US2886736A - Current rectifier - Google Patents

Description

y 1 1959 E. E. MARTIN I 2,886,736

CURRENT RECTIFIER Filed Feb. 2, 1954 INVENTOR. E ME/YE E. MARTIN #MT5M i i amt United States Patent CURRENT RECTIFIER Eugene E. Martin, McMinnville, 0reg., assignor to Research Corporation, New York, N.Y., a corporation of New York Application February 2, 1954, Serial No. 407,687

3 Claims. (Cl. 313-297) This invention relates to a novel high vacuum electron tube useful in the rectification of electrical currents and to rectifier circuits including the tube of the invention. The invention is particularly directed to a high vacuum tube, the electrodes of which include a field emission cathode and associated electronic circuitry for supplying a direct current at a steady potential from various nonsteady potential sources.

An object of the invention is to provide an evacuated electron tube which together with suitable external circuitry provides a direct current output at steady potential for any of a variety of non-steady potential inputs, the tube being particularly designed to minimize the thermal energy required by the cathode. The invention thereby minimizes or avoids the problems associated with energy supply to the cathode, dissipation of such energy, the insulation of the conventional thermal energy sources and the weight and size of such sources.

Another object of the invention is to provide an electronic vacuum tube rectifier with the foregoing characteristics which also has a cathode of minute size, surface and volume whereby it has correspondingly low electrical capacity to other electrodes and correspondingly improved mechanical characteristics such as strength and rigidity.

Another object of the invention is to provide an electronic vacuum tube rectifier with the foregoing characteristics which has a potential drop which is low compared to its useful output voltage.

Another object of the invention is to provide an electronic vacuum tube rectifier with the foregoing characteristics and in which the cathode emission current density is independent of the output load impedance connected to the tube.

Another object of the invention is to provide an electronic vacuum tube rectifier with the foregoing characteristics and which is designed to inhibit bombardment of the cathode by ions formed at other electrodes and/ or in the residual gas, thus improving cathode stability.

Another object of the invention is to provide an electronic vacuum tube rectifier with the foregoing characteristics and which, together with suitable external circuitry, protects the cathode from damage due to fluctuations in input voltage and at the same time allows use of an appreciable range in both input and output voltage for given electrode geometries.

Many forms of rectifiers are known. These can for the most part be divided into two classes:

(1) High vacuum tubes and those which are gas filled, all having sealed envelopes and utilizing thermal cathodes.

(2) Semi-conductors such as copper oxide, selenium, etc., whose rectifying action occurs because of the dependence of the electrical impedance of such materials, or their junctions with other materials, on the direction of applied electric field.

Rectifiers employing a thermal cathode require a source of cathode heating current, such as transformers or batteries, thus adding to the bulk, weight and power dissipation of the apparatus wherein they are employed. It

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is necessary in many instances to operate the cathode of such a tube at a high voltage relative to ground which adds the problem of providing adequate voltage insulation in the heating current source.

Rectifiers of the second type, utilizing the characteristics of semi-conductors do not readily withstand high peak inverse voltages, so that in cases where rectification of high voltage is desirable this can be done only by placing a number of such devices in series. Thus, the increase in expense, bulk and weight soon outweighs the advantages of this device, that is, the elimination of the need for filament transformers, and other features of rectifiers of the first type.

Previous efforts to use a field emission cathode in a rectifier have not been successful because of certain difficulties avoided by the present invention; however, a rectifier with field emission cathode would otherwise combine the best characteristics of the two foregoing types of rectifier. Two difficulties have prevented successful use of the field emission cathode in a rectifier of the conventional diode type. First, it is necessary to impress a considerable voltage (say 1 to 20 kv.) across a conventional field emission diode in order to develop sufficient electric field at the cathode to maintain electron emission. As a result there is appreciable heating and power loss at the anode of the tube due to the high kinetic energy of the impinging electrons. Also, since the load or filter condenser is placed in series with the rectifier, the usable output voltage is seriously reduced by the tube drop across the rectifier, V That is to say, that the output voltage V will at any time be given by V0: i' r 1 where V, is the instantaneous applied voltage. The second difficulty arises from the exponential dependence of field current density J, on the electric field F, given empirically as J=A exp (b/F) (2) A and b being constants. Because the electric field F depends directly on the tube voltage drop V,,, as follows,

