US2065997A - Gaseous discharge tube cathode - Google Patents

Description

Dec. 29, 1936. D. v. EDWARDS ET AL 2,065,997

GASEOUS DISCHARGE TUBE CATHODE Filed June 22, 1934 Fig.

INVENTORS ATTORN EYS Y Patented Dec. 29, 1936 UNITED STATES GASEOUS DISCHARGE TUBE CATHODE Donald V. Edwards, Montclair, and Earl K. Smith, East Orange, N. J., assignors to Electrons, Inc., of Delaware, a corporation of Delaware Application June 22, .1934, Serial No. 731,860

11 Claims.

This invention relates to gaseous discharge devices, and is particularly applicable to such devices having thermionic cathodes of the heatshielded type. It is also useful in connection 5 with tubes employing gaseous pressures of a low order.

The art has experienced difficulty in the use of gaseous discharge devices, especially those employing pressures below one millimeter of mercury and down to about five-thousandths of a millimeter (.005 mm.) and those in which the shields and glass walls are subjected to ion bombardment. The starting voltage and are drop in such devices have been found to increase with use and if the voltage is increased to force starting the difficulty is aggravated until finally only a glow discharge occurs. This amounts to a complete failure to function inasmuch as a glow, discharge has high resistance and cannot carry load currents. Such failures occur even though the electrodes are apparently in good condition and the generally accepted explanation has been that the gas has disappeared; hence the difliculty has been called hardening.

We have found that this so-called hardening is not caused by gas disappearance but, on the contrary, is due to cathode conditions brought about by a lack of sufficient free space for ionization immediately adjacent the cathode emissive surface, or surfaces, whereby the same cannot be utilized effectively. When there is actual disappearance of gas it is a secondary effect.

The object of the invention is to avoid the difliculty above-mentioned. To this end We arrange the cathode according to the pressure and kind of ionizable medium in a gaseous discharge device so that there will be sufiicient free and unobstructed space adjacent the emissive surface to utilize its emission efiectively for the generation of ions.

Further and related objects of the invention are the provision of a cathode which will operate satisfactorily in low-pressure tubes, and the provision of a high-voltage gaseous rectifier having longer life than has heretofore been practical.

The invention will be described with reference to the accompanying drawing in which Fig. 1 shows, partly in section, a gaseous discharge tube employing the invention; and Fig. 2 shows a modified form of cathode.

In Fig. l of the drawing, l is an anode, and a control grid 2 may or may not be provided for timing the starting of the discharge. 3 is the cathode which, in the form shown, consists of a cylindr cal hollow body open at the top and having its inner surfaces treated to render them electron emissive. The cathode is indirectly heated to operating temperature by a. heater 4. Heating energy is conserved by a heat shield surrounding both the heater and cathode, and consisting of a number of nested, heat-reflecting cans 5 having perforated covers 1. The cathode 3, heater 4, covers 1, and cans 5 are connected together near the top of the cathode. The above described electrodes are contained in an enlelope 8 which is provided with the usual re-entrant stems and exterior caps or bases. The anode l is supported from the upper stem by lead-in wires sealed therein and connected to a terminal 9 in the upper base. Connection is made to the grid! through an additional lead-in wire which is connected at one end to a terminal I!) and at its other end to a cross bar ll (shown in section) this cross bar being secured to two of four gridsupport wires I4 which are fastened at their upper ends to a collar (not shown) around the upper stem. A similar collar (not shown) around the lower stem, and wires l5 support the outer can 5 and the other parts secured thereto. Lead-in wires l6, secured to the outer can, provide additional support and also provide a common, cathode-heater-shield connection to a ter minal I2 in the lower base. The lower end of heater 4 is welded to a disc 6 in such manner as to allow for expansion of the heater, the disc being connected by insulated lead-in wires II to terminal l3.

