US3512030A - High intensity source of selected radiation - Google Patents

Description

HIGH INTENSITIISOURCE OF SELECTED RADIATION Filed June 16, 196'? M. E. LEVY May 12, 1970 2 Sheets-Sheet 1 INVENTOR- MAURICE E. LEVY his ATTORNEYS HIGH INTENSITY SOURCE OF SELECTED RADIATION Filed June 16, 1967 M. E. LEVY May 12, 19 70 2 Sheets-Sheet 2 INVENTOR. MAURICE E. LEVY gnaw/marl, 7m, mw/flwm his ATTORNEYS United States Patent 3,512,030 HIGH INTENSITY SOURCE OF SELECTED RADIATION Maurice E. Levy, Fort Lee, N.l., assignor to Vitro Corporation of America, New York, N.Y., a corporation of Delaware Filed June 16, 1967, Ser. No. 646,673 Int. Cl. H01j 61/88 U.S. Cl. 313-231 30 Claims ABSTRACT OF THE DISCLOSURE A plasma radiation source for producing selected high intensity radiation comprising a cathode having a forwardly converging surface, an anode providing "an annular surface surrounding an aperture on a common axis with the cathode and located forwardly of and facing the cathode converging surface in close proximity, and a plasma chamber enclosing the cathode tip and the annular anode surface. The area of projection of the aperture on a plane normal to the axis is substantially less than the area of projection on the plane of the cathode tapering surface such that only radiation from the central core of an arc discharge plasma column established between the cathode and anode passes through the anode aperture. The arc discharge is stabilized by a pressurized flow of gas through the aperture. Radiation from the central plasma core may be viewed through the anode aperture, and through apertures in one or more secondary chambers located forwardly of the anode aperture, each secondary chamber being at a differential vacuum with respect to adjacent plasma chambers. In a self-stabilized arc discharge configuration, a window, of radiation transmissive material covers the anode aperture, with th plasma chamber containing an ionizable gas.

This invention relates to improved plasma radiation sources, and in particular, to a new plasma radiation source in which radiation from the central core of a plasma arc discharge column may be observed without occlusion from the surrounding layers of the plasma.

Although high intensity plasma-forming arcs have been known for well over fifty years, it has been only relatively recently that a renewed interest has been shown in high intensity plasmas and their characteristics. In part, this renewed interest has evolved from a technological need for more information regarding extremely high temperatures and the characteristics of sources of intense radiation. High intensity arcs, for example, have been used to simulate solar radiation and as a source of propulsion in supersonic and hypersonic wind tunnels. The present invention deals generally with a high intensity radiation source which may be used for the laboratory production and analysis of particular types of radiation. It is particularly useful for the generation of radiation in particular spectral regions, such as the copious ultraviolet radiation yielded from certain areas of an arc discharge plasma. I

tion sources, however, has been the occlusion or absorp-,

tion of the radiation in the desired spectral region by the cooler layers of the arc discharge or plasma column. For example, in the fairly conventional type of plasma source in which a plasma plume or flame is produced,

3,512,030 Patented May 12, 1970 the selected radiation (e.g., vacuum ultraviolet) in the central portion, or core, of the plume cannot be detected by ordinary instruments because of absorption of that radiation in other regions of the plasma.

Accordingly, it is an object of this invention to provide an improved plasma radiation source in which plasma radiation from a selected portion of the plasma column may be observed or analyzed.

A further object of this invention is to provide new and improved anode and cathode configurations for plasma radiation sources.

Yet a further object of this invention is to provide an improved type of fluid stabilized plasma radiation source.

An additional object of the invention is the provision of an improved ultraviolet radiation source.