where a is a geometric factor depending on electrode geometry and spacing, it will be seen from Equations 1 and 2 that the cathode current density will be strongly dependent on the degree of loading of the device. Indeed any sudden large increase in load current could so increase the cathode current density that destruction of the cathode would result. (See Phys. Rev. 91, 1043 (1953).)

In the present invention the foregoing difiiculties are avoided and certain other advantages are gained, thus making possible the practical application of the field cathode in a rectifier tube.

The invention will be more particularly described with references to the accompanying drawing in which Fig. l is a sectional elevation of a vacuum tube structure embodying the principles of the invention;

Fig. 1a is a greatly enlarged fragmentary view of a typical field emission cathode;

Fig. 2 is a diagrammatic representation of a vacuum tube and associated circuitry illustrating one embodiment of the invention; and

Fig. 3 is a diagrammatic representation of a vacuum tube and associated circuitry illustrating another embodiment of the invention.

Fig. 1 shows a discharge vessel 1, containing a field emission cathode 2, encircled by, or adjacent to, an electrode 3, hereafter called field electrode, which is so designed that when a suitably high electric potential differencc exists between it and the cathode 2, those surfaces assaasa 3 of the cathode of minimum radius of curvature will be subjected to a very high electric field F (10" F l0 v./cm.). The field electrode 3, is so designed and positioned as to intercept a minimum fraction of the field electrons emitted from the cathode, to which emission it is therefore relatively transparent. A secondary electron suppressor grid 4 is positioned adjacent to an electron collector (or plate) 5. The suppressor 4 is so interposed between said collector 5 and the field electrode 3 as to inhibit transmission of secondary electrons formed at said collector to said field electrode.

The cathode 2 is supported on a filament 6 which is also useful for several purposes such as heating the cathode to smooth, clean, and degas it prior to or during operation of the tube. It is to be understood that during operation of the device the cathode and cathode support will be p at a temperature lower than that at which appreciable ermal emission will take place.

If the usual field cathode, shaped as a cone with an approximately hemispherical tip as shown in greatly enlarged view in Fig. la, is employed, an electrode geometry approximately that shown in Fig. 1 will be found to be advantageous. Under action of the high cathode field directed towards the surface, electrons are emitted from the hemispherical tip surface approximately normal to the 'surface and chiefly within a cone of vertex angle about 120 C. These electrons follow nearly radial trajectories outward from the cathode tip. Since operation of the de vice involves deceleration of these electrons, the opposing electric field should be radial also. That is to say, they should be decelerated in an electric field directly opposing their motion. This can be quite closely approximated if the collector 5 and field electrode 3 both have spherical symmetry; however, in practice, the approximation is sufficiently good if the emitter tip is surrounded by an electrode 3 in the form of a small ring in a position at least 90 off the axis of symmetry of the emitter tip from the center of curvature of the emitting surface, that is, the lumen of the ring subtends an angle of at least 180 at the center of curvature. This removes the structure 3 completely from the path of a great majority of the emitted electrons which therefore pass through the ring 3 and, because the ring structure 3 is relatively small, the electric field between it and the suppressor grid 4 is still suificiently radial to give good efiiciency of operation. In the event that a cathode geometry such as a razor edge having an approximately cylindrical emitting surface is used, a corresponding approximation to cylindrical symmetry would of course be desirable in the other electrodes also.

