US2219871A - Electric discharge device - Google Patents

Description

Oct. 29, 1940.

L. MALTER ELECTRIC DISCHARGE DEVICE Filed July 22, 1936 2 Sheets-Sheet l W WW m m-N a E Enventor NM m-m N h m @Malier h a D Louis (Ittomeg Oct. 29, -1940. L. MALTER 2,219,871

ELECTRIC DISCHARGE DEVICE Filed July'22, 1936 2 Sheets-Sheet 2 Ml/Tll/IL CONDUC'IHNCE r I I 3maentor Louis Malter PLHTL' 0017173? a v (Itforneg Patented Oct.-. 29, 1940 UNITED STATES "PATENT orrica ELECTRIC time I I I poration of America,

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a .corporation of Dela- Application July 22, 1936, Serial No. 91,807

Claims. (01. zsqg-zi) J This invention relates to electric discharge devices and methods of operating the same, particularly to grid controlled. and magnetically influenced electron discharge tubes, and has spe- 5 cial reference to the provision of improvements in the construction and operation of so-called magnetic-type electron-multipliers.

The novel features characteristic of the invention are set forth in particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, will be best understood by reference to the following description taken in connection with the accompanying drawings, wherein Fig. 1 is a view in perspective of a thermionic magnetic type electron-multiplier embodying the invention, portions of the container wall and the associated magnet structure being broken away to more clearly illustrate the disposition of the electrodes,

Fig. 2 is a diagrammatic sectional view of the cathode, control grid, and magnetic shield of the device shown in Fig. 1,

Fig. 3 is a diagrammatic sectional view of a magnetic shield enclosing a cathode and. a control grid, and shows a screen grid mounted exterior of the shield,

Fig. 4 is a diagrammatic sectional view of a magnetic shield enclosing a cathode, control grid, and a screen grid, r

Fig. 5 is a diagrammatic sectionalview of an assembly including the electrodes of Figs. 3 and 4, showing an alternative manner of supporting the screen grid, r

Fig. 6 is a diagrammatic sectional view exemplifying the manner in which the device shown in Fig. 1 may be operated,

Fig. '7 is a diagrammatic view illustrating the circuit connections and manner of operating a 40 multiplier having a screen grid similar to that shown in Figs. 3, 4, and 5, and

Fig. 8 is a graph exemplifying certain electrical characteristics of the improved device of the invention.

The mutual conductance of a tube is a good indication of the efiiciency of the tube as an amplifier, in that it is dependent upon the effect of the applied grid voltage upon the plate current, the greater the change in plate-current corresponding to any change of applied grid voltage, the greater the mutual conductance of the tube.

When it is recalled that electrons emitted from a thermionic cathode possess a Maxwellianvelocity distribution, it can be shown theoretically,

that the mutual conductance cannot be greater h n v where eIis the electronic charge, 70 is Boltzmanns 5 constant, and T is the absolute temperature of the cathode. Ip is the plate current measured in milliamperes. For a cathode operating at, say, 1000 K. the maximum value of this quantity is gmax:11,600 Ip micromhos Consequently, at a platecurrent of one millia' pere and a cathode operating at 1000 K. the maximum possible value of mutual conductance is 11,600 'micromhos. The mutual conductance of, commercially available radio tubes operating at .plate currents of one milliampere is usually no more than approximately 10% of this maximum value. v

A principal object, therefore, of the present invention is to provide an electron discharge tube the mutual conductance of which approaches substantially the theoretical maximum value of mutual conductance.

While, theoretically, the mutual conductance of a thermionic discharge tube may be increased by forming the grid of very fine mesh spaced very close tothe cathode, such construction carried to the extreme in practice, inhibits the passage of the electrons and thus prevents the desired increase in mutual conductance. I It would appear that if the relatively small number of electrons available on the anode side of a very fine mesh grid could be augmented, as by electron-multiplication, in the space between the grid andanode, then a high value of mutual conductance might be achieved. The present invention is predicated upon thisprinciple. j

The problems encountered in carrying the in vention into effect differ in electrostatic type and magnetic type? electron-multipliers. Thus, in electrostatic multipliers of the prior art, the field at each target drawing away the electrons is, rather weak and the multiplier becomes spacecharge limited at rather small current values. Inprior art magnetic 'multipliers,--the magnetic field interacts detrimentally with the low velocity electrons in the cathode-gridregion. The instant case has special reference to the solution of problems incident to carrying the in-" vention into effect with electron-multipliers of the .magnetic type, and. contemplates a tube wherein the low velocityelectrons in the oathode-gridwregiori 'or area are. shielded from the effect 0i the -magnetic,fieldywhereby they are j supplied with proper portion 3. The main body of the tube contains a plurality of secondarily-emissive lower electrodes 5 lying in the same plane and spaced apart along the long axis of rality of upper or accelerating electrodes 1 disposed in a plane parallel to and spaced from the plane in which lie the first mentioned elec-v trodes. The lower electrodes are preferably made of silver sensitized as by an application of caesium, and the upper electrodes of molybdenum,- tantalum, nickel, or any other metal which is easily degassed and not readily oxidized.