These and other objects of the invention are accomplished by a plasma radiation source which includes, in one embodiment, a more or less conventional cathode having a forwardly converging surface defining a forward cathode tip and an anode providing an annular surface in close proximity to the tip and surrounding an aperture aligned with the layer or the portion of the plasma column from which radiation is desired. The annular anode surface is located forwardly and facing the cathode converging surface, and the anode aperture has an area of projection on a plane normal to the axis of the cathode which is substantially less than the area of projection on that plane of the cathode tapering surface. For best results, the anode aperture is nominally one millimeter or less, and the cathode tip is spaced from 2 mm. to 5 mm. behind the anode surface. Preferably, the angle formed between the cathode converging surface and normal plane is between about 60 and A plasma chamber, pressurized with a flowing gas for stabilizing the arc discharge, encloses the cathode tip and annular anode surface in a principal embodiment.

The plasma radiation source may include one or more secondary chambers communicating with and located forwardly of the anode aperture. Each of the secondary chambers includes an aperture in alignment with the anode aperture and the core of the plasma column, and is maintained at a differential vacuum with respect to the adjacent secondary and plasma chambers.

For a better understanding of these and other objects of the invention, together with the further aspects and advantages thereof, reference may be made to the following drawings, in which:

FIG. 1 is an elevational view in cross-section of a plasma radiation source according to the invention;

FIG. 2 is a schematic rendition of the electrodes of the FIG. 1 apparatus, showing the cathode, anode and plasma column on an enlarged scale;

FIG. 3 is an enlarged cross-sectional view of a portion of the FIG. 1 apparatus, illustrating a modification of the anode assembly when used with a radiation transmissive window located forwardly of the anode aperture;

FIG. 4 is a cross-sectional view of a further form of catliode suitable for use with the plasma radiation source; an

FIG. 5 is a cross-sectional elevation of a further em bodiment of a plasma radiation source according to the invention.

The plasma radiation source of FIG. 1 is of the fluidstabilized type and includes a plasma chamber 10 defined by the cylindrical wall 12, flanged at each end, as shown at 12a and 12b. Axially centered in the chamber 10 is a cathode 14 including a pointed tip 14a defined by the forwardly tapering, or converging, conical surface 14b. The cathode 14 is supported at a rearwardly projecting portion 14c by a cathode holder and cooling jacket assembly v16 which comprises simply a hollow heat con- 3 ductive tube 161: of copper, for example, having a cooling passage 17.

The cathode 14 may be constructed of any material such as thoriated tungsten, demonstrating good refractory characteristics, and is either press-fitted or silver soldered onto the cathode holder cooling jacket assembly 16. Alternatively, the tube 16a and cathode 14 can be fabricated as a unitary piece.

Disposed inside the passage 17 is a conduit 18 of smaller diameter Which opens into the passage 17 at its end nearest the cathode. The passage 17 terminates in a T-shaped fitting 20 through which a suitable coolant, such as water, may be introduced, as indicated by the arrow 22. During operation of the plasma radiation source, coolant is flowed under pressure through the inner conduit 18, as seen at 22, to extract heat from the cathode 14 and cathode holder 16, whereafter the fluid exhausts to the atmosphere through the passage 17 and outlet at 21.

Supporting the assembly 16 and closing the rearward end of the chamber 10 is a cathode holder flange, or end plate 23, which abuts an O-ring seal 25 seated in grooves 34a to prevent leakage of the gas circulating within the chamber. To facilitate easy removal and adjustment of the cathode, a seal 25a is provided also between the cathode assembly 16 and a central aperture 23a in the cathode holder plate 23. An inlet port 26 and an exit port 28 in the plate 23 provide openings for circulating and discharging inert gas through the chamber 10 from a suitable source (not shown) for the purpose of purging the system. As will be explained shortly, the exit port 28 is open only when gas flow through the anode aperture is impeded, the port 28 otherwise being closed by operation of a valve 66 during operation so that the gas flows from the inlet port 26 through the chamber 10 and anode aperture.