It may be desirable to make the field electrode 3 in the form of a hairpin filament of a suitable refractory metal such as tungsten wire having a small looped portion at its extremity surrounding the cathode tip. This allows the passage of heating current through it as an aid to evacuation of the tube, and forms an effective source of thermal electrons for the bombardment heating of the suppressor grid 4 and plate 5 which may be required during evacuation of the envelope.

Since the function of the secondary electron suppressor grid 4 is the same as in certain thermal tubes such as pentodes, most of the art applying to construction of suppressor grids in those tubes will be found applicable here also. In the device illustrated the suppressor l is a rather 'Widely spaced, basket-like structure, advantageously of tantalum wire, internal to, and roughly conforming to, the geometry of the collector 5.

. The secondary electron suppressor grid 4 is mounted upon a shielding plate '7 which improves the efliciency of said grid by preventing passage of secondary electrons to the support structure of the field electrode 3, and serves the second purpose of shielding the cathode from stray electric fields which may result from the charging of the insulated surfaces of the envelope. The member 7 is in fact a part of the suppressor grid 4- and is not in itself cs- 4 sential to the operation of the tube, although it is useful because it improves the efiiciency of the device.

It will be apparent to those versed in the art that the envelope 1 may be of any suitable material such as glass, ceramic material, or a suitably insulated metal shell, and that the various electrodes may be of any suitable metal such as, for example, tantalum, tungsten, molybdenum, or nickel.

Since the electric fields at the surfaces of the various electrodes other than the cathode will be quite large during certain portions of the alternating current cycle, care must be taken to avoid local increases in the field due to surface roughness and geometric irregularity. Such areas, if the electrical field were sufiiciently high, would act as field emitters thus reducing the efficiency of the device, and in all probability resulting in a vacuum arc (see Phys. Rev. 91, 1043 (1953)) which would be injurious to the electrodes involved. Particular care should be taken to avoid such irregularities as clipped ends on wires and splattered spot welds. As would be expected under these conditions, surface cleanliness is also of the utmost importance.

The cathode (or cathodes) may be of any material which may prove to have a favorable combination of physical characteristics such as melting point, effective work function, conductivity, tensile strength, etc. The cathode likewise may have any geometry which may be proven desirable by reason of stability, emitting area, etc., provided that the radius of curvature be kept sufficiently small that the required electric field can be obtained. Examples of such geometries might be an approximate cone with a hemispherical cap, a multiplicity of such cones, or a razor edge. Such geometries may be formed by several methods including, for example, electrolytic or chemical etching, or by mechanical honing processes. The cathode of the tube shown in Fig. 1 is a single conical cathode of tungsten.

The emitting surface of the cathode may be either clean or coated with any material which may be desirable as a means of enhancing the emission from the cathode, improving the stability of same, or in any other way improving the operational characteristics of the device. In short, the cathode may be any emitter of electrons where the electron emission is given approximately by Equation 2.

The envelope may also contain one of the chemical getters known in the art such as, for example, a barium source or tantalum filament to be used for improvement of the vacuum following sealing off the tube. It is further to be understood that the envelope should be baked out and the various electrodes should have been subjected to such treatment as heating or bombarding to degas them during evacuation of the envelope.

Fig. 1 is not to be construed to constitute a restriction on the electrode or envelope geometries, spatial relationships, etc., since several conceivable combinations of these would result in successful operations.

The mode of operation of the device will be apparent upon examination of Fig. 2, in which is disclosed a schematic representation of the rectifier tube in conjunction with the simplest form of power supply circuitry, which is included for illustration of the operational characteristics only, and is not intended to limit the scope of the invention, which is readily adaptable to the more complex circuitry of cascade multipliers, voltage doubler circuits, full wave rectifiers, etc.

An alternating or pulsating electrical potential is impressed on the primary side 9 of a transformer of the proper construction and turns ratio to induce an alternating or pulsating voltage of suitable magnitude across the secondary winding 8. One end of the transformer secondary Winding 8 is connected to the field emission cathode 2 and the other end is connected to the field electrode 3. During that portion of the cycle in which.

