US2406850A

US2406850A - Electron discharge apparatus

Info

Publication number: US2406850A
Application number: US388043A
Authority: US
Inventors: John R Pierce
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1941-04-11
Filing date: 1941-04-11
Publication date: 1946-09-03
Anticipated expiration: 1963-09-03
Also published as: ES176051A1; CH260719A; DE845223C; GB592397A; FR938580A

Description

Sept .3,1946. HERCE 2,406,850

ELECTRON DISCHARGE APPARATUS A Filed April 11, 1941 FIG.

I "I 0.6- 74 OUTPUT Y AME INVENTOR By J .R. PIERCE ATTORNEY one, 22, of which the portion l1 of the beam forming electrode is fitted and to the other, 23, of which there are secured rigid, small diameter wires 24 afiixed to the cathode shell M. The cylindrical portion l9 of the accelerating anode is provided with a flange 25 extending between a pair of insulating washers 26, the anode being secured to the foundation member 2| by an annular clamping bracket 21 afilxed to the member 2|.

The electron gun, it will be noted, constitutes a unitary assembly which is supported from the stem H by a plurality of rigid wires 28 embedded in the stem, and to which wires the foundation member 2| is attached by rigid tie wires 29.

Mounted opposite the electron gun and coaxial therewith are a pair of electrodes, designated generally as 30 and 3|, one, 30, of which includes a central cylindrical portion 32 integral with an annular disc 33 extending through the wall of the enclosing vessel l and hermetically sealed thereto, and includes also a dished reticulated or grid end portion 34. The other electrode, 3|, includes a central cylindrical portion 35 integral with an annular disc 36 extending through and hermetically sealed to the wall of the enclosing vessel I0, and includes also a dished reticulated or grid end portion 31 conforming to and immediately adjacent the grid portion 34 of the electrode 30.

A dished circular reflecting electrode 38 is mounted opposite the electrode 3| and coaxial therewith and is supported from the stem l2 by a leading-in conductor 39.

Associated with the electrodes 30 and 3| and coaxial therewith are a pair of coaxial cylindrical conductors 40 and 4| which are joined at one end by an annular metallic spacer 42 and are slidably engaged intermediate their ends. by an annular conductor 43 or plunger the position of which may be adjusted by a drive screw 44 secured thereto and threaded to the metallic spacer 42. The inner coaxial conductor 40 engages and may be secured to the annular disc portion 36 of the electrode 3|. The outer coaxial conductor 4| is connected to the annular disc portion 33 of the electrode 30, as by way of metallic annuli 45 clamped to this disc portion and against the conductor 4| by clamping members 46 and 41 threaded to each other.

The electrodes 30 and 3| together with the coaxial conductors 40 and 4| define a resonant cavity which may be tuned to a desired frequency by adjusting the position of the plunger 43. Power may be taken from the cavity through a conductor 48 having a looped inner end as shown and extending through a cylindrical conductor 49 carried by the clamping member 41.

During operation of the device, the accelerating anode I9 is maintained at a positive potential with respect to the cathode and the reflecting electrode 38 is maintained at a negative potential with respect to the cathode. The electrodes 30 and 3| are maintained at the same positive potential with respect to the cathode. In one particular embodiment, operable in the 10 centimeter wave-length range, the accelerating anode I9 may be operated at the order of 330 volts positive, the electrodes 30 and 3| may be operated at the order of 250 volts positive, and the refleeting electrode 38 may be operated between 80 to 100 volts negative or between 22 to 32 volts negative, all with respect to the cathode.

The electrons emanating from the cathode l4, I are maintained in a substantially parallel beam by the beam forming electrod I1, l8 and are accelerated toward the gap between the grid portions 34 and 31 of the electrodes 30 and 3|, respectively, due to the positive potential of the accelerating anode l9. In passing across the gap between the grid portions 34 and 31, the electrons receive a velocity variation and the velocity varied electron stream then passes into the space between the electrodes 3| and 38, wherein a bunching of the electrons occurs and the velocity variations are converted into density variations. Because of the negative potential upon the reflecting electrode 38, the bunched electrons are reversed in direction and again cross the gap between the grid portions 34 and 31 of the electrodes 30 and 3 respectively, traveling toward the cathode, and deliver energy to the resonant cavity. The device, therefore, is self-excited and operates to generate oscillations, which are of extremely low wave-lengths, for example, of the order of a few centimeters, depending upon the electrical parameters of the system.