The anode assembly 30 includes an end plate 32 closing the forward end of the chamber and having formed in its center a small aperture 33 in coaxial alignment with the tip 14a of the cathode. A similar O-ring seal 34 abuts the flange 12b and forward end plate 32. The small anode aperture 33 opens into a larger cylin drical passage 35 away from the cathode tip and of greater cross-section than the aperture 33. Surrounding the small aperture 33 is an anodic disc 36 providing an inwardly tapering striking and operating surface 38 for the short are discharge formed between the cathode 14 and anode assembly 30 (see FIG. 2). Preferably, the angle a formed between the plane of the plate 32 and the surface 38 is less than the angle formed between the cathode surface 14b and that plane, the latter angle falling between 60 and 70 for best operation. The disc 36 fits into a complementary recess in the plate 32 and may either be press-fitted or silver-soldered in the recess, Alternatively, of course, the tapering surface 38 may be machined into the plate 32, in which case a separate disc 36 is not required.

For good results, the anode disc 36 should be fabricated from materials such as oxygen-free copper or thoriated tungsten, which demonstrate an ability to withstand high operating temperatures and have relatively good thermal conductivity for removing heat. Similarly the end plates 23, 32 may be constructed from any good heat conductive material, such as copper or brass. To this end, a passage 40 for coolant is formed in the plate 32 to extract heat from and around the anode surfaces and thus prevent excessive deterioration and erosion of the anode caused by the high temperatures generated in the arc discharge. This passage extends well into the center of the anode plate 32 to maintain the anode disc 36 as cool as possible, while the taper of the surface 38 presents a thin wall (i.e., the thinnest axial dimension) to the center of the plasma column, which is at the highest temperature. Flow of the coolant is from an inlet port 41a, through the passage 40 and out through the exhaust port 4 4111. (During operation, it will be understood that coolant is passed through the anode assembly 30 at a rate sulficient to prevent intolerable erosion of the parts of the anode near or exposed to the arc discharge.)

The forward side of the anode plate is closed by a plug 42, either threaded or pressed into a complementary recess in the plate 32 and silver-soldered in place, which seals the cooling channel 40 surrounding the wall of the passage 35. The plug 42 is seen to include an annular flange portion 44 forming an annular recess 45 between it and the end of the cylindrical wall defining the passage 35. Disposed in the recess 45 is a spacing ring 47 of suitable material, which may be replaced by a radiationtransmissive window for operation in a self-stabilized arc mode.

In the configuration shown in FIGURE 1, the exit port 28 is closed -by the valve 66 and the arc established between the cathode 14 and anodic disc 36 is stabilized by the flow of gas through the anode aperture 33. The shape of the plasma arc column will vary according to the flow mode employed. It is only when the discharge gas flows through the anode aperture 33 that it executes a stabilizing and constricting function on the shape of the plasma arc column. When no flow is permitted through the aperture 33, the arc column is of a different shape, being broader and self-stabilized (FIG. 3). Under such conditions, the plasma column shape is substantially independent of the flow between the inlet port 26 and open exit port 28.

FIG. 3 illustrates the arrangement of the anode assembly 30 for operation of the source in the self-stabilized arc mode. As seen there, the spacing ring 47 (FIG. 1) is replaced by a window 48 and annular seal 49 disposed in the recess 45, which blocks discharge of the gas through the aperture 33. As noted above, the exit port 28 is open during operation with this configuration so that the discharge gas is circulated internally of the chamber 12. The broader shape of the plasma column is observed under these conditions, since no fluid flow stabilization is employed. Radiation from only the central core of the plasma column is nevertheless realized, owing to the short arc configuration between the closely spaced cathode tip 14a and anodic disc 36. Among the materials suitable for construction of the window 48 are quartz and lithium chloride, both exhibiting suflicient transparency to radiation wavelengths found in the core of the plasma column.

