US2473031A

US2473031A - Electron multiplier for ultra high frequencies

Info

Publication number: US2473031A
Application number: US588315A
Authority: US
Inventors: Christian C Larson
Original assignee: Farnsworth Research Corp
Current assignee: Farnsworth Research Corp
Priority date: 1945-04-14
Filing date: 1945-04-14
Publication date: 1949-06-14
Anticipated expiration: 1966-06-14
Also published as: GB610662A

Description

June 14, 1949. c. c.

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IN ELECTRON-VOL ATTORNEY Patented June 14, 1949 ELECTRON MULTIPLIER FOR ULTRA HIGH FREQUENCIES Christian C. Larson, Fort Wayne, Ind., assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application April 14, 1945, Serial No. 588,315

6 Claims.

This invention relates to electron multipliers and particularly to electron multipliers arranged for amplifying ultra-high frequencies.

It is well known that electron multipliers, wherein a primary electron current is multiplied by secondary electron emission, have a definite upper frequency limit. This occurs at ultra-high frequencies when the reciprocal of the frequency of the input signal is of the order of the electron transit time between two successive stages. One explanation for the cut-off frequency of electron multipliers is the so-called electron transit time spread. The primary, as well as the secondary, electrons passing between the secondary electron emissive stages of an electron multiplier have a certain initial velocity range or spread which is of the order of a few electron-volts. At very high, frequencies this spread of the electron velocities masks the signal to be amplified because the output has a direct current component, as will be more fully explained hereinafter. Furthermore, the difierences in the paths of different electrons traveling between two successive secondary electron emissive stages also causes a transit time spread. This eifect, however, can be minimized ducing the distance between two successive mul- I tiplying stages. However, there are obvious limits to these expedients. In view of constructional difliculty the distance between two successive stages can not be made too short. Furthermore, cold emission from closely spaced edges of the plates of the multiplier makes it impossible to increase the accelerating potential beyond a certain limit. Therefore, it would be Very desirable to decrease the electron transit time spread, thereby to raise the upper frequency limit of the electron multiplier.

It is an object of the present invention, therefore, to provide an electron multiplier having a higher cut-off frequency than prior electron multipliers.

Another object of the invention is to provide an electron multiplier which has a reduced electron transit time spread thereby to raise the upper frequency limit thereof.

In accordance with the present invention, there is provided an electron multiplier comprising a plurality of secondary electron emissive electrodes and a source of primary electrons. Means are provided for directing primary electrons from the source towards the first one of the secondary electron emissive electrodes, as well as means for directing secondary electrons liberated from each of the electrodes towards the succeeding electrode. Means are also provided for controlling the number of electrons passing between the electrodes in accordance with an input signal. Further means are provided for passing only electrons within a predetermined velocity range between successive electrodes and means for collecting the electrons from the last secondary electron emissive electrode to derive an amplified output signal. Thus, the electron transit time spread is reduced and the frequency cut-off of the multiplier is raised.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing:

Fig. 1 is a schematic representation of two secondary electron emissive stages and electron clouds therebetween representative of a signal to be amplified.

Fig. 2 is a curve illustrating the velocity distribution of secondary electrons; and

Fig. 3 is a sectional view of an electron multiplier and an associated electric circuit embodying the present invention.

Referring now more particularly to Fig. l of the drawing, there are shown two secondary electron emissive stages I and 2 of an electron multiplier having electron clouds schematically indicated at 3. Electron clouds 3 may represent a signal to be amplified. We may assume that the electrons of which electron clouds 3 are composed have been liberated, for example, by the impact of primary electrons, not shown, from stage I and are accelerated towards stage 2. When the electrons of clouds 3 impact stage 2, they will liberate further secondary electrons. The electrons of each electron cloud 3 have a velocity distribution of the type illustrated in Fig. 2. In Fig. 2 curve 4 shows the number of electrons plotted against their initial velocity expressed in electron-volts. When the frequency of the input signal is increased, electron clouds 3 will be more closely spaced together. Now, the faster electrons of any electron cloud, such as 5, will overtake the slower electrons of the preceding electron cloud, such as 6. Accordingly, electron clouds 3 are no longer as sharply separated as illustrated in Fig. 1 and, hence, the alternating current component of. the signal output will be-reduced so that at least part of the input signal is lost.

This eflfect is known as the electron transit time.

spread and results in a definite upper frequency limit beyond which the electron multiplier. can not be operated. Experiments have shown that at frequencies of the order of 50 megacycles the frequency response of an electron multiplier shows an appreciable deviation from its static response, that is, the response at low frequencies. This value, however, depends upon the design of the multiplier and a number of other factors such as the transit time between individual multiplier stages and the like; The theoretical cut-off frequency ismuch higher, being of the order of 1000 megacycles.