.These separate sets of electrodes 5 and 7 may be mounted within the tube inany convenient manner. In the embodiment illustrated, each set of electrodes is supported by a sheet of mica 9, which serves to keep them accurately aligned in a common plane. Each electrode is provided with two straps II which lie in corresponding openings l3 in the mica sheet, straddling a small crosserod or wire l5 disposed against the upper surface thereof. During the application of weldingthe straps to an electrode, the rod I5 is forcibly biased against the mica sheet, thus locking the electrode firmly in position. Each cross-rod has a lead I! welded thereto, which lead extends through the wall of the tube to the exterior thereof. Each lower cross-rod is also provided with anoutput lead l9.

Anoutput electrode 2| is mounted in one end of the tube, preferably fairly close to the sets of electrodes and in tube axis. The output electrodeis provided with a terminal connection23 which extends through the wall of the container to the outside thereof.

The depending neck portion 3 of the tube has.

a press 25 of the'usual type sealed therein. The

press supports a virtual source of electrons con stituted by a filamentary heater element 2'! contained in a metal thimble 29 having alayer of electron-emissive oxides 3| (see Fig. 2) on its.

The thimble is surrounded and cylindrical structure 33 having an upper end. shielded by a orifice 35 therein and constituted of iron orof high magnetic permeability" which terminates'in a perforated-cap. The,

other metal upper face of the cap lies in the plane of the multiplying electrodesand, preferably, the per-( foration'39 therein is coaxial with the emitting portion 3| of thecathode and the orifice 35 in the magnetic shield33. I

A grid 4| of .yery fine mesh (say 60 mesh,-

.003" wire) is supported by the shield about from the primary-electron emitting surface and,

spanning the coaxial perforations 35 the shield and cap;- The grid is preferably spaced about 4, of an inch from the cathode. By reason of, its fine mesh and its close proximity to the primary-electron source 3!, grid 4|, when potentials, will exert a very high degree of control over the electrons, provided "they reach the grid without numerical diminutionsuch la's might be caused, in the abs and 39 in lower or multiplying electrodes.

the tube, and a plua plane transverse tonthe sence of the magnetic shield 33, by a deflection from their normal paths due to the influence of the magnetic field.

The magnetic shield about the cathode-grid space is mounted above the press on supports 33 which contact the shield adjacent its outer surface and are fixed thereto by a metal band 45. Durin assembly these supports are slidable in the band so that the top surface of the cap may be accurately aligned in the plane of the One of these supports 43 terminates in the press and the other one terminates in an external connection 4? to which the grid potentials are applied. The cathode heater leads are designated 49, and the single lead to the thimble-shaped cathode 5|.

Any convenient means may be utilized for establishing a magnetic field parallel to the surfaces of the electrodes 5 and 1. The device shown in perspective in Fig. 1 is constituted by a ,U-shaped element 53 of magnetically permeable material on which is mounted an energizing ,coil- 55 and to each upstanding portion of which is affixed a plate 51 also of permeable material. The tube is disposed between these plates in such position that a substantially uni-- form magnetic field is set up parallel to the opposed surfaces of the multiplying and the accelerating electrodes.

Referring now to Fig. 6, which shows one manner of utilizing the device-of Fig. l; the input circuit elements connected between the controlgrid 4! and the cathode 29 of the tube I are exemplified by an input resistor 6 I, a grid biasing potential source 63, and a potential divider 65.

Itis necessary to maintain each of the lower multiplying electrodes positive with respect to thecathode and with respect to any electrode between it and the cathode. For this purpose the cathode may be connected to the negative terminal of asource of unidirectional potential, exemplifiedin the drawing by a resistor 61; the output electrode 2| may be connected to the positive terminal of the source through an output'device such as a-resistor 69; and, starting f-rom'the cathode. the next adjacent electrode 5, or first multiplying electrode, may be connected to a point "H on the resistor somewhat more positive, and each of the remaining electrodes 5 connected to successively more positive points 13 and 15 on the resistor. h The first upper accelerating electrode 1, immediately above the disc 31 surrounding the primary electron source 3|, may be connected to a point [1 on the resistor more positive than the po nt 15 to which the last. multiplying eleceach accelerating (upper) electrode positive with respect to the'electrode directly beneath it, Excellent results are achieved by maintaining each multiplying electrode at the same potential as the'next preceding accelerating electrode.