When the arc is operated in the gas stabilized mode, it may be desirable to provide staged vacuum differentials downstream, or forwardly of, the anode assembly 30 to reduce absorption by the residual gases of the radiation viewed through the anode aperture. To this end, returning to FIG. 1, the radiation source may include one or more secondary chambers 50, 52 communicating with the anode aperture 33 and cylindrical passage 35 through apertures 54, 55 in respective end plates 57, 58 of the secondary chambers and in coaxial alignment with the anode aperture. The apertures 54 and 55 permit external viewing and transmission of radiation from the central core of the plasma adjacent the cathode tip 14a for utilization in laboratory instruments. Each of the chambers 50, 52 is connected through a respective conduit 60, 62 and a manifold 63 to a conventional pumping system 64 for exhausting the stabilizing gas from the chambers 10, 50 and 52. In this instance, the exit port 28- of the main plasma chamber 10 is sealed off by a valve 66 so that gas flows from the port 26 into the chamber 10, through the anode aperture 33, passage 35, secondary chambers 50, 52 and into the conduits 60, 62, as well as through the small apertures 54, 55. If desired, the pressures in the secondary chambers 50, 52 may be regulated, controlled or varied by any satisfactory orifice, valve or regulator in the manifold 63 between the exhaust conduits 60, 62.

In FIG. 3, where the temperature of the plasma column in the passage 35 exceeds the limit of such suitable window materials then the window 48 can be mounted on a plate further away from the arc column, as for example,

ever, the observation and analysis of radiation is desired in spectral regions such as the vacuum ultraviolet, where no optical material is known to transmit such radiation, the window 48 can be eliminated from use and the source operated with the anode configuration of FIG. 1. If a radiation transmission problem does not exist, then the are also can be operated with the anode aperture 33 and passage 35 directly open to the atmosphere, in which case the gas in the arcchamber 10 is kept at a pressure higher than atmospheric. 9

In accordance with the invention, the dimension of the small anode aperture 33 is such that the area of its projection on a plane normal to the cathode axis is substantially less than the projection of the tapering cathode surface 14b on that plane. For optimum performance in one radiation source, the diametric dimension of the aperture 33 was found to be about 1 mm., with the distance between the cathode tip 14a and anodic surface 38 ranging from 2 mm. to 5 mm. With this construction, the arc established is what may be termed a short are, and the outer layers of the plasma plume or column 68 (FIG. 2) caused by the high intensity arc discharge between the cathode tip 14a and anode portion 36 are blocked by the constricted configuration .of the anode, allowing the direct viewing of radiation from the central core 70 of the plasma. Since the highest (thermal) energy concentration occurs at the central core 70, intense radiation may be directly viewed and is made otherwise accessible through the anode aperture 33 and secondary chamber apertures 54 and 55, or through the window 48. In FIG. 3, depicting a self-stabilized arc, the column 68' is somewhat broader, and the plasma core 70 does not pass through the passage 35 because of the presence of the window 48. The desired selected radiation may nevertheless be transmitted through the window, subject to the transmission characteristics of the window material.

In operation, a voltage source, represented here by the battery 71, is connected between the cathode assembly 16 and the anode assembly 30 through a switch 72. Prior to closing the switch 72 the discharge gas is passed into the plasma chamber 10. When the window configuration of FIG. 3 is used, passage of the gas through the anode aperture 33 into the secondary chambers 50, 52 is prevented, and the valve 66 is opened to permit the inert stabilizing gas to exit from the port 28 after it has circulated through the chamber 10. If the window 48 is not used, the valve 66 is closed to block the exit port 28, and the stabilizing fluid is withdrawn through the anode aperture 33, passage, 35 and secondary chambers 50 and 52 by means of the pumping system 60-64. Also prior to closing the switch 72, a suitable coolant, e.g., water, is flowed through passage 17 and conduit 18 in the cathode assembly 16, and through the anode passage 40 to provide the amount of cooling necessary.

