US2974253A

US2974253A - Electron discharge apparatus

Info

Publication number: US2974253A
Application number: US806115A
Authority: US
Inventors: Robert L Jepsen
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1953-10-05
Filing date: 1959-04-13
Publication date: 1961-03-07
Anticipated expiration: 1978-03-07

Description

March 7, 1961 R. JEPSEN 2,974,253

ELECTRON DISCHARGE APPARATUS Original Filed Oct. 5, 1953 2 Sheets-Sheet 1 I 21 23 I'll |5 4; 4 1 8 a it 225 '11 8'" E DISTANCE DISTANCE FI|5 :2- A :EI I5 & A

NVENTOR. EaaERr L. E'PsE ATTORNEY March 7, 196 R. L. JEPSEN 2,974,253

ELECTRON DISCHARGE APPARATUS Original Filed Oct. 5, 1953 2 Sheets-Sheet 2 44 7 4 VII'IIIII/IA 43 Q as ea T6? 6/ o/srAucE- INVENTOR Ease-e r- L (/EPSEN FIE 2A ATTORNEV United States Patent fornia Original pp a 1 3,; Set-No: 3.84; .8 ew P te a a w. da d Max 9- Divi ed and this application Apr. 13, 1952, Ser. No. 806,115

1 Claim. (Cl. 315-519)" This invention relates, generally, toultr a' high frequency electron tube devices or apparatus and the invention has reference, more particularly, to noveljapparatus of this type which removes ions from the beam paths through the devices, such as klystrons, traveling wave and, other electron beam tubes, to thereby substantially eliminate perturbations of the beams thereof, and also to novel velocity modulation tubes of the reflex klystron type which are so constructed 'as to prevent reflected electrons from returning to the cathode region.

Several difficulties encountered in electron discharge devices such as, for example, klystrons and traveling wave tubes heretofore employed arise from an interaction be tween the electron beam and; ions trapped along. the path of the beam. These difiiculti'es may include:

(1 Modulation of output power, frequency, and beam currentat the frequency or frequencies ofcontinuous ion oscillation. These frequencies are, typically, in the range .1 megacycle per second'to 1 0 megaeycles per second.

(2) Modulation of outputpower; frequency andbeam current at both I a); Ion oscillation frequencies (in the megacycles per second range) and v (b) Low frequencies (typically in theaudio frequency range). The low frequency fluctuations are associated with the high frequency io-no'scillations. (3) Changes in output power, frequency, and; beam current as the frequency of an applied modulating volt-age is varied; These changes occur when the modulating frequency is near one of the frequencies of incipient ionoscillation and they may be abrupt orfluctuating. In many'heretofore existing reflex klystrons aqsource oftrouble encountered is theoccurrence of what has been termedmultiple transitsof the beam. Multipletransits refer to the action of the'electr'on 'be'am after passing through; the resonator gap subsequent to being reflected back by thenegatively charged reflector. After this re'- flecied or return trip through th e resonator gap, the bunched electron's haveperformed their useful function and t heir; elimination from further operation onthe tube is desirable. However, manyofthe returning electrons enter the cathoderegion of the reflex klyst-lfon andare turned about by the negative charge tobeginj va'se cond 2 the klystron of the electrons in eliminated. 1 Another object of the invention is to provide a novel reflex klyst'ron wherein ion trapping in the re-ent'rant tube is substantially eliminated.

Another object of this invention is to provide a. novel method and; means for draining ions out from fined by grided apertures.

Still another. object of the invention is to provide a novel method an'dmeans for draining ions from the rose; nator gap space in an electron discharge device such as, for example, a reflex klys'tron.

the'beam are substantially These and other objects. and advantages of the present I invention will be better understood from perusal of the following description wherein:

Fig. 1V is a schematic elevation view in section of a reflex klystron which-embodies the present invention whereby multiple transits of. the electrons in the tube are prevented. and whereby. the ions are continually drained from the re-entrant tube or drift space between the cathode and the first resonator grid to prevent the forma- Fig. 1 whereby the ions in the resonator gap space area three-cavityvelocity modulation tube which embodies the novel invention for preventing the formation of ion I i roundtrip through the resonator gap, These multiple transits of the electrons produce undesirable effects in the tube such as, for example, unstable power output.

Oneobject of the presentinyention is to provide a novel method and means in electron beam. discharge devices for preventing the trapping of ions in the beam path.