The electrode 38 is so spaced with respect to the gap and is operated at such potential that the drift time, that is the total time an electron in the stream is in the space between the electrodes 3| and 38, considering both its motion from the gap toward the reflector electrode and in the reverse direction, is substantially equal to (n%) cycles of the operating frequency, n being an integer, so that the returning electrons cross the gap in the proper phase to deliver energy to the resonant cavity. In practice, the resonant cavity is tuned to the desired operating frequency, by adjusting the position of the plunger 43, and the voltage upon the electrode 38 is then varied until the power output derived from the output loop 48 is a maximum.

The propriety of the requirement that the drift time for the condition of sustained oscillations shall be equal to (n%) cycles of the operating frequency may readily be verified by considering the history of electrons crossing the gap at various phases of the electric field in the resonant cavity. For convenience, consider an electron (which will be referred to as the reference electron) that crosses the gap at the instant when the electric field is changing from the direction which produces electron acceleration to the direction which decelerates an electron. The speed of the reference electron is substantially the same upon leaving as upon entering the gap. The electron will travel a certain distance, d say, toward the reflector before being halted and will reach the gap again after traveling a total distance equal to 2d. Such electrons as may have crossed the gap shortly ahead of the reference electron had their speed increased by the action of the electric field in the gap and hence penetrated further into the reflecting field than does the reference electron. Traveling a distance equal to more than 2d, the speedier electrons will tend to return to the gap at substantially the same instant as the reference electron. Similarly, such electrons as may cross the gap shortly after the reference electron will be slowed down, will penetrate a smaller distance into the reflecting field, and, traveling a total distance equal to less than 201, will tend to return to the gap at substantially the same instant as those that started earlier. Thus it will be seen that the reference electron may be regarded as the center of an electron group. In order that the group may contribute energy to the electric field to offset the natural .mentof a particular output power.

dampinginthe.- resonator, the group should: return to the gap at .aninstant when the-electric field is opposed to the motion. of the electrons inthe group and is of maximum intensity. Such 'an instant occurs substantially three-quarters of a cycle after the reference electron above described first crossed the gap and this and other such instants occur at the times determined by the value of (n /4) cycles of the operating frequency.

the gun, such electrons will again have their direction ofmoti'on reversed and be directed back across the gap. These electrons thus again reversedin direction of motion have substantially random velocities and also are debunched, so that in again crossing the gap they deliver little energy to the resonant cavity. Effectively they intermingle with and act similarly to the electrons emanating from the cathode of the electron gun.

It has been found that this reflection effect and double reversal of the direction of motion of the electrons is augmented-by the form of the grid vportionst l and 31 and of the electrode 38, that is the dished form thereof, with the inner or concavesurface thereof toward the electron gun and that thus shaping these elements reduces the magnitude of the current which must be drawn from the cathode to enable the attain- The dished form of the electrodes, furthermore, results in uniform electron transit times in the region between the electrodesfil and .38. I

It will be noted that in one sense the device shown and described is a velocity variation device wherein a single resonant cavity isutilized for bothinputand output. The transit timesin the space between the gun and the electrode 30 is not critical. Furthermore, it will be appreciated that the device is susceptible of simple operation inasmuch as only one tuning adjustment and one potential adjustment, that of the electrode 38, is necessaryto effect variation of the operating frequency over a fairly wide range. For example, in the specific construction shown and described, oscillations over the range of 8 to 13.4 centimeters have been obtained and, operating with a second mode in the cavity, oscillations of centimeters wave-length have been obtained. With the resonant cavity tuned to a particular frequency, the oscillator frequency maybevaried' by to 30 megacycles by varying the potential of the electrode .38.

It'may be noted also that the coupling conductor '48 need not have any direct current connection to the cavity. Hence, the cavity need not be grounded and the :device can be energized from a supply having a negative ground, such,

' for example, as thesupplyIfor an intermediate frequency amplifier. Also, it will be appreciated that but relatively low voltages, in comparison with the voltages employed in velocity variation devices of known constructionare necessary upon the electrodes of the device, this being due in .part to'the grid portions 34 and -37 utilized in the electrodes 39 and 3|.

