US2180279A - Method of operating electron multipliers - Google Patents

Description

Nov. 14, 1939. T FARNSWORTH 2,180,279

METHOD OF OPERATING ELECTRON MULTIPLIERS Filed, March 22, 1957 IN VEN TOR. PH/LO T. FAPNSWOPTH.

ATTORNEYS.

Patented Nov. 14, 1939 UNITED STATES PATENT OFFICE METHOD OF OPERATING ELECTRON MULTIPLIERS ware Application March 22, 1937, Serial No. 132,329

6 Claims.

' and more particularly to a method of operating electron multipliers energized by alternating current.

Among the objects otmy invention are: to

provide a novel method of operating electron.

multipliers energized by alternating current; to provide alternating current of special wave form for the operation of electron multipliers; and to provide a method of reducing shot effect in an electron multiplier.

My invention possesses numerous other objects and features of advantage, some of which,'together with the foregoing, will be set forth in 5 the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Referring to the drawing:

Figure 1 is a graph illustrating the electron multiplication in an electron multiplier supplied with a-voltage of sinusoidal wave form.

Figure-2 is a diagram, reduced to the simplest terms, of a diode electron multiplier and circuit.

Figure 3 is a graph showing-a special wave form which may be used in conjunction with the energization of the device of Figure 2.

Figure 4 is a diagrammatic drawing and circuit, in simplified form, of another modification of an electron multiplier and circuit.

Figure 5 is a drawing of a wave form which may be used in conjunction with the device of Figure 4.

In my prior applications, Serial No. 692,585,

filed October-7, 1933, Patent No. 2,071,515, dated 9 February 23, 1937, and Serial No. 61,042, filed January 27, 1936, Patent No. 2,137,528, dated November 22, 1938, I have described the operation of electron multipliers wherein voltages of sinusoidal wave form are used to energize the 4,5 cathode and anode. The fundamental principle involved in these electron multipliers isthat electrons are oscillated to repeatedly impact a surface capable of generating secondary electrons at a ratio greater than unity. The energy for 50 causing the oscillation of the electrons may be supplied by an outside source or the device may be made to self-oscillate, thus supplying its own alternating potential. When a source of electrons is provided external to the multiplier and 55 it is desired to multiply electrons from that (01.250-27) My invention relates to electron multipliers source, it is common to direct these electrons to be multiplied through an aperture in the device so that they may come within the influence of the oscillating fields.

' In this latter case, it is customary to drive the 5 device rather than allow it to self-oscillate, in

order that substantially linear amplification may be obtained. Further, to obtain linear amplification, it is highly desirable that each of the incoming electrons be multiplied by the same num- 10 her of impacts as every other incoming electron.

= Otherwise a shot effect is produced and perfect linear amplification is not obtained. My present invention comprises a method of reducing the shot effect by using'a non-sinusoidal driver. 15

My novel method may be more easily understood by reference to the drawing, which is diagrammatic in character and which will serve to illlustrate my method graphically.

Referring first to Figure 2, which shows a com- 20 mon A. C. multiplier circuit with a sinusoidal driver, an envelope I, is provided as a part of an envelope of another device, not shown, wherein electrons are emitted into space and thereafter travel into-the envelope I. The incoming elec- '25 trons to be multiplied are represented as arriving along a direction indicated by arrow 2. The

multiplier itself, in this case, comprises the wellknown Farnsworth diode having an exterior cathode cylinder 4 and an interior apertured 0. anode 5 in the electron path. The cathode cylinder 4 has its inner surface sensitized or otherwise treated so that electrons, will be produced at a ratio greater than unity when impacted by an electron traveling, for ex- 35 ample, with a velocity of 20 electron volts or more. Multiplication ratios up to 1 to 12 may be obtained.

An aperture 5 is provided in the cathode 4 through which the electrons to be multiplied may 40 enter the multiplier structure itself. It is obvious that the size of this aperture may be varied in accordance wtih the size of the electron beam.

