US3381162A - High frequency electron discharge devices having asymmetric self-biased ion drainingelectrodes - Google Patents

Description

SCHWARTZ ET AL HIGH FREQUENCY ELECTRON DISCHARGE DEVICES HAVING April 30, 1968 J ASYMMBTRIC saw-51mm) ION DRAINING ELECTRODES Filed 001;. 1, 1964 W O 1 m Mn m H S I 0mm f GZMAS GR S WW A 0 II .12 WJ O E W H m G M E w E C N l M A H T P R l A U T R S G E W C O m W R s w Y 6 S F ATTORNEY United States Patent 3,381,162 HIGH FREQUENCY ELECTRON DISCHARGE DE- VICES HAVING ASYMMETRIC SELF-BIASED ION DRAINING ELECTRODES James R. Schwartz, Mountain View, and Wayne G.

Abraham, Santa Clara, Calif., assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed Oct. 1, 1964. Ser. No. 400,679 12 Claims. (Cl. 315-538) ABSTRACT OF THE DKSCLOSURE A self-biasing ion draining electrode for use in eliminating AM and FM noise generated by ions and secondary (impact generated) electrons in klystrons, traveling wave tubes, etc. The electrode is positioned asymmetrically off the beam axis and is electrically floating with respect to the remainder of the tube. A portion of the electrode is made of a material having a secondary electron emission ratio of less than 1 so that this portion assumes a negative charge and serves to capture ions.

This invention relates in general to high frequency electron discharge devices, preferably to those operable in the microwave spectrum, and more particularly to such high frequency electron discharge devices incorporating asymmetric self-biased ion draining electrodes.

The rationale for providing high frequency electron discharge devices such as klystrons, traveling wave tubes, etc., with apparatus for reducing spurious noise therein are set forth cogently in US. Patent 3,293,481 by Curtis E. Ward and James R. Schwartz and assigned to the same assignee as the present invention. A citation of the pertinent prior art with respect to techniques for reducing noise, both AM and FM, in klystrons, traveling wave tubes, etc., is presented in the aforementioned copending application. The aforementioned Ward et al. application, primarily directed to asymmetrical ion draining electrodes having biasing means therefore, has been found to be satisfactory with regard to AM and FM noise reduction in klystrons, traveling wave tubes, etc.

The present invention is concerned with an improvement with respect to the elimination of the biasing source while retaining certain of the other novel aspects of the invention as set forth and claimed in said copending application of Ward et al. It has now been determined, as taught herein, that more than adequate ion drainage and reduction in noise levels, AM and PM, can be accomplished through the utilization of an asymmetrically positioned self-biased ion draining electrode. The terminology, self-biased or self-biasing, as used herein refers to an electrode which charges up either negatively or positively by virtue of electron impact on the electrode surface by either primary or secondary electrons or a combination of both as opposed to the utilization of an external biasing source, such as, for example, as used in US. Patent 3,293,481. Furthermore, the present invention teaches the additional concept of selecting particular materials having certain secondary emission ratios in conjunction with electrode position such that the self-biased electrode of the present invention can charge to a suitable negative voltage such that it will function as a means for effectively reducing noise levels in high frequency electron discharge devices.

The present invention in a preferred embodiment utilizes materials embodied in an electrically floating electrode disposed in an asymmetric configuration, said materials being selected such that the effective secondary emission ratio is less than 1 at the energy of the impacting electrons whereby the asymmetrically positioned electrode 3,381,162 Patented Apr. 30, 1968 charges to an adequate negative bias such as to functionas an ion draining probe and/ or as a means for deflecting low-speed secondary electrons from the central beam axis.

The present invention furthermore teaches the novel concept of the utilization of either primary (electron beam) electrons or secondary electrons (resulting from surface impact in region of electrode by primary beam electrons) or a combination of both to achieve the aforementioned purposes with regard to obtaining a negative bias on an asymmetrically positioned floating electrode disposed within said electron discharge device.

It is therefore an object of the present invention to provide improved means in conjunction with high frequency electron discharge devices for reducing noise levels therein which means do not require an external biasing source.

A feature of the present invention is the provision of a high frequency electron beam discharge device having a self-biased electrode means disposed therein asymmetrically with respect to the central beam axis, said electrode means having at least a portion thereof made of a material which has an effective secondary emission ratio less than 1 at the energy of the impacting electrons such that said element acquires a negative charge.

Another feature of the present invention is the particularization of the location of the self-biased electrode means recited in the aforementioned feature within the collector region of said high frequency electron discharge device.

