US 3808496 A
An improvement to a short arc gas discharge lamp is described. The anode is placed in the base of the lamp and the reflector is supported with reference to the cathode adjacent to the window. This allows more massive and simpler structures to be used for heat dissipation. The window edge is supported in compression. The improved structure includes a capacitive reactance which prevents sparking between the reflector and anode during start up.
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Description (OCR text may contain errors)
United States Patent 119 I McRae et al.
[ HIGH INTENSITY ARC LAMP Inventors: Russell C. McRae,
William R. Stuart, San Carlos, both of Calif.
Filed: Dec. 26, 1972 Appl. No.: 317,906
Varian Associates, Palo Alto, Calif.
Related US. Application Data Continuation of Ser. No. 109,537, Jan. 25; 1971,
us. c1 313/113, 313/44, 313/220 Int. Cl. Hlj /16 Field of Search 313/113, 220, 44
References Cited UNITED STATES PATENTS Marrison 313/1 13 Apr. 30, 1974 3,495,118 2 1970 Richter ..3l3/216 3,549,934 12/1970 Peacher 313/220 Primary Examiner-John Kominski Attorney, Ager t, or Firm-Stanley Z. Cole; Lee F. Herbert; John J. Morrissey ABSTRACT 15 Claims, 7Drawing Figures EH 2 I l Ii 1 HIGH INTENSITY ARC LAMP This is a continuation of application Ser. No. 109,537 filed Jan. 25, 1971, now abandoned.
BACKGROUND OF THE INVENTION This invention relates to a gaseous discharge device and particularly to a novel improvement in a high intensity short arc lamp structure.
In optical projection systems involving the generation and precisely controlled radiation of long pulses of non-coherent'light, such as in spectroscopy, microscopy, and solar simulation, in addition to the more conventional projection systems, there is a need for a light source capable of producing the highest possible light flux density, that is, the greatest total amount of light from the least possible volume. The ideal would be a point source of light with unlimited light output.
Of the electrical devices for the generation of noncoherent light in pulses of substantial length, gas discharge devices offer the possibility of generating the greatest total quantity of light from the least possible volume (i.e., light flux density). The light flux density which can be produced by incandescent or luminescent devices is limited by the amount of power that can be concentrated in the solid materials which serve as the light emitters before a change of state occurs in such material, whereas in a gas discharge device no such change of state can occur in the light emitting medium regardless of the concentration of power.
The amount of power which can be concentrated in a gas discharge maybe maximized by decreasing the spacingbetween the electrodes of the device and increasing the pressure of the gaseous medium, the voltage at which the discharge operates, and the current carried by the are. It has been found that for any given voltage and current the greatest light flux density will be obtained when the electrode spacing and gas pressure are adjusted to produce an arc discharge which is roughly spherical (that is, the length of the arc is approximately equal to its transverse dimensions). In this mode of operation the electrode spacing is less than two centimeters and usually less than one centimeter. Arc discharge devices designed to operate in this mode are called short arc devices to distinguish them from other forms of arc discharge such as medium arc and long arc devices which may produce larger total quantities of light but at much lower light flux density.
This invention is an improvement to the short are lamp disclosed in U.S. Pat. No. 3,502,929 which issued Mar. 24, 1970, to John F. Richter. This prior art lamp comprises a sealed envelope, a portion of which is ceramic. The envelope houses a cathode and an anode which are spaced apart a distance less than 2 centimeters to define a short arc gap therebetween. The envelope also houses an ionizable gas typically under approximately standard atmospheres of pressure. A sapphire window forms one end of the envelope and a reflector is attached to the other end of the envelope, either as part of the envelope or separate therefrom.
In the usual embodiment of this prior art lamp, the anode is suspended on the axis of the lamp adjacent to the window. As much as 70 percent of the energy of the discharge is converted into heat at the anode. This heat must be dissipated through the structure supporting the anode and then through flanges thermally connected to the support structure. The support structure is thin because light must pass by it. The flanges are thin to allow rapid dissipation of the heat in the critical area of the seal at the window. While highly effective at power consumptions up to approximately watts, at powers above this level heat cannot be properly dissipated and the seals break down. These seals themselves must be thin to allow for proper expansion as they absorb heat.