The frequency of oscillation can bevaried over a fairly wide range with but little resultant change in ,the :output obtainable. from thedevice,

so that ;"the .device ais-suitable for use for frequency modulation purposes. For example, if the device werequsedfor frequency modulation,

modulated. As a specific illustration, such elementcould be'the secondary Winding of an out- .put transformer theprimary winding of which wasin circuit with atransmitter or microphone.

Inasmuch'as the frequency of oscillation is'controllable by the potential of'theelectrode 38, the device disclosed is particularly suitable for use as'a beatingoscillatorwith automatic tuning, in which case the electrode 38 would be appropriate- .ly; associated with an automatic tuning circuit effective'to maintain the potential of the electrode 38 at thevalue requisite for the generation of oscillations of the desired frequency to thereby maintain-the correct relation between the beating oscillator andsignal frequencies.

High. frequencystability or, stated in another way, constant frequency operation of the device shown in Fig. 1, maybe realized in one way illustratedin Fig. 3. In the circuit illustratedin this figure, the "oscillator device I0 is associated with a highQ circuit, such as a resonant cavity .to the rectifier elements 12a defined by a cylindrical conductive member ill the frequency of which cavity maybe adjusted accurately by a screw H. Mounted adjacent one Wall of the member 10 are a pair of identical rectifierelements 12a and 12b which are connected to opposite ends of aresistance 13 in turn connected across the input terminals of a direct current amplifier 14, the mid-point of the resistance 13 being connected to the member 10.

The resonant cavity of the device It is coupledto theresonant cavity 10 by a coaxial line 15 having aloop end'lfia within the'cavity 10, the line also connecting the firstementioned cavity-totwo loops 16a and 16b in turn connected and "12b respectively.

In operation, the accelerating anode I9 and the electrodes "30 and f3! have applied thereto appropriate positive potentials as bybattery H, as described heretofore'in connection with Fig. 1, and thecavityof thedevice 10 and the potential of the electrode 38,=-supplied by a battery 18, are adjusted so that the device oscillates at the desired frequency. The resonant cavity 10 also is adjusted to this frequency.

When the discharge device [0 oscillates, energy .is conveyed by the line 15 to the two loops 16a and 151). Also, electromotive fOrceS will be pro- .duced in these loops, opposite in phase as applied will be produced by the rectifier elements.

Hence, substantially zero voltage exists across the resistance 13 and no voltage change occurs at the output terminals of the direct current amplifier 74.

However, ifthe frequency of oscillations generated by the device [0 changes from that to which the cavity 10 is tuned, due for example to temperature effects in the discharge deviceor fluctuations in the voltages supplied to the electrodes thereof, the alternating current voltages appearing across the rectifier elements 12a and 12b will be unequal and consequently the rectified voltages will no longer balance so that a direct current voltage will appear across the input of the amplifier 14. Hence, an amplified direct current voltage will be impressed across battery 18, in such direction as to change the potential of the electrode 38 in the manner requisite to bring it to the value whereby the frequency of oscillations generated by the device ill will be that to which the cavity is tuned.

In the electron discharge device illustrated in Fig. 4, the enclosing vessel comprises a metallic cylinder 50 having an internal annular shoulder portion 5|, and metallic

annular end portions

52 and 53 having sealed thereto insulating members 54 and 55, respectively. Within the vessel are mounted an electron gun l3 and electrodes 31!, 3| and 38 which may be of the same general construction as the corresponding elements in the device shown in Fig. 1 and described hereinabove. The annular disc portion 33 of the electrode 30 is seated against one end of the internal annular portion 5| of the enclosing vessel and has bearing thereagainst a cylindrica1 metallic spacer 56 against which there is seated in turn the annular disc 36 of the electrode 3|. The electrodes 30 and 3| and the spacer 56 are fitted slidably within the cylinder 50 and may be clamped in position by a cylindrical sleeve 51 fitted within the metal cylinder 50.

