICIENT RADIATION COOLBD BEAM COLLECTOR FOR LINEAR BEAM DEVICES Filed 0012. 10, 1965 (%)AONjOHJ3 milvmva DENNIS I. BOILARD ALBERT MlZUHARA United States Patent 3,448,313 EFFICIENT RADIATION COOLED BEAM COLLEC- TOR FOR LINEAR BEAM DEVICES Dennis I. Boilard, Sunnyvale, and Albert Mizuhara, San Mateo, Calif., assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed Oct. 10, 66, Ser. No. 585,459 Int. Cl. H01j 61/52, /02, 23/02 11.5. Cl. 313- 8 Claims ABSTRACT OF THE DISCLOSURE 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 radiation cooled beam collectors and, more particularly, to an improved radiative beam collector for linear beam devices -wherein the beam collector is a thin hollow shell of refractory material which is heated to incandescence by the collected beam power collected on the interior surfaces thereof. Beam collectors of the present invention are especially useful for, but not limited in use to, collecting and radiating the beam power of velocity modulation type microwave tubes.

Heretofore, linear beam tubes have employed relatively heavy and bulky beam collector structures. Typically, the collector is cooled principally by thermal conduction to a heat sink or by convection cooling to a fluid coolant, such as air or water. In cases where the collector is cooled by convection or conduction, it has been customary to make the beam collector of material having a high thermal conductivity such that the heat can be transferred to the fluid coolant or to the heat sink with a minimum of thermal gradient to prevent overheating the interior wall of the collector. As a consequence, the

hollow beam collector electrode is usually made of copper and is relatively large to spread the beam over a large internal surface to reduce the collected power density. Large copper collectors are bulky, heavy, and expensive.

In the present invention, the beam collector electrode is made of a thin hollow shell of refractory material which is heated to incandesceuce by the collected beam power. The beam collector is supported within the vacuum envelope of the device such that it does not have to hold off the atmospheric pressure. The beam collector is cooled by radiation of thermal energy through an infrared wave permeable portion of the vacuum envelope. In a preferred embodiment of the present invention, the beam collector is generally egg-shaped and disposed at the focal point of a generally paraboloid-shaped heat reflector for projecting the radiated energy through the wave permeable portion of the vacuum envelope and away from the device. Beam collectors of the present invention have demonstrated radiation cooling efficiencies of between and while being relatively light weight and of rugged construction.

Patented June 3, 1969 The principal object of the present invention is the provision of an improved radiation cooled beam collector for linear beam devices.

One feature of the present invention is the provision of a radiation cooled beam collector electrode formed by a hollow shell of refractory material having a characteristic wall thickness less than 0.050" and preferably between 0.003 and 0.005" thick, whereby the mass of the collector electrode is reduced to facilitate a rugged support thereof with a minimum of thermal conduction therefrom.

Another feature of the present invention is the same as the preceding feature wherein the beam collector is located at the focal point of a generally paraboloidshaped heat reflector for focusing the heat radiated from the collector into a beam directed away from the collector and preferably, through an infrared permeable portion of the vacuum envelope of the device.

Another feature of the present invention is the same as any one or more of the preceding features wherein the beam collector shell is supported from its surrounding structure by three non-thermally conductive, radially projecting, struts equally spaced at intervals around the periphery of the collector shell, whereby a rugged support for the collector shell is obtained.

Another feature of the present invention is the same as any one or more of the preceding features wherein the hollow beam collector electrode includes a paraboloidshaped end portion for receiving the beam which is situated at the focal point of a paraboloid heat reflector and which is oppositely directed to the paraboloid heat reflector, whereby heat radiated from the beam collector is collected by the heat reflector and reflected out of the device.

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:

FIGURE 1 is a longitudinal schematic line diagram of a linear beam tube employing features of the present invention,

FIGURE 2 is a detail view of that portion of the structure of FIGURE 1 delineated by line 22,

FIGURE 3 is a sectional view of the structure of FIG- URE 2 taken along line 33 in direction of the arrows, and

FIGURE 4 is a plot of radiation efliciency and collector temperature versus collector power for a collector of the present invention.

Referring now to FIGURE 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 e'lectrostatically 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 cumulative 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 an 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.

The output circuit comprises a double gap 1r mode cavity resonator 16 having conically shaped resonator end walls. 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 plunger, not shown. 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 efiiciency of 96.7 percent.