The distance (1 represents the free space for ionization adjacent the emissive surface of cathode 3; it is the distance an emitted electron may travel before it strikes the surface of a solid body, such as the opposite emissive surface. In the form of cathode shown in the drawing, d is the inside diameter of the cathode 3 and this distance is available to electrons emitted from any part of the cylindrical surface. An equal or greater distance is available to electrons emitted from the bottom surface of the cylinder. Sometimes a cathode is provided with vanes to increase the emissive area in a given space, or the cathode may have some other form or arrangement of its surfaces.

In such cases the distance contemplated by the present invention is that of the unobstructed space between oppositely disposed emissive surfaces, or it is the distance from an emissive surface, and perpendicular thereto, to another surface whether emissive or non-emissive, the latter including neutralizing and shielding surfaces which may or may not be connected to the cathode.

The art has endeavored to provide a maximum of emissive surface in a minimum of space in order to take advantage of the ability of electrons to travel in curved paths in an ionized medium. However, we have found that sufficient space should be provided for generating the ions and that there is a minimum value for the distance d in a given tube if the aforementioned difficulties are to be avoided. This minimum depends upon the kind of ionizable medium and its. pressure under operating conditions, and to some extent upon the cathode emissivity. It is preferable for practical reasons, such as conserving the heating energy and making the tube as small as possible consistent with other factors, to design the cathode for the minimum value of d plus a factor of safety, inasmuch as no advantage is gained by making d too large.

According to the invention we make the distance d not less than the sum of the distance an electron must travel to attain ionizing velocity plus the average distance an electron must travel after that to produce an ion. For brevity, we shall call the former the accelerating distance and the latter the mean-free-path for ionization. Their sum is a distance which allows the average electron to travel freely from the cathode surface to the first ionizing collision.

The accelerating distance varies with load and cathode activity and may be estimated mathematically. Its magnitude need not be known accurately as it is a small part of the above sum for low pressures and substantial load currents. The accelerating distance may also be defined as the perpendicular distance from an emissive surface to the outer boundary of its electronic space charge. Inasmuch asthe space charge is almost completely neutralized at full-load current, the boundary with such current is very close to the cathode surface. The space charge increases and its boundary recedes from the cathode as the current is decreased, from which it will be apparent that the accelerating distance approaches infinity as the current approaches zero. However, the currents which are small enough to make the accelerating distance large can usually be supported safely by a small portion of the cathode surface, such as the edge portion nearest the anode. In determining the accelerating distance for the type of tube shown in the drawing, it is sufficient to take the current at about 20% of full-load current. In some tubes a dark sheath is visible adjacent the cathode surface at low loads. If the emissive surface is visible through the tube envelope and if such a sheath appears, then all loads less than that at which the sheath becomes perceptible may be disregarded in determining the accelerating distance for that tube.

As an example of the length of accelerating distance which may be expected with a cathode of ordinary activity, let us assume a tube filled with argon at 0.1 millimeter pressure. The instantaneous peak or crest current in such a tube at full load would give an accelerating distance of about 0.01 centimeter, whereas at 10% load the accelerating distance would increase to 0.1 centimeter. It will be understood of course that, during each cycle of full-load current, the accelerating distance will vary from the above minimum of 0.01 centimeter to a maximum value corresponding to minimum current for the cycle. However, in the example given the accelerating above life tests.

distance will not exceed 0.1 centimeter for any load that would cause disintegration of the oathode.

The electron mean-free-path for ionization is the more important part of the distance d. After an electron has been accelerated to or above the velocity necessary for ionization in the p'articular medium, it usually travels a relatively large distance before it ionizes an atom, notwithstanding that it may collide with a number of atoms within this distance. Hence the mean-free-path for ionization should not be confusedqwith the mean-free-path of an electron at the same velocity. The latter is the distance such an electron travels before it collides with an atom, but, on the average, only one out of many such collisions generates an ion. In a sense the ratio of the mean-freespath of an electron to the meanfree-path for ionization is a measure of the efficiency of the emitted electrons in ionizing the gas. It is this low efficiency which requires sufficient unobstructed space immediately in front of the emissive surface to give the electrons ample opportunity to ionize atoms before they waste their energy by striking a solid body.

