GB1596732A - Xenon arc lamp with compressive ceramic to metal seals - Google Patents

Description

(54) XENON ARC LAMP WITH COMPRESSIVE CERAMIC TO METAL SEALS (71) We, VARIAN ASSOCIATES, INC., of 611 Hansen Way, palo Alto, California 94303, United States of America, a corporation organized under the laws of the State of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to high pressure gaseous discharge devices and in particular to improvements in the envelope structure and window seal for high pressure arc lamps.

In any arc lamp, various portions of the lamp structure will be at different potentials corresponding to one or another electrode. It is necessary, therefore, to provide insulation between these portions in order to maintain their mutual electrical isolation. Prior art high pressure arc lamps, for example, as described in U.S. Patents 3,876,908; 3,852,629; and 3,808,496, employ ceramic cylinders separating the lamp envelope into anode and cathode portions with a circumferential surface of the insulator as part of the lamp envelope exterior.

It will be apparent that as a consequence of this construction, the lamp design, operation and usage are subject to a number of critical specifications.

One such critical specification is related to the internal pressure and temperature to which the ceramic and the ceramic to metal bonds are subject. Ceramic cylinders or rings so employed are primarily under tensile stresses due to the high pressure gas filling of the lamp which tends to push apart the anode and cathode portions of the lamp envelope. The brazed metal to ceramic bonds of these prior art lamps are likewise under tensile or shear stress. Due to the properties of the brazing alloys and the metallic members, the stresses within the joint must be maintained within safe tolerances.

These stresses are thermally dependent and thus place limits on the lamp operation and associated cooling system.

Another critical consideration is the ambient environment wherein the lamp will be operated. Moisture dirt, grease, fingerprints and the like can seriously degrade the insulation characteristics of the ceramic, especially for high voltage starting conditions. Moreover, the exposed ceramic is subject to impact which can initiate a fracture thereof.

It will be clear also that failure of the ceramic or the metal-ceramic seals of a high pressure lamp can be hazardous to personnel or equipment nearby.

Another prior art arc lamp, described in U.S. Patent 3,715,613, employed a ceramic cylinder with a recessed plane surface for accepting a metal disk which in turn supported the anode structure. The outside surface of the ceramic was brazed to the inner surface of the envelope neck. Thus, the metal ring exerted compressive and shear stresses at one end of the ceramic due to the internal gas pressure while a brazement of the outer circumferential wall of the ceramic to the inner cylindrical surface of the envelope neck results in shear stresses due to the internal gas pressure.

An arc lamp is also characterized by an optical window which must withstand a very great pressure differential between the interior and exterior of the lamp. Prior art window seals in such lamps have normally employed a window disc of sapphire, metallized around its circumference and then brazed to one leg of a U-shaped metal flange, which is subsequently brazed into the window-cathode portion assembly, as shown for example in U.S.

Patent 3,852,629. This window sealing technique resulted in an improved seal but one which is not as good as the seal to be hereinafter described in respect to this invention.

According to the invention there is provided an envelope for an arc lamp, said envelope being capable of containing a gas therewithin at a pressure higher than atmospheric pressure, said envelope comprising: a first metallic member having an inwardly projecting flange portion; a second metallic member having an outwardly projecting flange portion; the outwardly projecting flange portion of said second metallic member being received within said first metallic member, the inwardly projecting flange portion of the said first metallic member being disposed to overlap the outwardly projecting flange portion of said second metallic member, said overlapping flange portions of said first and second metallic members being spaced apart to define a generally annular region between said first and second metallic members; a ceramic member disposed in said annular region, and means for hermetically sealing said ceramic member to said inwardly and outwardly projecting flange portions so that said ceramic member is under compression when said envelope contains a gas at a pressure higher than atmospheric pressure.

Examples of the invention will now be described with reference to the accompanying drawings in which: FIG. 1 is a section of high pressure arc lamp.

FIG. 2 is a detail of an alternative embodiment of the main body seal of the lamp.

FIG. 3 is a detail of an alternative embodiment of the window seal of the lamp.

FIG. 1 is illustrative of a high pressure sealed beam short arc lamp of the present invention. One electrode shown as cathode 10, is mounted in the forward part of the lamp adjacent to window 11. Cathode 10 is supported by metal struts 12 which are centrally attached to cathode 10. A getter 13 is provided to remove contaminants from the gas filling of the lamp. The struts 12 are attached to metal ring 14 adjacent the outer ends of the struts. Reflector 16 is suspended from ring 14. The exterior of the forward portion of the lamp is a cylindrical shell 18 made of a high strength metallic alloy, such as stainless steel.

