US3072865A - Gaseous discharge device - Google Patents

Description

Jan. 8, 1963 L. w. ROBERTS 3,072,865

GASEOUS DISCHARGE DEVICE ATTORNEYS Jan. 8, 1963 l.. w. ROBERTS 3,072,865

GAsEous DISCHARGE DEVICE Filed oct. 11, 1955 2 sheets-sheet 2 Flg. 5 IOY t7 OLTAGE DISTANCE ACROSS GUIDE DISTANCE ACROSS GUIDE ("4) Fig, 7

VII

A INVENTOR. LOUIS W. ROBERTS ATTOR N EYS ilited 3,072,865 GASEUS DISCHARGE DEVECE Louis W. Roberts, Roxbury, Mass., assignor to Microwave Associates, inc., Boston, Mass., a corporation of Massachusetts Filed Get. 11, 1955, Ser. No. 539,785 S Claims. (Cl. S33- 13) Subject invention relates to a gaseous discharge device and iu particular to a gaseous discharge tube having very wide band width at high frequencies.

Switching tubes of the TR, ATR and attenuator types, hereinafter referred to as TR tubes, are constructed for the most part in the form of sealed wave guide sections containing one or more resonant structures. In the case Where two or more resonant structures are utilized they are coupled together by appropriate spacing along the wave guide section. The use of resonant structures of this type results in an inherent limitation in the band width which the tube will encompass since the resonant structure will be tuned to a specified center frequency. In addition, the tubes of this type have been limited in band width to some extent by the characteristics of the Window utilized to seal the ionizable gas into the TR cavity.

It is the object of this invention to provide an improved TR tube having a very wide band width and capable of handling large amounts of microwave energy. It is a further object of this invention to provide a TR tube having an improved keep-alive electrode structure.

lt is a feature of this invention that it achieves Wide band response characteristics through the use of a capacity loaded wave guide which produces the high potential gradient necessary for ionization breakdown without the necessity for resonant iris structures.

It is a further feature of this invention that it incorporates a keep-alive electrode structure which provides an ionization path of substantially constant and unvarying length together with an abrasion resistant electrode tip permitting long life.

The subject invention will be more easily understood by reference to the drawings in which: i

FIG. l represents a plan View of the TR tube, 5

FIG. 2 represents a cross-sectional elevation along the tube axis,

FiG. 3 is an end view of the tube,

HG. 4 illustrates an enlarged detail view of the keepalive electrode fitted to its cavity,

FIG. 5 is a plan view of one-half of the guide showing an alternative ridge conguration,

FIG. 6 is a plot of the voltage across a wave-guide sec- 50 tion propagating the TEN mode, and

FIG. 7 is a plot showing the TR tube of this invention having opposed ridges providing a narrow gap. F

As illustrated by reference to FlGS. 12 and 3, this invention contemplates the utilization of a wave guide section lit having a rectangular cross-section and having flanges 12 at either end to facilitate attachment of the TR switch to other Wave guide sections. Two pyramidalshaped ridges ld and 16 project from the top and bottom surfaces of the rectangular wave guide section. Each of these fins is composed of three parts, namely, a knife edge center ridge and two tapered ramp-like transition sections. The two central longitudinally disposed knife edged ridges 18 are located adjacent the axis of the tube. These knife edges, in opposed relationship on the top and bottom faces of the tube, provide a narrow gap 2li across the axis of the tube. A high potential gradient will exist across this gap because of the difference in potential between the upper and lower wave-guide surfaces, and it is this potential gradient which will, upon receipt of a high power pulse by the tube, cause the ionizing breakdown of the Patented Jan. 8, 1953 gas in the tube thereby effectively short-circuiting the transmission of microwave energy.

On each end of the knife edged portion of each ridge there extends a tapering impedance matching ramp-like section 19 extending from the window at either end of the tube tothe knife edged ridge.

The ends of the wave guide section 10 are sealed by the use of a wide band window 24 preferably in the form of multiple iris windows having three openings of substantially the same length, 26, 28 and 30 in FIG. 3, the central opening being substantially wider than the two outer openings. Multiple iris windows for wide band performance have been described in the literature by Reingold, Carter and Garoif of the Signal Corps Engineering Laboratories in a report entitled Single and Multi- Iris Resonant Structures, published in the proceedings of the IRE. However, the multiple iris windows therein described were of uniform Width. The window utilized in this TR tube represents a substantial improvement in uniformity of performance over a wide band width. For example in the 2 centimeter to 3 centimeter wavelength region, when the middle slot is widened to .148 inch (by .7 inch long) and the two outer slots are maintained at .094 inch, the insertion loss of these windows is approximately .03 db over a band width of 4,000 rnc/s. centered at 10,000 mc./s. It is obvious that a wideband window is essential to a successful tube since the band width of the system will be limited to that of the poorest link.

