US2450602A

US2450602A - Thermionic discharge tube with electronic velocity filter

Info

Publication number: US2450602A
Application number: US549889A
Authority: US
Inventors: Levialdi Andres
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1944-08-17
Filing date: 1944-08-17
Publication date: 1948-10-05
Anticipated expiration: 1965-10-05

Description

Get. 5, 1948. A. LEVIALDI 2,450,592

THERMIONIC DISCHARGE TUBE WITH ELECTRONIC VELOCITY FILTER ATTORNEY Patented Oct. 5, 1948 THERMIONIC DISCHARGE TUBE WITH ELECTRONIC VELOCITY FILTER Andres Levialdi, Buenos Aires, Argentina, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application August 17, 1944, Serial No. 549,889

11 Claims.

This invention relates to velocity filters for electron beam and more particularly to thermionic discharge tubes utilizing monokinetic beams of low-velocity electrons.

It is already known to filter high-velocity electron beams in which high-intensity electromagnetic fields may be easily utilized for separating the electrons into substantially monokinetic beams.

However, in electronic devices or thermionic discharge tubes which utilize accelerating potentials approximately equal to the mean initial velocity of the generated electrons, i. e. in devices operating with accelerating potentials of the order of a few volts, filtering of the electrons constitutes a rather difiicult problem for which, so far as I am aware, no satisfactory solution has been found as yet.

One of the main difficulties encountered in filtering low velocity electrons consists in that the intensity of the electromagnetic fields which could be used for filtering purposes decreases to values falling within the range of the intensity of the terrestrial field during magnetic storms, so that even with special shielding materials and devices it is almost impossible to maintain the path-controlling magnetic field within the predetermined values.

However, I have now found that low-velocity electrons possess some inherent characteristics which may be used advantageously for filtering purposes. In fact, low-velocity electrons have relatively large associated wave lengths varying from 5.5 A. for accelerating potentials of the order of volts to 17.3 A. for accelerating potentials of the order of 0.5 volts in accordance wit the Broglie wave length formula.

Hence, even for electrons moving with nearly like velocities the associated wave lengths differ by considerable values, so that by directing the electron beam toward an adequate difiracting lattice structure, the electrons of the diffracted beam will be distributed according to the velocities thereof, thus making it possible to select a bundle of electrons having like velocities by means of an apertured diaphragm located in the path of the difiractedelectron beam. Consequently, the electrons pas-sing through the diaphragm aperture can be regarded as a monokinetic electron beam containing electrons of substantially like velocities determined by the glancing angle of the incident electron beam, the grating distance of the diffracting lattice structure, and the position of the diaphragm aperture within the area of the diffracted electrons, the velocity range of the monokinetic electron beam being further determined by the dimensions of the aperture.

It is therefore one of the main objects of the present invention to provide, in thermionic discharge tubes, a simple and efiective means for obtaining a monokinetic beam of low-velocity electrons.

A further object of the present invention is to provide, in thermionic discharge tubes, a velocity filter for low-velocity electrons, the filtering characteristics of which can be varied over a continuous range of electron velocities.

These and other objects and advantages of the invention will be apparent from a consideration of the following detailed specification taken in connection with the drawings which form part of the specification, and wherein:

Fig. 1 is a schematic longitudinal section of a thermionic discharge tube incorporating an electron-velocity filter according to my invention;

Fig. 2 illustrates a modification of the electronvelocity filter shown in Fig. 1;

Fig. 3 shows another modification of the electron-velocity filter allowing of a coaxial mounting of the discharge tube elements;

Fig. 4 is a cross section of the thermionic discharge tube taken at arrows 4-4 of Fig. 3;

Fig. 5 shows another modification of the electron-velocity filter illustrated in Fig. 2; and 1 Fig. 6 illustrates a thermionic discharge tube similar to that shown in Fig. 4 but utilizing the electron-velocity filter of Fig. 5.

The same reference characters or numbers in-' dicate like or corresponding parts or elements throughout the drawings.