The tube 1 is electrostatically focused by means of a plurality of ring electrodes 22 operated at cathode potential 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 l torr, and encloses the cathode 3, interaction circuit 8 and beam collector electrode 7. The output terminal end of the vacuum envelope 25 is sealed by a diskshaped infrared wave permeable window member 26 as of sapphire or quartz approximately 2. inches in diameter and 0.125" thick.

An outwardly flared, generally paraboloid-shaped, infrared reflector structure 27 is disposed between the beam collector electrode 7 and the interaction circuit '8 for reflecting 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 power supply 28 supplies the cathode-toanode operating potential as of -3 k.v. relative to the grounded anode and interaction circuit 8. An anode-tocathode cylindrical insulator 29 forms a portion of the vacuum envelope structure 25 and holds off the anodeto-cathode potential. A separate beam collector power supply 31 supplies the cathode to beam collector operating potential, as of +2 k.v. Thus, the beam collector 7 and reflector 27 are operated at a negative potential of 1 k.v. relative to the grounded interaction circuit 8 and anode portion of the envelope structure 25, thereby obtaining the increased RF. efliciency 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 tempera ture falling within the range of 1300 C. to 1500 C. At these temperatures the collector is incandescent and is an efficient thermal radiator, radiating the major part of its energy in the form of infrared rays out the end of the tube through the sapphire window 26.

In a typical example of an S-band tube 1, the tube provided R.F. power output of 100 watts with 35 mHz. bandwidth, with 42 db of gain at 36%--overal1 conversion efliciency. For this case, the radiation cooled collector must dissipate 200 Watts at 75% radiation efliciency. The other 25% of the energy is removed from the collector 4 region by thermal conduct-ion and radiation back down the beam path 6.

Referring now to FIGURES 2 and 3, the beam collector features of the present invention will be described in greater detail. The collector electrode 7 is formed by a hollow generally egg-shaped shell of refractory material, as of tungsten or carbon. The shell 7 has an inverted frusto-conical shaped beam entrance portion 35 closed by a generally paraboloid-sh'aped portion 36 which receives the beam of electrons incident thereon. This shape for the collector shell 7 produces uniform beam interception and, thus, uniform power loading or dissipation over the interior beam collecting surface portion 36 of the beam collector 7. It turns out that under R.F. modulation of the beam, as produced by the interaction circuit 8, the diameter of the beam expands to that as indicated by the outer beam boundary lines. The paraboloid-shaped portion assures uniform power dissipation on the collector surface 36 for both a modulated and unmodulated beam.

The inverted frusto-conical beam entrance portion 35 serves to shield the reflector 27 from being heated by ion and electron bombardment, such ions and electrons being generated in the collector shell 7. Also the conical portion 35 tends to inhibit back radiation of thermal energy from the beam collector surface 36 toward the reflector 27, thereby aiding forward thermal radiation through the Window 26. The inverted cone shape 35 conforms to the shape of the expanding beam and prevents unwanted beam interception.

The collector shell 7 is preferably made of chemically vapor deposited tungsten or carbon (pyrolytic tungsten or graphite) to a wall thickness certainly less than 0.050" thick, as of 0.010" and, preferably, 0.003" to 0.005" thick. This thin wall construction is desirable in that it permits a very low mass collector electrode 7. For example, a 200 watt egg-shaped collector shell 7 of 0.005 Wall thickness weighed about three quarters of an ounce and was 2" long and 1.25" wide at its widest point. Such a light weight collector 7 is relatively easy to support against shock and vibration. I

The collector shell 7 is situated at the focal point of the generally paraboloid-shaped heat reflector 27. Thus, heat radiated from the collector shell 7 and collected by the reflector 27 is focused into a beam which is directed thlrjough the infrared permeable window 26 and away from tu e 1.

The collector electrode 7 is carried from the surrounding heat reflector structure 27 by means of three struts 37. The struts are made of a refractory metal having a low thermal conductivity, such as tantalum. In addition, their thermal conductivity is further decreased by making the struts of a thin sheet metal formed into a tube having, for example, a A outside diameter and a 0.003 to 0.005" wall thickness and being about 1" in length. The struts 37 are fixedly secured at their ends to the collector shell 7 and to the reflector 27 as by beam welding. The plane of connection of the support struts to the collector shell 7 is preferably at a transverse plane whifih passes through the center of gravity of the collector she 7.