Another object of the present inventionis to provide a novel method and means for eliminating iontrapping in the beam. path of electron discharge devices byinsuring that the'electron beam occupies ajprogressively increasing proportion of the space within, the, confining walls of the beampassage way as the beam sage way. I p

Another object of the present invention is to provide a progresses alongthe pasnpvel reflex klystron wherein mpltipleytransitslthrough tionofan ion trap therein.

' Fig. 2 is a section view'of the reflex klystron of Fig. 1 taken along section line 22.

Fig. 3' is a sectional view of a portion of a reflex klystron of the type heretofore employed wherein. ions are trapped int'he drift space or re-entrant tube portion of thetube between the cathode and the first resonator grid;-

Fig. 3(a) is a plot of the potential alongthe beam axis versus the distance alongthe re-entrant tube space for the; reflex klystron shown in Fig. 3 showing the formation of an ion trap.

Fig.4 is a section view of. a portion of a reflex klystron which embodies the present invention wherein the'walls of the re-e'ntrant tube space between the cathode andthe first resonator grid are so formed as to prevent the forma-- tion of ion traps therein as occurs in the klystron of Fig; 3. This novel feature. is utilized in theklystron of Fig. 1". Fig. 4(a) is a plot of the potential along the beam axis versus the distance along the beam path in the re-entrant tube space of Fig. 4 showing. how the formation of ion traps is prevented.

Fig. 5 is a section view of. a portion of a reflex kly--' stron substantially identical to that shown in Fig. 1 which embodiesla novel additional feature to the klystron'of also continually. drained therefrom to prevent the forma tion of anion trap. between the grids.

Fig. 6 discloses in section view a two-cavity klystron employing an annular shaped. beam of electrons wherein a novel method and means is provided for continually draining ions from the re-entrant tube or drift space between the accelerating grid and the first resonator grid tov prevent the formation of an ion trap therein.

Fig; 7 shows in longitudinal section view a portion of traps at the resonator gaps as occurred in heretofore employed velocity modulation tubes of this type. a

. Fig. 7(a) isa plot of the potential along the be'anr axis versus the distance alongthe beam path for the novel beam device shown in Fig. 7 and also includesfaf plot of the beam potential. along the-axis of "a. beam. 1 versus" the distance along: the beam path for thehereto- I fore employedklystrons of .thisgeneral type to more, clearly illustrate the improvement brought about bythel,

' inventiondisclosed in Fig.7. 1 .=Referring now to Figs. 1 and Zthere is shown arefiepj ldystron, embodying the present invention, there being.

shown onlyasuch parts of aprefiex klys'tron: as are nec saryutogillustratelthe present-invention Itshama spaces de:

and. explanationof the. drawings understoodthat the beam path of the particular klystron would also be enclosed in a vacuum envelope or body and the klystron would include other elements such as, for example, an output window. The reflect klystron comprises an annular cathode 1 having a concave or recessed emitter surface, the cathode surface sloping radially inwardly and downwardlyas viewed in Fig. I to form substantially an acute angle with respect to the axis of the klystron tube. An annular heater 2 is shown positioned on the under side of the cathode to provide the necessary heat for electron emission. An annular, substantially cup-shaped focusing ring 3 encircles the annular cathode on both its'outer and inner peripheries to provide for focusing of the electron beam emitted from the cathode. Located within the center or frame opening in the annular focusing ring and cathode and in axial alignment therewith and also extending slightly above the cathode emitter surface is a metallic collector electrode 4. Positioned in axial alignment with the cathode focusing ring 3 and collector 4 is a cavity resonator 5 including a pair of mutually spaced

resonator grids

6 and 7. The drift space or re-entrant tube formed by the outer wall 8 of the re-entrant portion of the cavity resonator is of substantially truncated cone configuration, the sides of which have a slightly convex shape, this particular configuration being employed for a purpose to be subsequently described. Axially aligned with the

grids

6 and 7 and positioned above grid 7 is a metallic concave reflector 9.