Referring to Fig. 1, there is shown a thermionic discharge tube comprising an evacuated L-shaped tubular envelope If! the shorter leg of which includes an electron gun H connected by means of terminals l2 and I3 to an adequate power supply source not shown in the drawings. The electron beam, emerging from an electron gun II at the orifice I4, is accelerated and shaped into a convergent beam [5 by means of an electrostatic lens system It constituted by a first and second anode l6 and I 6", respectively, each connected to an intermediate potential of a direct current supply I1, the negative pole 11' of which is connected to terminal 13 of electron gun H, while anode I6" is also connected to ground. The lowvelocity electron beam I5 is focussed on a point l8 and penetrates into a diffraction zone l9 limited by the second anode l6" of electron lens system I6 and a diaphragm 20 also connected to 3 ground and provided with an aperture 2| so that substantially all points of diffraction zone I9 are at the same potential.

The convergent electron beam 55 is directed toward a diffraction element 22 of regular lattice structure located midway between orifice I 4 and focus [8 and having a substantially plane diffracting surface 22 placed at an angle of 45 with respect to the direction of the incident electron beam I 5, so that a glancing angle 01 of 45 is formed between diffraction surface iig' and the incident electrons. The diffraction element 22' can be formed by a nickel or quartz plate, the choice of the material depending' 'upon the lattice structure desired. Preferably, the reflecting The monokinetic and focussed electron beam surface of the plate is cut with certain crystal- I lographic orientation angles'with' respect to the crystallographic axes of the material.

It has been shown by de Broglie that electrons moving at a velocity much below that of light with kinetic energy equal to V electron-volts, have associated a wave length which is given in angstroms by the formula Now, since diffraction element 22, which may be formed of a crystal or metal having an adequate lattice structure, may be regardedas a plane gratmg of grating space at equalto the distance between the atom rows of the lattice structure, the incident electron beam i willbe diffracted in accordance with de Braggs formula n; \=d sin 01 where x is the wave length; 61 the glancing-angle of the incident electron beam I5, (I the grating space of the lattice structure and 12' any positive integer. With a fixed 'angle of incidence, the electrons of the diffracted beam 15 will be dispersed over a relatively wide area covering a definite range of anglesaz, formed between, the diffracted electrons of beam l5 and the plane diffracting surface 22'. Foreach angle 02 the diffracted electrons will have an associated wave length in accordance with'ide Braggs-fornrula. Hence, in the diffracted-beam t5 the electrons will be distributed in-accordancewitlr their velocity, electrons of lower velocity corresponding to larger angles 02' and vice versa. The distribution of the electrons within the dispersed diffracted electron beam i5 will depend naturally upon the distribution of the mean electron velocities present in the incident beam l5, so that the maximum and minimum values of 02 are determined by the minimum and maximum velocity of the electrons emerging from orifice I4 of gun I I. However, although scattered, the electrons of diffracted beam 15" will be focussed on a line which, in first approximation, can be represented by a circle 23 drawn with the distance between plane diffracting surface 22 and focus l8 as 'a radius; and aperture 2| of diaphragm'zll is located to coincide precisely with a point on the circumference of circle 23 which forms a minimum angle 0'2 withdiffracting plane 22, so that the electrons emerging fromdiaphragm aperture 2} will forma monochromatic and focussed electron beam l5" containing electrons o f very like velocities, since the'diameter of aperture 2| can be made as small as desired.

Itwill be understood that, in the position shown of aperture 2 i L electron gun i I together with diffraction element 22 and diaphragm 20, consti- 15' emerging'from diaphragm aperture 2| should be regarded asa tool prepared for further electronic work in devices requiring a practically perfect definition of electron velocity. In the ther- :mionic discharge tube shown in Fig. l the monokine'tic electron'beam l5" traverses modulation means indicated with the general reference numeral 24 in which the electron beam can be transformed or modulated in accordance with theparticular performance characteristics of the thermionicdisgharge tube in response to a modututes an electron velocity filter having a predetermiffed arid'fi'xed filteringcharacteristic, since the mean velocity of the electrons in monokinetic lation potential applied to

terminals

25 and 26 connected to modulation means 24. electrode 21, c onnected to the positive pole I1" of D.- C supply l 'l through a load impedance 23, may be formed of a fluorescent screen only, or may be constituted by an apertured target electrode, while load impedance 28 may be a pure; resistance,- a reso'na nt circuit, or a mere conductor the case of a thermionic discharge tube utilizing a fluorescent screen as indicating means,

