US2156625A - Electric discharge device - Google Patents

Description

May 2, 1939- I L. E. FLORY ET AL 2,156,625

ELECTRI C DISCHARGE DEVICE Filed March 50, 1935 www E TTOBNEL/ Patented May 2, 1939 UNITED STATES PATENT OFFICE 2,156,625 ELEc'rmo DISCHARGE nEvrcE Ware Application March 30, 1935, Serial No. 13,806

4, Claims.

stream, such, for example, as is emitted from a y thermionic cathode or from a photosensitive sur- -face exposed to light, is accomplished through utilization of the phenomenon of secondary emission.

If an electrode is subjected to electron Abom, bardment,it will emit secondary electrons. The ratio of the number of secondary electrons to the number of primary electrons depends, in part, upon the character of the bombarded surface and upon the potential difference between the surface y and the source of the electrons. This ratio can be made considerably greater than unity. For example, a ratio of five or more secondary electrons to one impinging electron is readily obtainable with metallic surfaces treated in known ways and subjected tovdischarges at potentials of 300 to 400 volts.

Ifthe secondary electron current, in turn,is caused to impinge with suillcient velocity upon a further electrode with a suitably treated surface, the ratio of secondaryV emission from the second multiplying electrode may also be greater than unity. Hence, one is able to obtain with "n multiplying electrodes in cascade, for example, an amplification of the original or pri-- mary electron current equivalent to the ampliiication per electrode raised tothe nth power. A million-fold amplification may be obtained in a single device.

Prior art devices, wherein the phenomenon of secondary emission is utilized for amplification,

have proved to be unreliable and ineflicient, principally because of incomplete and uncertain control and utilization of the secondary electron streams. For example, in the operation of a de- 40 vice constructed according to French Patent 582,428 (Dapsence et aL), the same electrostatic fields are utilized for electron acceleration and for determining the paths described by the primary and secondary electrons. It has been found 46 that, in general, it is verydiilicult to control the paths of the electrons in a tube of that type in such manner as to cause all of the electrons from any one source to fall upon the desired target. Another disadvantage of previous electron mul- 50 tipliers, of types exemplified by the French patent,

resides in the fact that the iield in the neighborhood of each multiplying electrode, which serves to draw off the secondary electrons, mustk necessarily be weak. Because of the small magnitude 55l of the eld, itis impossible to draw large cur- (Cl. 25o-166) rents from those electrodes. An attempt to overcome this defect by increasing the potential of the succeeding electrode, with the object of increasing the accelerating field in the neighborhood of the preceding multiplying electrode, generally re- 5 sults in a decreased gain since some of the primary eletrons which would normally strike the preceding multiplying electrode are drawn past it to `one of the following electrodes. As a-consequence, because of the space-charge limitation 10 of current, it is impossible to maintain linearity between the input excitation or the current from the primary source, and the output current, except for extremely we ak output currents.

It is, accordingly, an object of our invention to l5 provide an electric discharge device, utilizing secondary electron emission, wherein maximum gain per stage is obtained and linearity may be obtained between input excitation and output current.

Another object of our invention is to provide an 20 amplifier or electron multiplier of the secondary electron emission type in which the secondary electron Stream from each emitter is concentrated and is directed accurately to the desired 25 target, and interference between the various electron streams is minimized.

Another object of our invention is to provide an amplier or electron multiplier wherein there is practically no loss of secondary electrons and in 30 which the best conditions for amplification, or other desired results, can easily be obtained by external potential adjustments.

Another object of our invention is to provide an amplifier or electron multiplier that is eiicient 35 and reliable in operation and in which the ampliiication obtainable is very great as compared with the amplication obtainable with a thermionic amplier of usual type.

Another object of our invention is to provide a 40 device of the type described that may be used for substantially any purpose for which thermionic tubes of present types are used, Such, for example, as an amplier, a demodulator, an oscillator, a

combined oscillator and modulator, etc.

Another object of our invention is to provide a combined photocell and amplier that shall be responsive to the very highest frequencies encountered in television transmitting apparatus.

A still further and more specific object of our invention isto 'provide a device of the type described that lends itself readily to mass production methods.

