US2561768A - Thermionic cathode activation - Google Patents

Description

July 24, 1951 R. ADLER THERMIONIC CATHODE ACTIVATION Filed April 10; 1950 ROBERT ADLER INVENTOR. W

ms ATTORNE Patented July 24, 1951 2,561,768 THERMIONIC CATHODE ACTIVATION Robert Adler, Northfield, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois Application April 10, 1950, Serial No. 155,049

9 Claims.

This invention relates to the activation of thermionic cathodes and more particularly to a novel process for activating a thermionic cathode of the type which is not provided with an associated indirect-heater element.

In the copending application of Robert Adler, Serial No. 129,554, filed November 26, 1949, for Electron-Discharge Device and Circuits," assigned to the present assignee, there is disclosed and claimed a novel type of bilaterally conductive electron-discharge device. In its preferred form, such a bilaterally conductive device may comprise a pair of thermionic cathodes with a control grid disposed therebetween, one of the cathodes being provided with a conventional resistance-type indirect-heater element and the other cathode being adapted to be energized in operation partialhr by heat radiation from the first cathode and partially by virtue of its own plate dissipation during portions of each operating cycle when it functions as an anode or a collector for electrons originating at the first cathode. A device of this type is especially useful in a simple circuit for the generation of sawtooth currents and finds particular application, for example, in the line-frequency scanning system of a television receiver or the like, as disclosed in detail in the above-identifled copending application.

In the construction of conventional electrondischarge devices employing thermionic cathodes of the type utilizing indirect-heater elements for cathode energization, the process of rendering the cathode thermionically active is relatively simple of accomplishment. Such a cathode is usually prepared by coating a cathode base member with carbonates of barium. strontium, calcium, etc., heating the coated cathode member to a sufllcient extent to cause breakdown of the carbonates, outgassing the broken-down carbonates to leave an oxide coating, and seasoning the oxide-coated cathode to improve the emissivity of the cathode surface. The seasoning step is readily accomplished, for example, by heating the oxide-coated cathode above normal emission temperature by means of the indirect-heater element and by simultaneously applying a suitable accelerating field until substantially full electron emission is obtained.

Such conventional seasoning methods are not possible, however, in the activation of a cathode which is not provided with an associated resistonce-type indirect-heater element. Moreover, in a bilaterally conductive device of the type dis: closed in the aforementioned Adler application, activation of two cathodes at opposite ends of a single electron-discharge path must be accomplished simultaneously. Thus, both cathodes must be heated considerably above normal emission temperature, and both must be subjected to suitable accelerating fields.

It is a particular object of the present invention to provide a novel process for activating a thermionic cathode.

A further object of the invention is to provide an improved process for activating a thermionic cathode of the type which is not provided with an associated indirect-heater element.

Yet another object of the invention is to provide a new and simple process for simultaneously seasoning a pair of thermionic cathodes disposed at opposite ends of a, single electron-discharge path.

In accordance with the present invention, a new and improved process for activating a thermionic cathode comprises the steps of alternately heating the cathode by electron bombardment and subjecting the cathode to the influence of an accelerating field. The novel process comprises the further step of independently controlling the intensity of the electron bombardment and the strength of the accelerating field.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawing. in the several figures of which like reference numerals indicate like lements, and in which:

Figure 1 is a cross-sectional view of a bilateral- 1y conductive electron-discharge device constructed in accordance with the above-identified copending application and comprising a pair of thermionic cathodes disposed at opposite ends of a single electron-discharge path; and

Figure 2 is a schematic circuit diagram illustrative of one form of apparatus which may be used to carry out the novel cathode activation process of the present invention.

Figure 1 is a cross-sectional view of a bilaterally conductive electron-discharge device constructed in accordance with the above-identified copending application and comprising a thermionic cathode IQ of the type which is not provided with an associated indirect-heater element. Briefly, the device also comprises a second cathode, consisting of two parts II and I 2 provided with associated indirect-heater resistance-type 3 elements I3 and I4 respectively. A control grid I5 is provided between cathodes In and II, I2.