These and other features of the present invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional view, partly in elevation of a two cavity klystron oscillator embodying the teachings of the present invention;

FIG. 2 is a cross-sectional view, partly in elevation of a different two cavity klystron oscillator embodying the teachings of the present invention;

FIG. 3 is an illustrative graphical portrayal showing an illustrative example of secondary emission characteristic (a) of titanium and graphite forms vs. impacting electron voltages; and

FIG. 4(a) and (b) is another illustrative graphical portrayal showing secondary emission ratios cr for various hypothetical materials and numbers (N) of impacting electrons vs. impacting voltages V and secondary electron energy in volts expressed as a percentage of primary impacting energy of electrons bombarding the surface at a given impact voltage.

Turning now to FIG. 1, there is depicted a high frequency electron discharge device of the two cavity klystron oscillator type 5 incorporating an asymmetrically positioned self-biased electrode means 6 in the collector reigon 7 thereof, according to the teachings of the present invention. The tube main body 8, cavity resonator tuning means 9, electron gun assembly 10, output window assembly 11, etc., are advantageously constructed similarly to the equivalent structure shown and described in US. Patent 3,117,251, by I. M. De Pue, Jr., et al., which is assigned to the same assignee as the present invention.

Useful interaction between an electron beam traveling along a central beam axis Z takes place within the cavities 12 and 13 and the resultant output energy is derived through output window assembly 11 in a conventional fashion. Any suitable reference text may be referred to for an explanation of same. The asymmetrically disposed self-biased electrode means 6 will be described in greater detail hereinafter.

Directing your attention now to FIG. 2, there is shown therein another type of low noise high frequency electron discharge device of the two cavity klystron oscillator type 14, having an asymmetrically oriented selfbiased electrode means 16 disposed in the collector region thereof. A conventional electron gun structure 17 and interaction means including cavity resonators 18, 19, tuning mechanisms 20, 21, disposed within main body block 22., serve to define a vacuum envelope for the oscillator. An electron beam is generated from a conventional type cathode disposed within the electron gun mechanism 17 at the upstream end of said device and travels along a central beam axis Z to the downstream portion thereof and is dissipated in the collector region in a known manner. The collector 15 has cooling fins 23 and an outgassing region 24, and is provided with a suitable protective end cap 25 to complete the collector portion of the device.

Once again RF output energy is extracted through a conventional window assembly 26 according to techniques well known in the art.

Returning to FIG. 1, the self-biased asymmetric electrically floating electrode means 6 includes an alumina or the like insulator block 27, having a tip portion coated with titanium 28 which is supported in a vacuum sealed manner within the collector region by means of a molybdenum block 29 to which it is preferably fixedly secured by means of a plug of silica oxide 30 which is fired at a suitable temperature such that a portion of said plug is dispersed to form a bonded joint between the alumina and the molybdenum support block. The titanium layer 28 is preferably disposed on the alumina block by means of coating titanium oxide on the block end portion and firing the combination of alumina and titanium oxide at approximately 1900 C., thereby partially reducing the oxide and leaving a conductive layer of titanium as the surface material. Since it is relatively difficult to properly vacuum seal molybdenum to a copper body such as block 8, a preferably cold rolled steel end cap 31 is secured within the aperture and a silver braze is preferably utilized to obtain a vacuum seal between the copper main body block and the cold rolled steel end cap 31. Quite obviously, any suitable alternative insulating materials, such as, for example, sapphire, beryllia, etc., may be utilized as the insulator support member for the electrode means 6. Similarly, the particular titanium layer 28 may advantageously be a block of titanium rather than a layer of titanium and furthermore, other materials such as, for example, niobium may advantageously be utilized for the tip or end portion of the self-biased electrically fioating electrode means 6.

In the embodiment of FIG. 2, the asymmetrically disposed self-biased electrode means 16 preferably has a titanium tip 34 disposed and fixedly secured as by brazing to the insulation rod 35 which preferably once again is sapphire, alumina, beryllia, etc., which rod is disposed within a Kovar or the like plug 36 which is sealed within the collector block 15 by conventional means. In both the embodiments of FIGS. 1 and 2 it is to be noted that the material to be utilized for the end or tip portion of the self-biased electrically floating asymmetrical probe assembly has been selected such that an effective secondary emission ratio of less than 1 at the energy of the impacting electrons is achieved. This effective secondary emission ratio of less than 1 is preferably achieved with titanium since generally titanium over its entire impacting voltage, V range vs. secondary emission 0 characteristics as evidenced by FIG. 3, has a secondary emission ratio of less than 1. Therefore, it is to be noted that titanium easily charges up to a negative voltage which is sufficient to provide ion draining capabilities in the embodiments of FIGS. 1 and 2. In the embodiment of FIG. 1, the asymmetrical location of the titanium end or tip portion 28 is such that a small portion of primary beam electrons traveling along the central beam axis Z impact on the titanium tip such that a negative bias is achieved primarily through primary or high energy electrons having a narrow energy spectrum. Another matcrial suitable for the impact portion of the self-biased electrode would be carbon types such as graphite or carbon black which also, like Ti have secondary emission characteristics which are generally less than 1 over the impacting voltage range. See the illustrative characteristics of same in FIG. 3. The graphite can be painted, sprayed or disposed in bulk form on any suitable insulator block, rod, etc., in order to provide isolation relative to the body of the tube. It is to be noted of course, that the characteristics depicted in FIG. 3 are merely illustrative samples since quite obviously a varies as surface conditons vary which explains the many ditferent curves of 0' found in the literature for a given material.

