US2204479A - Means and method for producing electron multiplication - Google Patents

Description

June 11, 1940. 1 P. "r. FARNSWORTH 2,204,479 MEANS AND METHOD FOR PRODUCING ELEQTROIN MULTIPLICATION Filed May 1a, 1956 25heets-Sheet 1 OUTPUT IN V EN TOR.

P/ri/o T Fa rqswor/A 1' A TTORNEYS.

June 11, 1940. P. T. FARNSWORTH 2,204,479

nuns AND ammo FOR PRODUCING ELECTRON MULTIPLICATIQN Filed May 16, 1936' 2 Sheets-Sheet 2 I i. I 1

INVENTOR. Pbl'lo T Eu-hs wok/ll 6% W ATTORNEY Bjgffuk Patented June 11, 1940 I UNITED STATES PATENT mcs MEAnsANn Mn'rnon non rnonUcnvG ELECTRON uuurirucarion Philo T. Farnsworth, San Francisco, Calii'., as-

signor, by mesne assignments, to Farnsworth Television 8; Radio Corporation, Dover, DeL, a corporation of Delaware Application May 16, 1936, Serial No. 80,194

Claims. (01250-27) 1 My invention relates to electron multipliers. and with a velocity sufilclent to emit secondary and more particularly to that type or electron electrons at each impact. multiplier where electrons are directed to suc- Among other objects are: To provide ameans cessively impact a series of surface elements to and'method of segregating accelerating fields in produce a. current augmented by secondary emisa multistage secondary emission multiplier; to 5 sion at each impact. provide a means and method of insuring that Electron multipliers may be roughly divided electrons reach the proper target in a multistage I into two general types: first, the so-called "direct secondary emission multiplier; to provide, in a current multiplier, where the generation of secondary emission multiplien. a means and secondary electrons takes place by successive 'method of focusing electrons between electrodes 1 impact of an electron stream with a series of therein; to provide a means and methodof reemitting elements energized to successively in-- ducing space. charge limitations in electron multicreasing positive potentials; and second, that P r to provide a means and method of con- 7 type of multiplier where repeated "impacts are trolling electron paths in a multistage electron made between a pair of operatively opposed surmultiplier; to Pr v d a p r in System of face elements, such as has been described by me electrical fields in an electron multiplier to inin my prior applications, Serial Nos. 692,585 and sure eificient multiplication; nd to Provide a 733,837, Patents Nos. 2,071,515, dated Feb. 23, means and method of efllcient electron multi- 1937, and 2,071,516, dated Feb. 23, 1937, respecplication utilizing secondary emission. 0 tively. The present'application deals with the g y v ntion possesses numerous other obfirst type of multiplier mentioned above. iects and features of advantage, some of which.

When a series of surfaceelements are energized t sther w e foregoing, Will e Set forth n to successively increasing positive potentials h win rip n f sp fi apparatus and placed with relation to each other so that embodying and utilizing y novel e od- It is electrons may be drawn from one to another until therefore to be understood that y method is the entire series has been impacted, a number of applicable to other pparatus, and that I do not factors must be taken into account to produce limit y n ny W y, to the app of'the efiective electron multiplication. First, the elecp es t app cat on. as I may ado v o other trons must be accelerated toward eachsurface pp us embodiments.v utilizing the d. an

element with such a velocity that they will genwithin the Scope f the appended claimserate secondary electrons on impact; second, the Broadly S t method, y nv n comprises secondary electrons generated must be reformed d cting the electrons, in the yp o p er and usually changed in direction so that they dmribed. between p s in such amanner that may again be directed and accelerated toward theyv reach the next Surface. and y method m y a second and similar surface, and so on until a lso P vi h p of creating an accelerating sufficient number of impacts have occurred to profield in addition to the, fields created by the surduce the desired multiplication; third, electrons, face elemen themselves. the e nt during their passage from one element to another, field; and in this respect I may prefer to utilize must be sounder control that 'as many electrons the potential o e nex ucce din element to 40 40 as possible reach t t t; t t accelerate electrons between a pair or elements surface elementsthemselves must be so sensitized lower in potentiali as to produce secondary electrons with a maxily 8 to apparatus. y invention commum ratio of secondary electrons to primary p s a Series of surface elements energized to electrons; and fifth, space charge limitations v ly r as Positive potentials. and 45 must be avoided. means for directing electrons emitted from one My present invention ha for it main object element ontothe element of the next higher pothe provision of a 'means and method whereby tential. In order to ensure electrons impacting electron multiplication may take place with high the surfaces with the propervelocity, I may desire 3n efficiency and in which'electro'ns may be made to insert operatively between each pair of eleto follow predetermined paths between impacts ments an accelerating electrode energized to a with minimum collection, and it also provides a potential higher than either or that specific pair,