Once the switch 72 is closed, an arc is struck between the cathode 14 and the anode disc 36. This are is of very high intensity (i.e. high current density and energy content), and with the use of the stabilizing gas, the plasma column that would otherwise be narrow at the cathode and bell shaped toward the anode, is constricted to a cylindrical shape (FIG. 2) of approximately the same diameter as the anode aperture 33. The arc column constriction, effected through. the stabilizing gas flow, provides considerably enhanced radiation emission intensity from the core 70 of the arc column that is being observed through anode aperture 33, and the radiant power emitted by such a plasma exhibits good stability. Due to the stabilizing gas flow action, the central core of the arc plume carries substantially more current than the outer layers. As an additional aid to constricting the are a ring magnet (known in the art) may be placed around the plume.

It should be remarked that the dimensions and spacing of the anode and cathode are such that a short are discharge is established, the short are being characterized by the absence of a full plasma plume. In the plasma column created between the electrodes of the present invention, the outer layers of the plasma expanding outwardly are not allowed to converge forwardly of the tip, thereby avoiding occlusion of the plasma column core by the convergent outer layers in a fully developed plasma plume.

FIG. 4 shows an alternate form of cathode and cathode assembly which includes an open axial passage 75 from the tip of the cathode 14a to the rear of the assembly to provide a straight line of sight into the central core of the plasma. To accomplish this, the water cooling passages are rearranged such that the fluid flows in the passage 17' and exits from a concentric passage 74 defined between the inner wall 16b of the tube 16a and the central conduit 18.

FIG. 5 shows another form of the plasma radiation source in which a secondary chamber is formed downstream of the anode aperture by a surrounding chamber enclosure. In that embodiment, the plasma chamber 78 is formed by a cylindrical structure 80 including internal cooling ducts 81 for conducting cooling fluid to the anode plate 82 closing the forward end of the chamber. Cooling passages 84 in the plate 82 communicate with the ducts 81 so that coolant circulates around the wall surrounding the anode aperture 85. The anode striking surface 86 is similar to the surface 38 illustrated in FIGS. 1-3, except that it is smoothly contoured and machined in the anode plate itself. Seals 87 are provided at the abutting joints of the ducts 81 and passages 84, and are accessible by removing the fasteners (not shown) holding the plate 82 to the cylinder 80.

The cathode assembly 88, also, is similar to the assembly 16 shown in FIG. 1, comprising generally concentric conductive tubes 90, 91, the latter supporting the converging cathode end 92 which is concave at its rearward end 92a to promote smoother flow in the annular cathode cooling passage and better heat extraction from the cathode tip. At the rear end closure plate 93, here being unitary with the cylinder 80, a threaded cathode flange 94, constructed from a dielectric refractory material such as boron nitride, carries the cathode assembly 88 in a bushing 96 with'which the cooling fitting 98 is united. 0- rings 99, 100 provide pressure seals against gas leakage from the pressurized chamber 78.

Cooling ducts 81 communicate with bored passages, 102 in the rear end plate 93 through which cooling fluid from an external source (not shown) may be flowed, as shown by the arrows. Enclosing the primary chamber defined by the cylindrical wall 80 is a larger, coaxially mounted cylindrical chamber wall 104 which, at the rear end of the apparatus, abuts the annular flange 93a comprised of an extension of the end, or base, plate 93. The wall 104 includes cooling ducts 105 (only the inlet duct being shown) communicating with the passages 102 in the base plate 93 and also with passages 106 in a forward closure plate 108 suitably sealed and secured to the wall 104. Inside the forward plate 108, passages 106 (only one of which is illustrated) open into an annular duct 109 surrounding the wall defining an enlarged opening 110 aligned with the anode aperture 85. With this compact construction, a secondary chamber 112 is defined forwardly of the aperture 85, and both the forward plate 108 and plasma chamber are cooled through internal ducts communicating with the coolant inlet and outlet passages 102 in the base plate 93.

As noted briefly above, a window may be disposed forwardly of the anode plate so that the arc may be operated in the fluid-stabilized mode. In FIG. 5, a window 114 and seal 115 are positioned in a recess in the forward plate and held in place by a window cover plate 117 silver-soldered or otherwise fastened securely to the plate 108. A further seal 119 is provided between the outer and inner cylindrical walls 80, 104.