In the embodiment of FIG. 2, the titanium end or tip portion 34 is asymmetrically disposed such that the secondary electrons are the primary source of impactment rather than primary or high energy electrons from the main electron beam. In either case, since titanium has an effective secondary emission ratio regardless of the energies of the impacting electrons which is less than 1, a negative bias is obtained, such that ion draining capabilities are achieved. Furthermore, in both embodiments of FIGS. 1 and 2 due to the asymmetrical position of the electrically floating self-biased electrode within the collector region, maximum positive ion draining efficiency is achieved because maximum positive ion production generally occurs in the collector region due to the impacting of secondaries, primaries, etc., of the main beam therein. The asymmetry of the electrode means prevents slow secondary electrons as well as positive ions present in the collector region from being refocused or drained back into the interaction or cavity regions of the tube. It is to be noted that in both the embodiments of FIGS. 1 and 2, only a single asymmetric electrode has been shown, however, any number of electrodes as well as electrode configurations may be utilized as long as an effective asymmetrical relationship with respect to the beam axis is maintained.

Utilizing conventional noise measuring techniques it was not possible to measure any frequency or amplitude deviations in the main or generated signal when using the self-biased ion draining electrically floating electrode means as disclosed in the present invention. This of course, is the result of the inadequacies of present day crystal detector measuring devices. However, it is significant in the sense that the self-biased electrode means of the present invention have completely eliminated, as far as practical considerations are concerned, ion oscillations and frequency and amplitude deviations due to ion noise in conventional tubes constructed according to techniques disclosed herein.

The teachings of the present invention do not exclude the positioning of the self-biased asymmetrically disposed electrode means outside of the collector region of such high frequency discharge devices as klystrons, traveling wave tubes, etc. However, it is to be noted that maximum secondary emission and ion noise is present or generated in the collector region and that maximum efiiciency is obtained by such a positioning thereof as noted in the teachings in the aforementioned Patent No. 3,293,481 by C. E. Ward et al. For example, the asymmetrical location of the ion draining electrode means of the present invention, within the interaction cavities would, of course, cause perturbation of the electron beam in the interaction region which might be undesirable. Location of the asymmetrical self-biased electrode means as taught by the present invention within the cathode or drift tube regions could be accomplished easily enough and is within the purview of the present invention.

Referring now to FIG. 4, there is depicted in FIG. 4(a) an illustrative graphical portrayal of secondary emission ratios, 0-, for two hypothetical materials labeled A and B vs. impacting voltage V and in FIG. 4(b) there is depicted an illustrative graphical portrayal of a number of secondary electrons, N, emitted from any hypothetical surface as a function of secondary electron energy in volts,

expressed as a percent of primary impacting energy of electrons bombarding the surface for a given impact voltage. In other words, examination of 4(b) indicates that for primary electrons of a given energy impacting on any surface the resulting secondaries emitted from the surface will be distributed according to FIG. 4(b) and furthermore, experimental evidence indicates that approximately 90% of the secondaries will have energies of less than 20 volts for any primary impact energy greater than say 50 volts.

With regard to FIG. 4(a) characteristic A being representative of a material having a 0' of less than 1 over its entire V will, when embodied as an ion draining electrode as taught by the present invention, build up a negative charge if isolated from ground which is, in the vast majority of cases, body or collector potential for microwave tubes. The exact potential to which the electrode will charge negatively presents an extremely complex problem which is governed by, among other things, the material which the impact surface is made of, the surface condition of the material, the quality of the insulating material (in other words, how good an insulator is involved), the impacting voltages, the primary beam voltage, the number of ions attracted to the electrode, the energy spectrum of the ions, the distance of the electrode surface from the surrounding walls of the collector, etc. It has been found that using a titanium tip, as in FIG. 2, and secondary bombarding electrons that negative voltages of 20 volts can be obtained on the electrode which were more than adequate to reduce ion noise to an extent where conventional instrumentation was not able to provide an indication of positive ions within the interaction region of the tube.

