US2991389A - Cesium ovens - Google Patents

Description

July 4, 1961 E. F. GRANT ET AL CESIUM OVENS 2 Sheets-Sheet X Filed Jan. 16, 1959 FIG. 3

FIGZ

EUGENE F. GRANT GORDON E. SIMPSON ARTHUR O. MCCOUBREY ROBERT S. BURITZ INVENTORS BY W W, d/M HM 9 ATTORNEYS July 4, 1961 E, F, GRANT ETAL 2,991,389

\CESIUM OVENS Filed Jan. 16, 1959 2 Sheets-Sheet 2 EUGENE F. GRANT GORDON E. SIMPSON ARTHUR O. MCOOUBREY ROBERT S. BURITZ INVENTORS BY WW9 W g/ AM ATTOR N EYS United States Paten 2,991,389 CESIUM OVENS Eugene F. Grant, Marblehead Neck, Gordon E. Simpson, Melrose, and Arthur 0. McCoubrey and Robert S. Buritz, Topsfield, Mass., assignors to National Company, Inc., Malden, Mass., a corporation of Massachusetts Filed Jan. 16, 1959, Ser. No. 787,285 12 Claims. (Cl. 313-431) beam frequency standard which utilizes the interaction of microwave energy of a particular wave length with a beam of cesium atoms in a frequency-determining arrangement. Such apparatus is fully described in the copending application of Jerrold R. Zacharias, et al., Serial No. 693,- 104, filed October 22, 1957, for Molecular Beam Apparatus and assigned to the assignee of the present application. To understand the present invention, it is sufficient to state that during operation of the frequency standard, a flow of cesium atoms must be maintained through an elongated beam tube, certain portions of which .are illuminated by microwave radiation. The cesium atoms, in gaseous form, are obtained from a reservoir of liquid cesium and evaporated by heat from a suitable source such as an electrical heating element. After evaporation from the reservoir, the cesium molecules pass through a collimator which aligns the molecules into beam for passage through the beam tube.

Frequency standards of the above type have found increasing application in moving vehicles, such as airplanes, where they are subject to severe inertial forces including substantial vibration as well as complete change of orientation. Motion of this type has, in prior constructions, caused spillage of liquid cesium from its reservoir. Since the only supply of heat for evaporation of the charge is, in prior constructions, intimately associated with the reservoir, the cesium lost therefrom remains in liquid form or reverts to the solid state and is therefore of no further use in the frequency standard. The time during which the standard may be operated before being recharged with cesium is consequently materially reduced. The reliability of the apparatus is also adversely affected by the:fact the spilled cesium may lie in portions of the beam tube and associated components of the frequency standard in such manner as to cause serious malfunctions.

Accordingly, it is a principal object of our invention to provide an improved molecular beam source adapted to supply molecular cesium or the like in a molecular beam frequency standard. A further object of the invention is to provide a reservoir designed for incorporation in a beam source of the above character and adapted to retain a charge of liquid cesium therein under conditions of intense inertial forces and changes or orientation. Another object of the present invention is to provide an improved beam source of the above character adapted to prevent leakage of liquid cesium into the frequency standard. Since the frequency standard may be operated in moving vehicles such as airplanes or the like, the source should be small in size and capable of lightweight construction. Simplicity of design is also a desired feature 2,991,389 Patented July 4, 1961 objects and features of our invention will in part be obvious and will in part appear hereinafter.

Our invention accordingly comprises the features of construction, combination of elements, and arrangements of parts which will be exemplified in theconstructions hereinafter set forth, and the scope of the inventionwill be indicated in the claims.

For a fuller understanding of the nature and objects of the'inventiomreference should be had to, the follow ing detailed description taken in connection with the accompanying drawings in which:

FIGURE 1 is a sectional view of a molecular beam source made according to our invention;

FIGURE 2 is a sectional view taken along line 2-2 of FIGURE 1, showing in detail a collimator structure which may be used with the cesium ovens of our invention;

FIGURE 3 is a view,.partly in section, taken along line 3-3 of FIGURE 1 and illustrating in plan a baflie assembly used to prevent escape of the cesium charge from a reservoir of liquid cesium; and

FIGURE 4 is a sectional view of a second embodiment of a molecular beam source embodying the principles of our invention.