means and method of adding to the field produced and I thus provide an accelerating field graded as by the elements themselves an additional ileld the elements themselves are-graded, the acceler- 53 which ensures that electrons strike each element, ating field being higher at all points than the 55 other structures utilizing the method being deemed full equivalents.

Referring to the drawings:

Figure 1 is a diagrammatic longitudinal sectional view of a tube embodying one preferred form of my invention, together with a schematic circuit showing how it may be utilized.

Figure 2 is a diagram showing how the device of Figure 1 may be built, when as a multiplier of photoelectrons.

Figure 3 is a diagramatic sectional view of 1 another embodiment of my invention utilizing ode 4. A band 5 is placed around the stem, and v accelerating electrodes.

- Figure 4 is a sectionalview taken as indicated by the line 4-4 in Figure 3.

Figure 5 is a longitudinal sectional view and circuit of an embodiment utilizing a unipotential space.

Figure 6 is a view taken along a sectional plane, as indicated by the line 6 6 in Figure 5.

Figure 7 is a diagrammatic representation, partly in section, of a device utilizing a cone shaped accelerating electrode, and a magnetic deflecting field.

Figure 8 is a sectional view of the structure shown in Figure '7, taken as indicated by the line 8-8 in Figure 7.

Referring directly to the embodiment shown in Figure 1, an envelope 1 is provided at one end with an input stem 2 through which cathode leads 3 are sealed, supporting a filamentary cathsupports by means of risers 6, an input cylinder .1. A control grid 8 is placed in the opening to the cylinder 1. The cylinder 1 extends away from filament 4 and terminates at the opposite end. in a surface element 9 which is sensitized to readily emit secondary electrons at a ratio greater than unity when impacted by primary electrons traveling at the proper velocity.

One side of the cylinder is cut away to provide an opening toward which is directed the open end of a second cylinder Ii, also terminating in a secondarily emissive surface 9a. The second cylinder is supported by a lead I! through the wall of the tube I. Cylinder II is also cut away adjacent the emissive surface 9a, and a third cylinder l3 has its open end opposed to the opening, cylinder l3 also terminating in an emissive surface 9b. Cylinder I3 is provided with an opening exactly as before, and an output cylinder l4 hasits open end presented thereto so that electrons emitted from surface element 91) may pass to the cylinder l4, there tobe collected by an anode surface l5. Anode cylinder I4 is mounted on an output stem IS, a lead I! passing through the stem to provideexterior connection.

It is of course to be understood that while I repeated as often as is desired, the number of successive surface elements being limited only by the number of secondary-electron generating impacts desired.

I have found that the surface elements 9, 9a

and 91) may be sensitized for secondary emission by the deposit thereon in any well known manner of athin film of thorium, barium, potassium, caesium or other metals in the alkali and alkaline earth groups, and I have obtained a sensitivity with such surfaces so that a primary to secondary emission ratio of 1:1 can be obtained with an electron velocity of volts, maximum emission of a ratio of 1 :7 being obtained with caesium surfaces. However, any material capable of emitting secondary electrons on impact is satisfactory.

The setup can be made operative by energizing the filament 4 from a filament source 20 and attaching a resistor 2! across a potential source 22, taps 24, 24a and 24b being taken off and leading respectively to cylinders 1, H and 13, to produce progressively increasing positive potentials thereon, the end of the resistor 2| being connected directly to anode cylinder l4 through lead I! and output impedance 25, the latter being so connected that variations in current therethrough may be utilized.