The foregoing objects and other objects ancillary thereto we prefer to accomplish by/the provision of separate and independentilelds for the production of secondary electrons from one electrode andthe focusing of the electrons upon the next succeeding electrode. In particular, in a preferred embodiment of our invention we utilize an electrostatic field to cause electrons emitted from a primary source, such as a cathode, either photoelectric or thermionic in character, to be accelerated toward an electrode having a surface capable of secondary emission. The secondary electrons thus produced are directed against an output electrode or another similar surface at a still higher positive potential whereat further secondary electrons are produced. In each case electromagnetic focusing means are made use of for the purpose of focusing Vand directing the primary electrons toward the rst secondary emitter and the secondary electrons from the rst emitter toward the next succeeding electrode. 'I'his process may be repeated a number of times within the same container if desired, there being no limit to the number of secondary emitters interposed between the cathode and the output electrode.

The novel features which we believe to be characteristic of our invention are set forth with particularity in the appended claims. "Our invention itself, however, both as to its organization and method of operation will best be understood by reference to the following description taken in connection with the accompanying drawing, in which: Y

Figure 1 is a view in perspective of an embodiment of our invention, and

Fig. 2 is a diagrammatic view exemplifying the circuit connections utilized when our device is employed as an amplifier.

Referring now to Fig. 1` of the drawing, an electron. discharge device constructed according to our invention may be constituted by a Y-shape evacuated container I. Within the container adjacent to the extreme end of one of the arms 3 is mounted a photosensitive cathode 5 and an output electrode 'l is disposed Within the container adjacent the end of the other arm 9. An electrode II capable of emitting secondary electrons is disposed within the stein I3 of the container in such position that it is accessible to electrons emitted from the photosensitive cathode 5 and is also visible from the output elec-` trode 1.

It is to be understood, of course, although we have chosen a photosensitive electrode for purposes of illustration, that it may be replaced by any other suitably controllable electron source and, alternatively, that electrons may be introduced into the device from an outside source.l

It is also to be borne in mind that we are not limited to the single stage amplification shown in Fig. 1 since, merely for purposes of explanation, we have chosen the simplest possible form of our invention.

For purposes of convenience the electrode II mounted in the stem of the container will be referred to as a multiplying electrode.

Electrons emitted from the photosensitive electrode 5 or cathode leave the surface in random directions. For the purpose of focusing them upon the multiplying electrode II we provide an electromagnetic coil I5 which encircles the arm 3 of the container between the cathode 5 and the said multiplying electrode. Also, for the purpose of focusing and directing, upon the output electrode 1, secondary electrons from the multiplying electrode, a similar electromagnetic coil I1 encircles the arm of the container between the said output electrode and the multiplying electrode.

An accelerating electrode I9, preferably in the form of a grid, is interposed between the cathode and the multiplying electrode and a similar accelerating electrode 2l is interposed between the multiplying electrode and the output electrode. These accelerating electrodes also function to shield the input electrode from the output electrode. The accelerating electrode between the multiplying electrode and the output electrode also has an additional function of removing secondary electrons from the vicinity of the multiplying electrode and of urging them into the focusing field set up by the coil adjacent thereto.

In the manufacture of the device illustrated, the glass envelope was first fabricated in the form shown, the ends of the arms and the end of the stem being left open to permit the sealing therein of the respective electrode-supporting presses. The accelerating electrodes, which were preferablyy made of nickel, were next sealed in place, and the input, multiplying and output electrodes, each of which had previously been provided with an individual press, were next sealed in the open ends. 'I'he input and multiplying electrodes were made of silver, substantially ten-thousandths of an inch thick and the output electrode was made of tantalum or an analogous metal, approximately flve-thousandths 0f an inch thick.

After the electrode presses were in place, the tube was sealed to a high vacuum system by means of a tubulation (not shown), through which the tube could be evacuated. An appendage (not shown) containing pellets of a caesium compound, such as caesium chromate, and a reducing agent, such as aluminum powder or silicon powder, was sealed onto the tube by means of another tubulation (not shown) through which the caesium from the pellets could be admitted to the main body of the tube.

The tube was then baked at 450 centrigrade, being evacuated at the same time. 'I'he bake continued for approximately thirty minutes after the oven reached the final temperature. After the baking, the tube was cooled and a small amount of pure oxygen was introduced into it at a pressure of approximately 1 mm. of mercury. The cathode and multiplying electrodes were next oxidized by passing an electrical'discharge from these elements to some other element in the tube until the electrode surfaces acquired a bluishgreen tinge. The oxygen was then pumped out and the pellets of caesium compound and reducer were heated sufiiciently to start the reaction which yields metallic caesium. The metallic caesium was driven, by means of heating the appendage, into the main body of the tube. The tube was once more baked at 200 centrigrade for approximately ten minutes and allowed to cool. 'Ihe caesium appendage was then sealed off the tube and the tube sealed off the vacuum system.

