US2553184A - Electron discharge device - Google Patents

Description

May 15, 1951 F, E, GEHRKE 2,553,184

ELECTRON DISCHARGE DEVICE Filed Feb. l, 1947 INVENTOR.

Patented May 15, (1951 ELECTRON DISCHARGE DEVICE Forrest E. Gehrke, Kew Gardens, N. Y., assignor to Sylvania Electric Products, Inc., a corporation of Massachusetts Application February 1, 1947, Serial No. 725,770

11 Claims. l

This invention relates to electron discharge devices, and more particularly to grid-controlled gaseous electron discharge tubes such as those commonly referred to as thyratrons. An object is to improve the construction of electron discharge devices, especially of thyratrons, for more reliable and uniform performance. To this end the preferred embodiment of the invention includes an anode, a relatively long and slender cathode, and a so-called solid grid between anode and cathode, the grid having a conductive wall with a slot extending generally transverse to the length of the cathode. This construction assures uniform performance among the thyratrons assembled with like parts, even though the positioning of the slot in the solid grid may vary somewhat in its relationship to the cathode and to the anode.

The invention and further objects and features of novelty will .be better understood from the following detailed discussion of two embodiments thereof, in contrast to a prior art construction.

In the drawings:

Fig. 1 represents a fragmentary View of a prior art solid-grid thyratron, showing the grid and cathode in projection.

Fig. 2 is a fragmentary transverse section of the electrodes in that prior art thyratron.

Fig. 3 is a similar transverse section through the prior art thyratron, showing the elements imperfectly assembled in a manner to be expected with routine production methods.

Fig. 4 is an elevation similar to Fig. 1 showing the solid grid and cathode of a thyratron improved according to the present invention.

Fig. 5 is a transverse section through this embodiment of the improved thyratron.

Fig. 6 is an elevation in detail of another embodiment of the thyratron improved according to the present invention, the envelope being shown in section.

Fig. '7 is a transverse sectional view along the line I-'I in Fig. 6.

In the prior art it has become customary to use a so-called solid grid in thyratrons to control the discharge of electrons from cathode to anode. The helical wire or cage grids which are characteristic of amplifier-type vacuum tubes are inadequate to provide the desired control so that a conductive wall, breached only to provide a discharge path of limited cross-section, functions as the control grid. In Figs. 1 to 3 a solid grid I0 is shown in its relation to a llamentary cathode l2 and (Fig. 2) an anode I4. The aperture in the solid grid is shown centered about the line of centers from cathode and anode. Consequently the electrostatic field set up by control grid IE! (dashes representing equipotential lines) is symmetrical. Ordinarily the control grid Ill is operated at a potential made negative with respect to cathode I2 while anode i4 is considerably positive with respect to the cathode. Under these conditions the long and narrow aperture in the solid grid is effectively much narrower than it is physically, since the static field which the negative charge sets up reduces the gap through which the static lines of force between cathode and anode may act. When the potential on the solid grid is sufficiently negative, the aperture is in effect closed. When the negative potential is sufficiently reduced the effective slot may open slightly, permitting gaseous conduction between cathode and anode. In order that the thyratron of Fig. 2 may be controlled by a relatively low negative potential an additional negative grid in the form of negatively charged rods I6 is sometimes used.

It has been found that when tubes of this character are assembled with all reasonable care that there is a non-uniformity in performance; that, while it may be desired that the thyratrons Ishould fire when the negative potential of solid grid I0 is between 2.1 and 2.7 volts minus, one thyratron may iire at minus 3 volts whereas another of the same type may fire only when the negative potential on solid grid I0 is reduced to minus 1.2 volts.

Referring to Fig. 3 one explanation of this random performance is given. It is assumed in Fig. 3 that the relative spacing of the various elements is the same as in Fig. 2 and that their dimensions are also the same. It is of course difficult to maintain the aperture in numerous solid grids IB to absolute uniformity, and slight variations in ring potential among several thyratrons may be expected because of variations in the width of this aperture. The length of the aperture is not critical. In Fig. 3 the aperture in solid grid I0 is shifted laterally with respect to the line of symmetry of the other elements. The result of this difference from Fig. 2, even though it is caused solely by an improper positioning of the solid grid to the extent of half of its very narrow aperture width, is not only to increase the distance between cathode I2 and the center of the solid-grid aperture through which the discharge must commence, but also to upset the symmetrical electrostatic iield pattern that should be set up by the charge on solid grid 3 l and by negative grid I6. It is believed that these factors are responsible for the serious variations in firing voltage among prior-art tubes of a given type.