As in any TR tube, it is desirable to provide a keepalive electrode structure which will furnish a residual supply of electrons adjacent the discharge gap. In the structure of this invention two such electrodes 32 and 34 are provided along the length of the ridge 1S. Since the entire pyramidal ridge is a very wide band device which does not use resonant structures, the spacing of the electrodes is not restricted to any particular portion of a center frequency wave length. Alternative construction but one which is expensive and unnecessary for satisfactory operation of the tube is to provide a constant keep-alive discharge along the length of the gap by means of a longitudinal keep-alive gap adjacent the ridge instead of spaced circular gaps. In the construction shown, utilizing spaced conventional keep-alive electrodes, one or the other of the keep-alives will be predominately effective depending upon the frequency. However, both electrodes are connected at all times in order to maintain effective keepalive action over the entire range of frequencies utilized by the tube.

As illustrated by reference to the enlarged detail view in FIG. 4, the keep-alive electrode is composed of a conductive rod 36 surrounded by an insulating dielectric 33 fitted to a conical chamber 40 in the tube section l0, said chamber having an outer metallic wall 42. lt has been found that greatly improved performance, both in this TR tube and in TR tubes of conventional design, may be obtained by inserting adjacent the conical wall 42 a preformed insulating cone 44 of dielectric material. This dielectric insert has the effect of restricting the ionizing gap to a single path length running from the tip 46 of the rod 36 to the bottom edge 4S of the conical wall 42. This end is adjacent the gap 20 in the TR tube structure. The parts of the electrode are sealed to a metal sleeve 5d by means of a glass seal 52 and the sleeve in turn is soldered to the body l0.

A tip 46 of the electrode rod 36 is formed of a separate hardened pellet fused to the end of the electrode 36. This tip structure may be of stainless steel or of titanium dioxide reduced in hydrogen to give a content of 10% metallic titanium. In either event, the function of this hardened tip is to resist ion bombardment which in the case of the ordinary keep-alive electrode tends to wear the electrode away and change the over-all gap dimensions as well as the uniformity of the gap.

In the conventional TR tubes, a variety of paths are presented between the electrode tip 46 and the wall 42. The variation in discharge path has resulted in non-uniform tube action. Furthermore, insulating the inner surface of the wall 42 by means of a conventional brakedon insulating sheath is rendered ditiicult or impossible by the tendency for any such ceramic coating to withdraw from the end 48 of the wall 42 or to be perforated by the pin holes common to baked ceramic insulation. The insert 44 is preformed to extend over the entire inner surface or the cone thereby achieving greatly improved uniformity of keep-alive action.

For rectangular wave guides having a width to height ratio of 2.25 to l, it has been experimentally determined that the broad dimension of the guide should closely approximate 1.8 times the Width of the ridge base in the central knife edge section. It the ridge base width does not approximate .56 times the guide width severe spurious mode problems are created. If the ridge is too wide there is a tendency to create unwanted modes such as the T1521, T1343, and TE65 modes. If the ridge is too narrow the mode problems are accompanied by excessive leakage power.

In the preferred and illustrated embodiment, the width of the ridge base is uniform froml window to window. However, some furtherreduction in leakage power is possible at the expense of greatly increased problems of fabrication by increasing the width of the ramp bases from .56 times the wave guide width at the central knife edge section to the full width of the guide adjacent the windows. A plan view of this ridge configuration is shown in FIG. 5. The elevation view of such a ridge would be the same as that illustrated by reference to FIG. 2. The reference numbers are identical to those in FIG. l for like parts except that they are primed.

The preferred embodiment incorporating a ridge of uniform width at the base covers a range of from 2.4 to 3.6 centimeters and exhibits standing wave ratios not in excess of 1.1 over the entire range. The tube incorporates a .006 inch gap between knife-edge ridges .4 inch long. The tapered ridge sections are 1.3 inches long giving a compact overall dimension of 3 inches. The tube is capable of handling power levels of up to 100 kilowatts peak over this range without excessive leakage.

The ridge coniguration is in fact capable of satisfactory operation over several times the above band width up to approximately 25 centimeters. However, the length of the ridge is not critical with respect to a specific center wavelength since the tube operates over a wide band width. The lengths of the waves in the region of the ridge in fact differ markedly from free space or open wave guide propagation and these wave lengths become, in fact, irrelevant with regard to ridge dimensions. It has been experimentally discovered, however, that over the 8000 megacycles to 12,40() megacycles range the ridge need not be longer than the above .4 inch and that the standing wave ratios tend to increase if the ridge is as long as 2 inches. Because of the ridge distortions this .4 inch length is of the order of 1A wave length in the band width of interest and in any event, it may be said that the ridge length is preferably less than the shortest wavelength of interest.