In the thermionic discharge tube of Fig. l, the incident electron beam [5 is focussed by means of an electrostatic lens system 16, the electron velocity of monokinetic beam [5" being determined by the position of diaphragm aperture 2|. The thermionic discharge tube shown in the drawing of Fig. 2 di ffe s from that of Fig. 1 in that the incident andthe diffracted electrons are travelling in divergentbe arns and that focussing of tl fe -electrons is applied to the 'monokinetic beam emerging from diaphragm aperture 2 I. For

' purpose; a lens system, designated with general referericenumeral 29 and connected to an intermediate potential of D.-C. supply I1, is 1o cated between diaphragm 20 and modulation means 2 'l'of the tube, the diffraction zone 19 of which is limitedhyan accelerating anode 38 and the diaphragm 25.3, both connected to ground potential. Furthermore, diffraction zone id includes a pair of deflector plates 3! and 32 connected to

control terminals

33 and 34, respectively, so that by applying an adequate potential to these terminald theangle of incidence of electron beam 55 on diffracting surface 22 may be varied within certain limits, as shown by the dotted lines35 and 35. Consequently, the velocity of theelectronsin the monokinetic electron beam (5" will vary in accordance with the control potential applied to

control terminals

33 and 34, so that a monokineti'c electron beam of adjustable mean velocity can be obtained.

Although the electronic performance of thermionic discharge tubes shown in Figs. 1 and 2 is quite satisfactory, the L-shaped envelope i0 constitutes a constructive disadvantage which, under some circumstances, may restrict the utilization of the thermionic discharge tubes including the electron monochromator or velocity filter according to this invention.

Figs. 3 and 4 of the drawings show a longitudinal and cross section of a thermionic discharge tube, respectively, comprising a tubular envelope l0,

Collector wherein the difierent construction elements of the tube can be connected to the contact pins of a standard base, if desired. As can be seen in the drawing, electron gun I, electrostatic lens system l6, diaphragm 26, modulating means 24 and collector electrode or target 29 are coaxially mounted on a virtual axis parallel to that of tubular envelope I and coinciding with the axis of gun l I. Diffraction zone or chamber |9 is limited, similar to that of Fig. 1, by the second anode iii of electrostatic lens system it and diaphragm 20 and includes diffraction element 22 provided with a plane diifracting surface 22. However, difiraction element 22 is not located in the direct path of the convergent electron beam l emerging from orifice M of lectron gun H, but is placed in a plane parallel to the axis of envelope Hi, and convergent beam l5 is deflected toward diffracting surface 22' by means of a magnetic field perpendicular to the plane of the drawing and generated by a pair of coils connected in series to control terminals 33 and 34' to which an adequate D.-C. supply source (not shown in the drawing) is connected. The controlling magnetic field, formed as the sum of the individual magnetic fields schematically indicated with arrows 38 in the drawing of Fig. 4, also deflects the diffracted electrons towards diaphragm 20 and the difiracted electron beam I5 will be dispersed over a relatively wide area in a manner similar to that shown in Fig. -1. Hence, the electrons emerging from aperture 2| of diaphragm 20 form a monokinetic beam including electrons the mean velocity of which is determined by the position of the diaphragm aperture 2| within the area of the diffracted beam l5. However, by varying the amplitude of the magnetizing D.-C. voltage applied to control terminals 33? and 34', i. e. by varying the intensity of the electromagnetic field generated by coils 37, the glancing angle 01 of the incident electron beam l5 may be varied within certain limits, so that an adjustable velocity monokinetic electron beam will be obtained.

The position of diaphragm 20 in the thermionic discharge tube shown in Figs. 3 and 4 is elected so that the sum of the mean trajectories of the incident and difiracted electron beams 5 and l5, respectively, is substantially equal to the focal distance of electrostatic lens system it, so that monokinetic electron beam l5" emerging from diaphragm aperture 2| is focussed to obtain a maximum concentration of the electrons.