The electrodes themselves are energized by being connected through a series circuit comprising a resonant circuit 1 and a series condenser 9. Further, the cathode is connected to the negative terminal of a battery8, the positive terminal of which is at ground potential. The anode 5 is provided with a steady positive accelerating potential with respect to the cathode 4 by connecting one side of the tuned circuit to ground, thus completing the connection to the anode,

.as indicated in Fig. 2. .The output may be taken off across the resonant circuit 1. T

A driver oscillator I is coupled to the tuned circuit 1. I prefer to so adjust the constants of the tuned circuit 1 and the value of the accelerating potential between the cathode 4 and 5 the anode 5 that more than one electron impact may take place per cycle. If the anode voltage with respect to ground which is represented by the line g-t of Figure l is plotted against time,

a sinusoidal voltage wave A-E will appear. The distance between this curve and the horizontal cathode voltage line at the bottom of the graph of Figure 1 represents the difference of potential between anode and cathode, as shown in Figure 1. The operation of the device operating with such a sinusoidal potential will now be described.

If we assume the steady difference of potential between anode and cathode to be 1000 volts and the alternating potential difference to be 800 volts, an electron leaving the cathode at point A will be accelerated by about 1800 volts. During the time elapsing between the entry of the electron and its impact with the cathode, which may be called the flight time of the electron required for one trip or multiplication, the total potential difference decreases and the electron returns and strikes the cathode at the end of the trip with a velocity equal to the decrease in potential difference during its time of flight. As the difference of potential between anode and cathode is at its maximum at point A, the maximum acceleration of the electrons takes place in this part of the radio-frequency cycle, and hence the shortest time of flight occurs. The voltage, however, has changed very little between point A and point B; thus the multiplication ratio obtained will not be very great. As the potential decreaseathe times of flight become longer and the potential differences between start and end of the trips are therefore greater; hence the multiplication ratios become larger.

Referring now to electrons to be multiplied entering the multiplier chamber, it is obvious that an electron entering the multiplier chamber at time A will make the greatest number of flights, and hence maximum multiplication will be obtained. However, an electron entering the multiplier a short time later at time B will make one less trip and therefore lose one multiplication, although the loss of the first trip will not be serious because very little multiplication takes place at the first impact. The loss of multiplication, however, becomes more and more serious if an electron enters after a few trips have been made, particularly as the ratio of multiplication is continuously increasing. Thus, it may be seen that the electrons entering the multiplier over the entire multiplying period of the radio-frequency cycle from A to E experience a multiplication, but .the ratio of multiplication differs tremenduously with the time at which the electron enters the multiplier.

While this shot effect is not a great disadvantage under certain conditions, particularly where there is a profuse supply of incoming electrons, it is obvious that if the incoming electrons to be multiplied are relatively few in number, an output may be obtained which in no way perfectly resembles the input.

I have, however, devised a method such that substantially the same ratio of multiplication will be obtained for all electrons entering the multiplier during the greater part of the radio-frequency cycle.

To obtain this result, I drive the multiplier with a voltage of non-sinusoidal wave shape, one

preferred wave form being shown graphically in Figure 3. With this voltage wave, electrons entering the multiplier during the interval be tween times A and B are multiplied in the ratio of approximately one to one. This period may be conveniently called an accumulation period, and it should take up the greater portion of the radio-frequency cycle in order to fulflll the purpose of substantially equal multiplication of all electrons. Under this mode of operation an electron, starting from the cathode at time A, undergoes the maximum acceleration. The time of flight, therefore, is very short. During this time of flight the voltage may have fallen only about fifteen volts and after allowing forty trips, for example, the voltage will have dropped only 600 volts. So as not to lose too much of the acceleration voltage, the trips in the accumulation period between A and B are limited to a certain number. The voltage between B and C, however, decreases rapidly. The number of secondary electrons per impact will therefore increase up to approximately ten, for example.

If the time of flight is made short enough in comparison with the radio-frequency cycle, it is possible for the electrons to make three or four trips in the interval from B to C, utilizing the remainder of the potential difference. In other words, the radio-frequency cycle is divided into an accumulation period from A to B, and a multiplying period from B to C; collection takes place at all times, but collection alone takesplace in the period from C to D.

Thus there is only a small portion of the radiofrequency cycle where high multiplication ratios are obtained, and therefore the shot effect will be greatly reduced.

