US2228121A - Amplifying device for electron currents - Google Patents

Description

1941- o. KRENZIEN I 2,223,121

AMPLIFYING DEVICE FOR ELECTRON CURRENTS Filed Dec. 2, 1937 2 Sheets-Sheet 2 INVENTOR Patented Jan. 7, 1941 UNITED STATES AMPLIFYING DEVICE FOR ELECTRON CURRENTS Otto Krenzien, Berlin-Siemensstadt, Germany,

assignor to Siemens & Halske, Aktiengesellschaft, Siemensstadt,

near Berlin, Germany, a

corporation of Germany Application December 2, 1937, Serial No. 177,732 In Germany December 7, 1936 10 Claims.

The present application is a continuation-inpart of my application, Serial No. 148,451, filed June 16, 1937, and assigned to Siemens 8: Halske, Aktiengesellschaft. Y

The invention relates to an amplifying device in which an electron stream is amplified by applying the effect of secondary emission. In the known tubes of this type, the electron current supplied by an electron source is deflected to a plate maintained at a positive potential and provided with an active layer; from the latter plate secondary electrons are released, the number of which is a multiple of the number of primary electrons. This amplified current is then conducted to a further plate maintained at a higher positive potential than the previous plate and the process of secondary emission is repeated; from stage to stage, the electron current thus increases considerably.

In the known arrangements, however, the yield is not so large as may be expected from theoretical considerations. This is to be attributed to the fact thatthe large number of electrons leaving a plate do not impinge on the succeeding plate, and, therefore, are lost as far as amplification is concerned. An attempt has been made to remove this disadvantage by difl'erent arrangements. As a simple solution, it has been proposed to arrange between the secondary emission electrodes, guiding electrodes which deflect the electron current to the amplifying electrodes.

The present invention relates to an arrangement for a secondary emission amplifier in which the above-mentioned guiding electrodes are utilized and with which a considerable increase in the yield may be attained as compared to the known or the proposed arrangements.

According to the invention, the electrodes in an amplifying device for electron currentsin which the amplification takes place by means of secondary emissive electrodes consisting of parallel plates in cooperation with which guiding electrodes are associated-are so arranged opposite to each other and their potentials are so selected that the electrons in the last portion of accelerate the secondary electrons produced on the plate away from the plate. field in front of the secondary electrodes must accordingly be given such a form that the electrons which are emitted by the ,hot cathode, photo-cathode, or the previous amplifying stage, are actually deflected in the region of the plate, in which the electrons formed are subjected to accelerating fields which release them from the electrode and drive them to the next electrode. It is clear that in such a case the primary electrons in the latter part of their path and before impinging must cross equi-potential lines which correspond to a higher potential than that of the plate; that, accordingly, for a short portion of their paths, they must pass through decelerating fields. The field configuration in the tube must accordingly be so selected that the primary electrons, in accordance with the known principles of electron optics, attain such speed and direction distribution that they, to a large extent, reach the secondary emission electrodes and impinge on them in the proper position in spite of these decelerating fields. In particular, it is in this connection of advantage to maintain the gridshaped guiding electrode, that is negatively charged with respect to the associated secondary emission electrode, more positive than the most positive electrode of the previous stage. An arrangement thus results which corresponds in optics to the analogy of a number of deflecting prisms with refractive indices which vary along the path of the ray in a definite manner.

