Title:
MOUNTING RING AND METHOD FOR REFERENCING MEMBERS IN A SHORT ARC LAMP
United States Patent 3725714


Abstract:
A mounting ring and method is described for referencing the cathode to the reflector in a short arc lamp, and the cathode/reflector to the anode and the lamp exterior. A reference plane is formed on the mounting ring. One or more planes parallel to this reference plane are formed in a brazing fixture and spaced so that the tip of the cathode is at a controlled distance from the reference plane. The same reference plane or one parallel to it is used to control positioning of the anode and lamp exterior. Axial concentric surfaces are formed on the ring and on at least one cylinder in the brazing fixture to control concentricity of the ring, cathode and reflector, and of the ring, anode and lamp exterior. When two or more cylinders are used, surfaces on a first cylinder can be made to expand at the same rate as the ring, with one surface constrained at brazing temperature by a concentric surface on a second cylinder which expands at substantially the same rate as the electrode.



Inventors:
ANDERSON N
Application Number:
05/143165
Publication Date:
04/03/1973
Filing Date:
05/13/1971
Assignee:
VARIAN ASS,US
Primary Class:
Other Classes:
313/113
International Classes:
H01J61/86; H01J61/98; (IPC1-7): H01J17/06
Field of Search:
313/113,220,219,221,222,205,206
View Patent Images:
US Patent References:



Primary Examiner:
Brody, Alfred L.
Claims:
I claim

1. An arc lamp comprising a sealed envelope having a base, a window disposed in said envelope, said envelope containing an ionizable gas, means for supporting adjacent said window a concave reflector and a cathode having a tip precisely positioned relative to the focal point of said reflector, said support means comprising an annular support member having a first reference surface facing away from said window and a flange portion adjacent said first reference surface projecting away from said window, one side of said flange portion forming a second reference surface, the wide end of said reflector abutting said first reference surface, a peripheral portion of said reflector adjacent the wide end thereof abutting said second reference surface, said support means further comprising strut means secured adjacent one end of said strut means to the flange portion of said annular support member and adjacent the other end of said strut means to said cathode, said strut means received by slots in the periphery of the wide end of said reflector, an anode mounted within said envelope on said base, and means for electrically insulating said anode from said cathode.

2. The arc lamp of claim 1 wherein said strut means comprises a plurality of struts each secured to said flange portion at a position thereon spaced from said first reference surface in a direction away from said window.

3. The arc lamp of claim 1 wherein said first reference surface is perpendicular to the axis of said annular support member, said second reference surface is perpendicular to said first reference surface, and said cathode is elongate with its long axis coinciding with the axis of said annular support member.

4. The arc lamp of claim 1 wherein said flange portion is located radially outward of said first reference surface, said second reference surface is the radially inner surface of said flange portion, said annular support member has a third reference surface located radially outward of said flange portion parallel with said first surface, and the radially outward surface of said annular support member forms a fourth reference surface.

5. The arc lamp of claim 4 wherein said envelope comprises an annular metal wall portion surrounding said annular support member, said wall portion having an inwardly projecting annular lip facing said window, said annular support member being positioned in said wall portion with said third reference surface abutting said lip and said fourth reference surface abutting the inner periphery of said wall portion between said lip and said window.

Description:
BACKGROUND OF THE INVENTION

This invention relates to gaseous discharge devices and in particular to a mounting ring and a method of assembly which provides improved referencing of the reflector, electrodes and lamp exterior in a high intensity short arc lamp structure. The invention herein described was made in the course of or under a contract with the Department of Defense.

The brightest zone of a high pressure short arc in, for example, a xenon filled lamp is immediately adjacent to the cathode tip. The positioning of this zone relative to the focal point of the reflector is critical if the desired pattern of illumination is to be achieved in an optimum manner. For example, with a paraboloidal reflector maximum brightness is achieved when the brightest part of the arc is at the focal point of the paraboloid. There are also situations where it is advantageous to defocus the arc in order to achieve more uniform illuminance in the output beam. This can be done by using an ellipsoidal reflector with external optics but precise positioning of the zone of brightness is still required to give the particular defocusing desired.