Reversing the positions of the cathode and anode so that the cathode is adjacent to the window places the anode in the-base of the lamp where more massive structures can be used to dissipate heat. This allows for better dissipation of the heat produced but this effect is limited by the requirement that the reflector mounted in the base be thermally insulated from the anode. However, placing the cathode adjacent to the window with the anode adjacent to the base gives rise to a new problem which offsets the advantages of higher heat dissipation. The point of highest intensity in the arc discharge depends on the position of the cathode, not the anode. This point must be at the focal point of the reflector to give the greatest light flux density. Small errors in position greatly reduce light flux density. When the cathode is in the base, the positioning of the cathode and therefore the positioning of the point of highest intensity relative to the focal point of the reflector is fairly easy because the reflector and cathode are in the same. assembly. However, when the cathode is adjacent to the window, any error in the positioning of the point of highest intensity is difficult to detect. For this reason, the usual embodiment of the prior art lamp has the anode adjacent to the window, thus insuring the most light flux density.
SUMMARY OF THE INVENTION Briefly described, the present invention is an improvement to high intensity short arc lamps. The lamp is a sealed envelope comprising a base and anode thermally connected to the base, a window opposite the base, a cathode insulatively supported adjacent to the window, and a reflector supported adjacent the window so as to be referenced to the cathode. The window is supported so as to be under compression at the edge of the window.
Accordingly, it is an object of the present invention to provide an improved high intensity are lamp which can operate at higher power than the prior art.
Another object of the-present invention is to provide a lamp with higher acoustic resonance frequency so that the lamp can be modulated at higher frequencies.
Another object of the present invention is to provide a lamp which allows the use of simpler seals by placing these seals in areas of less stress.
Another object of the present invention is to provide a lamp which allows for stronger and more convenient front mounting of the lamp.
Another object of the present invention is to provide a lamp which has a longer external insulation between the cathode and anode.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a cross-sectional view of one embodiment of the prior art lamp disclosed in US. Pat. No. 3,502,929. FIG. 1A is a frontal view of the lamp in FIG. 1 along the line 1A1A in the direction of the arrows.
FIG. 2 is a cross-sectional view of one embodiment of the high intensity short are lamp structure of the present invention. FIG. 2A is a front view of the lamp in FIG. 2 along the line 2A2A in the direction of the arrows. FIG. 2B is a rear view of the lamp in FIG. 2 along the line 2B2B in the direction of the arrows.
FIG. 3 is a top view of one type of radiator which may be attached to the base of the lamp of this invention. FIG. 3A is a side view of this radiator along the line 3A-3A in the direction of the arrows.
DESCRIPTION OF PRIOR ART FIGS. 1 and 1A illustrate an embodiment of the prior art lamp. One end of the ceramic cylinder section 40, which is made of polycrystalline alumina, is brazed to a ductile metallic (copper, for example) ring 42, which in turn is brazed to a metallic (Kovar or stainless steel, for example) member 44 of the lamp envelope. The metallic member 44 may be spherical, ellipsoidal or parabolic. The ductile metallic ring serves as a stress relieving portion of the envelope. The inner surface of member 44 serves as integral reflector 46. The other end of ceramic member 40 is brazed to a ductile metallic ring 48, which in turn is brazed to one side of a rigid metallic terminal ring 50. The terminal ring is then brazed to another ductile metallic ring 52 which in turn is brazed to the flange of a tubular rigid metallic window support 54. As in the case of ring 42, the ductile metallic rings 48 and 52 serve to relieve stresses. The periphery of a disc-shaped window 56, which may be a sapphire, for example, is slightly recessed within and brazed to window support 54.
A rod-shaped metallic anode 58 (tungsten, for example) is supported along the axis of tubular ceramic member 40 and window 56 by three triangular, metallic supports 60 which may be of molybdenum, for example. Each support has a notch into which terminal ring 50 is brazed. Supports 60 provide electrically conductive paths between anode 58 and terminal ring 50. Each of the metallic supports 60 is bent into the shape of a spiral for stress release during high temperature states.
DESCRIPTION OF PREFERRED EMBODIMENT There is illustrated in FIGS. 2, 2A and 28 a high intensity short are lamp having a base and a ceramic cylinder section 11 of, for example, polycrystalline alumina. As will be more thoroughly described below in the discussion of heat dissipation, the base may be constructed of steel rather than more expensive material of high thermal conductivity. The ceramic cylinder is brazed to a ring 12 having a coefficient of thermal expansion approximating that of the ceramic. Such a material is an alloy of iron, nickel and cobalt sold under the trademark Kovar. The base is recessed at 15 to form a standing edge 16. A tungsten inert gas weld is made between this standing edge and the ring 12 at point 17. Spaces 13 and 14 are left to allow expansion of the base. The structure and purpose of raised portion 76 are discussed below.