In Figure 1 of the drawings, an electrode system is illustrated which is built up in accordance with the basic principles of the invention with respect to the spacial arrangement and the selection of the electrode potentials. Two amplifying steps are illustrated. l is a cathode of large surface, for example, a photo-cathode. The secondary emission electrodes of the first step are identified by the numeral 2, the guiding electrodes of this step by the numeral 3. As can be seen from the figure, all of the secondary emission electrodes of the first step have a potential of 150; the associated guiding electrodes have one of 100 volts. The guiding electrodes 4 of the second step are in a line with the secondary emission electrodes of the first step; secondary emission electrodes 5 of the second step are arranged in a line with the guiding electrodes of the first step. The secondary emission electrodes of the second step are maintained throughout at a potential of 250 volts; the guiding electrodes of the second step at a potential of 200 volts. It will, accordingly, be recognized that the potential of the guid- The electrical ing electrodes is always more positive than the potential of the most positive electrodes-in the present case, the secondary electrodesof the previous stage. From this selection of the arrangement of the potential relationships, the field line diagram shown in the figure results. It will be recognized that an electron emitted by the secondary emission electrode of the first stage is strongly accelerated in the direction of the secondary emission electrode of the second stage. Its path will, in accordance with the potential distribution, be such that it impinges on the following secondary electrodes at a point at which the secondary electrons formed after the impact at once find themselves in a positively accelerating field. In addition, the primary electron is also not lost for the formation of secondary electrons. The speed which this electron attains in its path from the first to the second secondary electrode is sufficient to permit it to pass over a small portion of its path against decelerating fields. These decelerating fields constitute, however, accelerating fields for the secondary electrons formed by this electron. Thus, all of the formed secondary electrons are drawn away from the secondary emission electrode. The anode comprises a set of parallel bars 6.

The arrangement according to the invention accordingly has the effect that on the one hand no electron passing from one stage to the other is lost as regards its function in the formation of secondary electrons and on the other hand, all of the formed secondary electrons are completely advanced and may be utilized for the release of new secondary electrons.

The guiding of the electrons takes place in this arrangement purely electrostatically; this is of great advantage as compared to arrangements with magnetic electron guiding. In electrostatic guiding, the radius of curvature of electron path is always proportional to the ratio of the potential through which the electron drops to the field strength. Therefore, the radius of curvature and the spacial current distribution are independent of potential fluctuations. Therefore, alternating current potentials may be used for deflecting the electrons. It is true that only small phase displacement, approximately determined by the conduction capacities, may exist in these alternating currents. In the guiding of the ray by a magnetic field, the radius curvature of the electron path is proportional to the U/H where U is the potential through which the electron has dropped and H is the strength of the magnetic field; i. e., a variation in the potential causes either only U alone to vary, or both U and H fluctuate at different powers with U when H is produced by the same potential, so that the electron path is never invariant as regards potential fluctuations. From this, there results an important advantage of the electrostatic beam guiding as compared to electromagnetic beam guiding.

In the building of electrode systems of secondary emission tubes in general, and the above described tubes in particular, care must, however, be taken that the yield of secondary electrons is not decreased by the fact that the electrons take an undesired path and then do not impinge at the position at which they should release new secondary electrons. This danger is also to a certain extent present in the secondary emission I amplifiers in'which the secondary emission elec-' trodes are of grid-shaped construction and are provided with a further grid for guiding the electrons.

Figure 2 shows such a general electron system which may be similar to the one shown in Fig. l, for example. In this figure, the numeral l represents a large surfaced cathode, 2 are the secondary emission electrodes of the first amplifying stage, 3 the associated guiding electrodes, 4 and 5 are the secondary emission and the guiding electrodes, respectively, of the second stage. The anode consists of a number of wires 6.

It is desirable that the electrons leaving one of the amplifying stages should be accelerated as much as possible in its path to the plates of the next amplifying stage. The positive potential of the secondary electrodes of the second stage must accordingly penetrate as strongly as possible in the field space of the first stage; the result of this might, however, easily be that the secondary emission electrodes of the second stage attract electrons to themselves which in the first, or, in general, in the previous stages, have not as yet released any secondary electrons.

This disadvantage is avoided in accordance with a further aspect of the invention by increasing the attractive power of the positive electrode parts through the grid parts (less positive parts) without any undesirable increase of the spacial region of current absorption; this object being accomplished by mounting preferably narrow gage wire electrode parts which by reason of their small extension, absorb few electrons, in the neighborhood of those electrodes in the surrounding region of which the attractive power of the more positive electrodes is to be increased and connecting these parts conductively with the associated positive electrodes.

In Figure 3, such an arrangement is illustrated. stage and between the electrodes of this stage, the thin wires 1, which are connected conductively with the positive electrodes 8 of the following stage, are located. In this manner, the accelerating efiect of the positive plate 8 on the electrons leaving the first stage is increased without the resultant condition that the electrons, because of the increase, are caught by the positive electrode 8 before they have released secondary electrons from the secondary emission electrodes 9 of the first stage. Accordingly, an extension of electrode parts is involved here and this is equivalent to a lengthening of the positive electrodes 8 in the direction of the electrode region of the first amplifying stage without the accompanying disadvantages of such a lengthening.