In a sealed beam short arc lamp with the reflector mounted within the lamp envelope, proper placement of the tip of the cathode relative to the focal point of the reflector requires accurate referencing of a number of different members. How many and which are the most critical depends on the particular design but one imprecise placement or dimension is enough to shift the cathode tip from its optimum placement. Furthermore, proper referencing must be maintained during the temperature cycling associated with assembly (typically high temperature brazing) and operation.

Prior art short arc lamp designs attack the referencing problem in a variety of ways but each has its drawbacks. In the lamp described in U.S. Pat. No. 3,502,929, issued Mar. 24, 1970, the reflector is part of the rear subassembly with the cathode either part of the same subassembly or part of the front (window) subassembly. When the cathode was part of the rear subassembly, both axial positioning and concentricity were good. The reflector can be formed so that the position of its focal point is precisely known relative to the plane of its forward opening. Similarly, the diameter and circularity of the forward opening can be precisely controlled. The reflector can then be placed face down in a brazing fixture for accurate placement of the cathode. Unfortunately, the placement of the anode in the forward subassembly limits the power of operation of the lamp because it is difficult to dissipate enough heat from the anode at powers above 150 watts.

Moving the anode to the rear subassembly and the cathode to the front helps to solve the heat dissipation problem but makes positioning of the cathode difficult. Axial positioning of the tip of the cathode can be good because the forward subassembly can be referenced to the same surface as the front of the reflector. In assembling the front subassembly a brazing fixture is used which locates the tip of the cathode precisely in axial position relative to a reference surface on the subassembly exterior, e.g. a ring or the bottoms of the electrode support struts, which will be seated in and referenced to a surface on the rear subassembly. The surface on the rear subassembly can then constitute the top of the reflector, if the reflector is part of the lamp envelope, or be a surface to which the top of the reflector has been referenced. While axial positioning is good, concentricity is at best 0.005 inch total indicated runout because it is difficult to precisely position the two subassemblies relative to one another in the plane of their meeting owing to cumulative concentricity tolerances of the several parts constituting front and rear subassemblies.

In the lamp disclosed in copending application, U.S. Ser. No. 109,537 filed Jan. 25, 1971, assigned to the same assignee as the present application, the concentricity problem was solved by placing both the reflector and cathode in the front subassembly. However, the axial positioning is poor in the embodiment disclosed because the tip of the cathode is first referenced via a braze to the bottom of the support struts while the reflector is referenced to a ring which is attached to top of the support struts. This makes the width of the support struts a key dimension. However, these struts are stamped from metal sheets, e.g. of molybdenum, and are therefore not precise enough to serve as a reference distance. To machine them to a precisely fixed width would be too expensive.

The problems described in all these prior art lamps carry over to the requirement for proper spacing of the cathode from the anode and for the proper positioning of any external lens or mirrors relative to the focal point of the reflector.

SUMMARY OF THE INVENTION

The present invention is a mounting ring and an improved method for referencing the cathode to the reflector in a short arc lamp of the type in which the cathode and reflector are mounted adjacent to the window of the lamp. A reference plane is formed on the mounting ring perpendicular to the axis of the ring. A surface concentric with the axis of the ring is also formed on the ring. Matching perpendicular and concentric surfaces are formed on a cylinder in a brazing fixture. This cylinder is of material with the same thermal expansion coefficient as the ring so that the matching surfaces are in contact throughout brazing. In one embodiment a second cylinder is used in the brazing fixture. The first cylinder has a second surface concentric with its axis. A matching inward-facing surface is formed on a standing portion of the second cylinder. This second cylinder is of material with substantially the same thermal expansion coefficient as the cathode, which is to be inserted in an axial hole in the second cylinder and supported there during the brazing. The matching concentric surfaces have diameters such that the surface on the first cylinder will expand out to just meet the surface on the second cylinder at brazing temperature. This insures that the first cylinder, and therefore the ring, is coaxial with the cathode. Planes parallel with the reference plane are formed on the cylinders to insure that the reference plane remains perpendicular to the axis of the cathode during brazing. These planes also allow the reference plane to be properly spaced from the tip of the cathode.