The lamp envelope includes a disc-shaped window 18, of for example, sapphire. This window is brazed to a sealing ring 19 of, for example, Kovar, which in turn is brazed to a support member 21 which may be of steel. The sealing ring 19 may be very thin because it is under very little stress. The J-shaped cross section is convenient because it allows for an expansion of the window in a radial direction. The support member 21 is constructed so that a portion 25 overlaps the front of the window. A ductile metallic ring 20 of, for example, copper, is placed between the window and the overhang 25 in unsealed contact with both. As will be discussed more thoroughly below, this overhang 25 serves to keep the window in compression while the ring 20 bears some of the stress. The support member 21 has a standing edge 23 which is tungsten inert gas welded at point 24 to a sealing ring 22 of, for example, Kovar.
Because the drawing in FIG. 2 is a cross section of many generally ring-shaped members, several lines appear parallel to the window 18 which may lead one to believe that some member other than the window fills the aperture of the lamp. Each of these lines results from one of the generally ring-shaped members. Line 101 is the edge of the top surface of the ring-shaped standing edge 23. Line 102 is the edge of the ringshaped surface of overhang 25 of member 21. Line 103 is the edge of the top surface of ring 20. Finally, line 104 is the edge of the ring-shaped surface of the bottom of sealing ring 19.
The three thin rectangular supports 31 for the rodshaped metallic cathode 32 are brazed to the bottom of support ring 30. These supports are of molybdenum, for example. The ring 30 is brazed to the sealing ring 22 at a point such that the supports 31 rest on the top of the ceramic ring 11. The supports 31, ring 30 and ring 22 provide an electrically conductive path to the cathode. The ring 30 has a standing edge portion 35. The reflector 33 contains slots 34 through which the cathode supports 31 pass when the top of the reflector is positioned at the standing edge 35 of ring 30. The standing edge 35 is joined to reflector 33 with a tungsten inert gas weld. The ring 30 must be sufficiently rigid to support the cathode and the reflector. A ring of Kovar has this property. The reflector is shown as a paraboloid but it may be, for example, a spherical or ellipsoidal reflector.
A rod-shaped metallic anode penetrates through the base 10. To maximize heat transfer from the anode, the portion which extends into the lamp should be as short as possible when compared with the thickness of the base. Specifically, the ratio of the portion of the anode within the lamp to the maximum thickness of the base in the area below hole 36 should be less than 0.6. This anode is preferably of tungsten while the cathode 32 is preferably of thoriated tungsten. The anode and cathode are positioned on the axisof the lamp, with the axis passing through a hole 36 in the bottom of the reflector. The anode and cathode are spaced apart by less than two centimeters, preferably less than one centimeter. The cathode is positioned so that the point of greatest light intensity in the arc discharge is at the focal point of the reflector 33.
After assembly the lamp is exhausted through tube 71 and then gas filled with, for example, xenon to a pressure approximately 25 atmospheres. The tube is then sealed at the pinch-off 72.
While a lamp of this type will operate at very low voltages, such as volts, once in operation, high voltages must be applied across the gap to start the lamp. Voltages such as 20,000 volts are required for this purpose. These voltages are tyically applied by a high frequency R.F. source. Because the reflector is in electrical contact with the support for the cathode, one path for the RF. current is along the reflector and across the gap at hole 36 to the anode. To prevent this arcing across the gap between reflector and anode, the base has a raised portion 76 whose inner surface is generally parallel to the outer surface of reflector 33. The outer surface of the raised portion 76 is parallel to the ceramic cylinder 11 and separated thereform by the gap 14. The gap permits expansion of the base. A capacitive reactance exists between this raised portion 76 and the reflector 33. This capacitance, in conjunction with the other electrical properties of the lamp structure considered as a passive network, especially the inductanceof the cathode and cathode support structure, makes possible a condition during starting of the lamp in which the main gap between the cathode and anode will break down as desired, but the gap between the edge of the reflector and the anode will not break down.