In the thermionic discharge tubes shown in Figs. 1 to 4, the focussing of either the incident or the monokinetic electron beam is obtained by means of suitable electron lens systems. The focussing of the electron beam can also be obtained by utilizing a concave diffraction element mounted in a manner similar to that used by Rowland for his spectroscope (A. C. Hardy and F. H. Perrin, The Principles of Optics, 1932, pg. 563). As can be seen in the drawings,- the thermionic discharge tube shown in Fig. 5 is similar to that disclosed in Fig. 2, with the only difference that a diffraction element 39 having a, concave diffracting surface 39 is utilized in the difiraction zone It" which is also provided with deflector plates 3| and 32 for varying the glancing angle of incident electron beam IS. The curvature of concave surface 39' corresponds to a circle 40 passing through orifice Id of electron gun H and diffracted as a plurality of convergent monokinetic beams focussed on different points located on the circumference of circle 40, as indicated by beams I51 and l52 in the drawing of Fig. 5.

The aperture 2| of diaphragm 20, being located at the focus of electron beam l5'1 will thus constitute a source of a concentrated monokinetic I electron beam I5, containing electrons the velocity of which is determined by the position of diaphragm aperture 2| with respect to difiracting element 39 and orifice M of electron gun II. 'By varying the glancing angle of incident electron beam Hi, the velocity of the monokinetic electron beam l5" can be also adjusted within certain limits, as already explained hereinbefore with reference to the electron velocity filter incorporated in the thermionic discharge tube shown in Fig. 2. In the thermionic discharge tube shown in. Fig. 6 the concave difiracting element 39 is mounted in a way similar to that of plane difiraction element 22 in the tube disclosed in Fig. 3, so that the electron velocity filter including concave difira-cting element 39 can also be used in a thermionic tube comprising a tubular envelope ill, the diffraction zone |9" of the tube being limited by an accelerating anode 3i] and the apertured diaphragm 20 both connected to ground. The deflection of the incident and diffracte-d electron beams l5 and I5, respectively, is obtained by means of an electromagnetic field generated by coils 31, as already explained hereinbefore with reference to Fig. 3.

While I have indicated and described several embodiments of thermionic discharge tubes incorporating electron velocity filters in accordance with my invention, it will be apparent to one skilled in the art that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

I claim:

1. A thermionic discharge tube comprising an evacuated envelope containing means for generating a. beam of electrons, electrode means for l r tin a directing said electron beam to a diffraction element having a substantially regular lattice structure to produce a difiracted beam of electrons dispersed in accordance with the respective velocities thereof. a diaphragm provided with an aperture and located in the path of the diffracted electron beam to produce an emergent beam of substantially monokinetic electrons the velocity of which is determined by the glancing angles of the incident beam and said monokinetic electrons with respect to the difiracting surface of said diffraction element, said accelerating elec trode and said apertured diaphragm being electrically interconnected to form a substantially equipotential space including said diffraction ele ment, means to vary the initial velocity of said accelerated electron beam to vary the velocity of said emergent monokinetic electron beam, and a target electrode to collect the said monokinetic electron beam.

2. A thermionic discharge tube including an evacuated envelope containing means for producing, accelerating and focusing a beam of electrons to a diifracti'on element, said element having a lattice structure arranged so as to disperse the said electrons with respective velocities approximately as a function of the glancing angles of the incident beam, electron barrier means provided with an opening positioned in the path of the said diffracted beam so as to pass monokinetic electrons of approximate like velocity, said bar- 7 rier mean's and sald accelerating means being electrically interconnected so as to. include. the said 'd'ifiralc'tion element: in a'substantiallyequi potential space, means to vary the initial velocity of the said beam-therebyto vary the velocity of said passed electrons, means to modulate th'esaid passed ele'ctrons in accordance with electric control energy; and atargct electrode to collect the said modulated pass-ed electrons.

3. A. thermionic discharge' 'tube comprising-an evacuated envelope and within'the envelope an" electron diffraction element; means for producing, accelerating, and focusingabeam of electrons to-' ward 's-ai'd diffractlon element, said element having a lattice' structure arranged"- to disperse the said electrons in divergent directions proportional to the respective velocities thereof;"electron barrier mean's provi'ded with an opening positioned in the path of the said dispersed electrons to pass a monokinet'ic beam of-electrons -of'sub'stantially like velocity, :s'aid barrier means-and said accelcrating means being electrically interconnected J" to provide a substantially e'quipotential space 'enclosed said diffracticn element; target elec-" trodetd collec't the said "passed electron beam, and means-intermediate said opening an-d said target electrode to modulate thesaid passed 'elec-s' tronbeam in accordance with'electricalcontrol energy. i 4. A thermionic-discharge tube' comprising an ,evacuatedxenvelope' and withincthe envelopeanelectron 'diiTra'cti-on element, means tolgenerate a beam of electrons and to rocusfiand 'directsaid electron beam toward said-diffraction "element;

