US3312853A - Flash tube construction - Google Patents

Description

April 4, 1967 MELA 3,312,853

FLASH TUBE CONSTRUCTION Filed Dec. 1, 1964 INVENTOR. RICHARD L. MELA BY w wh ATTORN EYS United States Patent Office 3,312,853 FLASH TUBE CONSTRUCTION Richard L. Mela, Arlington, Mass., assignor to Dynatech Corporation, Cambridge, Mass. Filed Dec. 1, 1964, Ser. No. 415,143 4 Claims. (Cl. 313-218) This invention relates to a novel flash tube. More particularly it relates to a flash tube of the type used to generate light by passing an electric current through a gaseous medium disposed in the region between a pair of electrodes. The electrodes are made of pyrolytic graphite, which exhibits characteristics providing an improved life span for the tube.

Generally speaking a flash tube of the type to which the invention is directed comprises a quartz tube with electrodes disposed at opposite ends thereof. The tube is filled with a rarefied gas. The gas is initially ionized by any one of several systems, for example, radiation by a high frequency signal, and the resulting low electrical resistance between the electrodes permits the passage of a large current through the tube to generate a flash of light. The life of a flash tube is adversely affected by a number of factors. Among these is electrode deterioration. The electrodes are subject to intensive electron and ion bombardment as a result of the passage of current through the tube. This, together with the heat generated in the gaseous plasma, substantially elevates the electrode temperatures. In fact, levels of several thousand degrees Kelvin may be reached in the tungsten electrodes conventionally used. At such temperatures further electrode bombardment causes displacement of significant amounts of material from the surfaces of the electrodes. This sputtering action results in the deposition of the material on the inner surfaces of the quartz tube. In turn the coating of these surfaces reduces the transmission of light from the tube.

Moreover, it appears that the larger droplets of electrode material deposited on the quartz surfaces concentrate thermal or mechanical stresses, thereby hastening the ultimate cracking of the tube.

The deterioration of the electrode surfaces resulting from removal of material therefrom may also have deleterious effects on the operation of the flash tube, depending on the particular electric field configuration and current distribution desired in the vicinity of the electrodes. In cases where these factors are largely dependent on electrode surface shapes, operation can be materially degraded by surface deterioration.

Accordingly, it is a principal object of the invention to provide a flash tube characterized by an improved life span.

A more specific object of the invention is to alleviate the life-shortening factors associated with electrode deterioration in prior flash tubes.

Yet another object of the invention is to provide a flash tube characterized by reduced rejection of material from electrodes, particularly in the form of relatively large droplets.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplifiedin the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing, which is a longitudinal section of a flash tube embodying the invention.

The invention makes use of pyrolytic graphite as elec- 3,312,853 Patented Apr. 4, 1967 trode material. This material has two properties which are materially better than those of the conventionally used tungsten and thereby significantly increase the life of the flash tube. These properties are the higher melting (or sublimation) point and the high thermal conductivity along the crystalline axis exhibited by pyrolytic graphite. The first of these properties provides a reduction in the amount of material lost from the electrodes during operation of the flash tube.

The higher thermal conductivity, an order of magnitude greater than that of tungsten in the 3,000 K. range, results in better transfer of heat from the surface of the electrode to the exterior of the flash tube where it is readily dissipated. Thus, there is a smaller thermal gradient along the electrode and a correspondingly lower temperature on the surface thereof at which the electron and ion bombardment takes place. The lower temperature combines with the higher melting point to provide a marked reduction in the amount of material removed from the electrode surface. This in turn has been found to increase significantly the operating life of the flash tube.

As shown in the drawing, a flash tube embodying the invention includes a quartz tube 10 having end caps 14 and 16 suitably sealed thereto. Extending through the cap 14 is an electrode assembly generally indicated at 18. The assembly 18 includes a metallic bar 20 on the inner end of which is an electrode 22. If desired, the bar 20 may be surrounded by a glass liner 24, aflixed to both the bar 20 and the cap 14 so as to help prevent leakage between the exterior and interior of the tube 10 along the assembly 18.