Some data as to the mean-free-paths for ionization have been given in publications but values for gas pressures greater than .05 mm. are'scarce. Values obtained by exterpolating such data vary considerably from the values which we have determined experimentally. This discrepancy is due to the increasing probability of cumulative ionization as the pressure increases. Our experiments with argon at 0.1 mm. pressure would indicate a mean-free-path for ionization of about 1.6 cm. for ionizing electrons of the velocity usually encountered in hot cathode discharges,

* whereas exterpolation of the published data would put the path at approximately 3.0 cm. for

argon at the same pressure. However, both val-.

ues are several times greater than the meanfree-path of an electron which, for argon under the same conditions, is about 0.16 cm.

The mean-free-paths above-mentioned should not be confused with the kinetic mean-free-path of the argon atom, which pathis also small relative to the mean-free-path for ionization at the same pressure. The kinetic mean-free-path is the average distance an atom moves due to thermal agitation before it collides with another atom.

The foregoing explanation of the distance d treats its two parts separately and is based on present electron theory. The sum or total value for d may be determined empirically by either of the following methods.

The first method consists in constructing a series of similar tubes having the same perpendicular distance d but with different mean-freepaths obtained by filling the respective tubes with the same gas at different pressures. These tubes are put on life test and the pressure found below which some of the tubes develop the effect heretofore referred to as hardening. The said distance d is then the minimum distance for that gas and pressure and it may be used for future designs, employing the same gas, pressure and cathode emissivity.

The second method is preferably employed after some experience has been gained with the It consists in operating a given tube on the pump at various gas pressures and plotting a curve of gas pressure versus arc drop. The are drop should be fairly constant with decreasing pressure down to a point at which it begins to increase rapidly. At this point the pressure is such that the distance d in the tube under test equals the mean-free-path for ionization plus the accelerating distance.

It is sometimes difficult to obtain definite and accurate results by the above methods unless the proper load current is employed, which in general should be relatively light, and some experience may be required in selecting the proper current. It should be large enough to shorten the life of the tube by hardening but not so large that an abundance of ions is generated by the small percentage of electrons which succeed in having ionizing collisions with atoms in less than the average distance. For this reason a tube which is operated at full-load current, or one in which the current wave has normal average value but a high peak value, may develop hardening less readily than if it were operated most of the time at about 30 to 50% of full-load current. The relative efi'ects of load currents vary in different tube constructions, and depend 'to agreat extent upon the amount of shielding and whether the distance d is only slightly below the minimum or very much smaller than it should be. In the latter case hardening may develop at any load greater than that which can be supported by the small percentage of cathode surface which is near the top and therefore substantially open to the tube atmosphere.

We believe that hardening is due fundamentally to a shortage of ions for the needs of the arc discharge and that the discharge current is uniformly distributed over the cathode surface so long as the free space meets the above-described minimum. If the distance d is less than this minimum for the particular gas and pressure used, there will be a shortage of ions under some load conditions. This causes the current to concentrate on the top part of the cathode and disintegrate it. The current then concentrates, if possible, on another part of the cathode with the same result, until the tube fails by, loss of emission instead of by loss of gas as has generally been supposed. With this type of failure the cathode is not physically destroyed as in the case of failure due to long life with load or due to overloads. On the contrary, a cathode which has failed due to hardening has a low emission which, if measured at a plate voltage below the ionization potential of the gas filling, will probably have the same value as that of a new tube, but the saturation emission of such a cathode is so low that it cannot carry load currents.