Figure 4 shows the field distribution of the above described electrode system. The effect of the extended electrode parts may be recognized from the position of the potential lines. From a consideration of this figure, something in addition follows, however. The electrons which leave the first amplifying stage impinge on the secondary emission electrode of the second stage, pricipally in its upper portion. Therefore, the lower portion of the secondary emission electrodes 5 may also be replaced by an extended narrow gage wire electrode part without changing the action very much. This is clarified in Fig. 5. In this figure, the numeral I0 identifies the cathode, H is the secondary emission electrodes of the second stage; the latter, however, have a smaller width than in the arrangement according to Figs. 2 to 4. The removed portion of the plates are replaced by the narrow gage wire electrodes In the region of the first amplifying l2 connected conductively with the plates II, as

is represented by the broken lines. In somewhat the same manner, the electrode system of the subsequent amplifying stage is built up. If the wires [3, which correspond to the wires 1 in Fig. 3, are added in addition, then the field distribution in the arrangement according to Fig.

- is practically the same as is illustrated in Fig. 4.

In Fig. 5, reference numeral 32 denotes a space charge grid and reference numeral 33 denotes a control grid for governing the emission from cathode H] which reaches secondary-emitting electrodes 9. Reference numeral 5| denotes the control grid which may be used to control the electron flow from electrodes 9 toward electrodes lll2.

The extended narrow gage wire electrode portions, by reason of their small cross-section, take up only very few electrons. The relationships become more propitious if the field is particularly strong in the immediate vicinity of these parts since in that case the electrons pass by the extended electrode parts over the largest portion of the Wire along a path similar to that of a comet without impinging on the extended parts.

If an increase in the portional capacity of the positive electrode portions is desired, the extended portions of the electrodes may be brought to the desired potential by a separate potential source, accordingly, perhaps by a battery which is connectedat one pole to the cathode and at the other pole to the extended electrode parts.

The construction arrangement of the described electrode system is apparent from Figs. 6- and 7. Fig. 6 shows an elevation of the tube. The electrode-plates are held by the mica discs I4 and I5 which are supported against the Wall l6 of the vessel. These two mica discs, the horizontal section of which is illustrated in Fig. '7, are connected and held by the wire stirrup H; the distance between the mica discs is maintained by tubular members I6 which are slipped over the stirrup. The fine-wired electrode parts [9 are held by the sheets 20 and 2| which are connected by the bolts 22 and 23. The anode wires, which are covered in Fig. 6 by the plates, are fastened in similar manner. Fig. 7 shows the mica plate by which the electrode plates are supported. The slots 24 hold up the secondary emission electrodes, the slots 25 the plate shaped guiding electrodes, and the holes 26 the thin-wire guiding electrodes. For the anode wires, a number of openings 21 are provided. The use of the extended electrode parts in all tubes operating with secondary emission electrodes independently of the specific construction of these electrodes is obviously of advantage.

The construction of electric discharge vessels which serve to amplify electric currents by causing an electron current supplied by a hot cathode to be influenced by the currents or the potentials to be amplified-for example, with the aid of a gridand by further amplifying this electron current in a directly connected electrode system functioning by electron multiplication by secondary electrons in one or more steps, is known. Since the degree of amplification depends on the emissive capacity of the electrodes at which secondary electrons are to be released, it is necessary to provide that the surface portions of these electrodes should manifest high secondary emissivity. For this reason, it has been proposed to provide the emissive surfaces with a coating of an alkali metal (e. g., caesium, rubidium or potassium, or a mixture of these metals).

For the same purpose, alkaline-earth metals, such as barium and strontium may also be utilized. In all of these cases, great care-must be taken that after the building in of the electrode system in the discharge vessel, the operating electrodes of theelectron multiplier should be activated as carefully as possible. This takes place almost exclusively by vaporizing the active layers on a suitable base, e. g., silver oxide. It has happened that in this treatment, the control grid is also given a high emissivity. The result .of this is that on this grid a harmful emission which limits the amplification takes place because the grid is raised to a relatively high temperature by heat radiation in particular on the side of the cathode. This phenomena is particularly disturbingwhen the cathode is activated by using alkali metals.