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tial With respect to the field electrode 3, electron emission will take place at the cathode 2 in accordance with Equation 2. The emitted electrons follow approximately radial trajectories away from the cathode and, with the exception of a small percentage (perhaps 5%), Penetrate the secondary electron suppressor grid 4 which is maintained at cathode potential, and impinge on the electron collector 5. These collected electrons induce a negative charge on the collector side of the filter condenser 10, the other side being electrically connected to the field electrode 3. No electron fiow will take place in the tube until that portion of the next cycle when the cathode again emits so that the charge on the filter condenser is retained unless reduced by some external load. During the period of emission of the next cycle, and succeeding cycles, more electrons will impinge on the collector 5, and thus add to the charge on the filter condenser. This action will continue until such time as the voltage drop across the filter condenser approaches the maximum voltage difference between the field electrode and cathode; that is to say, until the collector closely approaches cathode potential. When this condition is reached the electrons will no longer have sufficient kinetic energy to penetrate the secondary suppressor grid and impinge on the collector and so will be reflected and be collected by the field electrode 3. When the charge on the filter condenser is reduced by some external load (not shown) collection will again take place at the collector 5, thus tending to return the filter condenser to a fully charged condition. It will be apparent that either a positive or negative direct current electric potential is obtainable from this device depending upon whether the collector 5 or field electrode 3 is taken as a ground reference.

If the charge on the filter condenser 10 is sufficiently large, the cathode 2 will be positive relative to the collector 5 for an appreciable portion of the period of emission of the cathode in each alternating potential cycle. While this condition exists no collection will be possible at the collector 5, and so the electrons will be reflected and return to the field electrode 3; here they impinge with their maximum kinetic energy, thus being wasteful of power and causing unwanted heating. It is therefore desirable to hold emission to a minimum while this condition exists. This can be readily accomplished by placing a suitable resistance (resistor 11, Fig. 3) in the field electrode circuit. Undesirably high currents flowing in this circuit will produce a voltage drop across the resistor 11 as indicated by the polarity signs in Fig. 3. This potential drop tends to minimize the potential difierence between the cathode 2 and field electrode 3, which correspondingly reduces the electric field at the cathode and, in accordance with Equation 2, correspondingly reduces the current emitted from the cathode. The emitted current will in this Way be inhibited during electron collection by electrode 3, until such time as the cathode, due to variation of its potential relative to the field electrode and collector, becomes suificiently negative relative to the latter to improve collection efiiciency, that is, the percentage of emitted current collected at 5, thus reducing the fraction of the total current passing through the resistor 11, and allowing a corresponding increase in cathode field, hence emission.

Serious cathode damage can result if the field current density becomes excessive (see Phys. Rev. 91, 1043 (1953)). It is therefore desirable that the emitted current be restricted to a safe level. Due to the exponential dependence of current upon applied voltage of the field emitter, rather effective limitation of the emitted current can be obtained by placing suitable resistance in series with the cathode. It will be apparent, however, that inclusion of such a limiting resistor is otherwise undesirable in the present case since it would be both wasteful of power and tend to lower the obtainable outvoltage drop required for limitation of the field is developed over the resistor 11 in the field electrode circuit, both of the above mentioned ditficulties will be largely avoided. Since the voltage reference for the filter condenser 10 is taken at the junction 13, the output voltage will be unaffected by the voltage drop over the resistor 11, but cathode current will be controlled effectively. The drop over the resistor 11 is due to the passage of that fraction of the emitted current which is not collected at the plate 5. In the more favorable cases, namely the normal operation of the tube with collection of electrons primarily at 5, this fraction may be as small as 3% so that a considerable saving of power is realized over that required to achieve the same field limitation through use of a series cathode resistor alone.