As mentioned as much as 70 percent of the energy created in the discharge is converted to heat at the anode of a short are lamp. With the anode of the prior art lamp positioned adjacent to the window, as shown in FIG. 1, this heat had to be dissipated through relatively thin supports 60 and ring 50. This limited the operation of this'embodiment of the prior art to a power of approximately I50 watts. Placing the anode in the base of the prior art lamp does allow for better heat dissipation but because the reflector support 44 must in turn be supported in the base of the lamp and because the reflector cannot be permitted to exceed relatively low temperatures, even the relatively more massive heat transfer members which would be possible in such an embodiment would still dissipate heat less easily than the base of the current invention. Such a prior art lamp could be operated at powers up to perhaps 500 watts. However, it should be remembered that placing the anode in the base of the prior art lamp creates difficulty in positioning of the cathode relative to the focal point of the reflector. This difflculty serves to offset any advantage gained in heat dissipation. In the present invention the reflector is not attached to the base. This allows for a simple, more massive base with a relatively short portion of the anode within the envelope. Heat is transferred very quickly to the outer surface of the base so that the lamp may be operated in the range of 500 to l,000 watts even with a base made of steel, a relatively poor thermal conductor when compared with other metals which are available. The lamp will operate at even higher powers if one of these other high thermally conductive metals is used for the base.
The relatively massive and simple form of the base makes it possible to attach radiators of relatively simple construction. Such a radiator is shown in FIGS. 3 and 3A. The radiator consists of a cylinder 81, a ring 82 spaced radially from the cylinder, and a serpentine fin placed between and attached to the cylinder and the ring. The top of both the fin and the ring are flush with the top of the cylinder. However, as shown in FIG. 3A, the cylinder extends longitudinally below the fin while the fin extends longitudinally below the ring. Three holes 75 are threadedly bored into the base 10 of the lamp. Bolts (not shown) are then inserted through holes 76 in the cylinder and into the holes 75 in the base. The simple construction of the base allows the outer surface 79 of the base 10 of the lamp to be in thermal contact with the entire upper surface of the radiator. A hole 77 in the center of the cylinder 81 accommodates the anode and the pinch seal 72. Such a radiator is able to dissipate all of the heat generated in a lamp operated at powers in the range 500 to 1,000 watts. Other radiators of more efliciency can be used to dissipate the heat in higher power lamps.
In the prior art lamp, shown in FIG. 1, the window support 54 had to be relatively thin to allow for expansion and to properly connect with the other thin support members. Because the window is under high pressure during the operation of the lamp, there is severe stress on the support 54 and its seal with the window. In applicants invention heat dissipation in the area of the window is not critical, and the window support structure of FIG. 2 allows the window to be placed in compression. This means that a thin seal 19 is possible with this window, with this sea] under little or no stress.
The member 21 in FIG. 2 is relatively massive when compared with the thin flanges of the prior art. This allows for more convenient and stronger front mounting. In the embodiment shown in FIG. 2, holes are threadedly drilled into the member 21 to receive bolts (not shown) from any convenient mounting device.
In some applications using a short are lamp, it is desirable to modulate the current across the arc gap, thus modulating the light produced by the lamp. If the modulation frequency is at or near an acoustic resonance frequency of the lamp, the gas molecules in the lamp will oscillate. This causes the pressure at the arc gap to vary from near zero to maximums far greater than the normal operating pressure of the lamp. At these maximums the current in the arc gap. can no longer be maintained and the lamp will be extinguished. It is desirable to have the lowest of the lamps acoustic resonance frequencies as high as possible so that it will exceed any modulating frequency which might be used. This lowest acoustic resonance frequency increases as the volume of gas decreases.
Both the present invention and the prior art lamp consist-of a pair of coupled cavities. The lowest acoustic resonance frequency is a complex function of the resonance frequencies of each of these coupled cavities. However, the major contribution comes from the cavity in which the arc gap is located. In the present invention the distance from the bottom of the window 18 to the top of the reflector 33 is much less than in the prior art lamp. This is because the reflector is supported adjacent to the window and therefore adjacent to the cathode, requiring no insulation from the cathode in the area of the window. This means that the window can be quite close to the reflector. ln the preferred embodiment the distance from the window to the reflector is less than the thickness of the window. Thus for the same aperture size and shape of reflector, the volume of gas in the chamber which includes the arc is smaller in the present invention than in the prior art.
With the reflector attached adjacent to the window, the ceramic insulator 11 is much longer than in the prior art lamp. This means that higher starting voltages can be used, or alternatively, that there is less likelihood of arc-over at low external pressures, e.g. at high altitude.