It might be thought that the caesium would be deposited upon the accelerating and output electrodes. Such is undoubtedly the case, but, by reason of the greater afiinty of caesium for an oxide, particularly 'silver oxide, when the tube was heated in the final heating, most of thecaesium was driven oiilr the other elements and was taken up by I the oxidized silver electrodes. 'I'he caesium forms a chemical compound with the silver oxide which is reasonably stable, al-

though the actual chemical reaction that takes place ls not deflnitelyand accurately known.

The operation oi' our improved electron discharge device as anamplifler of fluctuating light,

, such as that provided by a moving sound track on4 a talking motion picture film or the like, is exemplified by Fig. 2 of the drawing. When utilized as such an amplifier the photosensitive cathode 5 may be connected to the negative terminal of a potential divider 23 connected across a source 25 of unidirectional lpotential .and the first accelerating electrode, the multiplying electrode, the second accelerating electrode and the output electrode, preferably, are connected to points on the potential divider successively more positive. 'I'he relative potentials shown in the drawing are to be construed solely as illustrative, and our invention is not to be limited thereby.

If desired, the primary winding 21 of an output transformer 29 may be connected between the output electrode and the potential source. The secondary winding 3l of the transformer may be connected rdirectly in series, or otherwise, with the voice coil 33 of a loudspeaker 35 or the terminals of the said secondary may be connected to the input terminals of any other indicating device or to an amplifier.

The several focusing coils may be provided with unidirectional potential from a battery 31 or the like. In the drawing these coils are exempliiied as being connected in parallel to the potential source, a potential divider 39 and a plurality of contact devices Il and 43 being utilized for the purpose of individually controlling the magnitude ofthe several iield currents. The polarity of the coils appears to be immaterial.

Any source of uctuating light may be utilized to energize the photosensitive cathode. -Such source is exemplified by a moving photographic sound record 45 which is transilluminated by an incandescent light 41 and the image of which is thrown upon the cathode by a lsuitable optical system 49.

When light, modulated by the film, strikes the cathode 5` photoelectrons are emitted therefrom in random directions under the iniiuence of the positive potential applied to the first accelerating electrode I9. In order that substantially all of these electrons may be utilized, the magnetic field set up by the first coil I5 (reading from right to left in the drawing) is so adjusted that they are concentrated and brought to foci upon the multiplying electrode. As the electrons impinge upon the multiplying electrode they cause the liberation of secondary electrons which are removed from the vicinity of the said multiplying electrode by the second accelerating electrode 2Iand brought under the influence of the electrostatic iield between the said accelerating electrode and the output electrode 1. concurrently, the magnetic field set up by the coil I1 adjacent to the output electrode functions as an electron lens to concentrate the secondary electrons and to direct them-upon the output electrode. The coil -I1 isnot absolutely necessary in the device i1- lustrated, since the accelerating electrode 2|, being maintained at high positive potential, will remove all secondary electrons emitted from the multiplying electrode. In a multi-.stage device, wherein a second accelerating electrode would be substituted for the output electrode, the focusing coil is necessary.

It should also be understood that'we are not limited to the use of a photosensitive cathode,

since it lies lwithin the scope of our invention to replace it by any suitable electron source, such as one of the thermionic type, and to provide one or more grids for the purpose of suitably controlling the electron emission. yThe manner in which a controllable thermionic source could be mounted in the container in lieu of the photosensitive cathode 5 will be `perfectly apparent to those skilled in the art and no necessity is seen for illustrating it. If further information, however, is desirable, attention is directed toward the copending application, Serial No. 4,049, led January 30, 1935, in the name of Louis Malter, and assigned to RadioCorporation of America.

Our improved device, in addition to its capability of providing an amplified fluctuating output j current in accordance with a uctuating light tional to input excitation but which follows some other curve. Non-linearity may be, for example,

obtained by interposing a'n impedance 'device 5| between the potential divider and the ilrst multiplying electrode. 'Ihis impedance device may be a resistor or an inductor having such electrical characteristics that, when the current therethrough changes, the potential on the multiplying electrode also changes to alter the gain in proportion thereto. An inductor, of course, would be preferable if a characteristic non-linear with respect to frequency is desired.

