Method And Apparatus For Extending The Useful Life Of An Arc Radiation Source

Abstract

An arc radiation source having a pair of concentric envelopes, the inner envelope of which defines the arc chamber, and including a cooling system comprising means for passing a gas coolant between the envelopes, and means for thereafter passing the gas into the arc chamber in a direction coaxial with the gas flow between the envelopes, the gas being continuously recirculated in a closed loop.

Description

This invention relates to arc radiation sources and more particularly to a method and apparatus for extending the useful life of such sources.

The invention provides an arc radiation source comprising an inner transparent elongated envelope defining an arc chamber, a pair of axially spaced electrodes located at opposite ends of said arc chamber, an outer elongated transparent tubular envelope surrounding said inner envelope and cooling means comprising means for passing a gas coolant between said envelopes from one end thereof in an axial direction, means for withdrawing said gas from the opposite end thereof, means for cooling said gas, means for redirecting said cooled gas into said chamber such that the gas flows coaxially with the gas flow between said envelopes, and means for withdrawing the gas from the chamber and recycling such gas to form a closed gas loop.

Arc radiation sources for generating high intensity light have been known for quite some time. Typically such sources comprise a pair of electrodes spaced apart in an arc chamber defined by an elongated transparent tubular envelope. An arc is established between the electrodes and constricted by means of a swirling gas introduced into the chamber.

One limitation upon the useful life of such arc radiation sources is the deterioration of the tubular envelope forming the arc chamber. Two significant causes of the deterioration are the heat generated by the arc and the pressure load within the chamber. Although the swirling gas in the arc chamber provides cooling of the envelope from its inner surface, auxiliary cooling of the envelope from its outer surface is preferred for high power operation.

One common method of providing auxiliary cooling is to pass a cooling fluid such as water or gas about and around the outer surface of the inner tubular envelope. Maximum heat transfer is obtained by passing the auxiliary cooling fluid in a direction opposite to the axial direction of the internal swirl gas flow. In following this procedure applicant has observed that the envelope was subjected to excessive thermal stress and the deterioration of the envelope was actually accelerated when cracks were formed. In accordance with the present invention such excessive thermal stress may be substantially avoided by maintaining a relatively small and uniform temperature differential between the inside and outside surface of the envelope along its entire length. This is accomplished without significantly impairing the increased effective cooling of the envelope thus increasing its useful life for high power operation.

It is therefore the principal object of the present invention to provide an arc radiation source having a cooling system which significantly extends the operating life expectancy of the envelope over that achieved in the prior art.

It is a further object of the present invention to provide a method of gas cooling an arc radiation source which substantially minimizes the accumulation of thermal stress within the envelope defining the arc chamber thereby extending the useful life of such envelope beyond that heretofore achieved.

These and other objects will become apparent from the following detailed description taken in connection with the accompanying single FIGURE drawing in which the arc radiation source and cooling system of the present invention is schematically illustrated.

Arc radiation source 10 comprises a pair of spaced hollow electrodes 12 and 14, respectively, located at opposite ends within an arc chamber 16 defined by the inside surface 18 of elongated tubular envelope 20. Tubular envelope 20, hereinafter referred to as the inner tubular envelope, is surrounded by an outer tubular envelope 22 radially spaced therefrom and in coaxial relationship therewith whereby an annulus 24 is defined therebetween. Tubular envelopes 20 and 22, respectively, are composed of any suitable transparent material such as quartz.

Each electrode has a central gas exit passage 26 and 28, respectively, in coaxial alignment with respect to one another. An arc 30 is established and maintained between electrodes 12 and 14, respectively, by electrically connecting the two electrodes to a power supply (not shown).

A suitable inert gas coolant preferably of argon, zenon, or krypton, is passed from gas supply source 32 through conduit means 34 into annulus 24 from the end 36 thereof. The gas flows axially downstream in the direction of the arrow and exits into conduit 38 at the end 40 of annulus 24. The passage of gas through the opening at end 40 is effectively restricted by the subsequent passage of the gas into chamber 16 via the swirl generating inlet ports 46. As a result the pressure in annulus 24 is greater than the pressure in chamber 16 so as to continuously maintain the inner tubular envelope 20 in mechanical compression. As is well known, a quartz tubular envelope is mechanically stronger in compression than in tension and by at least an order of magnitude. Hence, by maintaining envelope 20 in a state of compression its useful life is significantly increased. Moreover, should envelope 20 rupture it will implode rather than explode thereby providing a safety advantage over prior art systems.

Conduit 38 directs the gas into a conventional heat exchanger 42 which removes the heat taken from the tubular envelopes so as to cool the gas down to substantially the same temperature as supplied from gas supply source 32. The gas is thereafter redirected through conduit means 44 into swirl generating inlet ports 46 located at the end of chamber 16 adjacent the inlet end 36 of annulus 24. Swirl generating inlet ports 46 are tangentially arranged about the outer circumference of electrode 12 such that the gas passed therethrough will develop a swirling flow formation within arc chamber 16. The gas advances in a swirling manner axially downstream along the inside surface 18 of inner tubular envelope 20 toward electrode 14 where a portion of the gas flows out of the exit passage 26 thereof. The remaining portion of the swirling gas inverts, flowing back about the arc, internal of the swirling stream until it reaches electrode 12 where it flows out of exit passage 28.