What is claimed is:
1. An arc lamp comprising a sealed envelope having a base, an optical window disposed in said envelope opposite said base, said envelope including an annular insulating member between said base and said window, said envelope containing an ionizable gas at a pressure higher than atmospheric pressure, a first electrode within said envelope, a reflector within said envelope, support means supporting both said first electrode and said reflector on said insulating member adjacent the window end of said insulating member, a second electrode mounted within said envelope adjacent said base, and means supporting said second electrode from the other end of said insulating member.
2. The arc lamp of claim 1 wherein said envelope includes an annular wall portion adjacent said window end of said insulating member, and wherein said support means for said first electrode and reflector includes an annular ring the outer portion of which is attached to said annular wall portion of said envelope and the inner portion of which is attached to said reflector and to said first electrode.
3. The are lamp of claim 1 wherein said reflector is metallic and is electrically connected to said first electrode.
4. The arc lamp of claim 1 wherein said reflector is configured as a surface of revolution and said first electrode is cylindrical, the axis of said first electrode coinciding with an axis of said reflector.
5. The arc lamp of claim 1 wherein said reflector is a metallic surface of revolution, said first electrode is a cathode and said second electrode is an anode, said support means includes an annular ring upon which said reflector is mounted and to which said cathode is attached by at least one electrically conducting support member which supports said cathode generally coaxially with an axis of said reflector, said support member being elongated in a direction normal to said axis, said reflector and said cathode being electrically interconnected.
6. An arc lamp comprising a hermetically sealed envelope containing a first electrode and a second electrode, said envelope comprising an insulating member electrically separating said electrodes to form an arc gap, an optical window at one end of said lamp, said first electrode being positioned adjacent the window end and said second electrode being positioned adjacent the opposite end of said envelope, a reflector comprising a surface of revolution positioned to reflect light from said are gap through said window, said reflector having a large diameter cross section at one end, said large end of said reflector being positioned closely adjacent said window, and said insulating member being positioned to extend from said reflector toward said second electrode.
7. The are lamp of claim 6 wherein the distance from the surface of said window nearest said arc gap to said reflector is less than the thickness of said window.
8. The arc lamp of claim 6 wherein said opposite end of said lamp comprises an electrically conductive base electrically connected to said second electrode, and said first electrode is electrically connected to said reflector.
9. An arc lamp comprising a hermetically sealed envelope containing an ionizable gas at a pressure higher than atmospheric pressure, said envelope having an optical window portion, sealing means for hermetically sealing said window to the non-window portion of said envelope, said sealing means comprising a flexible member to accommodate lateral expansion of said window, and abutment means overlapping the outer surface of said window for compressively restraining said window from movement in response to the outwardly directed pressure of said gas without stress by said outwardly directed pressure upon said sealing means.
10. The are lamp of claim 9 wherein said abutment means is annular with a portion which extends outside said envelope to circumferentially overlap the outer surface of said window so as to form an abutment lip thereagainst.
11. The are lamp of claim 10 wherein said sealing means comprises a thin deformable sealing member one end of which has a first sealing portion connected to said window and the other end of which has a second sealing portion connected to said annular abutment means, said sealing member having a flexible portion between said first and second sealing portions which is deformable to accommodate the difference in coefficient of expansion of said window and said abutment means.
12. An arc lamp comprising a sealed envelope having a base, an optical window disposed in said envelope opposite said base, said envelope containing an ionizable gas at a pressure higher than atmospheric pressure, a cathode and a metallic optical reflector mounted within said envelope upon support means disposed adjacent said window, and a cylindrical anode mounted within said envelope upon said base and extending toward said cathode, said base comprising a heat conductive material in surface contact with said anode for at least sixty percent of the length of said anode within said envelope.
13. The arc lamp of claim 12 wherein said heat conductive material has a recess in its inner surface with the walls of said recess extending inwardly beyond the inner end of said anode.
14. The are lamp of claim 13 wherein the walls of said recess project inwardly to surround a portion of said reflector. 7
15. An arc lamp comprising a sealed envelope having a base, an optical window disposed in said envelope opposite said base, said envelope containing an ionizable gas at a pressure higher than atmospheric pressure, a first electrode mounted adjacent said window, a reflector, support means upon which said first electrode and said reflector are mounted within said envelope adjacent said window, a second electrode mounted within said envelope adjacent said base, and a dielectric member for electrically insulating said second electrode from said first electrode.