US3543076A

US3543076A - Aeorodynamic arc lamp electrodes

Info

Publication number: US3543076A
Application number: US772323A
Authority: US
Inventors: Ralph L Haslund
Original assignee: Boeing Co
Current assignee: Boeing Co
Priority date: 1968-10-31
Filing date: 1968-10-31
Publication date: 1970-11-24
Anticipated expiration: 1987-11-24

Description

Nov. 24, 1970 R. HASLUND I 3,543,076

I I AERODYNAMIC ARC LAMP-ELECTRODES 2 Sheets-Sheet 1 Filed Oct. 31, 1968 #vvavrm;

RAzP/ L. M51 u/m ATMA /YEV Nov. 24, 1970 R. L. HASLUND 3 AERODYNAMIC ARC LAMP ELECTRODES Filed Oct. 31, 1968 2 Sheets-Sheet 2 United States Patent US. Cl. 313184 2 Claims ABSTRACT OF THE DISCLOSURE Electrode shapes for a compact arc lamp of the type which contains a pressurized gaseous atmosphere. The disclosed system provides an improved and stable light source at high power levels for an extended operating lifetime. Aeorodynamic shaping of axisymmetric electrode elements is utilized to establish and control a recirculating flow pattern within the gaseous atmospher. The recirculating flow pattern optimizes heat transfer, stabilizes the arc, and controls deposition of vaporized electrode particles. The anode is shaped such that its radius of curvature increases going away from the tip. The cathode is pointed and its slope changes gradually from the tip into a generally cylindrical shank portion.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to an optical light source, and more particularly to an improved high power, compact arc lamp of the type used in solar simulators, search lamps and motion picture projectors.

Description of the prior art Compact arc lamps have been developed which offer relatively stable light sources and satisfactory operating lifetime for power levels up to about five kilowatts. However, these lamps usually will not produce satisfactory light sources or lifetimes when the power level is increased into the tens of kilowatts range.

In the manufacture of solar simulators for the aerospace industry there has long been a need for a compact arc lamp which could operate in the -30 kilowatt range while providing an approximation to a point light source. Considerable research and development activity has centered around improving existing compact arc lamp designs.

Low power compact arc lamps normally utilize arc producing electodes in which the anode has a blunt, large front surface area shape in order to maximize heat dissipation. Such anodes have a cylindrical body with tip geometries ranging from hemispherical to conical truncations. The cathodes paired with these anodes normally are cylindrical With a sharp or conical tip. A sealed bulb or lamp wall surrounds the electrodes and is filled with a gaseous atmosphere.

In operation, energizing the electrodes Will generate a light producing electrical arc and create an accompanying movement of the atmosphere within and surrounding the are. A cathode spot mode of operation can be created whereby the electrical arc beigns at a spot or point located at the cathode tip, and extends to the center of the anode, where it flares out slightly at the anode tip surface. This mode of operation can utilize relatively close electrode spacings to advantageously minimize the arc length and provide a good approximation of a point light source. However, due to high local current densities, this mode will create a magnetic pinch near the cathode tip which results in a hydrodynamic pressure gradient which, in turn, creates a gas jet or cathode jet directed toward the anode.

3,543,076 Patented Nov. 24, 1970 At low power levels the magnetically induced cathode jet is not strong, and jet velocities are low. So long as simple symmetry of the electrode is maintained, the energy of the cathode jet gasses does not become a disturbing factor. Prior art anodes having blunt or hemispherical forebodies will function adequately at these low power levels. While flow separation and turbulence are normally present, no significant net disturbance will be propagated back into the are region provided the separation remains symmetrical and the power levels remain low. The low velocity flow is effectively broken up at the anode and is dissipated without causing circulatory effects within the envelope surrounding the electrodes. At these low power levels the most significant influence on are stability is that due to heat conduction away from the arc region to the bulb wall. Therefore, the arc is said to be wall stabilized. Electorde centering, bulb symmetry, and external cooling are carefully controlled to maintain symmetry of gas thermal conduction paths and bulb surface temperature.

