Apparatus for maintaining vacuum conditions by molecular depletion of gas

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention has to do with control of gas pressure in vacuum devices which in use have exposure to higher pressure regions. In a particular aspect the invention is concerned with method and apparatus to buffer devices requiring continual vacuum condition for operation from a greater pressure region adjacent the device and on which region, in some instances, the device itself acts. Prominent among devices utilizing the invention are electron beam devices, as will be described, and accordingly the ensuing description treats of such devices in some detail, but the principles underlying the invention have applicability to various devices in which prevention of ambient gas incursion is necessary or desirable.

In general, an electron beam device comprises an electron beam source such as a cathode ray tube or electron gun and a path for the beamed electrons, leading to a target. An electron beam is a quantity of electrons moving in parallel paths in a suitable current density. An electron gun is a suitable arrangement of anode and cathode whereby electrons flow from the cathode toward and beyond the anode in parallel paths at a suitable density usually under the influence of a magnetic or an electrostatic field. Most electron beam apparatus generate and project beams in a substantially complete vacuum, i.e. pressures of 10—^!mm. Hg. Some applications, however, require electrons to be projected from a source in a vacuum to a target zone wherein pressure is greater than that in a substantially complete vacuum, e.g. pressures of 10—’mm. Hg or even at standard pressure, i.e. 760 mm. Hg.

In these last-mentioned applications, such as an electron welding or paint curing apparatus or a densitometer for high altitude air measurement, two problems arise which limit the effectiveness of the device. First, it is necessary to protect the electron source from contact with deleterious components of the ambient atmosphere which may react with and consume the electron emitter; this requires substantially complete vacuum to be maintained around the electron beam source. Second, it is necessary, for greatest efficiency and maximum electron beam intensity at the target area, that interference with the electron beam such as is caused by the presence of molecules of gas be minimized between the electron beam source and the target.

It is known that a gas molecule struck by an electron moving at sufficient velocity will give up an electron of its own, thereby becoming ionized. The gas ion carries a positive charge and exists in a mass of negatively charged electrons. While this presence tends to relieve mutual repulsion forces tending to spread the electrons constituting Ute beam, the ions, once captured in the beam, stay for long periods due to their relatively large mass and the small cathode attraction existing beyond the anode. The presence of gas ions and molecules in the electron beam is not desirable.

2. Prior Art

It has been known heretofore, e.g. U.S. Pat. No. 2,841,726 to Knechtli, to provide along the electron beam path in an electron beam apparatus a series of intercommunicating chambers provided with apertures in line which define the electron path. To each of these chambers or zones in past practice there has been connected a vacuum or diffusion pump which evacuates the particular chamber or zone to the desired (minimum) gas pressure level so that a gradient of gas content, and thus pressure, exists along the electron beam path from beam source to target. In addition to the expense involved in acquiring and maintaining a great many pumps, one being required for each chamber, there is a problem of mechanical vibration, too great weight and bulk in the electron beam apparatus since the pumping mechanism is, of necessity, large and heavy and composed of moving parts. Certain uses of electron beam apparatus, such as upper atmosphere densitometry, require extreme light weight and miniaturization; all beam applications, however, can benefit from increased handling ease and reduced capital cost resulting from elimination of cumbersome auxiliary pumps. There is accordingly, a need for an electron beam assembly operatively free of auxiliary vacuum pumping equipment wherein an electron beam is projected from the beam source at a high vacuum to a gas containing target which is at a lesser vacuum with minimum interference from gas in the beam path and without deleterious gas elements reaching the beam source.

SUMMARY OF THE INVENTION

Accordingly, it is an object to provide vacuum protection in devices requiring it without resort to mechanical devices. Another object is to provide such protection without the requirement of frequent or awkward reconditioning for reuse. A further object is to achieve by physical manipulation of a gas stream and strategic arrangement of a gas collector, molecular depletion of a gas in advance of a vacuum operating device. A further object is to provide a pressure buffer system which is repeatedly effective and restorable to original capabilities by simple expedients. It is a highly specific object to utilize both continuum flow and molecular flow characteristics of gases to achieve molecular depletion of a gas moving toward a vacuum region.