The result would be the same were solid grid I 0 assembled along the line of symmetry, with cathode l2 laterally displaced by an amount equal to so little as one half the width of aperture in the solid grid.A This prior-art aperture has been held to the width specified, within a tolerance of 0.0005 inch, in an effort to achieve the performance specified. Inaccuracies in the positioning of negative grid I6 are relatively unimportant, it is believed, because of the relatively slight control exerted thereby. The firing of thyratrons having two negative grids is usually relatively independent of the negative voltage on the wide-apertured second grid near the anode. The variable potential or control potential is applied to the first grid which is solid in this instance. Throughout this specication, where the terms rst grid or second grid are used, the terms refer to the order of the grids in the direction from the cathode to anode, as is usual practice in the industry.

From Fig. 3 it will be apparent that the positioning of a long and slender cathode in relation to a long and slender but generally parallel aperture in a solid grid is critical in that the positioning of both of these elements in accurate parallelism along the line of symmetry of the second grid and the anode results in a symmetrical field pattern whereas asymmetrical positioning produces a very different eld. Moreover., the effective `distance between the cathode and the aperture is considerably changed when one of these is shifted laterally from the line of symmetry. The length of electron path is believed to be a critical factor affecting the ring voltage of the solid grid.

The present invention aims at reducing the precision required in positioning the solid grid laterally of the electron path, at enlarging the allowable variating in aperture-width for reasonably uniform ring voltage, and at more uniformly determining the firing voltage of the thyratrons into which the grids are incorporated. In Figs. 4 and 5 there is shown diagrammatically, a solid grid i8, that is, a grid member having a wall portion, such as of conductive sheet material, and having apertures in the wall portion transverse (in projection) to cathode 20. In this construction, just as in the solid grid of Fig. 1, the width (rather than the length) of the individual aperture controls the firing voltage. While three apertures are shown, they are spaced `sumciently so that there is no important interaction among the elds of the three, and when the thyratron actually does re the resultant conduction is entirely through one of the apertures, the other two having no effect. When the emissivity of the cathode opposite one of the apertures decreases, another of the apertures becomes effective. The use of multiple apertures thus promotes long useful life of the thyratron without undesirably affecting its performance.

A salient distinction of the solid grid of Figs. 4 and 5 over that in Figs. 1 to 3, resides in the fact that the aperture or each of the apertures is not parallel to the long and slender cathode. Because of this construction and arrangement it now becomes relatively unimportant whether the cathode is positioned perfectly opposite the center line of the aperture or not.v Because of the very low internal resistance-drop there .is

low anode dissipation, and it is feasible to make the anode of small dimensions. I have found that a rod will serve admirably, both from a functional standpoint and for its adaptability to manufacturing practices, for small thyratrons of relatively low current ratings. In Fig. 5 rod 22 serves as the anode. A pair of connected rods 24 serve as the second grid, as in the thyrat'ron of Figs. 2 and 3. The' length of the apertures in solid grid I8 is of the same order of magnitude as the separation between rods 24, and this is made great enough to permit a proper breakdown current, Without increasing the width of slot in the solid grid.

In Figs. 6 and 7 there is shown a second form of thyratron embodying the principle of Figs. 4 and 5 in that the solid grid has a long and narrow slot o't' parallel to the 1ong dimension of cathode 28, which is coated with electron emissive material at its center only as indicated at 30. Solid grid 26 is supported onthree rods two of which 32' are sealed into the glass envelope 34 while the thirdv supporting rod 3B for solid grid 26 is welded to a conductor 31 which extends through envelope 34. The second grid 38, a desirable though not essential element of the thyratron, is in the form of a U-shaped rod Welded to a lead 40l extending through the glass envelope 34 parallel. to lead 31 of the firstgrid. Anode rod 42 is welded to its lead 44 which is similarly sealed through enveope 34. Cathode l28 is connected to .lead 46 (Which is sealed through envelope 34) by a tab 48. To avoid confusion two more leads have been omitted from Fig. 6 which are normally provided to energize a lamentary' heater (not shown) within tubular cathode 28. By limiting the emissive coating `3i! to the short length shown, it becomes possible to economize greatly in the amount of power required for heating the cathode. And because of the relatively great area of the indirectly hea-ted cathode in comparison te the lilamentary cathode, only one aperture for the firing current is regarded ample. I

rlfhe envelope 34 of the 4tube in- Figs. 6 and 7 is, as is usual, gas-filled. Cathode 28, anode 42, and the two rods of the second grid 38 all project through top mica 50 and bottom mica 52 which hold them in correct mutual spacing. Rods 32 and 3,6 of solid grid 26 also project through a top and bottom mica to fix that grid in proper relation to the otherelectrodes. yConductors 31, 40, 44, and 46 as well .as the dummy rods 56 which supportv rods 32 .are all lap welded to the several electrodes and butt against the lower surface of bottom mica 52,- thereby preventing that bottom mica from Ashifting axially of the thyratron. The top mica is vheld in place lby staking the rods above that mica, and by welding getterstrip 54 to rods 32` and in lateral contact withr the top `surface of mica 50.