In the cathode spot mode, the tip temperature of the cathode approximates 2500 C. The vaporization of electrode materials at this temperature is minimal for conventional low power operation as the current levels are low and emitting areas are small. The vaporized materials are predominately deposited on the inner surface of the bulb in the shadow region of the anode where they pose minimal problems in heat conduction and optical transmissivity.

Prior art lamps with conventional electrode shapes have been found to be unsatisfactory for use at high power levels when it is necessary to maintain reasonably close electrode spacing. At high power levels the magnetic pinch at the cathode tip creates a true magnetic pumping of the bulb atmosphere. Cathode jet velocities of many hundreds of feet per second are produced. The jet exerts a dominant influence on the character of the arc. This effect is made proportionately greater because space limitation and overall compactness requires that bulb size remain relatively constant even when power levels are increased several times. When prior art systems are used for high power operation, the kinetic energy of the gas is not dissipated and strong uncontrolled turbulent flow patterns develop. The flow has a greater thermal influence on the arc than does the gas thermal conduction to the wall. The high power are is, therefore, convectiton stabilized and is necessarily very sensitive to uncontrolled flow patterns within the bulb. At high power levels, the tip temperatures of the cathode are considerably higher than at low levels. High power levels cause the cathode tip emitting area to be increased over low levels due to higher current demand. The higher temperatures and increased emitting area causes an increased vaporization of the electrode material. The vaporized electrode materials have a much lower ionization potential than the gaseous atmosphere and can be a significant perturbing influence on the arc. If the bulb is cooled asymmetrically, vapor deposition is asymmetric and the reintroduction of vapor into the cathode jet region has the effect of creating olfaxis conduction paths and visible arc wander. This effect can be minimized only by the careful cooling of the bulb in the lamp housing. The vaporized material is not el'ficiently deposited out in the shadow region of the bulb if random flow patterns are present. The effect of turbulence at the anode shoulder region adds to the problem of instability through fluctuations in dynamic pressure and vapor concentration.

The blunt anode designs with inherent flow separation at the shoulders have a built-in random arc wander which tends to average out near the anode center. This lateral motion of the arc can lead to a significant drop in optical system efficiency. In addition, arc Wander can develop into permanent off-center arc attachment, ending the useful life of the lamp.

At high power levels the heat generated by the arc is transferred by circulatory effects to the lamp wall. In high power systems, temperatures of the order of 10,000 K. are attained within the arc region. In lamps utilizing prior art blunt anodes, excessive heating of the lamp wall occurs. Also, due to the turbulence in random flow patterns, vapor laden gas tends to deposit out on the central portion of the bulb, increasing radiation absorption at the wall and decreasing transmission of light and heat. The result of heating in such systems is a shortened lamp lifetime. The ultimate failure mode of the bulb may be explosive, with gas pressures typically approaching ten atmospheres. Accordingly, it is customary to require a conservative safety factor on bulb lifetime.

The new and unique compact arc lamp to be described here results in a cooler lamp operation, a more stable arc, and a longer operating lifetime as an intense optical source.

SUMMARY The general purpose of this invention is to provide a high power, compact arc lamp which is capable of sustained operation in the tens of kilowatts power range. The present invention teaches that the aerodynamic shaping of electrode elements with respect to a generally conventionally shaped bulb envelope will allow the cathode jet flow to be utilized for cooling and stabilizing purposes and that by such shaping, prior art difficulties generated by the kinetic energy of the cathode jet flow can be overcome.

In contrast to previous blunt anode configurations the anode utilized in this invention is shaped to maintain laminar flow over the anode body. The anode itself is internally liquid cooled. As a result, the hot gas flowing directly through the arc will be caused to come into intimate contact with a substantial portion of the cooled anode surface where efiicient gas cooling and deposition of vaporized materials will take place. In addition, the anode body is shaped so that the flow will not overexpand for flow deflection angles up to about from the longitudinal center line of symmetry. This prevents separation and promotes a recirculating dynamic pressure pattern which tends to recenter the flow and damp out minor perturbations of the cathode jet.