In accordance with the present invention the electron transit time spread is reduced by passing only electrons between the multiplying stages of an electron multiplier which have a predetermined velocity range. An electron multiplier embodying the' present invention is illustrated in Fig. 3. Electron multiplier 10 comprises evacuated envelope H and an electron source or primary emitter, such as indirectly heated cathode 1-2. It is to be understood that, for example, a photocathode may also be used as a source of primary electrons. Electron multiplier 10 further comprises control grid I3 and a plurality of secondary electron emissive stages or electrodes 14, l; l6, l and electron collector '20. The control grid l'3- is the means for controlling the number of primary electrons from the primary emitter to the first electrode l4.

Electron multiplier l0 preferably is of the magnetic type and provided with coil 2| supplied with direct current from a suitable source such as, for example, battery 22. Coil 2! is arranged to create amagnetic field, the lines of force of which pass perpendicularly through the: plane of the drawing.

For the purpose of supplying operating potentials tothe electrodes of multiplier I0, there is provided a voltage source such as, for example, battery 23 connected acrosspotentiometer 24 and having its positive terminal grounded". By means of lead25, cathode I2 is maintained at a potential that is negative against ground. Lead 26 connected through resistor 21 to a suitable tap of potentiometer 24 normally keeps control grid [3 at a potent al that is a few volts negative against that of cathode l2. Electron multiplying stages [4' to I! are kept at increasingly positive potentials with respect to cathode l2. Electron collector 20' is connected by lead '28 through load resistor 30 to ground and, hence, has a more positive-potential than multiplying stage". The input signal connected to terminals 3| may be impressed between

leads

25 and 26 through condensers 32. Hence, the input signal is impressed between control grid l3 and cathode l2. However; it is to be understood that the input signal may also be impressed, for instance, between. any two electron multiplyin stages I4, I5, 16 or H.

In accordance with the present invention a velocity filter is provided between electron source [=2 and first electronmultiplying stage l4, as well as between every two succeeding multiplying stages. Preferably, the velocity filter is of the magnetic type and includes coil 2| for generating a constant magnetic electron deflecting field. The magnetic velocity filter further includes

shields

33, 34, 35, 36 and 31 arranged between cathode l2 and first multiplying stage 14, as well as between every two succeeding multiplying stages. Shields 33 to 3 1' are each provided with a suitable slot or aperture 38. Shield 33, for instance, is maintained by means of a suitable tap of potentiometer 2.4 at a potential intermediate between that of cathode l2 and of multiplying stage Id. Shield 36 in turn is kept at a potential between'that of. multiplying stages I l and i5, and similarly shields 35, 36 and 31 are maintained at a potential intermediate between that of their associated. multiplying stages.

By virtue of the magnetic field created by coil 21* the primary electrons developed by cathode l2, as well as the secondary electrons liberated from each multiplying stage 14 to H, are deflect'edin accordance with their velocities. Thus, the radius of curvature of the electron paths is directlyproportional to the velocity of the electrons. Slots 38 in shields 33 to 3T are arranged in such a manner that only electrons within a predetermined velocity range are able to pass from. one multiplying stage to the succeeding stage.

Referring again to Fig. 2, it is, for instance, feasible to select those electrons which have a velocity corresponding to between two and three el'ectron-voltsfor passing them between two succeeding' multiplying stages. All other electrons, that is, those-which have a velocity ofless than two electron-volts or more than three electron volts are rejected, that is, they are collected by one of the shields 33 17031. It is of course to be understood that any other velocity range may be selected; The velocity of secondary electrons depends upon many factors, such as the energy of the primary electrons and the field gradient in the neighborhood of the secondary electron emissive surface. Hence, the shape of curve 4 as well as the values given on the abscissa are representative only of certain conditions. It will be obvious that the number of secondary electrons selected for passage between successive multiplying stages should be larger than the number of primary electrons which have liberated the secondary electrons, because otherwise the devi'cewill no longer function as a multiplier of electrons. However, inspection of Fig. '2 will show'that the-velocity spread of the electrons can be. considerably'reduced without unduly reducing the number of electrons allowed to pass between the multiplying stages.

The operation of. electron multiplier I0 is conventional' and, therefore, a short explanation thereof will'besufiicient. The primary electrons developed by cathode [2 are attracted by secondary electron, emissive stage I'd under the combinedinfiuence of. the magnetic field developedby coil 2| and the electric potential between cathode l2 and. multiplying stage 14. Only electrons within a predetermined velocity range are able to. reach multiplying stage 14. In a similar manner, secondary electrons are liberated from each of the succeeding multiplying. stages l5, l6 and H. Screens 34., 35, 36. and. 31. prevent electrons outside. the predetermined velocity range. from reaching. the succeeding stage. The multiplied electron current is collected by electron collector 20. The. output signal. is, developed across load resistor 30 and may be obtained from output lead 40.