Energized as described immediately above, electrons leaving the filament 4 enter the interior of cylinder 1. Cylinder I, being energized at a positive potential higher than that of the filament from which the electrons were emitted, tends to focus the electrons onto the central portion of the first element 9 in the series of surface elements, and secondary electrons will be emitted therefrom. These secondary electrons, having random velocities, are acted upon by the field of cylinder II and are drawn into that cylinder and accelerated toward surface 9a where more secondary electrons are emitted, these latter secondary electrons being drawn into cylinder l3 to impact surface element 9b where still further secondary emission takes place, the final multiplied current then being drawn into cylinder l4 and collected therein. Each cylinder is therefore a part of each emissive surface and is at the same potential as that surface.

The focusing is enhanced by the fact that the exits from the cylinders are close to the surface elements and the length of the cylinders toward the lower potential surface is such that electrons entering each cylinder are deflected a minimum amount by the field from the succeeding cylinder.

However, while my device is entirely operative to produce electron multiplication as described, space charge efiects occur which limit rather definitely the amplification which can be drawn from a device of this sort, and it is desirable, when cylinders alone are used, that a'fairly high voltage per stage be used, perhaps from 200 to 500 volts.

1 have also found that the multiplication effect may be greatly increased by the use of an accelcrating field in addition to the field produced by the surfaceelements themselves, or their connected cylinders, and produce this field in one manner as shown in Figure 3, where fine wire grids 26, 26a and 2617 are positioned immediately in front of surface elements 9, 9a and 9b, respectively, so that electrons entering the cylinders are subjected to the fields of these electrodes.

It is preferable that the potential on these accelerating electrodes will be greater than that of the surface element to which the electrodes are being accelerated thereby, and in order that the various accelerating electrodes be conveniently energized, I may prefer to attach, for example,

accelerating electrode 2i 'to'cylinder ll, thereby maintaining it at the same potential as surface element 911. In like manner accelerating electrode a is attached to cylinder l3 and at the potential of surface element 9b. Accelerating electrode 26b is attached to output cylinder II and therefore is at the same potential as anode l5. Of course these grids may be separately and independently energized directly from a potential source if desired.

It is relatively immaterial as to the direction in which electrons approach the surface elements, as they will pass through the accelerating screens to reach the emissive surface. They will, however, leave the emissive surface perpendicular thereto and therefore will be directed into the next succeeding cylinder to repeat the cycle.

In this manner I have also provided, in addition to the successively energized surface ele- 'ments, successively energized accelerating electrodes, and the potential of the accelerating electrodes is always higher than the surface to which the electrons are being accelerated thereby. I prefer to make the accelerating electrodes either of extremely fine wire so that few electrons will be picked up thereby, either approaching or leaving the surface elements, or to form the accelerating electrodes in any other well known manner so that the electrons approaching each surface will receive the benefit of the increased positive potentials thereon, with minimum pickup or collection.

vIn the manner described, I have been ableto obtain extremely high multiplication per stage. First, due to-the focusing, a majority of electrons leaving one surface element reach the next without collection; second, by the use of accelerating electrodes I ensure that the electrons will impact the surfaces with a sufilcient velocity to create secondary electrons and prevent formation of limiting space charges; and third, by continual focusing of the entire path from one end of the device to the other, I prevent gradual dispersion;

As the device is capable of extreme amplification, it is of course adapted to be used. either as a straight amplifying device, as indicated in Figure 1, where a grid is used to control the emission from the filament, or as a photoelectric device, as shown in Figure 2, where the grid 8 has been coated with a photosensitive material and connected to the first cylinder 1. Light projected against this grid by optical system 30 causes the emission of electrons which are then accelerated toward the first surface 9. The output in this case will then be proportional to the light falling upon the grid 8. g

It should also be pointed out that the device may be used in conjunction with any other apparatus where electrons from any source whatsoever may be passed into the input end of cylinder 1. It may be seen, therefore, from the foregoing description of this particular embodiment that I have completely surrounded the-entire path'ofthe electrons between successive surface elements,

thus obtaining electron focusing to ensure impact at each stage. In addition,'I have provideda means and method forreducing space chargelimitations within the device. 1 I