The basic plasma radiation source of FIG. 5, as thus far described, is adapted for operation in the fluid stabi lized arc mode by the flow of stabilizing gas from the lasma chamber 78 to the secondary chamber 112 through the anode aperture 85. Further, the source may be operated as a sealed system in the fluid stabilized mode, in which an ionizable gas is internally recirculated from the secondary chamber to the plasma chamber. In this event, small orifices 120 are provided in the cylindrical wall 80 of the plasma chamber, which communicate with the narrow annular channel 122 between the wall 80 and outer wall 104, and an exhaust port 124 in the secondary chamber 112 is sealed ofi after the chambers have been filled with any satisfactory inert stabilizing gas.

Once an arc discharge has been established between the electrodes, the stabilizing gas near the electrodes, heated and expanded by the high intensity are discharge, rushes through the anode aperture 85 to confine the lateral dimension of the plasma column. Depletion of gas from the plasma chamber 78 through the anode aperture 85 tends to create a slightly reduced pressure in the plasma chamber, which results in the influx of additional gas from the narrow annular channel 122 to continuously recirculate the stabilizing gas from the secondary chamber 112 to the plasma chamber.

In the second mode of operation, the orifices 120 are either plugged or omitted entirely, and a continuous supply of stabilizing gas is introduced through a conduit 125 and open valve 126 into an inlet port 128 in the cathode flange 94. The secondary chamber exhaust port 124 is vented to the atmosphere, as shown, so that the stabilizing gas flows from the inlet port 128 into the plasma chamber 7 8, and then through the anode aperture 85 into the secondary chamber 112 where it exhausts through the port 124. When internal circulation of the stabilizing gas is employed, the valve 126 is, of course, closed. In either case, only the selected radiation from the plasma column core is transmitted through the anode aperture and made accessible at the plasma source exterior through the window 114.

Although the plasma radiation source according to the invention may be used to directly view plasma radiation in different spectral regions, depending on whether the cathode is of the consumable or non-consumable type and the nature of the stabilizing gas, it has been found particularly useful in producing ultraviolet radiation. In a laboratory radiation source substantially identical to that shown in the drawings (FIG. 1), the chamber was operated in the windowless configuration (i.e., with the window 48 being absent) at a pressure in the chamber 10 of about 700 mm. Hg maintained by the circulation of an argon discharge gas. Secondary chamber 50 was pumped down to a pressure of 0.2 mm. Hg and secondary chamber 52 to a pressure of 10 mm. Hg. The vacuum ultraviolet instrumentation attached to secondary chamber 52 and monitoring radiation passing through apertures 35, '54 and 55 was at a pressure of about 'l0- mm. Hg. Under those conditions, a photon flux of .5 l photons/sec.-A. was measured with a power input of about 1.5 kw. A change in the power input level from 2 kw. to 4 kw. resulted in a doubling of the photon flux. Raising the arc chamber pressure to 1794 mm. Hg, the photon flux measured photons/sec.-A. with substantially the same power input (1.5 kw.). In general, radiation intensity levels of 10 photons/m. -sec. per angstrom unit have been observed from the apparatus in the spectral region between 1,000 A. and 2,000 A. Between 200 A. and 1,000 A., photon fluxes of 10 photons/mF-sec. per angstrom unit have been measured. Although in the foregoing examples, the radiation occupies the near ultraviolet, ultraviolet and vacuum ultraviolet regions of the spectrum, appreciable radiation is emitted in the visible infrared regions, as well.

From the foregoing, it can be appreciated that the invention provides a practical tool for laboratory and simulation purposes in producing and providing useful radiation near and below the visible wavelength spectrum. This is primarily made possible by the use of an electrode structure in which the area of the anode aperture is made considerably Smaller than the diameter of the working part of the cathode, thus permitting transmission of radiation from the central core of the plasma, but blocking radiation transmission from the cooler layers of the plasma which tend to occlude or absorb the desired radiation.