Window 11, preferably of sapphire, is face brazed with a knife edge braze process to annular sealing ring 21, the latter preferably of Kovar (Registered Trade Mark).

Although the window 11 remains subject to the same bending moment across its plane surface (face) as in prior art lamps, the outer peripheral region or rim of the window, together with the window to metal bond, is compressively loaded, with consequent reduction of tensile stresses. Since the ultimate yield strength for sapphire in compression far exceeds that in tension this seal has been found to withstand much higher internal pressures than the prior art peripheral seal for equal thickness sapphire windows. Whereas the prior art peripheral seals have been tested to seal failure at 1200 psi the compressive window seal of the present invention has not been.

observed to fail at 1900 psi. It is conjectured that exposure of the circumferential surface 19 of the window to internal gas pressure of the lamp results in a compressive stress across the diameters of the window, resulting in the higher observed resistance to fracture. It is also noted that the braze bond between the window 11 and the sealing ring 21 is in compression due to the high internal gas pressure.

Thus higher pressures may be achieved, or alternatively, a thinner sapphire window may be employed to maintain the same structural integrity as characterized by the prior art.

A second electrode, here anode 22, is supported in the rear of the lamp.

Surrounding and attached to anode 22 is a high strength metal alloy shell 28. Rear and front shells comprising the lamp envelope are connected by attaching metal shells 18 and 28 to a ring 25 of high strength ceramic. Sealing rings 29 of, for example, Kovar, are used to seal ceramic ring 25 to anode shell 28 and cathode shell 18, thereby forming a hermetic envelope. The cathode shell 18 and anode shell 28 are mutually configured to place ceramic ring 25 in compression and shield ceramic ring 25 from contamination and impact. This is accomplished by forming on cathode shell 18 an inward projecting flange 30 and forming on anode shell 28 an outward directed flange 32. These flanges overlap each other to intercept a common radial region de fined from the center line 34 and it is this common radial region which is occupied by ceramic ring 25.The internal pressure thus tends to hold the envelope together. The ex terior overlap of cathode shell 18 over the ceramic 25 tends to thus protect the ceramic from contamination which could promote short circuits and from impacts which could damage it. Should the ceramic fail, fragments are likewise prevented from causing damage to nearby personnel or apparatus. Moreover should the ceramic insulator or its seals fail, the cathodes and anode shells 18 and 28 are prevented from separating by the flanges 30 and 32.

The ratio of compressive strength to tensile strength for a typical ceramic as employed here is in excess of 10:1; therefore the ceramic insulator can withstand much higher com pressive stresses arising from the high pressure gas filling of the lamp than would be the case were the ceramic to be tensile loaded.

It will be observed that preparation of ceramic ring 25 for brazing reguires only grinding of the end plane surfaces rather than the more elaborate requirements for prepara tion of radial surfaces for brazing. Similarly, only plane surfaces (of flanges 30 and 32) need be prepared for brazing rather than circum ferential surfaces of shells 18 and 28. The seals are knife-edge brazements which are subject to substantially purely compressive stresses.

FIG. 2 is an alternative seal wherein two coaxial rings 29' and 29" are employed in place of the single ring 29. Similarly, dual sealing rings are illustrated in FIG. 3 for an alternative arrangement to seal the window 11 to the assembly ring 14.

Assembly of the lamp of FIG. 1 is accomplished by first performing the brazing operations required to seal cathode shell 18 to anode shell 28. The assembly ring 14 supporting the window and cathode is brazed to cathode shell 18 as the last braze. The lamp is then filled to high pressure through port 36, preferably with xenon and the port is then sealed off.

Figures 1 and 3 are described and form the subject matter claimed in copending application 8,013,603 (Serial No. 1,596,733) divided from this application.

WHAT WE CLAIM IS: 1. An envelope for an arc lamp, said envelope being capable of containing a gas therewithin at a pressure higher than atmospheric pressure, said envelope comprising: a first metallic member having an inwardly projecting flange portion; a second metallic member having an outwardly projecting flange portion; the outwardly projecting flange portion of said second metallic member being received within said first metallic member, the inwardly projecting flange portion of said first metallic member beeing disposed to overlap the outwardly projecting flange portion of said second metallic member, said overlapping flange portions of said first and second metallic members being spaced apart to define a generally annular region between said first and second metallic members;; a ceramic member disposed in said annular region, and means for hermetically sealing said ceramic member to said inwardly and outwardly projecting flange portions so that said ceramic member is under compression when said envelope contains a gas at a pressure higher than atmospheric pressure.