It has now been found that incursive gas flows in vacuumoperating devices are controllable by sequentially expanding the gas, first in a continuum flow state and adsorbing and second in a molecular flow state and adsorbing.

Thus in apparatus comprising a vacuum-operating device having in use an exposure to gas pressure, the invention provides fluid tightly secured thereto means for protecting the device from the gas during an operating period. This means includes generally a series of chambers, such as two in line chambers, defining the path of exposure to the device. The chambers are connected, e.g. with capillary tubes or otherwise to provide gas-restrictive intercommunication therebetween. Thus there is provided for incursive gas sequential expansion along the exposure path, first in a first chamber, followed by passage along the capillary or similar passageway and a second expansion in a second chamber and so on toward the device. Gas collector means are provided within the chambers including an adsorptive surface coating on the walls of the chambers.

A series of capillary passages, in coaxial alignment may be provided leading through the walls of the chambers from the gas pressure to the device e.g. for the conduct of electrons during use of the device and coincidently as a path for gas during such use. First chamber means associated with a first capillary passage to the gas pressure may be provided having circumferentially and radially disposed therein sufficient gas collector means to adsorb substantially all of the gas entering the first chamber. Downstream thereof there is provided a second chamber containing radially disposed sufficient gas collector means to adsorb all gas entering the second chamber from the first chamber for a second expansion. The second chamber communicates with the first chamber through a second capillary means extending between the first and second chambers and opposite the first capillary means across the first chamber.

The first chamber typically is larger than the second and may have a central orifice leading from the first capillary passage formed in a chamber wall which is dished to configure entering gas into a plume of continuum flowing gas. Sufficient gas collector presence in the first chamber e.g. to adsorb 99.9999 volume percent may be ensured by provision of radially arranged fins about the through passage through the chamber along the axis of the aligned capillary passages, the fins along with the walls affording adsorptive surface by virtue of the gas so collector thereon.

The gas collector typically is a thin layer of gas porous carbon granules, e.g. a monogranular layer adhered in a suitable manner to the walls of the chambers, e.g. by a polymer based adhesive resistant to oxidation and temperature variation. Gas adsorption being an exothermic process it is desirable to pro

3,555,331 vide a heat sink adjacent the gas collector to insure continuing adsorption. This may take the form of subcooled fluid e.g. liquefied inert gas such as nitrogen in heat transfer contact with the gas collector.

In its method aspects the invention contemplates protecting a vacuum operating device from gas incursion during operating periods in which the device is exposed to ambient gas pressure along a narrow passageway by repeatedly expanding and adsorbing the gas in a sequence of zones formed in the passageway to molecularly deplete the gas in advance of the device.

The term “molecular depletion” herein has reference to removal of gas from a space by adsorption to the extent of achieving vapor pressures of 10—⁵ or even lOAnm. Hg and lower.

The term “adsorption” herein refers to accumulation of a normal gas from the vapor state onto a surface from which it is removable by heating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of an apparatus according to the invention;

FIG. 2 is a transverse section thereof taken on line 2-2 in FIG. 1; and

FIG. 3 is a fragmentary view somewhat enlarged of a section of the wall of the apparatus shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Provision of relatively small or narrow passages, e.g. capillary tubes between intercommunicating chambers leading from a region of higher pressure to a one of high vacuum (10—“mm. Hg.), results in incursion of gas from the higher pressure zone being much reduced by the natural tendency of gas molecules to disperse beyond the passage orifice and for relatively few molecules to travel to the next succeeding orifice. While this effect serves to substantially lessen pressures between adjacent chambers, for example, by a factor of 10, unless supplementary means are employed, sufficient gas will be present after a reasonable number of chambers to interfere with an electron beam or to cause deterioration of an electron beam source. Thus, in electron beam apparatus, it is not sufficient merely to provide a series of chambers which are intercommunicated by a series of gas flow restrictive orifices. Means must be provided for removal from the chamber spaces of entering gases. As explained above, mechanical pumps have been used for ultimate evacuation with limited success.