It should also be pointed out that while I have shown the device in the form of cylinders, it is also operative with conduits or other cross sectional contours, and tubular in this application is deemed to mean a conductive surrounding .of

I 3 the path to produce an electrostatic focusing field, It will also be apparent that the device is a self oscillator if feedback from the output to inbut be used. 7

In the embodiment of my invention shown in Figures 5 and 6, surface elements 9, 9a, 9b, 9c and 0d, capable of secondary emission, are arranged opposite an accelerating electrode 21 which in this case takes the form of a wire screen extending parallel to each set of opposed surfaces, the screens being attached to side walls and end walls 3! also being'provided, the entire accelerating electrode structure being supported on leads 32, one of which is brought through the envelope wall for potential connection. The surface elements are connected to resistor 2| in exactly the same manner as in the device shown in Figure 1, so that there is a progressive increase in positive potential on each element.

As the electrons leaveeach element at right anglesthereto, the opposite element is positioned in the path of the outgoing electrodes, this giving rise to the divergence of the accelerating screens and the increasing distance between the elements on each side of the device. This arrangement provides certain advantages. The electrons, after they leave the accelerating screen on one side and until they pass through the accelerating screen on the other side, travel in a Faraday or equipotential space, and the electrons in one inter-element path are shielded, during their traversal of that path, from the voltages on the other surface elements', the voltage on the accelerating screens being, of course, at all times greater than any potential on any surface element. Furthermore, the arrangement provides a continually lengthening path between surface elements, the

electrons, therefore, being given more space as they increase in number. This has the effect of limiting the development of space charges in the de ice, and it may be seen that by proper coord ation of path lengths the space charge effects may be minimized. Electrons leavingthe last surface element Id will be collected either by the end wall ll or by the ends of the side walls 21.

In this embodiment also it is obvious that the entering electrons may be photoelectrons or those emanating from any source to be amplified.

The device shown in Figures 7 and 8 has the accelerating electrode 21 in the form of a truncated cone, all portions of which are preferably meshed. Here, the surface elements '9, 9a, 9b,-

9c and 9d, are arranged in spiral formation around the accelerating electrode, and a longitudinal magnetic field is provided to cause the electrons to take a curved path between one sur-- rive on the next succeeding element, and if it is desired the multiplication may be controlled by varying the deflecting new so that more or less of the electron stream will impact the surface elements. In this way a steady electron source may be used and the output modulated by a.

variation of the'magnetic field, or several frequencies may be imposed upon the device, one through the input end and the other through the magnetic field. The device, therefore, is adapted to wide use.

All of the devices described may be provided, therefore, in addition to their normal inter-element field, with additional fields which direct electrons leaving one surface element against the next; and in addition, the electron velocity may be added to by a separate accelerating field. Focusing the electrons from one surface onto the next may be accomplished in two manners: first, by the use of an electrostatic focusing system, as described with the tube of Figure 1; or, second, by means of an electromagnetic deflecting system which is the full equivalent of a focusing system, inasmuch as it regulates the path through which the electrons must travel, as shown in Figures 7 and 8.

In all of the devices utilizing additional fields there is the common attribute that space charge limitations are materially reduced, thus allowing greatly increased power outputs with a given number of impacts, and also reducing collection en route.

I claim:

1. In combination, an envelope containing means for producing electrons, an anode, a series of surface elements capable of producing electrons by secondary emission, said surface elements being progressively energized at increasing potentials, and additional accelerating electrode elements positioned to accelerate electrons during their passage from one element to the next.

2. In combination, an envelope containing means for producing electrons, an anode, a series of surface elements capable of producing electrons by secondary emission, said surface elements being progressively energized at increasing potentials, and additional accelerating electrodes positioned to accelerate electrons during their passage from one element to the next, said accelerating electrodes being progressively energized to increasing potentials.

3. In combination, an envelope containing means for producing electrons, an anode, a series of surface elements capable of producing electrons by secondary emission, said surface elements being progressively energized at increasing potentials, and additional accelerating electrodes cooperating with each element positioned to accelerate electrons during their passage from one element to the next, each of said accelerating electrodes being energized to a potential above that of the one of said elements on which electrons influenced thereby are due to land.