Although the invention has been described with reference to specific embodiments, certain variations and modifications in form and detail may be made within the skill of the art. Accordingly, all such variations and modifications'are intended to be included Within the scope and spirit of the appended claims.

I claim:

1. A plasma radiation source for the production of selected high intensity radiation, comprising:

a cathode having a forwardly converging surface defining a cathode tip;

an anode providing an annular surface surrounding an aperture located forwardly of the cathode tip; the aperture having an area of projection on a plane normal to its axis which is substantially less than the area of projection on the plane of the cathode converging surface;

a plasma chamber enclosing the cathode tip and annular anode surface;

means for establishing a continuous high intensity are discharge between the cathode tip and the annular anode surface, the cathode tip and anode annular surface being closely spaced sufliciently to establish a diverging short arc discharge therebetween producing a plasma column of high intensity radiation of which only direct radiation from the central core of the column passes through the anode aperture and cooler, outer plasma regions spread outwardly and contact the anode radially outwardly from the anode aperture.

2. A plasma radiation source as set forth in claim 1, in which:

the annular surface tapers inwardly and forwardly and is unbroken about the aperture;

said surface defining means for preventing convergence of cooler plasma regions and for providing a distribution of cooler, outer plasma layers surrounding the anode aperture radially outwardly therefrom.

3. A plasma radiation source according to claim 2, in which:

the angle formed between the anode tapering surface and the plane is less than the angle formed between said plane and the cathode converging surface.

4. A plasma source as set forth in claim 2, in which:

the axial thickness of the anode bordering the anode aperture is less than the axial thickness of the anode at points away from the anode aperture.

5. A plasma radiation source as defined in claim 2, in which:

the anode has a passage therein for coolant for extracting heat from and adjacent the annular anode surface.

6. A plasma radiation source as defined in claim 1, further comprising:

means for pressurizing the chamber interior with a flowing gas.

7. A plasma radiation source according to claim 6,

further comprising:

means defining a passage forwardly of and communicating with theanode aperture, but being of greater cross-section normal to the axis than the anode aperture.

1 8. A plasma radiation source according to claim 7, further comprising:

window means disposed in the :forward passage for preventing substantial gas flow through the anode aperture and having a property which transmits radiation from the central plasma core.

9. A plasma radiation source as defined in claim 1,

further comprising:

means defining a secondary chamber communicating with the plasma chamber and including an aperture therethrough in alignment with the cathode tip and anode aperture through which radiation from the central plasma core may travel.

10. A plasma radiation source as defined in claim 9,

further comprising:

means for exhausting from the secondary chamber the gas flowing thereinto from the plasma chamber through the anode aperture.

11. A plasma radiation source according to claim 9,

further comprising:

at least one further secondary chamber forward of the first secondary chamber and communicating with the aperture therethrongh, the further secondary chamber also having an aperture in alignment with the anode aperture for viewing the plasma radiation, the secondary chambers being eifective to establish a dilferential pressure between each thereof and the plasma chamber.

12. A plasma radiation source for the production of selected high intensity radiation, comprising:

a cathode having a forwardly converging surface defining a cathode tip;

an anode providing an annular surface surrounding an aperture located forwardly of the cathode tip, the aperture having an area of projection on a plane normal to its axis which is substantially less than the area of projection on the plane of the cathode converging surface;

a plasma chamber enclosing the cathode tip and annular anode surface;

means for establishing a high intensity are discharge between the cathode tip and the annular anode surface, the arc discharge producing a plasma column of high intensity radiation of which only radiation from the central core of the column passes through the anode aperture;

an enclosure surrounding the plasma chamber defining a secondary chamber communicating with the first plasma chamber and including an aperture therethrough in alignment with the cathode tip and anode aperture through which radiation from the central plasma core may travel.