In general, a molecular beam source embodying the features of our invention includes a reservoir containing a charge of liquid cesium and a heating coil which evaporates the cesium to provide the molecules comprising the beam these being collectively referred to as an oven. The reservoir has a baffle plate isolating it from other portions of the source to prevent the flow of liquid cesium into these portions. The baffle plate is provided with narrow tubular members extending therethrough to permit the flow of cesium vapors from the reservoir, but the tubes are so dimensioned and positioned as to prevent flow of the liquid metal through them even when the beam source is completely inverted or subjected to intense vibration. The source also includes a valve adapted to cut olf the flow ofcesium vapors from the oven when the frequency standard is not in use and a collimator which confinesthe molecules issuing from the source to a narrow well-defined beam. It will be understood that while the apparatus tobe described below in greater detail has been incorporated in a molecular beam frequency standard using. an atomic resonance of the cesium atom, the present invention is not limited to use with this particular element. Thus, our beam source may in many cases be used with frequency standards utilizing the characteristics of others of the alkaline metals or other materials having the proper molecular characteristics for use in frequency standards, as noted in the above-identified application, Serial No. 693,104. I

:FIGURE 1 illustrates a molecular beam source generally indicated at 2. The source includes a cesiumreservoir generally indicated at 4, a valve generally indicated at 6, and a collimator assembly generally indicated at 8. During operation of the source, a cesium charge 10 is slowly evaporated by a heating coil '11, and gaseous cesium atoms freed thereby pass to the left (FIGURE 1) to the valve 6 and thence upwardly through the collimator assembly 8 where they are formed into a narrow beam projected into other portions of the frequency standard (not shown).

7 More particularly, the source 2 has a cylindrical block generally indicated at 12 recessed at one end to house the cesium reservoir 4. Achamber 13 in theother end of block 12 accommodates the valve 6. The collimator i assembly 8 is secured to block 12 and includes a collimainasmuch as it facilitates low cost fabrication. Other tor 14 communicating with chamber 13. The latter is connected to reservoir 4 by a passage 16. A plug 18 brazed to block 12 at its right hand end seals the end 0 reservoir 4 and accommodates a filler tube 20.

Reservoir 4 comprises three chambers 22, 24 and 26 separated by bafiie plates 28,30 and 32. The latter may i i be suitably located in position by annular spacers 34, 36 and 38. As seen in FIGURES l and 3, communication between chambers 22 and 24, 24 and 26, and between chamber 26 and valve 6 is provided by a series of tubes 40 extending through and secured to the respective baffles. The charge of liquid cesium is retained in chamber 22 and gaseous cesium is evaporatedtherefrom by heat from the coil 11, which, as seen in FIGURE 1, is disposed around the collimator 14.

Valve 6 includes a valve stem 42 whose comically shaped nose 44 engages a valve seat 45 comprising the end of passage 16. The head 46 of stem 42 ridesin and is guided by a collar 48 secured in block 12, and a spring 50 urges head 46 and stem 42 into'the open position against a diaphragm 52'secured in collar 48. Diaphragm 52 transmits actuating forces to stem 42 to close the valve and also provides a vacuum seal. Actuating forces are provided in the construction illustrated by an actuating screw 54 threaded through a cap 56 fastened to collar 48. A cup 58 of nylon or the like serves as a buffer between screw 54 and diaphragm 52 to minimize wear on the diaphragm from turning of the screw. The flow of cesium vapors through the valve 6 may thus be controlled by turning screw 54 to move the valve nose 44 toward or away from the seat 45.

Both the diaphragm 52 and collar 48 are preferably of stainless steel, and proper care should be taken in securing the diaphragm to the collar to ensure the required vacuum-tight connection. This problem is complicated by the thinness of the diaphragm-about .0006 inch. Accordingly, the collar 48 includes two portions 48:: and 48b provided with cooperating flanges 48c and 48d; the outer portions of the diaphragm 52 'aredisposed between these flanges. To assemble these parts, the portions 48a and 48b are first clamped together with the diaphragm between them and preferably extending at least to the outer edges of the flanges. Next, an electric arc is struck between an electrode and these outer edges, and the unit is rotated to permit the arc to move along the entire circumference thereof. The diaphragm S2 and flanges 48c and 48d are thus fused to form a unitary vacuum-tight structure.

The collimator assembly 8 is housed within a sleeve 60 brazed to block 12 and includes a pair of opposed support members 62 and 64 which are of a generally semicylindrical shape. As best seen in FIGURES 1 and 2, the collimator is disposed between and clamped in position by members 62 and 64. The latter conform to the inner dimensions of sleeve 60 and are secured within the sleeve by a retaining ring 66 brazed to these parts. In its preferable form, the collimator comprises alternate flat strips 68 and corrugated strips 70 of nickel foil (FIGURE 2). Members 62 and 64 are preferably of a good heat conducting material such as copper to maintain the collimator 14 at a relatively high and uniform temperature. Sleeve 60 may be provided with a recess 72 (FIGURE 1) to facilitate connection of the source to the beam tube of the molecular beam apparatus.