In operation the inner cathode I0 is adapted to be energized partially by heat radiation from outer cathode members I I and I2. To this end, a pair of heat-reflecting members I6 and I! are provided to concentrate reflected heat from outer cathode II, I2 at inner cathode II). In order to prevent overheating of control grid I5, heat-radiating fins I8 and I9 are aflixed to the supporting rods for control grid I5. Preferably, the outer surfaces of fins I8 and I9 are blackened to provide eflicient outward heat radiation, and the inner surfaces are polished to prevent substantial heat radiation inwardly toward the control grid. All of the elements are supported in any conventional manner within an evacuated envelope 20.

Sincethe inner cathode l0 is not provided with an associated indirect-heater element, it is readily apparent that conventional seasoning processes may not be employed in the activation of the inner cathode. In fact, in the preferred embodiment represented by Figure 1, the inner cathode may comprise a member 2| of conductive material such as nickel or the like which is provided with a pair of thermionically emissive coatings 22 and .2 3. No means are provided for heating the inner cathode I0 independently of the other electrodes of the tube.

In the processing of such a tube, the inner cathode II) is provided with emissive surfaces 22 and 23 in a conventional manner, as for example, by applying suitable carbonate materials to the surfaces of cathode sleeve 2| and outgassing the carbonates to leave thermionically emissive oxide deposits on the sleeve. The outgassing operation comprises raising the temperature of the inner cathode, by induction heating or the like, to a sufficient extent to cause breakdown of the carbonates and drive off carbon dioxide which is then removed in the evacuation of the envelope. It has been found undesirable to raise the temperature of the inner cathode l0 during the outgassing process by electron bombardment from outer cathode members II and I2 since electron bombardment of the carbonate coating before the breakdown process is complete may result in poor emission characteristics. However, once the outgassing process is completed, the inner cathode may be subjected to electron bombardment without encountering detrimental effects of this type.

Following the outgassing process, the device is evacuated, gettered, and sealed in any conventional manner, and the inner and outer cathodes I0 and I I, I2 are subjected to the novel seasoning process of the present invention. Briefly, the seasoning process for inner cathode It) comprises the steps of alternately heating the inner cathode by electron bombardment from the outer cathode II, I2 and subjecting the inner cathode I 0 to an accelerating field. In accordance with the invention, the intensity of the electron bombardment and the strength of the accelerating field are independently controlled during the seasoning process. The frequency of alternating between the electron bombardment and the application of the accelerating field is maintained at a sufliciently high value to render the inner cathode continuously electron-emissive. In other words, the inner cathode is preferably not permitted to drop at any time below the minimum temperature required for electron emission. In practi e, it has been found that the thermal time constant of the inner cathode may be of the order of one second or less; therefore, the frequency of alternation between electron bombardment of the inner cathode and application of the accelerating field at the surface of the inner cathode is preferably maintained at a value greater than one cycle per second. In practice, it'has been found quite convenient to use an alternation frequency corresponding to the local power-line frequency, which is 60 cycles per second in most locations in the United States at the present time.

In other words, the process of activating the inner cathode comprises the steps of intermittently heating the inner cathode by electron bombardment during spaced time intervals, which intervals are preferably of sufliciently great time duration and of sufficiently high repetition frequency to maintain the cathode continuously in an electron emissive state, establishing an accelerating field at the surface of the cathode during intervening time intervals, and independently controlling the intensity of the electron bombardment and the strength of the accelerating field. The repetition frequency of the spaced time intervals during which the cathode is intermittently heated preferably has a period shorter than the thermal time constant of the inner cathode and may conveniently assume a value equal to the local power-line frequency.

As an additional feature of the invention, the outer cathode I I, I2 may be seasoned at the same time as the inner cathode l0. Preferably, the seasoning process is accomplished by applying an alternating voltage between the inner and outer cathodes and changing the amplitudes of the positive and negative half-cycles at different rates, so that the intensity of electronbombardment of the inner cathode I0 and the strength of the accelerating field formed adjacent emissive surfaces 22 and 23 of inner cathode II) are independently controlled. The'seasoning process is continued until substantially full electron emission is obtained and until the emissivity of the inner and outer cathodes is substantially uniform over the respective emissive surfaces.