The thyratron illustrated in Figs. 6 and '7 has flexible 'external conductors intended for direct wiring into the circuit in which the thyratron is to be used. The external diameter of the glass envelope is approximately 3/8 of an inch. The width of the slot in the solid grid is approximately one millimeter -to -.00l inch tolerance, uand with standard xed voltages on the anode and the second grid, the first soli-d `grid will control firing of a number 'of thyratrons to within approximately w0.37 `volt above or below minus 2.6 volts. This latitude is due largely to variations in emission, .gas pressure and processing. The thyratron current is about l'25 ma. VWere .it not for the transverse arrangement of the slot in the solid grid such uniform performance could not be realized. With a given standard of precision the construction described considerably improves the thyratrons forrperformance in accordance with their specifications.

The use of multiple slots in the solid grid, made possible by the skew or transverse slot arrangement, is an additional feature enhancing the utility of this invention. Furthermore, the satisfactory performance of a rod-shaped anode substantially parallel to the cathode is in large measure attributable to this form of solid grid. With a rod-like anode, there is merit in the transversely slotted grid even were the cathode relatively broad.

While a speoic construction of thyratron has been discussed in detail, and certain of its features have been especially emphasized, it will be recognized that the invention possesses other features of utility, and is subject to other modifications than those shown. Thus, while it is preferred to use the second grid, it might be considered satisfactory to omit the second grid and rely solely on the single solid grid for control, and for determining the static voltage at which the tube would i-lre. Also, while the aperture of the solid grid appears in projection perpendicular to the cathode, a substantial angle although less than 90 would also serve the purpose. Therefore the specic disclosure should be considered merely illustrative, and not by way of limitation.

What is claimed is:

1. An electron discharge device having, within an envelope containing an ionizable gas, a relatively long and slender cathode, and anode, and a solid grid between said cathode and said anode, said solid grid having a relatively long and narrow aperture having its length arranged at an angle to said cathode.

2. A thyratron having, within an envelope containing an ionizable gas, a relatively long and narrow cathode, and anode, a solid grid positioned between said cathode and anode, said solid grid having an aperture skew to said cathode.

3. An electron discharge device including, within an envelope containing an ionizable gas, a straight lamentary cathode, a rod-like anode parallel to said cathode and a sheet metal grid interposed between cathode and anode, said anode and cathode being parallel to each other, said grid being provided with a long and narrow aperture substantially transverse to the plane of said cathode and anode.

4. In a thyratron having, within an envelope containing an ionizable gas, a relatively long and slender anode and a relatively long and slender cathode lying in a common plane with said anode, a grid between said cathode and said anode in the form of a conductive wall having an aperture which is long in the direction perpendicular to the plane of said cathode and said anode, and which is narrow in its other dimension.

5. A thyratron having, within an envelope containing an ionizable gas, a long and slender cathode, a long and slender anode parallel to said cathode, a conductive-wall grid between said cathode and said anode having plural rectangular apertures narrow in the direction of the plane common to said cathode and said anode, and widely spaced apart in comparison to their width, said apertures being long in the direction transverse to said plane all of said apertures intersecting said common plane.

6. A thyratron according to claim 5 including a second control grid having a long and wide aperture the length of which is approximately coextensive with the eiective length of said cathode and said anode, and which is positioned between said rst-mentioned grid and said anode.

7. An electron discharge device having, within an envelope containing an ionizable gas, a long and slender cathode, a grid of conductive sheet material effectively constituting an electrostatic shield around said cathode, and a rod-like anode external of said solid grid, said grid having an aperture at a substantial angle to said cathode.

8. An electron discharge device having, within an envelope' containing an ionizable gas, a long and slender cathode, a grid of conductive sheet material effectively constituting an electrostatic shield around said cathode, a rod-like anode external of said solid grid, said grid having an aperture at a substantial angle to said cathode, and a second grid in the form of a U, the sides of which are substantially parallel to said cathode and which are spaced apart approximately the length of the apertures in said rst-mentioned grid.

9. A thyratron having, within an envelope containing an ionizable gas, a cathode, a long narrow rod like anode parallel thereto, and a solid grid for effectively separating said anode and said cathode and provided with a number of apertures at a substantial angle to said anode.

10. A thyratron having, within an envelope containing an ionizable gas, an electron emitting element, an electron receiving element, said elements being relatively long and narrow and lying in a common plane, a solid grid for isolating said elements electrostatically during non-conducting intervals, said grid having a long and narrow aperture skew to said long and narrow element and intersecting said common plane to provide a current path at other intervals.

11. A thyratron according to claim 10 wherein said aperture is close to said emitting element.

FORREST E. GEHRKE.

REFERENCES CITED The following references are of record in the lle of this patent:

UNITED STATES PATENTS Number Name Date 2,080,235 Smith May 11, 1937 2,201,880 Bruce May 21, 1940 2,296,324 Bahls Sept. 22, 1942 2,391,967 Hecht et al Jan. 1, 1946 2,404,920 Overbeek July 30', 1946 FOREIGN PATENTS Number Country Date 475,106 Great Britain Nov. 12, 1937

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