The beam source of FIGURE 1 is preferably encased in a cover 73of suitable heat insulating material and a second heater schematically indicated at 74 is disposed downstream (upward in FIGURE 1) fromthe collimator 14 and in contact with the sleeve 60. Accordingly,,the temperatures within the beam source are dependent upon the temperatures of the heating coil 11 and heater 74 and are substantially independent of the actualexternal ambient temperature.

The molecular beam frequency standard, of which the source 2 is a component, is internally evacuated to a hard vacuum, preferably mm. mercury or better. Therefore the chamber 22 and succeeding: portions of the source communicating with the recess 72 must be effectively sealed from the atmosphere. This may conveniently be accomplished by brazing these parts together. Accordingly, the material of which the various parts are made should be susceptible of joining by suitable brazing techniques. It should also be relatively inert, particularly with respect to cesium, and it should emit little or no contaminating material into the apparatus. Certain nickel copper alloys, specifically, the material sold by International Nickel Company under the same 403 Monel have been found to meet these requirements. Accordingly, sleeve 60, valve stem 42 and baffle plates 28, 30 and 32 are preferably of this material. The block 12 is preferably of oxygen-free, highconductivity copper in order to facilitate conduction of heat to the cesium charge 10 in the chamber 22.

The tube 20, extending through plug 18 into reservoir chamber 22, is used to exhaust the apparatus and supply the cesium charge to the reservoir 4. After the source 2 is exhausted, it is preferably baked at high temperature to drive out impurities, and then the temperature of the source is reduced to a low level, e.g., 0 C., and still under vacuum conditions, cesium from a supply thereof is distilled through the tube 20 to condense in chamber 22. After a charge 10 of approximately 0.5 gram, sufficient for many years operation, has accumulated in the chamber, the tube is pinched off as indicated at 20a.

During operation of the frequency standard, the cesium in chamber 22, which is maintained above its melting point by coil 11 and heater 74, rests on the inner wall of the block 12 when the source 2 is in the position illustrated in FIGURE 1. Prior to the present invention, inversion or tilting of a source would cause fiow of the liquid from the reservoir into other parts of the apparatus. However, if the source 2 is stilted with the cesium thereby lying on baffle 28, escape of the liquid from the chamber 22 will be prevented by the tubes 40. These tubes project into chamber 22 a sutncient distance so that their ends will be above the liquid charge therein during tilting of the source 2. The tubes are also disposed inwardly of the walls of the chambers a sufiicient distance to maintain them above the liquid level when the axis of reservoir 4 is horizontally oriented, as in FIGURE 1. Accordingly, the source may undergo complete rotation about any axis without appreciable loss of liquid cesium from the chamber 22 while still permitting the passage of cesium vapors fiom the reservoir by way of the tubes 40.

By cascading the baifles 28, 30 and 32 and their associated tubes 40, we have eliminated practically all possibility of loss of liquid cesium from the reservoir 4. The small amount that may escape from chamber 22 into chamber 24 during vibration of the source will find it still more diflicult to escape from the latter to chamber 26. An even smaller fraction of the liquid finding its way to chamber 26 will be able to escape from reservoir 4. Furthermore, as shown in FIGURE 1, the tubes 40 project a substantially greater distance in the direction of chamber 22 than they do into the chambers on the opposite sides of the respective baffles. The probability of liquid entering the shorter ends of the tubes is thus increased relative to the probability of entrance into the longer ends. Accordingly, under conditions of vibration, inversion, etc., there will be a tendency toward net migration of liquid back through the tubes 40 to the chamber 22. For maximum flow discrimination by the tubes 40, the downstream ends 40a thereof should be as short as may be compatible with eflicient fabrication of the baflle-tube assemblies.

The liquid retention characteristics of the reservoir are further enhanced by the location and operation of the heating coil 11 and heater 74. The temperatures of these heating units are controlled by thermostatic switches schematically indicated at 76 and 78, respectively. By way of example, coil 11 may be operated at a higher temperature than heater 74, the temperatures of these units being such as to maintain the chamber 22 and the cesium charge 10 therein at a temperature of approximately 70 C., a level high enough to supply sutficient cesium vapor to the molecular beam apparatus. The temperature of the collimator 40 will then be somewhat hotter, say 75 C., than that of the cesium charge. Thus, the coolest location which the cesium may contact is within the chamber 22 in the reservoir 4, and the temperature of the reservoir 4 progressively increases as one moves from chamber 22 to the left (FIGURE 1).