Apeculiar effect of making the distance 11 less than the minimum as above described, in a tube having a heat-shielded cathode, is a pronounced tendency to generate parasitic oscillations in arcdrop voltage at a frequency of approximately 10 kilocycles per second. This is probably due to the inability of the cathode to generate suflicient ions inside the shield whereupon ions are generated outside thereof and, due to the high mobility of ions in this region, they suddenly diffuse into the space within the shield thereby momentarily lowering the arc drop. When the electrical charges on these ions are dissipated there is another shortage and the process repeats itself at the frequency above-mentioned. The provision of a proper free space for ionization within the cathode shield avoids this difiiculty.

In the event directly-heated emissive filaments are used, such as where a number of spiral filaments are arranged with their axes parallel to one another about a circle inside a cylindrical heat shield, the effective cathode surface for a large range of load will be the same as if the emissive surface were a continuous cylindrical surface passing through the axr s of the spirals.

Such a cathode is illustrated in Fig. 2 which shows a horizontal section of a cylindrical heatshield 20 containing a number of spiral emissive filaments 2| disposed in a circle of diameter d with their axes parallel to each other and to the cylindrical surface of the shield. The filaments may have their upper ends connected to the shield and their lower ends connected to an insulated disc, such as 6 in Fig. 1. According to the invention the distance d which, as stated, is the diameter of the eiTective cathode surface, should be determined as described above. Under normal loads no ionization goes on between ad jacent spirals, and there may be other arrangements whereby part of the emissive surface is ineffective, hence the effective cathode surface will be smaller in such cases than the total emissive surface. However, the effective surface should be suificient for the operating requirements of the tube and should have suflicient free space for ionization adjacent thereto.

Where a directly heated filament has a material voltage drop along its length, it is possible to reduce the free distance somewhat provided the filament voltage is phased, relative to the anode voltage, so that the more distant end of the filament is negative, relative to the end nearer the anode, on the half-cycles when the anode is positive.

The invention is particularly advantageous in making high-voltage tubes, for it is known that the permissible anode voltage for most rectifiers increases as the pressure of the gas filling is reduced. The permissible anode voltage remains fairly constant with decreasing pressure down to a critical pressure below which it increases rapidly. This critical pressure can be raised by decreasing the physical size of the tube. However, former low-pressure tubes intended to take advantage of these relations have had very short life. We have found that this is due to a lack of a sufficient space for ionization which, according to our invention, may be made large enough so that the pressure at which the arc drop suddenly increases is below the critical pressure at which the permissible anode voltage suddenly increases.

The following is a specific example of the results thereby obtainable. A tube having only 1.2 cm. of free space may be filled with not less than 0.4 mm. of argon or roughly 0.2 mm. of xenon, and have a permissible plate voltage of volts per anode. By merely increasing the free distance for ionization to 2.8 cm. the same tube may be filled with 0.05 mm. of argon or 0.025 mm. of xenon and have the same current rating but seven times greater plate voltage rating. Thus with slightly more than two-fold increase in the cathode free space and without increasing the size of the tube, a seven-fold increase in permissible tube output is obtained.

From the foregoing it will be apparent that the invention is particularly applicable to tubes containing metal vapor, such as mercury vapor tubes. In such tubes the vapor pressure is not constant but varies over a wide range due to variations in load and ambient temperature. The cathode in such a tube should be designed to have a free distance for ionization, as above described, for the lowest ambient temperature and the smallest load for which the tube is designed. If the distance is large enough under these conditions it will be ample for higher temperatures and greater loads.

The-invention may be employed with other rare gases and vapors than those specifically mentioned and also with common gases if they are inert to the envelope and the electrodes.

We claim:

I 1. A discharge device comprising an envelope containing an ionizable medium, a hollow thermionic cathode having a fixed electron emissive surface therein, and another electrode outside the cathode, said cathode having an unobstructed space adjacent its effective emissive surface for a distance which allows the average emitted electron to move from said surface to its first ionizing collision in said medium without striking a solid body.

