US3525896A

US3525896A - Arc lamp envelopes

Info

Publication number: US3525896A
Application number: US747729A
Authority: US
Inventors: William J Soule; Martin L De Poy
Original assignee: Sanders Associates Inc
Current assignee: Lockheed Corp
Priority date: 1968-07-25
Filing date: 1968-07-25
Publication date: 1970-08-25
Anticipated expiration: 1987-08-25

Description

Aug. 2

AMPLITUDE TIME FIGZD.

w. J. SOULE ETAL ARC LAMP ENVELOPES Filed July 25, 1968 FIG! FIGZA.

FIGZB FIGZC.

JNVENTOI? WILLIAM J. SOULE MARTIN L. DE POYII United States Patent US. Cl. 313-221 5 Claims ABSTRACT OF THE DISCLOSURE A metallic halide envelope is provided for a high temperature alkaline vapor arc lamp whereby a reduction in the recombination time of dissociated molecules in the arc plasma is produced. This reduction further provides an improvement in the modulation index of such lamps.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to are lamps and more particularly to protective envelopes for are discharge phenomena in high-temperature alkaline vapor arc lamps.

Description of the prior art The alkaline vapor arc discharge lamp has become the primary laboratory source of far-infrared radiation. Such lamps are now commercially available, filled with sodium, cesium, rubidium, and lithium to name but a few. It will be readily apparent that some sort of protective envelope is required to contain the arc discharge phenomena. These envelopes must maintain their structural integrity at elevated temperatures, and, on exposure to hot plasma constituents, have a coefficient of thermal expansion which is compatible with that of the material to which the envelope must be sealed and also must be highly transmissive of the infrared energy in the desired wavelength bands.

Previously available alkaline vapor arc lamps have employed such materials as quartz, sapphire and calcium aluminate as protective envelopes. These lamps unfortunately have a relatively short lifetime due to envelope fracture under conditions of lamp-induced thermal shock; i.e., the extremely high rate at which the envelope is heated when the lamp is started. In many applications, it is desirable to provide an arc lamp which may be electronically modulated at relatively high frequencies. An additional limitation of the prior art lamps is their low modulation index at pulse repetition frequencies above 100 to 1000 pulses per second. This shortcoming is due to the relatively long time which is necessary for the molecules in the arc plasma to recombine when the lamp is turned oti and hence the long fall time of the recombination radiation. Thus, when it is desired to electronically modulate the lamp output at relatively high frequencies, the recombination radiation precludes a depth of modulation much in excess of 80 percent.

OBJECTS OF THE INVENTION Accordingly, it is a primary object of the present invention to provide a new and novel protective envelope for alkaline vapor arc lamps.

It is another object of the present invention to provide an envelope of the above-described character which is suitable for high temperature operation.

It is an additional object of the present invention to provide an envelope of the above-described character which enhances the lamp modulation index.

It is still a further object of the present invention to 3,525,896 Patented Aug. 25, 1970 provide an envelope of the above-described character which is resistant to thermal shock.

SUMMARY OF THE INVENTION According to this invention, these objectives are achieved by providing an arc lamp envelope fabricated of a metallic halide. These materials are particularly well adapted to use as an arc lamp envelope due to their high melting temperature, resistance to structural failure on exposure to hot alkaline plasma constituents, coeflicients of thermal expansion which are well matched to envelope seal metals and excellent transmission of intrared energy to beyond nine microns. In operation, the arc plasma attacks the metallic halide envelope to a limited extent thereby freeing a limited quantity of the halogen. The halogen atoms act as a plasma dopant which decreases the recombination time of the dissociated molecules in the arc plasma by two to three orders of magnitude. Thus the range of lamp modulation frequencies is greatly widened and the lamp modulation index approaches percent.

These and other objects, features and advantages of the present invention will become more apparent from the detailed discussion considered in conjunction with the accompanying drawing. The scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view of an arc lamp fabricated according to the present invention.

FIG. 2 is a graph illustrating the advantages of the present invention with respect to the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to FIG. 1, there is shown a crosssectional view of a typical vapor arc lamp structure 10 in which the present invention may be embodied. Positive and

negative electrodes

12 and 14, respectively, between which there is an arc gap 16, are enclosed in an alkaline vapor filled envelope 18 which is transparent in a desired wavelength region. The envelope is sealed to

end caps

20 and 22 which are electrically coupled to the

electrodes

12 and 14, respectively. The

end caps

20 and 22 are of a configuration which facilitates the insertion of the lamp in a holder (not shown) which is coupled to an external power supply. It is to be understood that the lamp configuration illustrated is not in any way to be construed in a limiting sense. The envelope could as well be, for example, of a bulbous shape in the vicinity of the arc gap 16. The present invention contemplates the use of a metallic halide as the material of which the lamp envelope 18 is fabricated.