Merely by way of illustration and in no sense by way of limitation, there is illustrated in Figure 2 a schematic diagram of one form of apparatus which has been found suitable for performing the novel process provided by the present invention. In the apparatus of Figure 2 a pair of continuously variable alternating-current trans formers 30 and 3| are connected in parallel across a suitable alternating-voltage source 32, which may for example constitute a conventional volt-60 cycle A. C. power line. The variable outputs of transformers 30 and 3| are differentially applied to the primary windings 33 and 3! of a pair of transformers 35 and 36 respectively. A pair of rectifier devices 31 and 38 are differentially connected in circuit with the secondary windings 39 and III of transformers 35 and 36 respectively; as shown, rectifier devices 31 and 39 may conveniently comprise thermionic diodes, the anodes of which are connected with oppositely polarized terminals of secondary windings 39 and 40. Of course, any type of unilaterally conductive device may be used in place of diodes 31 and 38. A pair of load resistors II and 42 are connected in series between the cathodes of diodes 31 and 38, and a connection 43 is provided between the junction of resistors 4| and 42 and the free ends of secondary windings 39 and 40.

The device of Figure 1 is connected in parallel with series-coupled resistors II and 42, the inner cathode l being coupled to the cathode of diode 31 and the outer cathode II, II being connected to the cathode of diode 33. During the seasoning process, the control grid l is'connected to outer cathode ll, l2, and indirect-heater elements I3, i4 associated with outer cathode Ii, l2 are connected to a suitable energizing current source (not shown). In order to provide separate indications of the respective currents drawn from the inner and outer cathodes, a pair of oppositely-polarized direct-current milliammeters H and 45, respectively connected in series with oppositely-polarized rectifier devices 46 and 41, are connected in parallel between inner cathode Ill and the cathode of diode 31.

The apparatus of Figure 2 constitutes a convenient circuit for providing independent control over the intensity of electron bombardment of the inner cathode l0 and the strength of the accelerating field produced adjacent the inner cathode. Essentially, the apparatus 01' Figure 2 operates to app y an alternating voltage between the cathodes I8 and II, l2 so that the inner cathode II is alternately bombarded by electrons from the outer cathode ll, l2 and subjected to an accelerating field or positive potential gradient which tends to draw current from the inner cathode. The amplitudes of the positive and negative half-cycles of the alternating voltage applied between the cathodes are independently controllable by means of the variable taps associated with variable A. C. transformers and 3|. Thus, variable transformer 3| controls the intensity of electron bombardment of the inner cathode ID by controlling the amplitude of the positive half-cycles of the alternating-voltage wave from source 32, while variable transformer 30 controls the strength of the accelerating field produced adjacent inner cathode I II by controlling the amplitude of the negative half-cycles of the alternating-voltage wave from source 32. Milliammeters M and indicate respectively the magnitudes of the emission currents drawn from outer cathode ll, 12 (the bombarding current) and from inner cathmle ll (the emission current).

The operational details of the circuit of Figure sion temperature. Variable transformer ii is adjusted to provide sufficient emission current from outer cathode II, I! during the positive halfcycles of the supply voltage to maintain inner cathode lflabove its normal emission tempera ture. Variable transformer 30 is then adjusted to provide a relatively small accelerating field adjacent inner cathode l0 during the negative halfcycles of the supply voltage from source 32.

The seasoning process is completed by gradually increasing at different rates the peak amplitudes of the positive and negative half-cycles of the alternating voltage from source 32 by means of variable transformers 3| and 30 respectively; generally, it has been found most desirable to increase the emission current from inner cathode ID at a slower rate than the bombarding current from outer cathodes i I, I2, and it may even be desirable to decrease the bombarding current slightly toward the end of the seasoning process. Of course, it is contemplated. that the changes in the peak amplitudes of the positive and negative half-cycles, and hence the changes in the bombarding and emission currents, may be effected either continuously at diiferent rates or in unrelated steps until the emissivity of both cathodes is uniform and complete.