In this construction, the resonant cavity is entirely within the enclosing vessel, being defined by the electrodes 30 and 3|, the spacer 56 and the portions of the cylinder 50 between the discs 33 and 36. Energy may be taken from the cavity through the conductor 59 which extends through an eyelet 58 secured to the cylinder 5!] and is hermetically sealed to the eyelet by a glass or other insulating bead. The cavity may be tuned through a coaxial line 60 of adjustable effective length in shunt with the cavity, the inner conductor of the coaxial line being connected to the disc 33 and the outer conductor of th coaxial line contacting and being secured to the metal cylinder 50.

The device illustrated in Fig. 4 may be operated in the same manner as the device shown in Figs;

1 and 2 as described heretofore.

A particular feature of the construction illustrated in Fig. 4 resides in the fact that the cavity does not include any joints so that power losses due to contact resistance losses in such joints or to radiation losses through gaps therein are avoided. Furthermore, the cavity is free from insulating material so that losses due to insulation in other constructions are avoided as are also extraneous capacities of such insulators. Also, it will be noted that this construction enable the very accurate alignment and spacing of the electrodes so that the construction of devices for operation at very high frequencies, at which accurate alignment and spacing of the electrodes is very important, is facilitated.

Although specific embodiments of the invention have been shown and described, it will be understood that they are but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is 1. Electron discharge apparatus comprising a pair of spaced electrodes having juxtaposed reticulated portions defining a gap, an electron gun opposite one of said electrodes and in alignment with said gap, and a reflecting electrode opposite the other of said electrodes and in alignment with said gap, said reticulated portions being dished in the direction away from said electron gun.

2. Electron discharge apparatus in accordance with claim 1 wherein the face of said reflecting electrode toward said gap is dished in the direction away from said electron gun.

3. Electron discharge apparatus in accordance with claim 1 wherein said electron gun includes an accelerating electrode having a reticulated portion opposite said one electrode and dished in the opposite direction to the reticulated portion of said one electrode.

4. Electron discharge apparatus comprising a pair of coaxial electrodes having juxtaposed grid portions defining a. gap, an electron gun opposite one of said electrodes and coaxial therewith, and a reflecting electrode opposite the other of said electrodes and in alignment with said gap, said grid portions being concavo-convex and uniformly spaced with the concave surface thereof toward said electron gun, and the surface of said reflecting electrode toward said other electrode being concave and uniformly spaced from the grid portion of said other electrode.

5. Electron discharge apparatus comprising an enclosing vessel having a cylindrical metallic portion provided with an internal annular shoulder, an electrode having an annular portion seated on said shoulder and having a central electron permeable portion, a second electrode having an outer annular portion and a central electron permeable portion in juxtaposition to said first electron permeable portion, a cylindrical metallic spacer member engaging said annular portions of said electrodes and bounding therewith a resonant cavity, an electron gun opposite one of said electron permeable portions, an auxiliary electrode opposite the other of said electron permeable portions, said annular portions and said spacer member being slidably fitted Within said metallic portion of said vessel, and means securing said portions and spacer member against said shoulder.

6. Electron discharge apparatus in accordance with claim 5 comprising a variable coaxial line coupled to said cavity.

'7. Electron discharge apparatus comprising an enclosing vessel having a cylindrical metallic portion, a pair of electrodes within said vessel and defining a resonant cavity with a section of said metallic portion, said electrodes having inner reticulated portions in juxtaposition and defining a gap and having also outer portions conductively connected to said metallic portion, an electron gun opposite one of said electrodes, and a reflecting electrode opposite the other of said electrodes, said reticulated portions being concavo-convex and having the concavo face thereof toward said electron gun.

8. Electron discharge apparatus in accordance with claim '7 wherein the surface of said reflecting electrode opposite said other electrode is coaxial with and-curved in the same direction as the reticulated portion of said other electrode.

9. Electron discharge apparatus comprising a pair of electrodes having juxtaposed grid portions defining a gap, means cooperatively associated with said electrodes defining a resonant cavity therewith, an electron gun including a cathode opposite one of said grid portions, a refleeting electrode opposite the other of said grid portions, said grid portions being dished and having the inner faces thereof toward said electron gun, reflectin electrode having a dished surface facing the grid portion to which it is opposite, means for maintaining said pair of electrodes at a positive potential with respect to said cathode, and means for maintaining said reflecting electrode at a high negative potential relative to said cathode.

' JOHN R. PIERCE.