The electrode can also be supported directly on the axis of the first cylinder in the brazing fixture, eliminating the need for the second cylinder.

Additional perpendicular and concentric surfaces are formed on the mounting ring to provide referencing for other parts of the lamp, such as the anode, and for external optics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of one embodiment of the mounting ring of the present invention.

FIGS. 2A and 2B are a top view and a cross-sectional view, respectively, of a cathode-strut assembly usable with the method of the present invention.

FIG. 3 is a fragmentary side view of one type of strut attachment means usable with the ring in FIG. 1 in the method of the present invention.

FIGS. 4A and 4B are a top view and a cross-sectional view, respectively, of a brazing fixture usable with the method of the present invention, showing the ring of FIG. 1 in place by dotted lines.

FIG. 5 is a cross-sectional view showing the cathode-strut assembly of FIG. 2B and a reflector in place with the ring of FIG. 1, as assembled with the method of the present invention.

FIG. 6 is a cross-sectional view, partially in schematic of a short arc lamp assembled using the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates one embodiment of the mounting ring of the present invention. A reference plane B is defined by the lower surfaces 10 and 11 of ring 14. This plane B must be precisely formed and must be perpendicular to the axis A of the ring 14 because plane B is the basic plane of reference for the cathode, the reflector and the anode, as well as any other structures requiring accurate positioning relative to the focal point of the reflector. Ring 14 can be of any convenient shape but it requires a lower surface forming the reference plane B. The first portion of plane B, surface 11, is adjacent to an inward facing axial surface 12, which is concentric with the axis A. The second portion of plane B, surface 10, is adjacent to an outwardly facing axial surface 13, which is also concentric with axis A. While discussed in more detail below, surfaces 11 and 12 control the axial placement and concentricity, respectively, of the tip of the cathode relative to the focal point of the reflector, and surfaces 10 and 13 control the axial placement and concentricity, respectively, of the anode and any structures attached to the lamp exterior. It should be noted that the placement of the various surfaces relative to one another is a matter of convenience. For example, surfaces 10 and 11 need not both be in plane B; they could be in different parallel planes. Further the adjacency of the surfaces noted above is not required so long as each surface is controlled as indicated.

FIG. 2A and 2B illustrate a typical cathode-strut assembly with three straight struts 23 abutted to a cathode 21. The slotted sleeve 22 aids in positioning during assembly. The use of straight struts is a matter of convenience but any of the electrode support means described in previously mentioned U.S. Pat. No. 3,495,118 could be used. The use of straight struts is made possible by the flap portions 31 of ring 14 illustrated in FIG. 3 as discussed below but described in detail and claimed in copending application U.S. Ser. No. 143,377 filed May 14, 1971 and assigned to the same assignee as the present invention.

One embodiment of the assembly method of the present invention employs a brazing fixture such as the one illustrated in FIGS. 4A and 4B. Certain of the surfaces must be carefully formed and the materials used must have certain coefficients of thermal expansion relative to materials used in other parts of the fixture and in the structure being assembled, as discussed below in connection with the actual method of assembly. The fixture illustrated comprises a cylindrical base 42 with a central hole 43 formed therein and a standing portion 44. The functions of the spacer 49 and cylindrical sleeve 47 with central hole 50, both placed in hole 43, will be discussed below. A cylinder 41 of generally T-shaped cross section and having a central hole 51 rests on cylinder 42. Three slots 59, equally spaced around the circumference, are cut through cylinder 41 and standing portion 44 of cylinder 42 into central hole 51. These slots will accommodate the cathode support struts 23. Other shapes could be employed for the cylinders and more than two cylinders could be employed. The fixture illustrated is a convenient form.