With reference now to the drawing, in FIG. 1 there is shown a vacuum-operating device 10 representative of an election beam source, plasma gun or other gas sensitive device which is typically exposed, if only briefly, e.g. 20 seconds, in use to a higher pressure region, e.g. 100 mm. Hg indicated at 12. A buffer against the disparate pressure causing loss of vacuum is provided in the form of pressure buffer apparatus 14 secured fluid tightly to the vacuum operating device 1®.

The pressure buffer apparatus 14 is seen to include a first capillary tube 16 extending between the higher pressure ambient gas at 12 and the interior of chamber 18 through transverse wall 18α, forming orifice 16«. The capillary tube 16 in a typical device will measure 4 mm. in diameter and 40 mm. in length. Such a capillary tube will pass about 0.5 liter/second from a 100 mm. Hg. pressure region such as indicated at 12. The chamber 18 is referred to herein as the first or outer chamber; it is the continuum flow chamber.

Coaxial with the first capillary tube 16 is a second capillary tube 20 extending between the first chamber 18 and the second somewhat smaller inner chamber 22. As shown the capillary tube may take the form of a narrow bore in a common wall 18ft between chambers 18 and 22. Coaxial with capillary tubes 16 and 20 is a third capillary tube 24 leading directly to the vacuum region, not shown, in device 1®. The several capillary tubes and the chambers 18 and 22 define the exposure path or through passage for gas to move toward the device.

It will be readily appreciated that in general the gas entering at capillary 1.6 and emerging into chamber 18 at orifice 16α will disperse or expand, with a consequent drop in pressure. A small portion of the gas will flow to the second capillary 20 and emerge into chamber 22 at orifice 20α. A second expansion will take place, further lowering the pressure.

Nonetheless, without active removal of gas molecules from the chamber volumes gas will enter capillary tube 24 and reach the device 1®. To prevent this the present apparatus adsorbs the emergent gases in chambers 18 and 22 following their expansion. That is gas molecules are not permitted to wander aimlessly within these chambers 18 and 22 but are positively removed by adsorbing contact with gas collector.

To make the effectuation of molecular depletion herein more clear, the invention may be described in terms of gas dynamics. The extremes of the spectrum of gas density are represented by continuum flow at the greatest density and molecular or free-molecular flow at the lowest or least density. In a continuum flow chamber such as chamber 18, a gas plurne, shown by dotted line 26, is formed on entry thereinto of the gas from the moderate pressure region 12. By reason of the gas moderate density, the constituent gas molecules have very small mean-free-paths or, statistically speaking, short distances on average between molecular collisions. These collisions drive the molecules eventually to the chamber wall where continuing removal of the molecules, if provided as herein, reduces the number of molecular collisions. The nature of physical adsorption is such that the ratio of adsorbed gas mass to the mass of adsorbent material increases exponentially with pressure. By maintaining a moderate pressure in the continuum flow chamber 18, this adsorption mass ratio can be made as high as 0.1. That is 10 parts of adsorbent can adsorb 1 part of gas. For example, 10 grams of adsorbent could adsorb 1 gram of air which is about 10 liters of air at a pressure of 100 mm. Hg. Assuming capillary 16 to have the dimensions given above, about 0.5 liter/second of air will enter chamber. Therefore, for each gram of adsorbent in the chamber, the gas plume 26 entering can be adsorbed for 2 seconds, and so 10 grams of adsorbent would sustain (adsorb) the plume for 20 seconds.

Continuing the gas dynamics desiderata of the invention, it is known that the gas density (pressure) of the plume 26 in chamber 18 decreases with distance from entrance orifice 16α. Thus at a distance on the order of 20 orifice 16α diameters, the gas density will be reduced by a factor of 10,000 (10⁵) provided gas is continually removed from the chamber 18 by adsorption.