4. An electron multiplier comprising a series of surface elements progressively energized to increasing potentials and capable of emitting secondary electrons on electron impact therewith, and a cooperating series of accelerating electrodes each positioned adjacent a surface element and progressively energized to increasing potentials, said latter potentials being higher throughout than said former potentials.

5. In an electron multiplier wherein a series of surface elements are energized to cause electrons contacting one of said surfaces to impact each of the others in succession, accelerating electrode means connected to each of said elements and extending adjacent the previous ele ment in said series for insuring electron contact with said elements.

6. In combination, an' envelope containing means for producing electrons, an anode, a series of elements having surfaces capable of producing electrons by secondary electron emission, means for positioning said elements with said surfaces arranged to be successively impacted by a stream of electronsfiowing from said first named means to said anode, said elements and said anode being energized in said successive relationship progressively'to higher potentials with respect to said first named means, and means for imparting to said electron stream during a relatively small initial period of its movement between each pair of said successively arranged surfaces an electron velocity substantially equal to that which said stream. ultimately attains while moving between said each pair of said surfaces.

7. In combination, an envelope containing means for producing electrons, an anode, a series of elements having surfaces capable of producing electrons by secondary electron emission, means for positioning said elements with said surfaces arranged to be successively impacted by a stream of electrons flowing from said first named means to said anode, said elements and said anode being energized in said successive relationship progressively to higher potentials' with respect to said first named means, and means for imwhile moving, between said each pair of saidv surfaces, said last mentioned means including electron permeable means positioned across the path of said electron stream and spaced relatively closer to the preceding surface than to the succeeding surface of said each pair of said surfaces.

8. In combination, an envelope containing means for producing electrons, an anode, a series of elements having surfaces capable ofproducing electrons by secondary electron emission, means for positioning said elements with said surfaces arranged to be successively impacted by a stream of electrons flowing from said first named means to said anode, said elements and said anode being energized in said successive relationship progressively to higher potentials with respect to said first named means, and accelerating electrodes so positioned and energized as to impart to said electron stream during a relatively small initial period of the movement betwen each pair of said successively arranged surfaces an electron velocity substantially equal to that which said stream ultimately attains While moving between said each pair of said surfaces.

9. In combination, an envelope containing means for producing electrons, an anode, a series of elements having surfaces capable of producing electrons by secondary electron emission, means for positioning said elements with said surfaces arranged to be successively impacted by a stream of electrons flowing from said first named means to said anode, said elements and said anode being energized in said successive relationship progressively to higher potentials with respect to said first named means, tubular conductive electrodes enclosing substantially the entire path of said stream of electrons between successive pairs of said surfaces, each of said conductive electrodes being respectively energized at substantially the potential of the succeeding surface of each said pair of said surfaces, and means including said tubular electrodes for mechanically supporting electron permeable accelerating electrodes in position in proximity to the preceding surface of each said pair of surfaces, said accelerating electrodes imparting to said electron stream during a relatively small initial period of its movement between each said pair of successively arranged surfaces an electron velocity substantially equal to that which said stream ultimately attains while moving between said'each pair of said surfaces. v

10. In combination, an envelope containing 'means for producing electrons, an anode, a series of elementshaving surfaces capable of producing electrons by secondary electron emission, means for positioning said elements with said surfaces arranged to be successively impacted by a stream of electrons flowing from said, first named means to said anode, said elements and said anode beingenergized in said successive-relationship progressively to higher potentials with respect to said first named means, tubular conductive electrodes enclosing substantially the entire path of said stream of electrons between successive pairs of said surfaces, each of said conductive electrodes being respectively energized at substantially the potential of the succeeding surface of each said pair of said surfaces, and means including said tubular electrodes for supporting electron permeable grids in position in proximity to the preceding surface of a respective pair of said successively arranged surfaces and for energizing said grids to impart to said electron stream in its initial movement between said pair of surfaces an electron velocity substantially equal to that which said stream ultimately attains while moving between said pair of surfaces.

- PHILO T. FARNSWORTH.

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