13. The plasma radiation source of claim 12, further comprising:

a base plate secured to the plasma chamber and having an internal passage for flowing a liquid coolant thereou the plasma chamber and anode having an internal duct therein communicating with internal passage to be cooled by a coolant flow through the base plate.

14. The plasma radiation source of claim 13, in which:

the secondary chamber enclosure has an internal duct communicating with the internal passages in the base plate to thereby be cooled by coolant flow through the base plate.

15. The plasma source defined in claim 14, in which:

the secondary chamber aperture is located in a plate closing the forward end of the secondary chamber,

the plate having an internal cooling passage in com- 10 l munciation with the internal duct to be cooled by the coolant flow therethrough.

16. A plasma radiation source as set forth in claim 9, further comprising:

window means closing the aperture in the secondary chamber and being transmissive of the selected radiation from the core of the plasma column. 17. The plasma radiation source of claim 16, further comprising:

means for internally recirculating a stabilizing gas between the secondary and plasma chambers. 18. The plasma radiation source of claim 9, further comprising:

an ionizable gas filling the plasma and secondary chambers, the plasma chamber including an orifice communicating with. the interior thereof and the secondary chamber to permit internal recirculation of the gas between the secondary and plasma chambers under the in- I fluence of the arc discharge.

19. A plasma source as defined in claim 1, in which:

the cathode has an axial aperture extending therethrough through which radiation from the central plasma core may be viewed from a position behind the cathode tip.

20. A plasma source as defined in claim 11, further comprising:

means supporting the cathode in the chamber and having inlet and outlet passages for flowing a coolant in contact with a portion of the cathode.

21. A plasma radiation source for the production of selected high intensity radiation comprising:

a plasma chamber;

a cathode having a forwardly converging surface defining a cathode tip and a rearward extension having an axial aperture therethrough communicating with a region of the chamber forwardly of the cathode tip to provide a line of sight thereto through the cathode from outside the chamber;

an anode having an annular surface forwardly of the cathode tip;

means for establishing a short high intensity arc discharge between the cathode tip and the annular cathode surface to produce a plasma column of high intensity radiation in said region.

22. A plasma radiation source according to claim 1,

in which:

the anode aperture has a cross-sectional dimension which is less than one half of the distance between the anode surface and cathode.

23. A plasma radiation source according to claim 1,

in which:

the distance between the cathode and anode surface is between about two millimeters and five millimeters and the largest cross-sectional dimension of the anode aperture is less than two millimeters.

24. A method for producing selected high intensity radiation, comprising:

establishing a short electrical arc discharge between a pair of electrodes;

operating said are discharge to produce a column of plasma radiation in the vzone of the arc discharge; and

restricting radiation from the plasma column, at points remote from the arc discharge in at least one direction, to radiation directly from the central core of the plasma column.

25. A method as set forth in claim 24, in which:

radiation to said remote points is restricted by preventing convergence, forwardly of the arc discharge, of the plasma column boundary.

26. A method according to claim 24, in which:

the arc discharge is established between a cathode tip and an annular anode surface defining an aperture forwardly of the cathode tip; and

the step of restricting includes establishing a cooler plasma region distribution surrounding the aperture where the cooler plasma meets the annular anode surface.

27. A method as defined in claim 26, further comprisstabilizing the arc discharge by flowing gas in the vicinity thereof and through the aperture.

28. A method according to claim 26, in which:

the annular anode surface tapers inwardly and forwardly of the cathode tip.

29. A method as set forth in claim 26, in which:

the cathode tip is defined by a. forwardly tapering cathode surface and is generally concentric with the annular surface on a common axis.

30. A method as set forth in claim 26, in which:

UNITED STATES PATENTS 2,911,567 11/1959 Fischer 3l5111 X 3,172,000 3/1965 Rosener et a1. 315111 X 3,225,245 12/1965 Takei et a1. 313231 RAYMON-D F. HOSSFELD, Primary Examiner US. Cl. X.R.

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