The cathode is shaped to promote laminar flow over a portion of its body surface. The flow is preferably forced to remain attached until just upstream of the cathode tip. In this manner, the gas is uniformly introduced at the cathode tip region, thereby eliminating fluctuations due to turbulence which exists in this region in prior art configurations. Electrode elements which are aerodynamically shaped according to the teachings of this disclosure may be used with conventional bulb shapes and sizes to effect a recirculating flow pattern which oevrcomes difficulties encountered in previous attempts to extend the operation of conventional low power lamps into the tens of kilowatts range.

A further benefit obtained by an arc lamp apparatus constructed according to the teachings of this disclosure is that reliable non-vertical lamp operation is possible. The flow patterns achieved by the aerodynamic shaping of the lamp elements are strong enough to overcome the so-called buoyancy velocity component which has restricted operation of prior systems to certain alignment positions with respect to the gravity vector.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a compact are lamp bulb and electrode system constructed according to the teachings of this disclosure.

FIG. 2 presents a point-by-point plot and tabulation of a preferred surface configuration for an aerodynamically shaped anode.

FIG. 3 shows a point-by-point plot and tabulation of a preferred surface configuration for an aerodynamically shaped cathode.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows in section, a bulb type envelope structure 10 which contains a gaseous atmosphere denoted generally by 12, an anode body 14, and a cathode body 16. The envelope is sealed to contain the gaseous atmosphere under a pressure exceeding several atmospheres. The gaseous atmosphere is preferably an ideal gas such as Xenon. The envelope 10 is preferably of generally spherical shape and is constructed of a material such as fused silica or blown quartz, selected for its high melting point, physical strength, optical transmissivity and ultra-violet transparency. The anode body 14 in this embodiment comprises a thin copper shell 18, an interior body 20, and a cooling passageway 22.

The cathode body 16 may be constructed of thoriated tungsten and includes a cooling passageway 24 with coolant delivery means shown at 26. In this embodiment a thin sleeve 28 of getter material is incorporated into the cathode body for the purpose of removal of free oxygen from the surrounding gaseous atmosphere in the manner of a getter material of vacuum tube technology. The sleeve 28 is made of titanium or a like material which will absorb oxygen and withstand high temperatures. The sleeve 28 should be positioned so that it will occupy a re gion of temperature :which maximizes oxygen absorption for the particular material selected.

In operation, an electrical potential imposed upon the electrodes will cause an illuminating arc to be drawn from a cathode spot 30, at the tip portion of the cathode body, to the surface of the tip portion 32 of the anode 14. At high power levels, high local current densities generate a cathode jet stream 36, composed of portions of the gaseous atmosphere 12 which are entrained at the cathode tip and propagated through the arc to achieve a velocity of several hundred feet per second in the direction of the anode. As the cathode jet impinges upon the anode, a stagnation streampoint is formed at 32. When the anode body 14 is aerodynamically shaped with surface contours such as shown generally in FIG. 1 and more precisely in FIG. 2, the cathode jet and the accompanying contiguous atmosphere will form a laminar flow pattern over the anode for a substantial portion of its length. As an examination of FIGS. 1 and 2 will indicate, the shape preferred for the purpose of establishing this laminar floW has a radius of curvature which is a minimum at the tip and smoothly increases going away from the tip. The preferred contour is from a mathematical family of contours which maintain the continuity of higher order surface derivatives, such contours being especially adapted to promote continuing subsonic laminar flow. The particular shape shown in FIGS. 1 and 2 was selected on the basis of wind tunnel testing using Reynolds number scaling and aspect ratio similarity. In addition to its aerodynamic benefits, the relatively small tip radius of curvature acts to help electrically stabilize the are by increasing the olfaxis arc length more rapidly than for prior art blunt or hemispherical shapes. The laminar flow over a substantial portion of the anode maximizes heat transfer from the cathode jet to the cooled anode body and promotes deposition of vaporized electrode materials thereon, thereby selectively cooling the hottest portions of the gas and removing a perturbing influence on the recirculating flow.