The electrode 22 is made of pyrolyticgraphite, with the axis thereof parallel to the axis of the tube 10, i.e. extending between the tip 22a of the electrode and the bar 20. The electrode 22 is fastened to the bar 20 by any suitable technique providing a low thermal impedance at the juncture of the two parts, for example by silver soldering. The bar 20 is of a high conductivity material such as copper which readily conducts heat from the electrode 22 to the exterior of the tube 10. It is preferably oxygen-free copper in order to minimize contamination of the interior of the flash tube. Alternatively, the bar 20 may be an integral extension of the pyrolytic graphite electrode 22, i.e. the entire structure 20-22 may be of this material.

The opposite end of the flash tube 10 is fitted with a second electrode assembly generally indicated at 26. The assembly 26 includes a copper bar 28 extending through the cap 16 and joined to the end of a pyrolytic graphite electrode 30. A glass liner 32 surrounds the bar 28. The

various parts of the electrode assembly 26 are similar to, and serve the same function as, the corresponding parts of the assembly 18.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficient attained and, since certain changes may be made in the aboveconstruction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. In a flash device of the type comprising an enclosure defining a chamber containing a gas to be ionized to provide a flash, electrodes spaced apart within said chamber for passage of an electric current through said gas between tips of said electrodes and means for conducting ing substantially between the tip of the electrode and the electrical conducting means connected thereto.

2. A flash device comprising (A) enclosure means providing a chamber containing a gas to be ionized,

(B) first and second electrode assemblies, each of said electrode assemblies including (1) an electrode having a tip thereon,

(2) electrical conducting means extending into said chamber from the exterior of said enclosure means and connecting with an end of said electrode remote from said tip, 7

(C) said first and second electrodes being located and oriented with respect to each other to provide for passage of an electric current between said tips thereof by way of said gas,

(D) at least one of said electrodes being of mon-ocrystalline pyrolytic graphite with the axis of maximum thermal conductivity of said graphite extending substantially in the direction from the tip of said electrode to the electrical conducting means connecting therewith.

3. In a flash device of the type comprising an enclosure defining a chamber containing a gas to be ionized to provide a flash, electrodes spaced apart within said chamber for passage of an electric current through said gas between tips of said electrodes and means having a relatively high thermal conductivity for conducting heat from said electrodes to the exterior of said chamber, the improvement in which at least one of said electrodes is of pyrolytic graphite having an axis of maximum thermal conductivity, said axis extending from the tip of said electrode to the thermal conducting means conducting heat therefrom.

4. The combination defined in claim 3 in which both electrodes are of pyrolytic graphite having an axis of maximum thermal conductivity, said axis of each electrode extending from the tip of said electrode to the thermal conducting means conducting heat therefrom.

References Cited by the Examiner UNITED STATES PATENTS 2,154,317 4/1939 Suits 313-184 2,459,579 1/1949 Noel 3l3184 2,703,374 3/1955 Fruengel 313-484 2,716,713 8/1955 Noel 313--218 3,107,180 10/1963 Diefendorf 117226 JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

Claims (1)

1. 2. A FLASH DEVICE COMPRISING (A) ENCLOSURE MEANS PROVIDING A CHAMBER CONTAINING A GAS TO BE IONIZED, (B) FIRST AND SECOND ELECTRODE ASSEMBLIES, EACH OF SAID ELECTRODE ASSEMBLIES INCLUDING (1) AN ELECTRODE HAVING A TIP THEREON, (2) ELECTRICAL CONDUCTING MEANS EXTENDING INTO SAID CHAMBER FROM THE EXTERIOR OF SAID ENCLOSURE MEANS AND CONNECTING WITH AN END OF SAID ELECTRODE REMOTE FROM SAID TIP, (C) SAID FIRST AND SECOND ELECTRODES BEING LOCATED AND ORIENTED WITH RESPECT TO EACH OTHER TO PROVIDE FOR PASSAGE OF AN ELECTRIC CURRENT BETWEEN SAID TIPS THEREOF BY WAY OF GAS, (D) AT LEAST ONE OF SAID ELECTRODES BEING OF MONOCRYSTALLINE PYROLYTIC GRAPHITE WITH THE AXIS OF MAXIMUM THERMAL CONDUCTIVITY OF SAID GRAPHITE EXTENDING SUBSTANTIALLY IN THE DIRECTION FROM THE TIP OF SAID ELECTRODE TO THE ELECTRICAL CONDUCTING MEANS CONNECTING THEREWITH.

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