So far, I have described my novel method as applied to the cylindrical diode structures shown in Figure 2. If, however, the type of multiplier shown in Figure 4 is utilized, having an anode not in the electron path, an even more advan tageous mode of multiplication may be obtained. In this type of. structure the cathode 4 is in the form of two opposed plates, one of which is provided with aperture 6 in the path 2 of the electrons to be multiplied, and in this case the accelerating anode 5 is in the form of a ring surrounding the space between the two cathodes. A

, solenoid II is provided around the tube and is energized by solenoid source l2 so that a strong magnetic field perpendicular to the cathode surfaces is obtained. This magnetic field prevents the electrons from being immediately collected by the anode ring 5, and they will not be collected until their velocity is low.

Generator I0 is then connected to energize the anode and the two cathodes connected together, and drives the device with a voltage of nonsinusoidal wave form of the shape, for example, shown in Figure 5. The usual steady acceleration potential is provided on the anode. The output may be taken oil across the resonant cir- 'cuit 1.

In this case, an electron to be multiplied enters the multiplication chamber through cathode aperture 6 at time A on the voltage wave shown creased again. Thus, the entering electrons travel back and forth without striking the oathodes, nor are they collected by the anode 5. The maximum potential difierence is at time B, and the time of flight, therefore, will be the shortest at that time. Beyond point B in the time cycle, the electrons will be able to strike the cathodes as the potential difference rapidly decreases. Multiplication, therefore, takes place at high ratios until the potential difference reaches its minimum value at time C. From to D the electrons are decelerated and collection occurs by the anode 5.

This mode of operation has an advantage over the previously described mode in that the entire swing of the potential difference can be made useful for the multiplication of the accumulated electrons.

Furthermore, all electrons accumulated between times A and B in this latter instance are multiplied by the same ratio, namely, the ratio of. multiplication when impacting the first cathode surface after entry into the multiplier.

It is to be noted that as connected both multiplier structures are operated as diodes, the only difference being in the use of an anodein Figure 2 positioned in the electron path, and an anode intFigure 4 positioned outside of the electron pa h. I

Summarizing, it may be seen that what I have done is to accumulate electrons with substantially uniform, low multiplication during a large part of a multiplying cycle, and then to multiply them at high ratios during an extremely short part of the multiplying cycle. Thus I have been able to obtain a great diminution in the shot effect and have been able to give a far more equal multiplication of electrons entering a multiplier during a given period of time than can be obtained by the use of a voltage of sinusoidal wave form to energize the multiplier.

It is further to be understood that the particular wave forms here exhibited are illustrative only and first while I have shown no particular apparatus for obtaining these wave forms, it is well known in the art how to modify sinusoidal waves or to provide special oscillators in order to obtain wave forms of practically any shape desired. As such specific apparatus is no part of the present invention, it will not be described in this application.

It is also obvious that I do not desire to be limited to any particular source of incoming electrons to be multiplied, nor does my method I necessitate any outside source. For example, the. cathode may be photo-electric and itself generate, under the influence of light, the initial electrons. My method, therefore, is applicable to all arrangements wherein multiplication of elecratio multiplication for a relatively long portion of the cycle and then to a relatively high-ratio multiplication for a relatively short portion of the cycle.

3. In an electron multiplier wherein electrons are oscillated against and away from a surface adapted to emit secondary electrons upon electron impact therewith, the method of operation which comprises oscillating the electrons by the application of a slowly changing field to produce low-ratio multiplication, and continuing the oscillation by the application of a rapidly changing field to produce high-ratio multiplication.

4. In an electron multiplier wherein electrons are oscillated against and away from a surface adapted to emit secondary electrons upon electron impact therewith, the method of operation which comprises oscillating the electrons by the application of a slowly changing field to produce low-ratio multiplication, continuing the oscillation by theapplication of a rapidly changing field to produce high-ratio multiplication, and

collecting the electrons after said high-ratio multiplication. a

5. In an electron multiplier wherein electrons are oscillated against and away from a surface adapted to emit secondary electrons upon electron impact therewith, the method of operation which comprises oscillating the electrons without impact for a predetermined time and thereafter continuing theoscillation with impact to produce multiplication.

6. In an electron multiplier wherein electrons are oscillated against and away from a surface adapted to emit secondary electrons upon electron impact therewith, the method of. operation which comprises supplying electrons in a continuous. stream, accumulating electrons by oscillating the electrons without impact for a predetermined -time and thereafter continuing the oscillation with impact to produce multiplication.

- PHILQ T. FARNSWORTH.

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