If primary beam electrons are used to bombard the selfbiased electrode of the present invention which have an effective energy greater than level C in FIG. 4(a) then an electrode made from material having hypothetical a characteristic A will indeed attain some negative value which is a complex function of the above indicated parameters while an electrode made of a material having a 0 characteristic equivalent to B will stabilize at some negative voltage around crossover level D where the 0' characteristic becomes greater than 1. The exact potential involved will, of course, depend on the number of positive ions being collected and thus will deviate accordingly from crossover level D.

If secondary electrons are being utilized to bombard the self-biased ion draining electrode surface and a material having a a per characteristic A or B is selected then a negative voltage will be acquired by the electrode which is less than crossover point E in the case of B. Once again, the exact stabilizing voltage will depend upon the number of positive ions collected as well as their energy and all of the aforementioned parameters.

If a combination of primary beam and secondary electrons are utilized to bombard the self-biased electrode the resulting negative charge on the electrode will be a function of the effective energy level of the bombarding electron and will once again in the case of a material such as B stabilize at a level either around crossover D or below crossover E while a material such as typified by A will be dependent on the effective energy.

Since the surrounding collector walls are generally kept at ground potential a positive charge of any appreciable level will not be obtained on a floating electrode even if material such as B is selected and the impact energy level is at level P since the excess secondaries will be returned to the electrode in view of the ground potential level of the surrounding collector walls.

While we have found titanium to be the most suitable material for the impact portion of a self-biased ion draining electrode because of its inherent gettering functions, the present invention contemplates the selection of any suitable material disposed in an asymmetrical relation to the beam axis on insulator material which has an effective secondary emission ratio which is less than 1 for the impacting energy of the impacting electrons on the electrode such that a negative bias on said electrode results.

Since many changes may 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 high frequency electron discharge device having a central beam axis including a. self-biased electrode means asymmetrically disposed within the vacuum envelope of said device with respect to the beam axis, said means having at least a portion thereof made from an electrically conductive material which has an effective secondary emission ratio less than 1 for the energy of the electrons impacting on said electrode means generated within said device, said means acquiring a negative charge thereon through a self-biasing mechanism during operation of said device.

2. The device as defined in claim 1 wherein said selfoiased electrode means is disposed within the collector region of said electron discharge device.

. 3. A high frequency electron discharge device defining an electron beam axis and including a collector assembly disposed about said axis, said collector assembly including a self-biasing asymmetrically positioned electrically floating electrode means disposed therein spaced from said beam axis, said electrode means having at least a portion thereof made of a material which has an effective secondary emission ratio which is less than 1 for the energy of the impacting electrons such that said floating electrode means acquires a negative charge thereon.

4. The device as defined in claim 3 wherein said electrode means is positioned within said collector assembly such that said electrode means is bombarded primarily by secondary electrons.

5. The device as defined in claim 3 wherein said electrode means is positioned within said collector assembly such that said electrode means is bombarded primarily by primary electrons from said electron beam.

6. The device as defined in claim 3 wherein said electrode means is disposed within said collector assembly such that said means is bombarded by a combination of primary and secondary electrons.

7. An electron discharge device comprising an electron gun adapted and arranged to generate and direct an electron beam along a predetermined beam axis, said gun being disposed at the upstream portion of said device; a collector structure disposed at the downstream portion of said device; interaction means disposed along said axis between said gun and collector for interaction with said electron beam; and a self-biased electrode disposed within said device and insulated therefrom, said electrode being asymmetrically disposed with respect to said beam axis and having at least a portion thereof made from an electrically conducting material which has an effective secondary emission ratio which is less than 1 for the energy of the impacting electrons whereby said electrode will charge up to a negative self-biasing voltage such that positive ions present in said device in the vicinity of said electrode are attracted to said electrode.

8. The device as defined in claim 7 wherein said electrode includes an alumina block having a coating of titanium on the end portion thereof, said alumina block being vacuum sealed within said collector region.

9. The device as defined in claim 7 wherein said electrode includes a block of titanium mounted on an insulating member and disposed within the evacuated portion of said electron discharge device such that secondary electrons are essentially the only electrons bombarding said titanium block.

10. The device as defined in claim 7 wherein said electrode is disposed within said collector such that said electrode is bombarded by a combination of primary and secondary electrons.

11. The device as defined in claim 7 wherein said electrode is disposed within the collector region of said device and adapted and arranged to be bombarded by secondary electrons resulting from primary beam electrons impacting on the walls of said collector.

12. The device as defined in claim 7 wherein said electrode is disposed Within the collector region of said device and insulated therefrom and adapted and arranged to be bombarded by primary beam electrons such that a negative bias is acquired by said electrode.

References Cited UNITED STATES PATENTS 3,201,630 8/1965 Orthuber et al. 313106 X HERMAN KARL SAALBACH, Primary Examiner.

10 PAUL GENSLER, Examiner.

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