The effect of the ridge configuration on the tube operation may be shown by reference to FIGS. 6 and 7. In FIG. 6, the voltage intensity across an ordinary rectangular guide propagating the TEN mode is shown. It may be shown that this intensity is a function of the cosine of the distance across the guide. The effect of the opposed ridges is to concentrate the field intensity in the region of the ridges, and the resulting plot of field intensity is shown schematically in FIG. 7. The intensity may be shown to be a function of the cosine to some power n where n is greater than 2. The intensity of the electric field in the region of the gap is therefore very high and is in fact more than sufficient to produce an ionizing breakdown upon receipt of a high power pulse.

While this invention has been described with respect to a single embodiment, it will be apparent that the same design principles may be applied at other frequencies, and that the invention may be modified in a variety of ways without departing from the scope of this invention which is encompassed in the following claims.

I claim:

1. A wide band gaseous discharge device comprising a wave guide section containing a pair of pyramid shaped ridges having sharp edges defining a narrow gap along the guide axis, a plurality of ionizing electrodes mounted in said section adjacent the gap, and two triple iris windows one at each end of the section enclosing the ridges, the iris openings being of equal length, the outer irises being of decreasing width.

2. A gaseous discharge device comprising a wave guide section having a cross-sectional width to height ratio of approximately 2 to l, a wide band window to seal each end of the wave guide section, two indentical pyramidal ridges raised in opposed relationship from the broad sides of the section and extending substantially the length of the section, said ridges having a base width which is uniormly of the order of one half the wave guide width, said ridges having a central sharp edged section to provide a narrow gap across the guide axis, said gap having a length many times the ridges displacement, said ridges decreasing in height from the central section to the windows to provide a transition ramp, and electrode means adjacent the gap.

3. A gaseous discharge device comprising a rectangular wave guide section having two broad and two narrow walls, a window to seal end of the waveguide section, a central ridge section in each broad wall, each ridge having a pyramidal cross-section providing a narrow edge adjacent a substantial portion of the guide axis, the base width of each pyramid being substantially less than the guide width, whereby the two ridges are displaced by a narrow gap, and ramp-like transition ridges tapering downward in height and increasing in width from the ends of the ridges to the wave guide walls adjacent the window.

4. A gaseous discharge device comprising a waveguide section having two broad and two narrow sides defining a rectangular waveguide passage, windows to seal the ends of said sections, each window having at least one sealed iris opening on the guide axis of a length less than the guide width and parallel to the broad sides of the section, first and second ramp-like tapering transition elements projecting toward each other respectively from each of said broad sides, the width of the base of each of said elements being in excess of one-half the width of the broad side from which the element projects, each of said transition elements having two longitudinally directed sides sloping toward each other from the longitudinal sides of its base to form a ridge, the ridges of said elements confronting each other to provide a longitudinally extended narrow gap in the region of the guide axis, each of said transition elements having two ramp surfaces each extending from one of the transverse sides of its base to the ridge thereof and tapering from said transverse side essentially to a point at said ridge, and keep-alive electrode means adiacent said gap.

5. Device according to claim 4 in which said waveguide has a width to height ratio of substantially 2 to 1 and the base width of each of siad transition elements at least in the intermediate ridge portion is approximately 0.56 times the guide width.

6. Device according to claim 4 in which said gap has a length in the direction of the guide axis which is several times the distance between said confronting ridges.

7. Device according to claim 4 in which the length of the base of each of said transition elements is substantial- 1y equal to the length of said waveguide section.

8. Device according to claim 7 in which the width of the base of each of said transition elements is nonuniform in the axial direction, said width of each base being substantially the same as the width of said waveguide at the ends thereof, and tapering gradually to a value in excess of one-half the guide width in a region substantially lo equal in length to the ridge of said transition element.

References Cited in the file of this patent UNITED STATES PATENTS Noel Ian. 18, Stavro Mar. 10, Booth June 1, Roberts Dec. 21, Heins Feb. 7, Varnerin May 29,

Claims (1)

1. A WIDE BAND GASEOUS DISCHARGE DEVICE COMPRISING A WAVE GUIDE SECTION CONTAINING A PAIR OF PYRAMID SHAPED RIDGES HAVING SHARP EDGES DEFINING A NARROW GAP ALONG THE GUIDE AXIS, A PLURALITY OF IONIZING ELECTRODES MOUNTED IN SAID SECTION ADJACENT THE GAP, AND TWO TRIPLE IRIS WINDOWS ONE AT EACH END OF THE SECTION ENCLOSING THE

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