The capillary 16 itself effects a pressure drop typically by a factor of 10 so that the pressure at orifice 16α where the capillary 16 is open to 10 mm. Hg. gas pressure will be on the order of 10 mm. Hg. At the outer portion of the gas plume 26 then the pressure will be reduced by the factor of 10—“, as explained, or the pressure at that portion will be 10—¹ (=1®—“ X I®¹). This portion of the plume 26 is at the second capillary tube 2® and, if we assume no pressure drop across this capillary, at orifice 2©a in chamber 22. At such pressure, freemolecular flow or expansion takes place, thus chamber 22 is a free-molecular-flow chamber. Most molecules proceed directly to the walls 18b, 28 and 3® of the chamber 22 without intermolecular collisions, hence the term “free-molecular” flow. Intermolecular collisions if they occurred might tend to drive molecules through the capillary tube 24 and to the device 1® and therefore it is important to preserve a freemolecular flow condition in the second chamber 22. This can be achieved by insuring a nearly total adsorption or near unity probability of gas molecules striking any of the walls of chamber 22. Such high probability of capture of gas molecules requires a very low incidence of molecules in chamber 22. For example, to maintain a pressure of 10-⁰ mm. Hg. at the absorbent surface, i.e. a back pressure there of 10—⁰, the mass ratio of adsorbed mass to adsorbent mass must be kept low, e.g. no higher than about 10-“ (c.f. the 0.1 or 10—’ ratio for chamber 18). Thus the chamber IS should remove 99.9999 volume per cent of all incoming gas to the apparatus. Then, if the original

3,555,331 air intake rate, through orifice 16α, is 0.5 liter/second or 0.05 gm./sec. at 100 mm. Hg. the intake through orifice 20α would be at the rate of 5 x Hg—⁷ gr./sec. With the desired adsorption ratio of 10—⁵ one gram of adsorbent in the molecular flow chamber 22 can sustain a pressure no higher than 10—⁶ mm. Hg for 20 seconds, matching the flow through continuum flow chamber 18 and protecting the device 10 from an increase in pressure during a 20 second exposure.

The arrangement of chambers having been described, we turn to the gas collector. It has been noted that proper operation of the continuum flow and free-molecular flow chambers 18 and 22 is largely dependent on continual removal of gas molecules at the chamber walls. To achieve efficient removal, the buffer apparatus is provided with a gas adsorbing lining or gas collector means, typically taking the form of highly gas adsorbing material, e.g. a highly gas porous granular material such as a low density form of carbon for example, charcoal, lamp blacks, channel blacks, acetylene blacks, furnace blacks and the like. The effective portion of such a liner is the surface portion, so in practice a monogranular layer, i.e. a single layer of granules about 1.5 mm. diameter is utilized and preferably of charcoal form carbon. These granules are adhered to the chamber walls in any desired manner with fluid settable adhesives offering oxidation resistance and retention of adhesion through a wide temperature range being preferred. Most suitable adhesives will be synthetic polymers containing combined olefinically unsaturated monomers having polar groups, e.g. carboxyl, ester, carbonyl, hydroxyl and like groups copolymerized with olefin monomers such as ethylene, vinyl and styrene monomers. Preferred adhesives will have a negligible vapor pressure for obvious reasons.

Entry of adsorbent grains into the capillary passages is prevented by funnel-shaped screen 32 extending around the exposure path axis 34 and through the chambers 18 and 22.

There must of course be an adequate surface amount of the gas collector. Under the gas flow conditions outlined above, full gas adsorption can be achieved with 0.05 gm./cm.² of granulated natural charcoal. In the continuum flow chamber 18, 200 cm.² of surface will be required to support 10 gms. of sorbent. In the free-molecular chamber 22, to support the required 1 gm. of solvent 20 cm.² of surface will be required. The walls 186, 28 and 30 of chamber 22 generally will afford sufficient wall area for the required adsorbent. Thus, the chamber 22 is typically cylindrical having transverse circular walls 186 and 30 and a cylindrical wall 28. All are coated monogranularly with the charcoal.