A step in the assembly of the cathode support structure is to choose a cylinder 41 of the proper size and material. Cylinder 41 is chosen with concentric outer axial surface 52 on its upper portion 57 having a diametric equal to the diameter of ring 14 at surface 12, with concentric outer axial surface 53 on its lower portion 58, and with top and bottom surfaces 55 and 56 perpendicular to the axis of cylinder 41. As can be seen in FIG. 4B, upper surface 55 is in reference plane B when ring 14 is placed over cylinder 41. The diameter of hole 51 in cylinder 41 need only be large enough to accommodate the slotted sleeve 22 of the cathode-strut assembly shown in FIG. 2B. Cylinder 41 is made of the same material as ring 14 or of a material having the same coefficient of thermal expansion, typically stainless steel. As discussed below, the use of such materials means that cylinder 41 and ring 14 will remain in contact at surfaces 52 and 12 during brazing.

Another step is to choose a cylinder 42 of a material with a coefficient of thermal expansion very close to that of cathode 21, and therefore less than cylinder 41. With cathode 21 of tungsten, cylinder 42 can be of molybdenum. The standing portion 44 has a concentric inner axial surface 46 with diameter equal to the diameter of cylinder 41 at surface 53 after cylinder 41 has expanded relative to cylinder 42 at brazing temperature. The necessary calculation is easily done and is based on the coefficients of thermal expansion of the materials. The upper surface 62 of cylinder 42 is perpendicular to axis A of cylinder 42. Therefore, when cylinder 41 is placed on cylinder 42 with surface 62 in contact with surface 56, surface 62 is parallel to surface 55 of cylinder 41, which is in reference plane B. This insures that reference plane B will be perpendicular to axis A of cylinder 42 when the cylinders are in place.

Another step is to choose sleeve 47 of material with coefficient of thermal expansion the same or substantially the same as cylinder 42. This sleeve 47 should fit exactly in hole 43 in cylinder 42, while cathode 21 should fit exactly in hole 50 of sleeve 47 and be coaxial with cylinder 42. A sleeve such as 47 is used so that cathodes of various diameters can be accommodated by the same cylinder 42. It would, of course, be possible to form hole 50 directly in cylinder 42.

Another step is to choose a cylindrical spacer 49 of material with coefficient of thermal expansion the same or substantially the same as cylinder 42. The thickness of spacer 49 is such that when it is placed at the bottom of hole 43 in cylinder 42, the axial distance from spacer 49 to reference plane B after cylinders 41 and 42 have expanded axially during the heat up of brazing is that which gives the desired distance from the tip of cathode 21 to reference plane B at ambient temperatures. Spacer 49 could be eliminated by precisely forming hole 50 but this would preclude subsequent adjustments of cathode-reflector positioning.

Having chosen the parts of the brazing fixture with the properties described, the fixture is assembled but cylinder 41 need not be coaxial with cylinder 42. They will become coaxial as the brazing fixture is heated, as described below. Next the cathode-strut assembly of FIGS. 2A and 2B is placed in the brazing fixture with the tip of cathode 21 resting on spacer 49 and each of the three struts 23 placed in a slot 59 of the fixture. The struts 23 should not rest on any surface of the brazing fixture until brazing temperature is reached. Otherwise the tip of cathode 21 could be lifted off spacer 49 because cathode 21 typically has a lower coefficient of expansion than the materials in the brazing fixture and therefore expands less. Thus the slots 59 cut into cylinders 41 and 42 should be deep enough so that the bottom surfaces 60 and 61, respectively, never reach struts 23 during brazing.

Next ring 14 is placed on cylinder 41 so that lower surface 11 of ring 14 rests on upper surface 55 of cylinder 41. These surfaces, as mentioned, are in reference plane B. If the type of strut-to-ring attachment illustrated in FIG. 3 is used, ring 14 is positioned so that struts 23 each meet the end of a flap 31. If some other attachment means is used, the struts and ring are positioned as appropriate. When the strut-to-ring attachment of FIG. 3 is used, it is convenient to modify the brazing fixture of FIG. 4B somewhat. By making the diameter of hole 51 in cylinder 41 larger than the diameter of hole 43 in cylinder 42, a standing portion can be formed on cylinder 42 extending into hole 51. By matching the diameter of the outward facing axial surface of this standing portion to the diameter of hole 51, cylinder 41 can be made coaxial with cylinder 42 at the beginning of assembly. This eliminates any shifting of the struts 23 once they have been positioned at the ends of flaps 31.