said element having a lattice "structure arranged to disperse the said-electrons indivergent directions proportional tothe velocities 'thereofi 'a 'diaphragm" provided withan aperture positioned in the p'athliof 'the said dispersed electrons to pass a monokineti'c' beam of" electrons of substantially like velocity, said diaphragm and said focusing I.

and directing means being electrically interconheated-and enclosing th-e'said 'diffractiorrtelement-i in a substantially 'equipotential" space, a targeti electrode-tot collect the said passed electron beam said generat ng; focusing, "and .directingmeans: being adaptedto varyth'e velocity of theelectrons; of the passed electron'bearrr, and means-,iintermen di'ate r said aperture and said :tar'getelectrodei'to modulate said passed electron beam;

5. A thermionicdischarge tube comprisingan" evacuated-envelope and within'the envelope an electron diffraction element; means -for' generata beam-oi: electronsymeans to-accelerate and direct said electron beam-toward said (infraction:- elemenizsaid element having 'a'substantia'lly re'gular lattice structure arranged to disperse-by 'diffraction the said electrons in divergentdirections according-"to: the respective" velocities thereof, electron barrier means provided with anropening positione'diin the path of the said dispersedelectrons to pass amonokinetic'bea'rn of'electron's of substantially like velocity; said barrier'meansand said accelerating and directing means being eleccollect' the said passed electron beam,and means intermediate said opening and said target 'elec-' trode to mo'dulate said passed electron beam.

6. A thermionic discharge tube comprising an static'cb'eam deflecting 'means'tovary the angle 7 of incidence of said generated electron beam on said difiracting surface in accordance With'ele-ctnicalicontrol energy, a target electrode to collect the said passedelectronbeam, and means intermedia'te'said openin and said target electrode to modulate-said passed electron beam.

7. A' thermionic discharge tube comprising an evacuated envelope'and within said envelope an electron diffraction element, means for generat-' ing a beam of electrons, means to'accelerate and direct said electron beam toward said diffraction clement, said element having a substantially reg- 1lular -lattice' structure arranged to disperse by dilfraction the said electrons in divergent direcntions -:p-roportional to the respective velocities thereo'f', electron barrier means provided with an opening positioned in the path of the said disapfirse'd electrons to pass a monokinetio beam of electrons'of substantially like velocity, said barrier means'and said generating means being electrica-lly interconnected and enclosing the said ldiffract'ion element in a substantially equipotenzm'tial space; said accelerating and directing means to vary the angle'of incidence of said generated ':electron beam on said 'difiracting surface in aci-":- :cordan'ce'=with electrical control energy, a target 45. electrode to collect the said passed electrons, and 1 means intermediate said opening and said target electrode to'rnodulate said passed electron beam. 8. A thermionic dis-charge tube comprising an evacuated'envelope, and within said envelope an electron diffraction'element, an electron gun pro vided with an electron lens system to produce a beam of electrons directed toward said diffraction element; said element having a lattice structure arranged to disperse the said electrons in diver- 5-51 gent directions proportional to the respective velocities -thereof, electron barrier means provided .with'an opening positioned in the path of the said dispersed electrons to pass a monokinetic ."uAbearr'r-of electrons of substantially like velocity, 60."; said-barrier means and said electron gun means being-electrically interconnected and enclosing 2;. the said 'difiraction element in a substantially aliequipotentlal space; the sum of the mean paths 4. of the incident electron beam and the diffracted '65 electron bearnto said opening being substantially equal-'to the focal distance'of said electron lens system',- a target electrode to collect the said passed electron beam, and means intermediate isaid-opening and said target electrode to modula'te said passed" electron beam.