2. A discharge device including a gas filling, an anode, shields, and a thermionic cathode having a fixed electron emissive surface, said cathode having an unobstructed space in front of its effective emissive surface for a distance which bears the same ratio to 1.6 centimeters as the mean-free-path for ionization in said filling bears to the mean-free-path for ionization in argon at 0.1 millimeter of mercury pressure.-

3. A gaseous discharge tube having an anode, a thermionic cathode having a fixed electron emissive surface, and a gas or vapor filling at a lower pressure for the particular filling than the pressure which gives a mean-free-path for ionization in said filling corresponding to the meanfree-path in argon at 0.2 millimeter of mercury pressure, and a heat-shield cooperating with said cathode, the cathode having free perpendicular distances from substantially all of its effective emissive surface to other surfaces of said cathode and shield, said distances being greater than the average distance an electron must travel from the cathode to its point of "ionization for all loads down to that at which the cathode has a perceptible dark sheath.

4. In a discharge device containing an ionizable medium at low pressure, a hollow cathode structure having a fixed electron emissive surface and unobstructed space therewithin, measured .perpendicularly to substantially all of the effective emissive surface, sufficient to allow the average emitted electron free travel to its first ionizing collision in said medium at any load great enough to affect the cathode life, said cathode structure having a discharge opening which maintains the ionizable medium inside the hollow cathode at the said low pressure.

5. The combination in a gaseous discharge tube of an anode, a thermionic cathode having a fixed .electron emissive surface, a filling of gas or vapor at low pressure, and shields, said cathode having a space in front of substantially all of its effeccathode, and another electrode, said cathode having oppositely disposed fixed,'electron emissive surfaces spaced apart, over substantially their entire area, a distance at least as great as the sum of the electron accelerating distance for substantial load currents in said medium plus the electron mean-free-path for ionization therein, the space between said emissive surfaces having free communication with the remaining space in said envelope.

'7. A gaseous discharge tube comprising'an envelope, an ionizable medium at a pressure of about 1 millimeter of mercury or less and a pair of cooperating electrodes in said envelope, one electrode being a thermionic cathode having a fixed electron emissive surface and an unobstructed space, immediately adjacent its emissive surface, at least as great as the sum of the electron accelerating distance plus the mean-free-path for ionization in said medium, said unobstructed space having free communication with the remaining space in said envelope.

8. A gaseous discharge tube comprising an envelope, an ionizable medium and a pair of cooperating electrodes in said envelope, one electrode being a hollow thermionic cathode containing fixed electron emissive surfaces, the distances between said surfaces being fixed according to the kind and pressure of said medium so as to provide sufilcient free space for the majority of the emitted electrons to ionize said medium within the cathode, said cathode being open to said medium whereby the pressure within the cathode is not increased relative to the pressure outside thereof.

9. A discharge device comprising an envelope, an ionizable medium therein, and electrodes including a thermionic cathode having a fixed electron emissive surface, a heat-shield therefor and another electrode, said electrodes and shield being so disposed within said envelope relative to the effective emissive surface of the cathode that there is suflicient distance to permit the average electron to travel perpendicularly from said surface to its first ionizing collision with an atom of said medium before striking one of the said electrodes, the space between the cathode and said other electrode being unobstructed.

10. A discharge tube comprising an envelope containing argon at approximately .05 millimeter of mercury pressure, an anode, a thermionic cathode having fixed electron emissive surfaces and a free distance for ionization of about 2.8 centimeters between said emissive surfaces, and a heatshield surrounding said cathode, said tube having a permissible anode voltage of at least 750 volts.

11. A discharge tube comprising an envelope containing a rare gas, an anode, a thermionic cathode having fixed electron emissivesurfaces, and a heat-shield surrounding said cathode, the said emissive surfaces being spaced apart a distance greater than the electron mean-free-path for ionization in said gas, the pressure of said gas being above the pressure at which the arc drop in said tube increases rapidly and below the critical pressure at which the permissible anode voltage suddenly increases.

DONALD V. EDWARDS. EARL K. SMITH.

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