In operation, an annular shaped beam of electrons is emitted from the cathode 1 and is focused into a beam by the focusing ring 3, the stream of electrons being accelerated toward the

resonator grids

6 and 7 by the positive potential of the cavity with respect to the cathode as provided by the source of potential represented by battery 11. Because of the angle of the emitter surface of the cathode 1 with respect to the axis through the tube, the annular electron beam is directed in such a manner as to tend to form a hollow substantially conical beam, the apex of which coincides with the axis of the klystron and is located within the re-entrant tube portion 8, the sides of the beam being somewhat concave and conforming to the convex surface of the tube re-entrant portion 8. However, the electrons in the annular beam, as they approach the axis, repel each other due to space charge and this interaction between the electrons tends another trip through the tube and thus the problem of multiple transit is eliminated. A recess or pocket 13 is shown in the upper surface of the collector 4 and serves the purpose of retaining any secondary emission electrons which may be emitted from the collector by the striking electrons. A decided advantage in the use of the collector electrode for catching the electrons after they have performed their useful function in the tube is that the electrons are collected on a separate electrode, the collector, rather than on the walls of the tube and the cavity resonator, and thus the heat generated by the striking electrons does not cause expansion of the cavity resonator or other critical tube parts with a resultant change in the operating frequency of the tube.

The collector is connected to a source of potential slightly negative with respect to the potential of the cavity resonator by the variable resistor 12. The collector, be cause of its slight negative potential, attracts the positive ions which are produced in the re-entrant tube or drift space 8 of this klystron due to the collision of the electrons with gas molecules in the space. The collector thus continually drains the ions from the space and prevents the formation in the beam path of a deleterious ion trap. The particular configuration of the re-entrant tube walls 8 also serves to prevent the formation of an ion trap as will be more readily understood from the following description of this novel feature disclosed in Fig. 4.

In heretofore existing reflex klystrons wherein attempts were made to prevent the beam from returning into the cathode region after passing through the resonator gap, one example of which employs a spike in the center of V the reflector to produce an umbrella-shaped reflected beam, the electron beam does not travel parallel to the electric field vectors across the resonator gap during both passages thereacross and, therefore, the optimum interchange of energy between the beam and resonator field does not occur. In this present invention, the electron to bend the annular beam within the re-entrant tube portion was to form the beam into a straight substantially hollow cylindrical beam. The electrons in this hollow cylindrical beam configuration pass through the cavity resonator gap between the

grids

6 and 7 with the electrons traveling in paths perpendicular to the grids or, as stated in another way, parallel to the electric field vectors across the resonator gap. The electrons are velocity modulated by theradio frequency voltages across the gap in a well known manner and are repelled by the reflector 9 which may carry a negative potential with respect to the cathode. The electrons are turned about and again pass through the resonator gap in bunches parallel to the electric field vector across the gap to give up energy to the field in the cavity resonator. The stream of electrons continues axially through the reentrant tube 8 and are collected on the collector 4 which is connected to a source of potential positive with respect to the cathode, the potential of which may be made variable as represented by the variable resistor 12. As noted, the returning electrons are in the form of a cylindrical beam and, since the diameter of this beam is determined in part by the diameter of the resonator grids, the collector electrode upper surface is shown having a diameter equal to or slightly larger than the diameter of the grids. Since the returned electrons are all collected on the positive collector electrode. 4, few, if any, will, re-enter the. region of the negatively charged oath ode Lwhere. they would beturned about and started .on

beam travels parallel to the electric field vectors across the resonator gap on both passages therethrough to give the maximum energy exchange even though the electron beam does not re-enter the negative cathode region on its return trip through the klystron.

Referring to Figure 3 there is shown a portion of a reflex klystron tube of the heretofore employed type including the cathode 16, the focusing ring 17, the walls 18 forming the cylindrical re-entrant or drift space and the first resonator grid 19. The outer periphery surface of the beam emitted from the cathode and focused by the focusing ring is shown in dotted lines. It is noted that at the entrance to the re-entrant tube space the beam occupies substantially the entire opening, the beam occupying a progressively smaller portion of the cylindrical drift tube space as it proceeds toward the resonator grid 19. The beam may spread again as it approaches the grid 19 but, in any case, the beam occupies a smaller proportion of the drift space at some point within the space defined by walls 18 than at the left-hand end of the drift space. This particular relationship between the walls of the drift tube space and the beam space produces an ion trap within the drift tube. The amount of potential depression within a drift tube due to the passage of an electron beam through the drift tube depends on the beam voltage, on the beam current, on the distribution of current across the beam, and on the geometry of the drift'tube. In particular, increasing the diameter of a cylindrical drift tube, keeping all other quantities constant, results in an increased potential depression.