The outer chamber 18 requiring considerably more charcoal surface is generally not conveniently enlarged to provide the needed area. Rather, internal walls shown in FIGS. 1 and 2 as fins 36 are provided within chamber 18. Fins 36 are mounted within chamber 18 in cruciform fashion, if four in number, as shown. Two or six to eight fins may be used. The fins are shaped to provide increasing surface area toward the common wall 186. In cross section the fins are truncated triangles with the base adjacent the common wall 186. This provides a corresponding apparent conical gas expansion area 18* in the chamber 18 defined by the inward edges 38 of the fins 36. The base of the conical area is formed by the chamber wall 18α and is desirably configured to encourage plume-type flow in the chamber 18. To this end, wall 18α is dished about the orifice 16α therein to have recess 40 which aids in shaping the plume 26. Walls 18α and 186 are monogranularly coated like fins 36 and the walls of chamber 22.

Gas adsorption is an exothermic process and accordingly heat removal is important to effective adsorption. To effect heat removal, the buffer apparatus 14 is provided with a coolant coil 42 extending spirally about the exteriors of chambers 18 and 22. The chamber walls are of metal typically and thus good heat transfer is secured between the coil 42 and the chamber interiors. Referring to FIG. 3, coil 42 is shown secured abutting wall 28 of chamber 22. Adhesive 44 carries a monogranular layer of adsorbent 46. On gas adsorption this arrangement, providing a thin heat path, carries heat from the granular adsorbent 46 through the adhesive layer 44 and the chamber wall 28 to the coolant coil. Suitable coolants are subcooled or liquefied gases such as CO₂, N₂ and the like.

It is a signal feature of the present apparatus that restoration of adsorptive characteristics is readily accomplished by simply heating the adsorbent material following a use and evacuating the erstwhile adsorbed gases.

To prepare the present apparatus for use, then it is connected fluid tightly to a vacuum requiring device, evacuated e.g. to 0.1 Hg. with a mechanical pump and of desorbed gases liberated by heating the adsorbent. Once thus purged, the orifice 16α is sealed ready for use. Coolant is piped into coil 42 and the apparatus is ready for operation.

Upper atmosphere densitometry is an illustrative use of the just described apparatus, but other operations where electrons are beamed from a source to a target at higher pressure such as electron beam welding and paint curing may be similarly carried out.

The preconditioned buffer apparatus 14 is fitted to the device 10 and still sealed is placed in a support such as a rocket nose provided with sensing means. When the altitude at which gas density measurements are to be taken is reached, an electron beam is generated in and projected along the exposure path axis 34 defined by the capillary tubes 16, 20 and 24. Outer orifice 16α is opened by the electron beam burning through its closure. At this point, ambient gas rushes into the evacuated chamber 18 as the electron beam passes countercurrently out of the chamber. The entering gas from area 12 passes through orifice 16α and disperses in the chamber 18 just beyond the orifice. Some gas penetrates to the next chamber 22. The quantity of gas admitted to that chamber is dependent on the size of orifice 20. Gas molecules entering the chamber 18 and dispersing collide with the chamber walls 18α, 6 and c and fins 36. The adsorptive layer adsorbs these molecules, effectively depleting the air or other gas of its constituent molecules, lowering the pressure within the chamber.

The electron beam projected through outer orifice 16α collides with gas molecules present dislodging electrons and forming ions. The resulting ionized gas has a familiar luminescent glow. The intensity of the glow is of course proportional to the number of ions, which is proportional to electrongas molecule collisions and thus to gas density.

The luminescence is measured by receiving the light generated and sensing its strength in a photomultiplier. Results may be telemetered to the ground or stored in the device 10 for later recovery. Other electron beam induced changes in the gas or other target material such as conductivity can be measured in addition to or in place of luminescence.