Should some of the liquid cesium escape to the left through the tubes 40, it will tend to evaporate faster than the liquid within the chamber 22, and gases contacting the escaped liquid will tend to condense at a slower rate than those in contact with the charge 10. Consequently, there is a tendency for the escaped cesium to migrate back into the region of lowest temperaturechamber 22. Thus, over a substantial period of time, there will be essentially no net flow of liquid from the chamber 22, even under severe environmental conditions encountered in aircraft use.

By suitable choice of materials, the reservoir 4 may be adapted for still further protection against loss of the liquid charge. For example, copper, the preferred material of the block 12 is wetted by liquid cesium. Therefore, the charge spreads out to form a thin, adherent layer on the walls of chamber 22, as shown in FIGURE 1. This layer will be dislodged from the wall only by relatively intense vibration. The tubes 40, on the other hand, are preferably of nickel (grade A or better), which is not wetted by cesium. Liquid cesium entering the tubes will therefore remain in globular form rather than spread through the tubes by capillary action. To make full use of this feature, the tubes should have small interior diameters, e.g., .02 inch. To provide the same action in chambers 24 and 26, spacers 34 and 36 should also be of copper.

In FIGURE 4, we have illustrated another embodiment of our invention in which a folded or reflex construction operates to conserve space and facilitate the desired temperature distribution. The beam source generally indicated at 80 in FIGURE 4 has a cesium reservoir generally indicated at 84, a valve generally indicated at 86 and a collimator assembly generally indicated at 88. A cesium charge 90 in reservoir 84 is evaporated by heat from a heating coil 92, and gaseous cesium atoms from the reservoir pass upwardly (FIGURE 4) to the valve 86 and thence downwardly through the collimator assembly 88 where they are formed into a narrow beam projected through an exit tube 94 communicating with other portions of the frequency standard (not shown).

More particularly, the source 80 is enclosed by a cylindrical tube 96 to which is brazed an end plate 98. Closure of the source is completed by a flexible diaphragm 99 secured between the tube 96 and the tubular housing 100 of the collimator assembly 88. The reservoir 84 retains the liquid cesium charge 90 in a chamber 102 defined by tube 96, housing 100, diaphragm 99 and a second diaphragm 104 also connected between tube 96 and housing 100. A plurality of tubes 106 extending through diaphragm 104 permit effusion of gaseous cesium from the chamber 102 while insuring retention of the liquid material therein in the manner described above. Tube 96 accommodates a filler tube 107 whose function is similar to that of tube of FIGURE 1.

The valve 86 includes a valve nose 108 which is aflixed to end plate 98 and is conically tapered to mate with a valve seat 110 at the upper end of the tubular housing 100. The relative movement of the nose and seat required for valve operation is provided by a dilferential screw mechanism generally indicated at 112. The latter comprises rings 114 and 116 affixed to tubes 96 and 94, respectively, the rings being provided with threaded portions 114a and 116a, one of which has a right hand thread and the other a left hand thread. The threaded portions engage similarly threaded portions of a nut 118. Thus, as the nut is turned, rings 1.14 and 116 move toward or away from each other depending upon its direction of rotation.

Such relative movement is imparted by way of tubes 94 and 96 to the valve nose 108 and seat 110, to open or to close valve 86. It will be apparent that flexibility of the diaphragms 99 and 104 is a desirable feature to permit relative movement of their outer and inner edges with the tubes 94 and 96 during valve operation. Accordingly, the diaphragms are provided with the corrugated configurations shown in FIGURE 4.

The collimator assembly 88 includes a collimator 120 secured within the housing and through which cesium atoms must pass on their way to the tube 94. It is preferably of the same general construction as the collimator 14 of FIGURES 1 and 2.

The materials used in the beam source 80 should be the same as those mentioned above for the source 2. Thus tubes 94 and 96, plate 98, diaphragm 99 and valve 86 may be of a nickel-copper alloy such as theaforementioned 403 Monel. The upper diaphragm 104, however, is preferably of copper in order to minimize the temperature gradient across it. The tubes 106 are of nickel. The various parts may be joined by brazing.