This ion trap is better illustrated in Fig. 3(a) which is a plot of the potential along the beam axis between the entrance to the re-entrant tube 18 and the resonator grid 19. .The ion trap is formed where the potential curve drops below the positive value which is present atthe left-hand end of the re-entrant tube or drift space due to the space charge eflects of the electronbeam.

This ion trap is represented by the cross-hatched area 21. The positive ions in excessnof the number necessary to fill the trap'or', in other words, to balance .thedecreas'e in positive potential of the beam will drain out the lefthand end of the re-entrant tube. The present inventors have devised a 'novel drift tube structure whichprevents the formation of ion traps, this novel structure employed in Fig. l is being shown in Fig. 4.

Referring to this Fig. 4 and also to Fig. 4(a) it :will benoticed that the drift tube space, rather' than being of a cylindrical shape, is now of an approximately truncated cone shape with convex walls, the walls tapering towardthe right-hand or grid end; The cathode 22 and focusing ring 23 maybe of the same construction as shown in Fig. 3 and the beam produced thereby of the same shape as that in Fig. 3. The drift tube walls are so shaped with relation to the beam perimeter that as the beam progresses toward the resonator grid 24 it occupies a progressively increasing portion of' the drift tube space. This particular relationship between the drift tube walls and the 'elect'r'oiibeam prevents the formation in the beam path of any ion traps. This is better illustrated in Fig. 4(a) which shows a plot of the potential along the beam axis between the left-hand end of there-entrant tube and the resonator grid. As

can be seen there is no depression or decrease in the positive potential along this drift space length so that substantially all of the positive ions formed in the drift tube space drain out the left-hand end thereof.

It is this particular feature of the re-entrant tube portion of the klystron in Fig. l which aids in preventing the formation of ion traps in the re-entrant tube space of this tube as mentioned above.

Referring to Fig 5 there is shewn therein in section view a portion of a reflex klystron tube of the type shown in Fig. 1, there being disclosed only the reflector electrode 26, the two

resonator grids

27 and 28 and a portion of the walls 29 defining the re-entrant tube and cavity resonator. It will be noted that the second resonator grid 27 is thinner than the first resonator grid 28 and also that the openings or interstices defined by the vanes of the grid 27 are substantially larger than those of the first resonator grid. This particular type of second rescnator grid permits a portion of the negative field produced by the reflector electrode 2.6 to penetrate through this grid into the resonator gap space between the two grid, thus providing for the draining of positive ions from the resonator gap to the reflector and thereby preventing the formation in the resonator gap of an ion trap.

In Fig. 6 is disclosed still another novel structure for use in electron beam apparatus for draining ions from drift spaces. There is shown just that much of a two-cavity resonator klystron which suffices to explain the present invention, it being understood that the apparatus shown, or at least the beam path, is enclosed within a vacuum type envelope and may include other structural features not shown. An annular cathode 31 having a slightly concave emitter surface produces an annular beam of electrons when heated by a heater 32 extending under the cathode. This stream of electrons is focused by a circular focusing member 33 having a substantially -shaped cross section, the upwardly extending cylindrical central portion 34 of this focusing member extending within the central opening of the annular cathode 31 and in axial alignment with the klystron. An annular accelerating grid 35 is positioned in the path of the electron beam for accelerating the electrons therein to a constant velocity within the space formed by walls 36 of the tube. The beam passes through the gap formed 7 by the resonator grids 37. and 38 of the first or buncher cavity resonator 39 where radio frequency energy acts on the beam for velocity modulating the electrons therein, the electrons then passing through the drift space 64 where they become density modulated and then passing across the second resonator gap formed by the output resonator grids 141 and .42.where .the bunchedelectrons give up energyto the output cavityresonator 43. The electrons in the beam then collect on the collector electrode 44. Since in customary operation of beam discharge .tubes of .this type the focus electrode 33 is at a negative potential with respect to the walls .of-the drift tube spaceand the cavity resonators and since in this particular embodiment the central upwardly extending portion 34 of the focusing electrode 33 extends slightly above the cathode 31 and is aligned with the central opening in the annular accelerating grid 35, .ions will be drained out from the drift ftubespace'by. the negative 7 potential on the focus electrode through the central opening in the annular accelerator grid to thus prevent formation of ion traps in the drift .tube spaceQ Fig. 7 discloses another embodiment of the present inventionwherein the ion draining feature disclosed in Fig. 4 is applied to a multicavity gridless klystron tube which maybe, for example, of the .high power amplifier class such as disclosed in the US. patent application, Serial No. 370,568, of Wayne G. Abraham and Sigurd F. Varian entitled High Frequency Tube filed on July 27, 1953, now US. Patent No. 2,879,440. The electron beam emitted from the cathode 51 is transmitted down the series of

drift tubes

52, 53, 5.4 and 55, the ends of which define resonator gaps in the three cavity resonators 56,