Although a magnetic velocity filter as described hereinabove is preferred, it is also feasible to utihas a velocity filter of the electric type. Such an electric velocity filter has been described, for example, in United States Patent 2,271,985 to Morton. In this case the electrons are deflected in accordance with their velocities by means of an electric field. However, the advantage of a magnetic velocity filter is that a uniform magnetic field created by one coil, such as coil 2!, is sufficient to deflect the electrons from all multiplying stages of the multiplier.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electron multiplier system comprising a plurality of secondary electron emissive elec trodes, a primary emitter of primary electrons mounted adjacent said electrodes, voltage means coupling said primary electron emitter and the first one of said electrodes for directing primary electrons from said primary emitter towards the first one of said electrodes, means magnetically coupling said electrodes for directing secondary electrons liberated from each of said electrodes towards the succeeding electrode, means coupled to one of said electron emitters for controlling the number of electrons passing between said electrodes in accordance with an input signal, means including members mounted between said electrodes for passing only electrons within a predetermined velocity range between successive electrodes, and means mounted adjacent the last one of said electrodes for collecting the electrons from the last one of said electrodes to derive an amplified output signal, thereby to reduce the electron transit time spread and to raise the frequency cut-ofi of said multiplier.

2. An electron multiplier system comprising a plurality of secondary electron emissive electrodes, a primary emitter of primary electrons mounted adjacent said electrodes, voltage means coupling said primary electron emitter and the first one of said electrodes for directing primary electrons from said source towards the first one of said electrodes, means magnetically coupling said electrodes for directing secondary electrons liberated from each of said electrodes towards the succeedin electrode, means coupled to one of said electron emitters for controlling the number of electrons passing between said electrodes in accordance with an input signal, a velocity filter device including members mounted between said electrodes for passing only electrons within a predetermined velocity range between successive electrodes, and means mounted adjacent the last one of said electrodes for collecting the electrons from the last one of said electrodes to derive an amplified output signal, thereby to reduce the electron transit time spread and to raise the frequency cut-oif of said multiplier.

3. An electron multiplier system comprising a plurality of secondary electron emissive electrodes, a source of primary electrons mounted adjacent said electrodes, voltage means coupling said source and the first one of said electrodes for directing primary electrons from said source towards the first one of said electrodes, means coupled to said source for controlling the number of electrons passing between said electrodes in accordance with an input signal, a magnetic velocity filter device including shields mounted between said electrodes for passing only electrons within a predetermined velocity range between successive electrodes, and means mounted adjacent the last one of said electrodes for collecting the electrons from the last one of said electrodes to derive an amplified output signal, thereby to reduce the electron transit time spread and to raise the frequency cut-off of said multiplier.

4. An electron multiplier system comprising a plurality of secondary electron emissive electrodes, a source of primary electrons mounted adjacent one of said electrodes, voltage means coupling said source and the first one of said electrodes for directing primary electrons from said source towards the first one of said electrodes, means mounted adjacent said source for controlling the number of electrons passing between said electrodes in accordance with an input signal, a magnetic velocity filter device including a constant magnetic field producing means mounted adjacent said electrodes and a plurality of apertured shields mounted between said electrodes for passing only electrons within a predetermined velocity range between successive electrodes, and means mounted adjacent the last one of said electrodes for collecting the electrons from the last one of said electrodes to derive an amplified output signal, thereby to reduce the electron transit time spread and to raise the frequency cut-01f of said multiplier,

5. An electron multiplier system comprising a plurality of secondary electron emissive electrodes, a source of primary electrons mounted adjacent an end one of said electrodes, a unidirectional power supply, means including said power supply coupling said source and the first one of said electrodes for directing primary electrons from said source towards the first one of said electrodes, means mounted in the path of said primary electrons for controlling the number of electrons passing between said electrodes in accordance with an input signal, a magnetic velocity filter device including means surrounding said electrodes for creatin a constant magnetic field extending between said source and said electrodes and a plurality of shields each being provided with a slot, one of said shields being mounted between said source and said first electrode and the remaining shields being mounted between succeeding electrodes in such a manner that only electrons within a predetermined velocity range can prass through the slot of each of said shields, and means mounted adjacent the last one of said shields for collecting the electrons from the last one of said electrodes to de- "ive an amplified output signal, thereby to reduce the electron transit time spread and to raise the frequency cut-off of said multiplier.

6. An electron multiplier system comprising a plurality of secondary electron emissive electrodes, a source of primary electrons mounted adjacent the first one of said electrodes, voltage means coupling said source and the first one of said electrodes for directin primary electrons from said source towards the first one of said electrodes,-means including a coil surrounding said electrodes to produce a constant magnetic field for directing secondary electrons liberated from "7 each of said electrodes towards the succeeding electrode, means mounted between said source of primary electrons and the first one of said electrodes for controlling the number of electrons passing between said electrodes in accordance with an input signal, a magnetic velocity filter device including a plurality of apertured shields mounted between said source and said first electrode and between succeeding electrodes for passing only electrons within a predetermined velocity range between successive electrodes, and means mounted beyond the last one of said shields for collecting the electrons from the last one of said electrodes to derive an amplified output signal, thereby to reduce the electron transit time 5' 2,264,269

spread and to raise the frequency cut-off of said multiplier.

CHRISTIAN C. LARSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 10 Number Name Date 1,903,569 Jarvis Apr. 11, 1933 2,138,928 Klemperer Dec. 6, 1938 2,147,756 Ruska Feb. 21, 1939 2,227,062 Brett Dec. 31, 1940 Banks Dec. 2, 1941