The collection efiiciency for a given value of the potential on the collector 5 is reduced if the cathode becomes positive relative to the suppressor grid. Therefore, it may be desirable to place sufficient resistance (resistor 12-) in the cathode circuit to cause a reduction in collection efficiency and a corresponding increase in the fraction of the total current passed through the resistor 11, thus sharpening the cut-ofi characteristics of the circuit as the critical current is approached.

Limitation of the peak emitted current as outlined above allows the emission of currents near the maximum safe current over a considerably greater portion of an alternating current cycle than would otherwise be possible. This, of course, allows a substantially greater direct current power output and greater fluctuation of the input voltage than would otherwise be possible.

The maximum safe voltage that can be applied between the cathode and field electrode of the tube will be determined by the electric field at the cathode and resultant cathode current density. This limit will be strongly dependent upon the geometry and work function of the cathode, and to a lesser degree upon the geometry and spatial relationship of the various other electrodes. For any given combination of these, there then exists a limit to the maximum no load voltage which can be developed on the filter condenser of the circuit of Fig. 2. This limit is approximately equal to the aforementioned maximum safe applied voltage. Inclusion of the resistors 11 and 12 (Fig. 3) in the circuit will allow an appreciable increase in output voltage above the otherwise safe limit, because of the voltage drop over the resistor 11. It may in some instances be desirable to use the tube in connection with a device delivering direct current at a steady output voltage substantially higher (say by a factor of 2 or more) than the maximum safe voltage permitted between electrodes 2 and 3. This can be readily achieved by the inclusion of an additional source of alternating potential such as additional turns of the transformer secondary winding or the secondary winding of a second suitable transformer between terminal 13 and the filter condenser Fig. 3. The existing connection between these points is understood to be removed, and said winding to be so connected as to reinforce the alternating potential of the secondary winding 8. The only limit imposed on the peak output voltage obtainable in this manner is the peak inverse voltage that can be held off between the plate 5 and the secondary electron suppressor 4. This will be dependent on such things as length of the path along the insulated portions of the envelope, cleanliness and geometry of the electrodes, etc., as discussed more fully above.

The advantages of the invention'are:

The device will withstand high peak inverse voltages.

The device can be operated with a cold cathode thus reducing heat dissipation, power used, weight, and size, and eliminating the problem of providing a suitably insulated heating current source.

Because of the small emitting area involved (for example, of the order of 10- cm?) it is possible to achieve put voltage for a given applied voltage. If, however, the extreme miniaturization of the cathode structure thus reducing the mass involved and providing great mechanical stability. Miniaturization of the cathode structure also reduces to a minimum the energy required to heat the emitter. i

The invention provides a safe and reliable method for operating a field cathode in a rectification circuit. This is achieved by the direct application of the alternating potential between the cathode and field electrode of the tube, thus eliminating the dependence of the cathode field on the condition of loading of the device, which loading is connected to a third electrode, the collector.

By decelerating the field electrons and using their kinetic energy to charge a storage condenser, a much lower effective tube potential drop is achieved than would otherwise be possible. This reduces power loss in the tube and greatly increases the useful output voltage obtainable for a given applied input voltage.

The characteristics of the device are such that efficient control of the emitted current can be obtained by methods involving the collection of reflected electrons on the field electrode together with use of appropriate external circuitry. This allows a considerable increase in the efficiency of the device, by (a) inhibiting emission during portions of the alternating input potential cycle when collection efiiciency is poor, and (b) limiting the cathode current density to a safe level without serious reduction of output voltage and without so large a resistive power loss as would be required if limitation were obtained by use of a cathode resistor alone.

The electric field between the field electrode and suppressor grid is in a direction such that passage of positive ions, which may be formed at the collector, to the cathode is prevented.

It will be seen that in the device of the invention provision is made for application of an alternating or pulsating electric potential directly between the cathode and field electrode of a rectifier tube, utilizing a field emitter of electrons as cathode, said electrons being collected at a third electrode, the collector, with the external load connected between collector and field electrode. A useful feature of the device is that the cathode electric field and hence its emission current density are independent of the load impedance and/or charge on the output filter condenser. This provides protection of the cathode from dangerously high field current densities due to changes in load impedance.