With such potential distribution, the number of wire to glass seals may be reduced by the provision ofl'jumpers (not shown) connecting the several mentionedpairs" of electrodes within 75 he tube.

When the cathode heater '2'! is energized, thermionic electrons will be given off by the emissive coating 3|. Many of these primary electrons will be lost by reason of the barrier presented to their passage by the relatively large effective area of the wires constituting the fine mesh grid 4!. Those electrons which do pass through the interstices in the grid are subject in passing to its controlling action. Once past the grid they will be accelerated toward the upper electrode 1 directly above the emitter, because of the electrostatic field between it and the surface of 31, and, if no magnetic field were present, they would impinge thereon. However, the magnetic field (not exemplified in Figs. 6 or 7), which is parallel to the planes of the electrodes, subjects the electrons, moving toward the accelerating electrode, to a force component at a right angle to. their instantaneous direction of travel. The direction of the component depends upon the polarity of the field. If the electric and magnetic fields are adjusted to proper values, the electrons will describe trochoical paths and will strike the first multiplying electrode, which is the second from the left end of the device, as illustrated in the drawing.

If this first multiplying electrode is maintained at a potential positive with respect to the emitting electrode, the primary-electrons striking it will cause the emission of secondary electrons, the number of which is dependent, in part, upon the magnitude of the potential difference. The secondary electrons, as shown in the drawing, will in turn be accelerated toward the second upper electrode but, by reason of the magnetic field, they will be diverted and be focused upon the third lower electrode. Here again, a multiplication. by reason of the secondary emission, is secured and this process is repeated in any number of stages until the amplified stream of secondary electrons is collected by the output electrode and caused to fiow in a utilization circuit exemplified in the drawing by the resistor 69 included between the output electrode 2| and the positive terminal of the potential source 61.

It will be apparent that if the primary electrons in the cathode-grid space were permitted to be influenced by the magnetic field, they would be given trajectories similar to the trochoidal trajectories exhibited by the electrons in their travel from one multiplying electrode to another.

This in itself would be most undesirable, since it would direct the electrons which might ordinarily pass through the grid, by reason of their velocity of emission, away from this electrode, back to the cathode. As the number of electrons which pass through the very closely spaced grid wires will in any event be a relatively small per cent of those originally emitted, a further reduction might render the device inoperative, or at least require that this additional loss be made up by increasing the gain in the multiplying stages.

Of greater importance is the effect of the magnetic field upon the mutual conductance of shielded and unshielded devices. It has been found experimentally that the relative change in current transmitted through the grid for a given change in grid voltage is less when the magnetic field is present than when it is not present. By way of example, without the magnetic field, let the current transmitted through the grid at a fixed value of grid voltage (Eg) be, say, 10 microainperes. Let the grid voltage now be changed to Eg0.1 volt, then the current transmitted through the grid may be 5 microamperes. Here this change in grid voltage has caused a two to one change. in. the current transmitted through the grid. With the cathode-grid space unshielded and the-magnetic field applied in the manner illustrated, it was found that the available currents corresponding to each of the above values of grid voltage were 5 microamperes and 4 microamperes, respectively. Thus with the magnetic field present, the relative change of plate current caused by a change of .1 volt is only in the ratio of 5 to 4 instead of 2 to 1. current transmitted through the grid in each case may be stepped up to the same final value by electron-multiplication, but in the former case the change in the output current caused by a certain change in grid voltage will be greater than in the latter case, where the magnetic field is present. Thus it is clear that with the cathodegrid space shielded against the magnetic field, the mutual conductance of the device is materially increased.

This increase in mutual conductance is graphically shown in Fig. 8, which shows-a plot, A, of

the mutual conductance theoretically attainable;

a plot C of the mutual conductance obtained with a tube having a magnetically unshielded cathode-grid region; and a plot B of the mutual conductance obtained with the same tube when the cathode-grid region is magnetically shielded in accordance with the principle of the invention. An inspection of these plots shows clearly that the mutual conductance achieved in a tube having a magnetically shielded cathode-grid space more nearly approaches the theoretically attainable valuethan that obtained in the absence of a magnetic shield.

The final output current from an electronmultiplier. of the typedescribed, in addition to being proportional to the number of electrons available adjacent the side of the grid remote from the cathode, is also dependent upon the potential of any one of the electrodes with respect to the next adjacent electrode which is normally maintained at a lower potential. of the multiplier is changed by changing the voltage per stage, alteration of the potential on the first upper or accelerating electrode afiects the control characteristic of the grid due to the fact that the potential distribution in the oathode-grid region is affected.