In accordance with a further'aspect of the invention, this undesirable condition. is suppressed by selecting the distance of the hot cathode and the grid so large that the temperatures of the grid which arise during operation are not sufficient to produce the disturbing electron emis sion and by providing, in addition, between the control grid and the cathode a space charge grid which compensates for the field weakening arising by the increase of the distance between the cathode and the control grid in the region of the cathode. It is possible in this manner to maintain the grid emission within permissible limits without finding necessary the use of particularly high potential for the operation of the first step of the discharge vessel.

Exemplary embodiments of the invention are illustrated in the drawings. Fig. 8 shows schematically the structure of a discharge vessel according to the invention. The numeral 3! identifies the hot cathode, 32 is a space charge grid maintained at a suitable potential and 33 is a control grid. Above the control grid lies the electrode system of the electron multiplier. This system consists of tWo boxed sets of plates 2% and 35. potential or at a potential above cathode potential. The plate 35 is supplied with a higher positive potential. 36 is the anode. The electrons leaving the control grid are deflected in such manner between the sets of plates that the largest portion of these electrons impinge on the set of plates 35 above the electrodes of the set of plates 34. Here they release secondary electrons which then move to the anode it. The main path of the electrons is illustrated in Fig. 8 by dotted lines 31. As has been already mentionedyit is possible, by reason of the arrangement according to the invention, tomaintain the emission of the grid so small that it does not disturb the operation. In addition the field losses between the control grid 33 and the cathode 31! are wholly or in part eliminated by the space charge grid 32. 1

Fig. 9 shows schematically, a structure of a discharge vessel in which the electrodes are arranged concentrically within each other. 50 is a hot cathode which may be heated directly or indirectly. 38 is the cylindrical space charge grid, and 39 the control grid. The numeral 48 identifies the electrodes corresponding to the plate sets of the electrode system 36 according to Fig. 8, while the plates corresponding to the electrode system 35 carry the number 41. 1 42 is the anode. The potentials of the individual electrodes are selected in the manner given for the Fig. 8 struc- The set 34 is maintained at cathode ture. The electrodes 40 and 4| are ring-shaped discs which enclose the cathode 50 and the grids 38 and 39. The anode 42 is a cylinder.

In the illustrated exemplary embodiments, only 5 a single step amplification takes place by the production of secondary electrons. It is, however, easy to see that the amplification of this type may be repeated a plurality of times. Also, additional control grids may be inserted between the individual systems of electron multipliers to influence the electron current flowing to the final electrode (anode) for example, to modulate it. In this manner, for example, high frequency oscillations produced with the aid of the first control grid may be modulated by speech or the like with the aid of one of the following grids.

While for purposes of illustration, I have shown and described specific embodiments of my invention, it will be apparent that many changes and modifications can be made therein without departing from the true spirit of my invention of the scope of the appended claims.

I claim as my invention:

1. A vacuum-tight container enclosing an electron-emissive cathode and an anode separated by a space containing a plurality of additional electrode systems, one such electrode system comprising a first plurality of parallel bars, a second plurality of bars electrically insulated from said first plurality, the bars of said second plurality bein positioned between the bars of said first plurality, a third plurality of bars positioned between said first plurality of bars and said anode, a fourth plurality of bars electrically separated from said third plurality of bars, the bars of said fourth plurality being positioned between the bars of said third plurality, said bars of said second plurality being adapted to be maintained at a positive potential relative to said cathode, said 40 bars of said first plurality being adapted to be maintained at a more positive potential than said second plurality, said bars of said fourth plurality being adapted to be maintained at a more positive potential than said first plurality, and 45 said bars of said third plurality being adapted to be maintained at a more positive potential than said fourth plurality.