The applicants have found that metallic halides, and in particular, magnesium fluoride or Irtran I which is a hot pressed polycrystalline form of magnesium fluoride manufactured by the Eastman Kodak Company of Rochester, N.Y., possesses the physical properties which are desirable in a high temperature arc lamp envelope. As established by the manufacturer, this material has a melting point of 1255 degrees centigrade and a coeflicient of thermal expansion of 11.0 10 over the temperature range 25-200 degrees Centigrade. This coefficient is well matched to the coefficient of thermal expansion for annealed stainless steel which is 10.7 10" over approximately the same temperature range. Further, the thermal shock characteristics of Irtran I are particularly well suited to the thermal environment of an arc lamp envelope.

The use of magnesium fluoride as an alkaline vapor arc lamp envelope provides a very significant improvement in the modulation characteristics of the lamp. It

has been found that the alkaline constituents in the arc plasma attack the magnesium fluoride envelope material to a limited degree, thus freeing minute quantities of fluorine. The fluorine atoms act as a dopant in the arc plasma which results in a decrease in the recombination time of the dissociated molecules in the arc plasma. By this means, the recombination radiation fall time; i.e., the time required for substantially complete molecular recombination, is reduced by two to three orders of magnitude. The halogen atoms freed by the limited alkali attack have an oxidation state of minus one and are thus highly reactive with the free alkali metal atoms of the arc plasma which have an oxidation state of plus one. When the power to the lamp is cut off, the reaction between the halogen and the alkali occurs much more rapidly than when the alkali atoms recombine with oxygen, for example. The drastic reduction in the recombination time provided by the free halogen atoms results in a dual benefit in the modulation characteristic of the arc lamp. First, for a given depth of modulation, the present invention permits the lamp to be operated at a higher pulse repetition rate; i.e., more pulses per second. Second, for a given pulse repetition frequency, the present invention permits a greater depth of modulation to be achieved. The advantages of the present invention are illustrated in FIG. 2. Plot A illustrates the input energy amplitude applied to an arc lamp as a function of time. An ideal arc lamp would have instantaneous response and thus have an output which would correspond directly to the input energy. Plot B illustrates the performance of prior art are lamps which is characterized by a fast rise to the peak amplitude I, a substantially flat-topped pulse during the application of the input energy, and a slowly decaying output resulting from the recombination radiation. It will be noted that for pulse repetition frequencies on the order of 100 pulses per second or more, the recombination radiation does not have suflicient time to decay before the start of the next pulse. Since the pulses overlap, the depth of modulation which is achievable generally does not exceed 80 percent. Plot C of FIG. 2 illustrates that through the use of the present invention the recombination time is substantially reduced thus resulting in a much improved depth of modulation which approaches. 100 percent. Plot D further illustrates that if the depth of modulation available with the prior art lamp of Plot B is satisfactory in a given application, the number of pulses which may be transmitted in a unit time is significantly increased. Here, there is shown an improvement of three and one-half times in the available pulse repetition rate. In actual practice, an improvement by a factor of about may be provided. Although magnesium fluoride has been described in the foregoing dis cussion for the purposes of illustration, it is to be understood that the halides of lithium, sodium, magnesium, calcium, barium, aluminum, zirconium, and yttrium are also suitable for use in the practice of the present invention.

The extent of the attack of the metallic halide envelope by the alkaline constituents of the arc plasma is not precisely known. It is known, however, that the amount of halogen necessary to achieve the reduction in recombination time is very minute and that the phenomenon occurs to such a limited extent as to have no material effect on the lamp lifetime. This is to say that the total power output of the lamp over its lifetime, the number of pulses times the peak pulse power, is not noticeably reduced.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efliciently attained, and, since certain changes may be made in the above construction without departing f5om the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings 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 therein described, and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween.

Having described what is new and novel and desired to secure by Letters Patent, What is claimed is:

' 1. An alkaline vapor arc lamp comprising an envelope containing an alkaline vapor, means for sealing each end of said envelope, a plurality of electrodes sealed Within said envelope and in electrical contact with said sealing means, and

said envelope being formed of a material which releases a halogen when attacked by said alkaline vapor during operation of said lamp.

2. Apparatus as recited in claim 1 where said envelope is formed of a halide of a metal selected from the group consisting of lithium, sodium, magnesium, calcium, barium, aluminum, zirconium, and yttrium.

3. Apparatus as recited in claim 2 wherein said halide is a fluoride.

4. Apparatus as recited'in claim 2 wherein said envelope is formed of magnesium fluoride.

5. Apparatus as recited in claim 4 wherein said magnesium fluoride is in a hot-pressed polycrystalline form.

References Cited UNITED STATES PATENTS RAYMOND F. HOSSFELD, Primary Examiner US. Cl. X.R. 2202.l; 313-412