In the last step the brazing fixture, cathode-strut assembly and ring 14 are heated to brazing temperature. Because ring 14 and cylinder 41 are of materials with the same coefficient of thermal expansion, they expand as if one piece and they remain in contact. Because the coefficient of thermal expansion of cylinder 41 is higher than that of cylinder 42, cylinder 41 expands radially relative to cylinder 42 until surface 53 exactly meets surface 46. Because both of these surfaces were carefully formed as concentric with the axes of their respective cylinders, cylinders 41 and 42 are coaxial at brazing temperature. Therefore, surface 52 on cylinder 41 is concentric with axis A of cylinder 42, and therefore so is surface 12 on ring 14. Because sleeve 47 was chosen so as to make cathode 21 coaxial with cylinder 42, i.e., with axis A, surface 12 on ring 14 is concentric with axis A of cathode 21 at brazing temperature. Thus when the bonds form between struts 23 and ring 14, cathode 21 is on the axis of ring 14 as desired.

Further as cylinders 41 and 42 expand radially during the heating to brazing temperature, ring 14 and reference plane B move axially away from spacer 49. As indicated earlier, when the bonds form between struts 23 and ring 14 at brazing temperature, the tip of cathode 21 is in a proper position relative to reference plane B such that after the assembly cools down, contraction of the electrode brings the tip of cathode 21 into the desired ambient temperature position.

Once the cathode-strut assembly is properly attached to ring 14, the next step is to attach the reflector 71 as shown in FIG. 5. The dimensions of the cathode 21 and reflector 71 were chosen so that the tip of cathode 21 could be placed in the desired position relative to the focal point or other convenient reference point of reflector 71. Upper surface 74 of reflector 71 is formed perpendicular to the axis of reflector 71. Outer surface 73 is formed concentric with the axis of reflector 71 and with the same diameter as inner surface 12 of ring 14. Three slots 72 are formed in reflector 71 to accommodate struts 23. When the reflector 71 is placed in position with upper surface 74 in contact with lower surface 11 of ring 14, surface 74 is in reference plane B, thus placing the reference point of reflector 71 in its proper position relative to reference plane B, and therefore relative to the tip of cathode 21. Reflector 71 is concentric with axis A of cathode 21 because surface 12 of ring 14 is concentric with axis A.

The brazing fixture of FIGS. 4A and 4B is a convenient one to use because it insures that ring 14 and cathode 21 will be coaxial. However, cathode 21 could be supported directly on the axis of cylinder 41 (such as by sleeve 47 and spacer 49 placed in a hole in cylinder 41), eliminating cylinder 42. In this case the brazing fixture primarily controls axial spacing with concentricity controlled by initial positioning at the start of assembly, as by using the embodiment of the above-mentioned application U.S. Ser. No. 143,377 in which struts 23 are attached to the inside surface of flaps 31 in FIG. 3.

In discussing the outer surfaces 10 and 13 of ring 14 in connection with FIG. 1, it was noted that surface 10 is in reference plane B while surface 13 is concentric with axis A. These surfaces are used to reference the remainder of the lamp and parts attached thereto, such as the anode and external optics.

FIG. 6 illustrates a short arc lamp showing the cathode-strut-ring-reflector assembly of FIG. 5 in place. Details of construction of the lamp are not shown, aside from those of the assembly of FIG. 5. Such a lamp is described in more detail in copending U.S. Pat. application Ser. No. 143,166 filed May 13, 1971 and assigned to the same assignee as the present invention. The lamp has a cylindrical member 62 which, together with window 64 and base 63, forms a sealed envelope filled with gas under high pressure. An anode 61 is supported by appropriate means from base 63. Surfaces 65 and 66 perpendicular to and concentric with the axis of cylinder 62, respectively, match surfaces 10 and 13 of ring 14. These surfaces 65 and 66 allow anode 61 and any external optics (not shown) to be referenced to plane B.