The recirculating flow diagram of FIG. 1 shows the flow remaining laminar and attached to the anode body until it approaches region 42 where the flow turns, as indicated at 44, in the direction of the envelope 10 to come under the influence of the generally spherically shaped envelope wall. The flow pattern continues down the envelope wall to be redirected toward the cathode body, as at 46, in a steady state flow configuration. Centrifugal forces caused by the curvature of the flow pattern at 44 and 46 will promote deposition of vaporized material and erosion products in these regions, particularly in the shadow region of the anode near location 42. As the flow diagram indicates, the cathode jet 36 comprises a stream tube of gas which creates motion of the entire gaseous atmosphere along the streamlines which are established by the aerodynamically shaped electrodes and the bulb surface. On each cycle of motion the gases are cooled and cleansed of erosion products.

The cathode body 16, has been shaped with an aerodynamic contour along the lines of a boat tail configuration, having a slope which gradually decreases from a maximum at a generally cylindrical shank to a minimum at a pointed tip which will tend to round with use. This particular contour is well adapted to promote laminar flow in a stream which is initially directed substantially at right angles to the contour center line, as is the case in region 46. A preferred cathode shape, selected from wind tunnel data, is precisely set forth in the plot and tabulation of FIG. 3. A cathode which is shaped according to FIG. 3 will cause laminar flow over a subtsantial portion of the cathode body and will induce continuity of the flow pattern into the cathode spot area 30 where the cathode jet is initiated. The resulting axial flow field symmetry and retained directed kinetic energy combine to dynamically center the arc.

As the foregoing description would indicate, aerodynamically shaped electrode elements, in combination with a generally conventionally shaped bulb or envelope, can achieve a controlled recirculation pattern which promotes cooling and deposition of vaporized particles, and which creates pressure gradients tending to recenter the arc and allow an improved non-vertical operation. While a particular embodiment has been described here, variations appropriate to the other applications, such as the case of an unusually shaped envelope, will be obvious to persons skilled in the art of aerodynamics in view of the teachings of this disclosure. It is also to be noted that while the initial purpose of this invention was to extend the operation of arc lamps into powers of the tens of kilowatts range, the teachings of this disclosure can be utilized to show significant improvement in low power lamp operation and lifetime. Additionally, it should be noted that if it is not required to approximate a point light source and an extended light source is acceptable, power levels even higher than discussed above are achievable with the system described here simply by increasing the electrode spacing and thereby decreasing local current densities.

What is claimed is:

1. A compact arc lamp apparatus for operation at power input levels of 20 kilowatts or higher comprising:

a gaseous atmosphere,

an envelope means having a generally spherical portion for containing said gaseous atmosphere under a pressure exceeding one atmosphere,

an electrode means comprising oppositely facing aerodynamically shaped anode means and cathode means for drawing an electrical arc within said envelope and for cooperating with said spherical portion to establish and control a continuously recirculating axissymmetrical flow pattern which is characterized by a moving cathode jet stream tube of gas surrounding said are, wherein said anode means comprises:

a cylindrical shank portion extending beyond and away from said flow pattern and out of said generally spherical portion of said envelope means.

a curved portion having a minimum radius of curvature at the tip end facing said cathode, and a radius of curvature which smoothly increases going away from said tip end to a maximum at said cylindrical shank portion, the variation of said radius of curvature being established by a mathematical family of curves which maintains the continuity of high order surface derivatives, and

wherein said cathode means comprises:

a pointed tip portion facing said anode means,

a cylindrical shank portion, extending beyond and away from said flow pattern and out of said generally spherical envelope means,

a shaped portion interconnecting said tip and shank portions characterized by an aerodynamic boattail shape in which the surface has a slope which smoothly decreases from a maximum value at the shank portion where it fairs into and is parallel to the slope of said shank portion to a minimum value at said pointed tip portion,

wherein said power input level and the spacing of said anode and cathode means are established to create cathode jet velocities in excess of one hundred feet per second and said continuously recirculating flow pattern tends to recenter the flow and damp out minor perturbations of the cathode jet. 2. The apparatus of claim 1 which additionally provides liquid means for cooling the interior of each of said anode means and said cathode means and wherein means are provided for removal of oxygen from said envelope means.

References Cited UNITED STATES PATENTS 2,965,790 12/1960 Ittig et al. 313-217 3,412,275 11/1968 Thouret 313-30X RAYMOND F. HOSSFELD, Primary Examiner US. Cl. X.R.