As seen in FIGURE 4, we have provided the beam source 80 with an outer casing 122 of heat insulating material. The source also has a second heater 124 located at the bottom thereof in contact with the ring 116. The temperatures of the heating coil 92 and heater 124 are controlled by thermostatic switches 126 and 128, respectively, set to operate in like manner to the switches 76 and 78 of FIGURE 2. Thus the coil 92 may be heated sufiiciently to maintain the collimator at a temperature of 75 C. The heater 124 is at a somewhat lower temperature than the coil 92 so as to keep the cesium charge 92 at a temperature of approximately 70 C. Accordingly, with the collimator 120 hotter than the charge 90, there will be a net heat-induced migration of liquid cesium back to the chamber 102 in the manner described above. This eifect is aided by the fact that the highest temperature in the source 80 is that of the upper diaphragm 104 on which liquid cesium displaced from chamber 102 by vibration will come to rest.

The tubes 106 of FIGURE 4 extend a sufiicient distance into the chamber 102 and are disposed far enough from the tube 96 to inhibit flow of liquid cesium therethrough. The ends of the tubes projecting on the other side of the diaphragm 104 are as short as practicable. Accordingly, the tubes 106 and diaphragm 104 function in the manner of the tubes and baffle-plates of the source 2 to offer much greater resistance to flow from the chamber 22 than to reverse flow back into it.

'It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained and, since certain changes may be made in the above constructions without departing from 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 herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

We claim:

1. An improved molecular beam source adapted to supply gaseous molecules of a substance from a liquid charge thereof, said source comprising, in combination, means forming a reservoir for said liquid charge, outlet means and means forming a chamber between said reservoir and said outlet means, a first wall between said reservoir and said chamber, a narrow tube passing through said wall and projecting into said reservoir beyond said wall adapted to pass said gaseous molecules to said chamber while substantially impeding escape of said liquid from said reservoir.

2. The combination defined in' claim 1 including means for heating said liquid in said reservoir to facilitate evaporation thereof.

3. The combination defined in-claim'2 including means for maintaining the temperature in said chamber at a higher level than that of said liquid charge in said reservoir whereby there is a tendency for liquid in said chamber to evaporatae and recondense in said reservoir.

4. The combination defined in claim 2 in which said tube projects farther into said reservoir than into said chamber, to thereby increase the probability of a net liquid fiow from said chamber to said reservoir.

5. The combination defined in claim 1 in which an interior surface of said reservoir contacted by said charge is of a material subject to wetting by said charge, and said tube is of a material not subject to wetting by said charge.

6. The combination defined in claim 1 in which said reservoir includes a second wall disposed between said first wall and said outlet means, said second wall having a narrow tube passing through said second wall and projecting beyond said second wall toward said first wall, whereby said tube in said second wall may pass gaseous molecule toward said outlet while substantially impeding passage of any liquid condensed in said reservoir between said first wall and said second wall.

7. The combination defined in claim 6 in which said tubes project farther in the upstream direction toward said first wall than in the downstream direction toward said outlet, to thereby increase the probability of net liquid flow upstream through said tubes.

8. An improved molecular beam source adapted to supply gaseous molecules of a substance from a liquid charge thereof, said source comprising, in combination, a housing, an end plate closing one end of said housing, outlet means including an exit tube disposed within said housinggand a valve having cooperating closure members second spaced walls extending between said exit tube and said housing, a narrow gas tube projecting from said first wall into the interior of said reservoir and adapted thereby to provide communication between said reservoir interior and said valve through the interior of said housing, whereby vapors may readily pass from said reservoir through said valve and exit tube while said liquid charge is substantially sealed in said reservoir.

9. The combination defined in claim 8 in which said spaced walls are deformable to permit said housing to move with said end plate during said relative movement of said plate and said exit tube.

10. The combination defined in claim 8 in which said gas tube projects a sufficient distance from said first wall and said housing to lie above the surface of said charge in all orientations of said source.

11. The combination defined in claim 8 including a collimator in said exit tube downstream of said valve.

12. The combination defined in claim 8 including a heating coil around said housing and between said first wall and said end plate, said heating coil being adapted to heat said liquid charge to facilitate evaporation thereof and provide a higher temperature between said first wall and said end plate than in said charge whereby there is a tendency for any said liquid between said first wall and said end plate to evaporate and recondense in said reservoir.

References Cited in the file of this patent UNITED STATES PATENTS 2,621,296 Thompson Dec. 9, 1952 2,714,665 Tunnell Aug. 2, 1955 2,808,510 Norton Oct. 1, 1957

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