Our improved device, either of the photosensitive cathode or the thermionic cathode type, is capable of generating sustained oscillations. This may be accomplished through utilization of any of the conventional regeneration circuits which permit of feeding back a portion of the output potential to the input circuit in proper phase. g

Sight should not be lost, also, of -theiact that our device is capable of use as a demodulaton Such purpose may be accomplished by applying the signal to be demodulated either across the impedance device 5I, shown in Fig. 2, or in series therewith. g

Many other uses of our device will be apparent to those skilled in the art, such, for example, its use as a combined oscillator-modulator through the application of modulating potentials to the multiplying electrode while the carrier frequency is applied tothe input circuit or while the device is in the`self-oscillatory condition. Since it is substantially impossible to list herein all of the possibilities of our improved device, it is to be distinctly understood that our invention is not to be circumscribed by the examples given.

' By reason of the shape of our improved electric discharge device and the presence of the electron lenses, electrons from any given source are .prevented from being driven past their preallocated target and are caused to impinge thereon.

Because of the fact that the acceleration of the electrons is produced through the action of an electrostatic field which does not interfere with the focusing action of the electromagnetic electron-lenses, the space charge limitations of prior devices are avoided. The eciency thus obtained is much greater than where dependence is placed solely upon electrostatic fields between adjacent electrodes for the purpose of both accelerating the electrons and directing them to the secondary emitters. By providing the electron lenses, the impinging electron stream is concentrated and directed to the emitting surface and at the same time there may be maintained at the surface an electrostatic potential gradient which is favorable for removing the emitted electrons with maximum eillciency. Substantially no interference or interruption between the high velocity impinging electrons and the low velocity secondary electrons has been observed and it seems highly probable that there is no actual interference between the electromagnetic concentrating iields and the electrostatic accelerating iields.

It follows from the foregoing enumerated facts that, in our improved device, the output current is not limited by space charges. As a result, the device appears to have no saturation point, the amountvof output current which it can provide being only dependent upon the amount of heat which the electrodes can dissipate, upon their resistance to destructive electrostatic forces, the potentials applied to the electrodes and the adjustment of the focusing action of the electron lenses through suitable control of the potentials applied thereto.

We are aware of many physical modifications of our device and many other possible uses thereof that at once will be apparent to those skilled in the art. Our invention, therefore, is not to be limited except insofar as is necessitated by the prior art and by' the spirit of the appended claims.

We claim as our invention:

1. An electric discharge device including an evacuated envelope, an electron source, a secondary emitter electrode and a collector electrodemounted in the order named within said envelope, said emitter electrode being accessible to electrons from said source and said collector electrode being accessible to secondary electrons from said emitter electrode, an accelerating electrode mounted in the path of the electrons intermediate said source and said collector electrode and means whereby an electromagnetic electron-lens may be established in the path of the electronsy which are subject to the influence of said accelerating electrode whereby said electrons may be concentrated and brought to foci on the electrode toward which they are directed.

2. The invention as set forth in claim 1 wherein an accelerating electrode and means for establishing an electromagnetic electron-lens are provided intermediate said electron source and said emitter electrode.

3. An electric discharge device including an evacuated envelope, an electron source, a secondary emitter electrode and a collector electrode mounted in the order named within said envelope, said emitter electrode being accessible to electrons from said source and said collector electrode being accessible to secondary electrons from said emitter electrode, means whereby an electromagnetic electron-lens may be established intermediate the emitter electrode andthe collector electrode for concentrating the secondary electrons and bringing them to foci upon the collector electrode, and an accelerating electrode in the path of the secondary electrons for drawing the secondary electrons Within the inuence of said electromagnetic electron-lens.

4. An electron multiplier comprising a source of electrons, a multiplying electrode capable of emitting secondary electrons, an output electrode, leads connected to said source and electrodes whereby an electrostatic field may be established between the source and the multiplying electrode, and whereby an electrostatic eld may be established between the multiplying electrode and the output electrode, means for establishing two electromagnetic fields, each parallel respectively to the electrostatic fields, and means comprising a grid-like electrode mounted in the path of the electrons for shielding the electron source from the output electrode. n

LESLIE E. FLORY. GEORGE A. MORTON.

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