In the particular application of the activation process of the present invention to the manufacture of iii-directional switch tubes of the type disclosed in the above-identified copending application and illustrated in Figure 1, particularly good results are obtained by maintaining the control grid l5 and the outer cathode l I, II at a common potential throughout the seasoning process, as by a galvanic circuit connection therebetween. When the grid is so connected, the amount of bombarding current from the outer cathode H, H required to keep the inner cathode ill at the desired seasoning temperature is maintained at a low value, since the device functions as a grounded-grid triode. Consequently, during such intervals when the inner cathode l0 serves as an anode or collector, plate dissipation at the inner cathode I0 is obtained with high voltage and low current, and the current-collecting capacity of the inner cathode is not exceeded. However, when the device is conductive in the opposite direction, from inner cathode III to outer cathode the outer cathode.

Thus, the invention provides an important new process for activating a thermionic cathode of the type which is not provided with an associated indirect-heater element. The invention provides a simple and reliable method of seasoning a cathode-of this type and is particularly useful in connection with the processing of a bilaterally conductive electron-discharge device of the type disclosed and claimed in the above-identified copending application.

While a particular embodiment of the present invention has been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. The process of activating a thermionic cathode which comprises: alternately heating said cathode by electron bombardment and subjecting said cathode to an accelerating field; and independently controlling the intensity of said electron bombardment and the strength of said accelerating held.

2. The process of activating a thermionic cathode which comprises: alternately heating said cathode by electron bombardment and subjecting said cathode to an accelerating field; and changing the intensity of said electron bombard ment and the strength of said accelerating field at diiferent rates until substantially full electron emission from said cathode is obtained.

' 3. The process of activating a thermionic cathode which comprises: alternately heating said cathode by electron bombardment and subjecting said cathode to an accelerating field; and gradually increasing the strength of said accelerating field independently of the intensity of said electron bombardment until substantially full electron emission from said cathode is obtained.

4. The process of activating a thermionic cathode which comprises: intermittently heating said cathode by electron bombardment during spaced time intervals to maintain said cathode continuously in an electron-emissive state; establishing an accelerating field at the surface of said cathode during intervening time intervals; and independently controlling the intensity of said electron bombardment and the strength of said accelerating field.

5. .The process'of activating a thermionic cathode which comprises: intermittentlyheating said cathode by electron bombardment during spaced time intervals occurring at a frequency greater than one cycle per second to maintain said cathode continuouslyin an electron-emissive state; establishing an accelerating field at the surface of said cathode during intervening time intervals; and independentlycontrolling the intensity of said electron bombardment and the strength of said accelerating field.

6. The process of activating a pair of thermionic cathodes disposed at opposite ends of a single electron-discharge path within an evacuated envelope which comprises: impressing an alternating voltage between said cathodes; and independently controlling the peak amplitudes of the positive and negative half-cycles of said alternating voltage.

7. The process of activating a thermionic cathode oi. the type adapted to be energized in subdevice of the type comprising a pair of thermionic cathodes arranged at opposite ends of a common electron-discharge path and having a control grid intermediate said cathodes, the process of activating one of said cathodes which comprises: maintaining said control grid and the other of said cathodes at a common potential; impressing an alternating voltage between said cathodes; and independently controlling the peak amplitudes of the positive and negative half-cycles of said alternating voltage.

9. In the manufacture of an electron-discharge device of the type comprising a pair of thermionic cathodes arranged at opposite ends of a common electron-discharge path and having a control grid intermediate said cathodes, the process of activating one of said cathodes which comprises: galvanically connecting .said control grid to the other of said cathodes; impressing an alternating voltage between said cathodes; and independently controlling the peak amplitudes of the positive and negative half-cycles of said alternating voltage.

ROBERT ADL No references cited.

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