In addition, the three struts 37 project away from the collector shell 7 .at intervals about the periphery of the shell 7. Moreover, the struts fall on a common conical surface generated by revolution of the collector shell 7 with one of its struts 37 around the collector shells axis of revolution. This inwardly converging strut arrangement is advantageous in that only three struts are required, thereby minimizing thermal conduction from the collector electrode 7. The struts 37 provide a rugged nonmicrophonic support for the collector 7, while flexing to accommodate different thermal expansions of the collector 7 and its surrounding support structure.

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, molybdenum, 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 from the end of a cylindrical support 38, as by spot welding.

A pair of thermal shields 39, 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 39 are centrally apertured to a diameter of about 0.350" to provide a beam entrance port to permit passage of the beam therethrough. The thermal shields 39 inhibit back streaming of heat from the reflector 27 .to the interaction circuit 8. The reflector 27, heat shields 39, and collector electrode 7 are supported from the output cavity 16 via the intermediary of a cylindrical ceramic insulator 41 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 helium arc 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 and a collector shell thickness of 0.005, dissipated 250 watts of beam power with 80% radiation efliciency (see FIGURE 4). The structure will dissipate about 1 kw. beam power with the same efficiency with about a 50 to 100% increase in the diameter of the beam collector and radiation structure. 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.

Although the beam collector shell 7 and its associated support and heat reflector structure have been described as it would be employed for collecting and dissipating the beam energy of a high power microwave tube, it may also be employed for collecting the beam of other types of linear beam tubes. In addition, it is noted that the radiating collector 7 gives off substantial light and heat and has a long operating lifetime, as of 20,000 hours. Therefore, when the collector 7 is operated at white hot temperatures, as of 2200 C., the linear beam device will serve as a source of light. When operated at lower temperatures the device will provide a beam of infrared energy for signaling. Modulation of beam power into the collector will produce both frequency and amplitude modulation of the infrared output signal.

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 device having a radiation cooled beam collector electrode including, means for forming and projecting a beam of electrons over a linear path, means disposed at the terminal end of the beam path for collecting the electrons of the beam, means forming an evacuated envelope structure enclosing said beam forming and collecting means, said envelope including an infrared permeable window adjacent said cooling means, said beam c01- lecting means comprising a hollow shell of refractory material to be heated by the collected power of the beam to incandescence for dissipating the energy of the collected beam by radiation of thermal energy to its surrounds, wherein said beam collector shell is generally eg -shaped with the axis of revolution of the egg-shaped shell being coaxially disposed of the longitudinal axis of the linear beam path and having an aperture at one end to form a beam entrance port, whereby the inwardly converging terminal end wall of the egg-shaped collector experiences generally uniform power density distribution of the intercepted beam power over its interior surface.

2. The apparatus of claim 1 wherein said beam collector shell has a characteristic wall thickness less than 0.015" thick, and is made of a material selected from the class of pyrolytic tungsten and graphite.

3. The apparatus of claim 1 including, means forming a generally paraboloid-shaped heat reflector, and said beam collector shell being situated substantially at the focal point of said heat reflector, whereby heat waves radiated from said beam collector shell toward said reflector are focused into a beam and directed away from the beam collector for cooling thereof.

4. The apparatus of claim 1 including a plurality of support struts affixed at their inner ends to said collector shell and projecting away from said collector shell to surrounding support structure for supporting said collector shell therefrom, and said support struts being made of a refractory material and dimensioned of sheet wall construction to minimize thermal conduction from said beam collector through said struts to the surrounding support structure.

5. The apparatus of claim 4 wherein there are three struts spaced at intervals about the periphery of said collector shell, and wherein said three struts lie substantially on a common conical surface generated with one of said struts by rotation of said beam collector about its axis of revolution, whereby a rugged support for said collector electrode is obtained which allows for differential thermal expansions of said collector and its support parts.

6. The apparatus of claim 1 including, means disposed along the beam path intermediate said beam forming means and said beam collecting means for modulating the beam to produce a microwave output signal.

7. The apparatus of claim 6 wherein the interior wall portion of said collector shell which is opposite said beam entrance port and which collects the beam is generally shaped in the form of a paraboloid in order to obtain a uniform power density distribution of the collected beam power on the interior beam collecting surface of said beam collector.

8. The apparatus of claim 7 wherein that portion of the hollow collector shell which interconnects the marginal edge of the beam entrance port to the outer periperal margin of said paraboloidal shaped beam collecting wall is generally frusto conically shaped to conform to the shape of the beam as it expands in said beam collector structure.

References Cited UNITED STATES PATENTS 2,947,905 8/ 1960 Pierce 315--5 X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

US. Cl. X.R.