2. The structure specified in claim 1 provided with a fifth plurality of bars positioned between 50, the bars of said first plurality and electrically connected with the bars of said third plurality. 3. A vacuum-tight container containing an electronemissive cathode and an anode separated by a space containing a plurality of additional 55 electrode systems, one such electrode system comprising a first plurality of bars, a second plurality of bars, the bars of said second plurality being positioned between the bars of said first plurality, a third plurality of bars between said first 60 plurality of bars and said anode, a fourth plurality of bars having their lower edges positioned between the upper edges of said third plurality of bars and their mid points nearer to said anode than are the mid points of said third plurality 5 of bars, and a fifth plurality of bars positioned between the lower edges of said third plurality of bars and electrically connected to said fourth plurality of bars.

4. A vacuum-tight container containing an 70 electron-emissive cathode and an anode separated by a space containing a plurality of additional electrode systems, one such electrode system comprising a first plurality of bars, a second plurality of bars, the bars of said second plu- 7 rality being positioned between the bars of said first plurality, a third plurality of bars positioned between said first plurality of bars and said anode, a fourth plurality of bars, the bars of said fourth plurality being positioned between the bars of said third plurality, and a control grid positioned between said first plurality of bars and said second plurality of bars.

5. A vacuum-tight container containing an electron-emissive cathode, and an anode separated by a space containing a plurality of additional electrode systems, one such electrode system comprising a first plurality of parallel bars each having a dimension parallel to the axial line joining the center of said cathode with the center of said anode which is larger than its dimension in one direction transverse to said line, a second plurality of bars each having its dimension parallel to said line less than the corresponding dimension of the first-mentioned bars, the bars of said second plurality being positioned between the bars of said first plurality, said second plurality of bars being adapted to be maintained at a positive potential relative to said cathode, and said first plurality of bars being adapted to be maintained at a more positive potential relative to said cathode.

6. A vacuum-tight container enclosing an electron-emissive cathode and an anode separated by a space containing a plurality of additional electrode systems, one such electrode system comprising a first plurality of parallel bars, a second plurality of bars electrically insulated from said first plurality, the bars of said second plurality being positioned between the bars of said first plurality, a third plurality of bars positioned between said first plurality of bars and said anode, a fourth plurality of bars electrically separated from said third plurality of bars, the bars of said fourth plurality being positioned between the bars of said third plurality.

'7. A vacuum-tight container enclosing an electron-emissive cathode and an anode separated by a space containing a plurality of additional electrode systems, one such electrode system comprising a first plurality of parallel bars, x

a second plurality of bars electrically insulated from said first plurality, the bars of said second plurality being positioned between the bars of said first plurality, a third plurality of bars positioned between said first plurality of bars and said anode, a fourth plurality of bars electrically separated from said third plurality of bars, the bars of said fourth plurality being positioned between the bars of said third plurality, and a fifth plurality of bars positioned between the bars of said first plurality and electrically connected with the bars of said third plurality.

said second plurality being positioned between the bars of said first plurality, a grid between said cathode and said first plurality of bars, and a second grid between said grid and said cathode.

9. A vacuum-tight container containing an electron-emissive cathode, and an anode separated by a space containing a plurality of additional electrode systems, one such electrode system comprising a first plurality of parallel bars each having a dimension parallel to the axial line joining the center of said cathode with the center of said anode which is larger than its dimension in one direction transverse to said line, a second plurality of bars each having its dimension parallel to said line less than the corresponding dimension of the first-mentioned bars, the bars of said second plurality being positioned between the bars of said first plurality, a third plurality of bars each of which has a dimension parallel to said axial line which is greater than its dimension in one direction. transverse to said axial line interposed between said first plurality of bars and said anode and being substantially in the plane of the bars of said third plurality, and a fourth plurality of bars positioned between the 20 bars of said third plurality.

10. A vacuum-tight container enclosing an electron-emissive cathode and an anode separated by a space containing a plurality of additional electrode systems, one of said electrode systems comprising a first plurality of parallel bars, a second plurality of bars, the bars of said second plurality being positioned between the bars of said first plurality, a third plurality of bars positioned between said first plurality of bars and said anode, a fourth plurality of bars, the bars of said fourth plurality being positioned between the bars of said third plurality and a fifth plurality of bars, the bars of said fifth plurality being positioned between the bars of said first plurality and being electrically connected to the bars of said third plurality.

O'I'I'O KRENZIEN.

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