The provision of a screen grid on the anode side of the control grid has been found to prevent variations in the control characteristic when the potential distribution among the accelerating and multiplying electrodes is altered. As shown in screen-grid, designated 85, is maintained at a When the gain Now, the

55 Fig. 7 (which is otherwise similar to- Fig. 6) the fixed potential, positive with respect to cathode 29 by a connection to a suitable point 81 on the potential divider Bl. Due to the shielding action of this screen-grid, alteration of the accelerating potential on the electrode immediately thereabove does not react upon the control grid-cathode region.

Referring now to Fig. 3. The auxiliary or screen grid 85 need not necessarily be shielded from the effects of the magnetic field, but may be mounted upon a separate support 89 with the surface of the screen above the magnetic shield and covering the orifice 35 in the shield 33, which is spanned by the control grid 4 I.

In Fig. 4 the screen grid, 85, rests upon an annulus of mica 9| or other insulating material which is supported on the rim about the orifice 35 covered by the control grid 41. The walls of the shield 33 are extended as at 33*, to protect the space about the screen-grid from the influence of the magnetic field and to provide a spaced support for the electrostatic disc 31. The conductive lead 89 for the screen grid 85 enters the magnetic shield 33 through an insulated bushing 33 in the extended position 33 of the shield. As in Figs. 2 and 3, the shield itself forms part of the conductive lead to the control grid.

An alternative construction is shown in Fig. 5. Here the magnetic shield 33 forms part of the conductive connection to the screen grid 85, which is mounted directly thereon and encloses the end of the electron-conduit through which the electrons must pass. The control grid 4| which, as in the other embodiments of the invention, is of very fine mesh, is here supported on a hollow cylinder 93 surrounding the thimble 29 containing the heater element 21.

As a number of possible embodiments may be made of the above invention, and as changes may be made in the embodiments set forth without departing from the spirit and scope of the invention, it is to be understood that the foregoing is to be interpreted as illustrative and not in a limiting s'ense, except as required by the spirit of the appended claims and by the prior art.

I claim as my invention:

1. Method of operating an electron multiplier which comprises generating a stream of primaryelectrons, subjecting said electrons to a controlling potential adjacent their point of origin and causing said controlled primary-electrons to produce a stream of secondary-electrons under the influence of a magnetic field while shielding the region of origin and control of said primary-electrons from the influence of said magnetic field.

2. Method of operating an electron multiplier which comprises generating a stream of primaryelectrons, subjecting said electrons to a controlling potential adjacent itheir point of origin, subjecting said controlled electrons to the accelerating influence of an electrostatic field, and causing said controlled accelerated primary-electrons to produce a stream of secondary-electrons under the influence of a magnetic field while shielding the region of origin and control of said primaryelectrons from the influence of said magnetic field and from variations in the intensity of said electrostatic field.

3. An electric discharge device comprising a cathode, a grid and an anode mounted in spaced relation in an evacuated container, means for producing a magnetic field through said container, and means for shielding said cathode-grid space from the influence of said magnetic field.

4. An electric discharge device comprising an electron emissive cathode, a control electrode mounted in the path of electrons from said cathode, a shield composed of a magnetically permeable material immediately surrounding said path and a third electrode mounted remote from said shield and accessible to said electrons.

5.- An electron-multiplier comprising a cathode for generating a stream of primary electrons, a grid mounted adjacent said cathode, an electrode surface adapted to liberate electrons by secondary emission, an anode, a magnet for directing said primary electrons tosaid emissive surface, whereby secondary-electrons are released from said surface and for directing said secondary electrons from said surface to said anode, and a conduit of magnetically permeable material about the space intermediate said cathode and said grid through which said primary electrons pass.

6. An electron tube comprising a primary-electron emitter, an electrode surface adapted to liberate electrons by secondary emission, a conduit of magnetically permeable material extending part way between said emissive electrodes, a control electrode within said conduit, an accelerating electrode for directing primary electrons from the terminal of said conduit to said secondary-electron emitting surface, and an auxiliary grid for shielding said control grid from said accelerating electrode.

'7. The invention as set forth in claim 6, wherein said auxiliary electrode is mounted within said conduit.

8. The invention as set forth in claim 6, wherein said auxiliary electrode is supported by said conduit and is electrically connected thereto.

9. The invention as set forth in claim 5 wherein said grid is electrically connected to said conduit.

10. The invention as set forth in claim 5 Wherein said grid is of fine mesh and is mounted in close proximity to said cathode.

LOUIS MALTER.

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