A means is provided for obtaining a relatively low effective tube potential drop in a tube utilizing a field emitter as cathode, thus allowing an output voltage only slightly less than the peak applied voltage, if the tube is used in a rectifier circuit and materially reducing the power loss of such a tube and attendant electron bombardment energy at and heating of the various other electrodes.

Suitable external circuitry (the addition of resistors 11 and 12, Fig. 3) improves the operation of the device by reducing the cathode current during those portions of the alternating potential cycle when collection of electrons by the collector is inefficient or impossible, and making the cathode current self limiting and thus safeguarding the cathode. This is achieved with only a small reduction in the available output voltage as compared with the rather large voltage loss that would occur if limitation were achieved through use of a series resistor in the cathode circuit alone. There is also a considerably reduced resistive power loss by utilization of this system. Both of these actions are dependent on changes in the collection efficiency of the tube and are achieved by the reflected fraction of the current which returns to the field electrode.

, I claim:

1. A space discharge device including an evacuated envelope and an electrode assembly supported within said envelope comprising a field emission cathode, a collector electrode providing a collecting surface positioned to intercept the major portion of the emission from said cathode, said collecting surface being substantially equidistant from the electron emitting surface of the cathode over a major portion of the effective collecting area thereof, a field electrode adjacent to said cathode between said cathode and said collector electrode substantially equidistant from the electron emitting surface of the cathode and transparent to the major portion of the emis sion therefrom, and a secondary discharge suppressor electrode positioned adjacent to said collector electrode between said collector electrode and said field electrode transparent to the major portion of the emission from said cathode.

2. A space discharge device including an evacuated envelope and an electrode assembly supported within said envelope comprising a substantially hemispherical field emission cathode, a substantially hemispherical collector electrode spaced from said cathode in symmetrical relation to the electron emitting surface thereof and positioned and shaped to intercept the major portion of the emission therefrom, a ring-shaped field electrode adjacent to said cathode and between said cathode and said collector electrode shaped to be symmetrical to the electron emitting surface of said cathode and transparent to the major portion of the emission therefrom, the lumen of said ring-shaped member subtending an angle of at least at the center of curvature of said electron emitting surface, and a secondary discharge suppressor electrode positioned adjacent to said collector electrode between said collector electrode and said field electrode and shaped to be transparent to the major portion of the emission for the electron emitting surface of said cathode.

3. A space discharge device including an evacuated envelope and an electrode assembly supported within said envelope comprising a substantially hemispherical field emission cathode, a substantially hemispherical collector electrode spaced from said cathode in spherically symmetrical relation to the electron emitting surface thereof and positioned and shaped to intercept the major portion of the emission therefrom, a ring-shaped field electrode adjacent to said cathode and between said cathode and said collector electrode shaped to be axially symmetrical to the electron emitting surface of said cathode and transparent to the major portion of the emission therefrom, the lumen of said ring-shaped member subtending an angle of at least 180 at the center of curvature of said electron emitting surface, and a secondary discharge suppressor electrode positioned adjacent to said collector electrode between said collector electrode and said field electrode spherically symmetrical to the electron emitting surface of the said cathode and shaped to 0 be transparent to the major portion of the emission from the electron emitting surface of said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 1,559,714 Lilienfeld Nov. 3. 1925 1,738,960 Mutscheller Dec. 10, .929 2,121,615 Vatter June 21, 1938 2,122,269 Wagner June 28, I938 2,158,564 Meier May 16, 1939 2,217,448 Muller Oct. 8, 1940 2,223,040 Mahl Nov. 26, 1940 2,271,990 Ramberg Feb. 3, 1942 2,272,353 Ruska F b. 10, i942 2,276,861 Penney Mar. 17, 1942

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