Thus the gas passing through annulus 24 and chamber 16 along the outer and inner surfaces of envelope 20 presents a heat sink of substantially equal temperature at each surface. Since the gas flows are substantially equal, the heat transferred by envelope 20 to the gas in annulus 24 and chamber 16 is substantially the same at each surface and uniform with length resulting in a small and uniform temperature differential across envelope 20 along its entire length.

The gas exiting from exit passages 26 and 28, respectively, is passed through heat exchangers 47 and 48, respectively, to remove heat generated within the arc and then returns through conduit means 50 to gas supply source 32 from whence the gas cycle is renewed. Gas supply source 32 may include an additional heat exchanger and appropriate filters to restore the gas to its original temperature and purity.

In as much as the same gas supplied to annulus 24 is cycled through chamber 16 in the same flow direction from approximately the same starting point with relatively equal temperatures it follows that the temperature differential between the inside and outside surface of envelope 20 will be minimized along its entire length. Hence, the tendency for thermal stress accumulation across the thickness of envelope 20 is substantially reduced thereby extending the useful life of the envelope.

Although the invention has been described with reference to spaced hollow electrodes it is equally applicable to a combination of a stick electrode and a hollow electrode. Moreover, although the gas flow sequence shown and described is highly preferred, the invention is applicable to an inverted flow sequence wherein the gas is initially injected into the chamber and then redirected into the annulus between the envelopes after appropriate cooling; although in such instance inner envelope 20 will be in tension and the advantage derived from maintaining the inner envelope in comparison will be lost. It is still necessary to maintain coaxial flow and to introduce the gas into the chamber and annulus at substantially equal temperatures.

Claims

What is claimed is:

1. An arc radiation source comprising: an inner transparent elongated tubular envelope defining an arc chamber; a pair of axially spaced electrodes located at opposite ends of said arc chamber; an outer elongated transparent tubular envelope surrounding said inner envelope; and cooling means comprising; means for passing a gas coolant between said envelopes from one end thereof in an axial direction and for discharging said gas from the opposite end thereof, means for cooling said discharged gas, means for redirecting said cooled gas into said arc chamber such that the gas flows in the same direction the gas flow between said envelopes, means for withdrawing the gas from the chamber, and means for cooling and recycling such gas to form a closed gas loop.

2. An arc radiation source comprising: an inner transparent elongated tubular envelope defining an arc chamber; a pair of axially spaced electrodes located at opposite ends of said arc chamber; an outer elongated transparent tubular envelope surrounding said inner envelope; and cooling means comprising; means for passing a gas coolant into said arc chamber from one end thereof in an axial direction, means for withdrawing the gas from the chamber, means for cooling said withdrawn gas, means for redirecting said cooled gas into the area between said envelopes in the same direction with the gas flow in said chamber, means for discharging the gas from the area between said envelopes, and means for cooling and recycling such gas to form a closed gas loop.

3. An arc radiation source comprising: an inner transparent elongated envelope providing an arc chamber; a pair of axially spaced electrodes located at opposite ends of said arc chamber for establishing an arc therebetween, an outer elongated transparent tubular envelope radially spaced from and in coaxial relationship with said inner envelope for defining an annulus therebetween, said annulus having an inlet gas passage at one end thereof an an outlet gas passage at the opposite end thereof, and a cooling system comprising; a gas supply source, conduit means for directing the gas flow from said supply source to said inlet gas passage, means for cooling the gas exiting from said outlet gas passage, means for redirecting said cooled gas into said arc chamber at the end thereof adjacent the inlet passage of said annulus such that the gas flow in the arc chamber is in series with the gas flow in said annulus, means for draining said gas from said arc chamber, means for cooling said drained gas and conduit means for returning said drained gas to said supply source to form a closed gas recirculating system.

4. A method of gas cooling an arc radiation source having an inner tubular envelope providing an arc chamber in which an arc is established and an outer tubular envelope surrounding said inner tubular envelope and radially spaced therefrom which comprises; continuously passing a gas between said inner and said outer envelope from one end thereof in an axial direction; withdrawing said gas from one end thereof in an axial direction; withdrawing said gas from the opposite end thereof; cooling said withdrawn gas; redirecting said cooled gas into said arc chamber from one end thereof an in a flow direction coaxial with the gas flow between said envelopes, said gas being passed through said chamber in a swirling flow pattern; withdrawing said gas from said arc chamber; recooling said gas; and recirculating said gas thereby forming a closed gas loop.

5. A method of gas cooling as defined in claim 3 wherein the temperature of the gas when injected into said arc chamber is substantially the same as the temperature of the gas when passed between the inner and outer envelopes.