9. A'thermionic discharge tube comprising an H evacuated envelop-e, and Within the envelope an electron-diffraction element having a surface the cross se'ction of which'is a concave substantially circular'linefimeans' for generating a beam of i evacuatedenvelope and within theenvelope a dif -fraction element, means for generating a beam of tice'structure arranged-to disperse by difiraction i-ncludin'g 'm'agnetic field beam deflecting means electrons, an electron lens system for focusing said beam of electrons and directing said beam toward said surface, said element having a substantially regular lattice structure at said surface elastically curved in the plane of said cross-section with a radius of curvature substantially equal to the diameter of the circle of said diffracting line, said beam being diffracted by said element to disperse the said electrons with respective velocities approximately as a function of the glancing angles of the incident and diffracted beams, electron barrier means provided with an opening positioned in the path of the said diffracted electrons to pass a monokinetic beam of electrons of substantially like velocity, said barrier means and said lens system being electrically interconnected and enclosing the said diffraction element in a substantially equipotential space, the sum of the mean paths of said incident beam and the diffracted beam to said opening being substantially equal to the focal distance of the said electron lens system, the said circle passing substantially through the source of said incident beam and through said opening, means to vary the velocity of the electrons of said passed beam in accordance with electrical control energy, a target electrode to collect the said passed electron beam, and means intermediate said opening and said target electrode to modulate said passed electron beam.

10. A thermionic discharge tube comprising an evacuated envelope, and within the envelope an electron diffraction element having a surface the cross-section of which is a concave substantially circular line, means for generating a beam of electrons, an electron lens system for focusing said beam of electrons and directin said beam toward said surface, said element having a substantially regular lattice structure at said surface elastically curved in the plane of said cross-section with a radius of curvature substantially equal to the diameter of the circle of said diffracting line, said beam being diffracted by said element to disperse the said electrons approximately as a function of the respective velocities thereof, electron barrier means provided with an opening positioned in the path of the said diffracted electrons to pass a monokinetic beam of electrons of substantially like velocity, said barrier means and said electron gun means being electrically interconnected and enclosing the said diffraction element in a substantially equipotential space, the sum of the mean paths of said incident beam and the diffracted beam to said opening being substantially equal to the focal distance of the said electron lens system, the said circle passing substantially through the source of said generated beam and through said opening, said electron lens system being adapted to vary the velocity of the electrons of the passed beam in accordance with electrical control energy, a target electrode to collect the said passed electron beam, and means intermediate said target electrode and said opening to modulate said passed electron beam.

11. A thermionic discharge tube comprising an evacuated envelope, and within the envelope an electron diffraction element having a surface the cross-section of which is a concave substantially circular line, means for generating a beam of electrons, an electron lens system for focusing said beam of electrons and directing said beam toward said surface, said element having a substantially regular lattice structure at said surface elastically curved in the plane of said crosssection with a radius of curvature substantially equal to the diameter of the circle of said diffracting line, said beam being diffracted by said element to disperse the said electrons with respective velocities approximately as a function of the glancing angles of the incident and diffracted beams, electron barrier means provided with an opening positioned in the path of the said diffracted electrons to pass a monokinetic beam of electrons of substantially like velocity, said barrier means and said electron gun means being electrically interconnected and enclosing the said diffraction element in a substantially equipotential space, the sum of the mean paths of said focused beam from said lens system to said element and the diffracted beam from said element to said opening being substantially equal to the focal distance of the said electron lens system, the said lens system being adapted to vary the angle of incidence of said focused beam on said element in accordance with an electrical control quantity, the said circle passing substantially through the source of said focused beam and through said opening, a target electrode to collect the said passed electron beam, and means intermediate said opening and said target electrode to modulate said passed electron beam.

ANDRES LEVIALDI.

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

UNITED STATES PATENTS Number Name Date 2,126,286 Schlesinger Aug. 9, 1938 2,158,314 Von Korshenewsky May 16, 1939 2,197,033 Diels Apr. 16, 1940 2,260,041 Mahl et al Oct. 21, 1941 2,281,325 Ramo Apr. 28, 1942 2,372,422 Hillier Mar. 27, 1945 OTHER REFERENCES Davisson and Germer: Diffraction of Electrons by a Crystal of Nickel, page 4, Bell Telephone Laboratories Reprint B-281, January 1928.

Germer: Optical Experiments with Electrons, page 4, Bell Telephone Laboratories Reprint B-351, October 1928. (Copies of both publications in Div. 54.)