57 and 58. The inner diameter of each succeeding drift tube along the beam path is slightly smaller than the inner diameter of the preceding drift tube, the drift tube inner diameters of each drift tube being constant. The electrons in the beam are expended in the collector end of the tube. In heretofore employed velocity modulation tubes of this type the drift tubes had equal and constant inner diameters throughout the length of the tube as shown in the above cited patent application. In these heretofore employed multicavity resonator tubes a potential depression is produced at each of the resonator gaps, these depressions producing ion traps at each gap. This is better illustrated in Fig. 7(a) which is a plot of the potential along the beam axis versus the distance along the beam path. The potential curve labeled 61 is typical of the heretofore employed multicavity klystrons while the curve 62 which is vertically transposed on the graph with respect to curve 61 is that of the novel tube structure shown in Fig. 7. As seenin curve 61, the potential depressions at the resonator gaps produce ion traps 63 which are designated by the cross-hatched area. As indicated by the curve 62 representing the novel struc ture shown in Fig. 7, these ions traps are eliminated due to the fact that as the curve is traced from left to right there occur no potential depressions where ion traps may be formed. In addition to preventing the formation of' ion traps at the resonator gaps, this novel configuration also permits the rapid draining of ions formed throughout the entire length of the beam path due to the fact that the beam potential curve 62 decreases in its positive value from right to left, whereas the beam potential curve 61 is a fiat or unchanging curve. This illustrates another embodiment where an increased portion of the drift space is occupied by the electron beam as it passes through the tube.

Although the klystron disclose in Fig. 7 has each succeeding drift tube of a smaller inner diameter than the preceding drift tube, the greatest benefit in tube operation is derived from the elimination of the ion trap at the first resonator gap and a lesser benefit by eliminating the ion trap at the second resonator gap. This is due to the fact that any ambiguities in operation that appear early in the stage of this amplifier tube are amplified to a great extent as the beam proceeds down, the tube. If such a high degree of perfection in operation is not necessary, the drift tube 55 may have the same sized inner diameter as the drift tube 54 or, to carry it a step further,

7 the inner diameters of

drift tubes

53, 54 and 55 may be of the same value or drift

tubes

54 and 55 may be made with larger inner diameters than drift tube 53 in certain cases, for example, to accommodate spreading of the beam. In these latter examples, ion traps would occur at the resonator gaps in

resonators

57 and 58 but not at the resonator gap in resonator 56. These latter examples were given to illustrate the flexibility of this particular embodiment of the invention.

As an example of the application of this invention, the inner diameters of the drift tubes of a power amplifier tube of the type disclosed in the above cited patent are each 1.2". To achieve the improvement described herein this power amplifier tube would be modified such as, for example, changing the inner diameter of the first drift tube to 1.5", the inner diameter of the second drift tube to 1.3" and the inner diameter of the third and fourth drift tubes to 1.1".

Since many changes could be made in the above construction of the novel invention and many apparently widely diflerent embodiments of this invention could be made without departing from the scope thereof as, for example, its utilization in other types of klystrons, in traveling wave tubes and in other electron beam devices, 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.

This application is a division of my application Ser. No. 348,018 filed October 5, 1953, and entitled Electron Discharge Apparatus, now Patent No. 2,888,599.

What is claimed is:

A reflex klystron including a cavity resonator having a pair of resonator grids therein, each grid having a plurality of electron-permeable interstices, and a reflector aligned with the resonator grids wherein the resonator grid nearest to the reflector is substantially thinner than the other resonator grid and wherein the interstices of the first mentioned grid are substantially larger than the interstices in said other grid, to thereby cause ions formed in the gap between the resonators to be drained through the thinner, larger-intersticed grid to the reflector.

References Cited in the file of this patent UNITED STATES PATENTS 2,518,954 Steele Aug. 15, 1950 2,567,674 Linder Sept. 11, 1951 2,645,739 Fremlin et a1. July 14, 1953 2,758,245 Varian Aug. 7, 1956