2. An envelope as claimed in claim 1 wherein said sealing means comprises a first seal interposed between a first surface portion of said ceramic member and the outwardly projecting flange portion of said second metallic member, and a second seal interposed between a second surface portion of said ceramic member and the inwardly projecting flange portion of said first metallic member.

3. An envelope as claimed in claim 2, wherein said ceramic member is of annular configuration surrounding a nonflanged portion of said second metallic member and is contained within said first metallic member.

4. An envelope as claimed in claim 3 wherein said annular ceramic member is spaced apart from said first and second metallic members.

5. An envelope as claimed in any one of claims 2 to 4 wherein said first and second surface portions of said ceramic member are planar, and wherein each of said first and second seals comprises a metallic alloy member formed to an annular seal.

6. An envelope as claimed in claim 5, wherein said planar surface portions of said ceramic member are metallized, and wherein said first and second annular seals are brazed, respectively, to said first and second metallized surface portions of said ceramic member.

7. An envelope as claimed in claim 6, wherein said first and second annular seals are brazed, respectively, to the flanged portions of said second and first metallic members.

8. An envelope for an arc lamp substantially as herein described with reference to and as illustrated in Figure 1 alone or as modified by Figure 2 of the accompanying drawings.

9. An arc lamp comprising an envelope as claimed in any one of claims 1 to 8, wherein in said first metallic member is a cylindrical member, the lamp comprising a window supported from said first metallic member adjacent one end thereof, an arc electrode supported from said first metallic member adjacent said window and projecting toward the other end of said first metallic member, said second metallic member being positioned adjacent the other end of said first metallic member, and another arc electrode supported from said second metallic member and projecting toward the first said arc electrode.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. operations required to seal cathode shell 18 to anode shell 28. The assembly ring 14 supporting the window and cathode is brazed to cathode shell 18 as the last braze. The lamp is then filled to high pressure through port 36, preferably with xenon and the port is then sealed off. Figures 1 and 3 are described and form the subject matter claimed in copending application 8,013,603 (Serial No. 1,596,733) divided from this application. WHAT WE CLAIM IS:

1. An envelope for an arc lamp, said envelope being capable of containing a gas therewithin at a pressure higher than atmospheric pressure, said envelope comprising: a first metallic member having an inwardly projecting flange portion; a second metallic member having an outwardly projecting flange portion; the outwardly projecting flange portion of said second metallic member being received within said first metallic member, the inwardly projecting flange portion of said first metallic member beeing disposed to overlap the outwardly projecting flange portion of said second metallic member, said overlapping flange portions of said first and second metallic members being spaced apart to define a generally annular region between said first and second metallic members;; a ceramic member disposed in said annular region, and means for hermetically sealing said ceramic member to said inwardly and outwardly projecting flange portions so that said ceramic member is under compression when said envelope contains a gas at a pressure higher than atmospheric pressure.

2. An envelope as claimed in claim 1 wherein said sealing means comprises a first seal interposed between a first surface portion of said ceramic member and the outwardly projecting flange portion of said second metallic member, and a second seal interposed between a second surface portion of said ceramic member and the inwardly projecting flange portion of said first metallic member.

3. An envelope as claimed in claim 2, wherein said ceramic member is of annular configuration surrounding a nonflanged portion of said second metallic member and is contained within said first metallic member.

4. An envelope as claimed in claim 3 wherein said annular ceramic member is spaced apart from said first and second metallic members.

5. An envelope as claimed in any one of claims 2 to 4 wherein said first and second surface portions of said ceramic member are planar, and wherein each of said first and second seals comprises a metallic alloy member formed to an annular seal.

6. An envelope as claimed in claim 5, wherein said planar surface portions of said ceramic member are metallized, and wherein said first and second annular seals are brazed, respectively, to said first and second metallized surface portions of said ceramic member.

7. An envelope as claimed in claim 6, wherein said first and second annular seals are brazed, respectively, to the flanged portions of said second and first metallic members.

8. An envelope for an arc lamp substantially as herein described with reference to and as illustrated in Figure 1 alone or as modified by Figure 2 of the accompanying drawings.

9. An arc lamp comprising an envelope as claimed in any one of claims 1 to 8, wherein in said first metallic member is a cylindrical member, the lamp comprising a window supported from said first metallic member adjacent one end thereof, an arc electrode supported from said first metallic member adjacent said window and projecting toward the other end of said first metallic member, said second metallic member being positioned adjacent the other end of said first metallic member, and another arc electrode supported from said second metallic member and projecting toward the first said arc electrode.