CA1066818A - Cesium beam tube - Google Patents
Cesium beam tubeInfo
- Publication number
- CA1066818A CA1066818A CA318,219A CA318219A CA1066818A CA 1066818 A CA1066818 A CA 1066818A CA 318219 A CA318219 A CA 318219A CA 1066818 A CA1066818 A CA 1066818A
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Abstract
ABSTRACT OF THE DISCLOSURE
An atomic beam tube provides a single sealed envelope structure that serves both as vacuum envelope and as structural member to which the operative components are attached. The envelope is composed of a heavy and relatively rigid frame and a relatively thin and flexible coves sealed to the frame. The operative elements are separately assembled in independent sub-assembly units which are secured to the frame as a minimum of locations to provide fixed alignment and thermal isolation of the operative elements, and easy disassembly of the tube. C-field means in the improved molecular beam tube-apparatus for producing a weak magnetic field comprise a conductor in-cluding a plurality of equally spaced helical turns forming a generally closed loop lying generally in a beam plate including the path of the beam.
The loop including two end sections transverse to the beam path and parallel to one another, each of the end sections including a beam aperture having first and second opposed edges interrupting the conductor in at least two adjacent helical turns to provide two internal ends adjacent each of the opposed beam aperture edges. There are at least two conducting jumpers adjacent a beau aperture, each said jumper having a first jumper end connect-ed to the helical turn internal end adjacent the first aperture edge, and a second jumper end connected to the internal end of the same helical turn adjacent the opposed second edge, whereby the conductor provides a continuous conducting path through the helical turns and the jumpers around said beam apertures, and the magnetic field adjacent an aperture has greater strength than at other portions of the loop.
An atomic beam tube provides a single sealed envelope structure that serves both as vacuum envelope and as structural member to which the operative components are attached. The envelope is composed of a heavy and relatively rigid frame and a relatively thin and flexible coves sealed to the frame. The operative elements are separately assembled in independent sub-assembly units which are secured to the frame as a minimum of locations to provide fixed alignment and thermal isolation of the operative elements, and easy disassembly of the tube. C-field means in the improved molecular beam tube-apparatus for producing a weak magnetic field comprise a conductor in-cluding a plurality of equally spaced helical turns forming a generally closed loop lying generally in a beam plate including the path of the beam.
The loop including two end sections transverse to the beam path and parallel to one another, each of the end sections including a beam aperture having first and second opposed edges interrupting the conductor in at least two adjacent helical turns to provide two internal ends adjacent each of the opposed beam aperture edges. There are at least two conducting jumpers adjacent a beau aperture, each said jumper having a first jumper end connect-ed to the helical turn internal end adjacent the first aperture edge, and a second jumper end connected to the internal end of the same helical turn adjacent the opposed second edge, whereby the conductor provides a continuous conducting path through the helical turns and the jumpers around said beam apertures, and the magnetic field adjacent an aperture has greater strength than at other portions of the loop.
Description
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This invention relates, in general, to atomic beam apparatus, and, more particularly, to atomic beam tubes which utilize magnetic hyperfine re-sonance transitions.
Atomic beam tubes are the basic frequency determining elements in extremely stable frequency standards. Fundamentally, an atomic beam frequency standard detects a resonance within a hyperfine state of the atom to obtain a standard frequency. To utilize this resonance, atomic particles, such as cesium atoms, in a beam interact with electromagnetic radiation in such a manner that when the frequency of the applied electromagnetic radiation is at the resonance frequency associated with a change of state in the particular atoms, the atoms in selected atomic states are deflected into a suitable detector. The frequency of the applied radiation is modulated about the precise atomic resonance frequency to produce a signal from the detector cir-cuitry suitable for the servo control of a flywheel oscillator. Control cir-cuitry is thus employed to lock the center frequency of the applied radiation to the atomic resonance line.
When cesium atoms are employed ln an atomic beam tube, the particular resonance of interest is that of the transition between two hyperfi~ne levels resulting from the interaction between the nuclear magnetic dipole and the -spin magnetic dipole of the valence electron. Only two stable configurations of the cesium atom exist in nature, in which the dipoles are either parallel or anti-parallel, corresponding to two allowed quantum sta~es. Thusg in the absence of an external magnetic field, there are two hyperfine energy levels, each of which may be split by an external magnetic field into a number of Zeeman sublevels.
The hyperfine resonance transition ~sèd in the atomic beam tube of the present invention occ~s between the ~(F=4, mF-0) and ~F=39 mF-0) states, where the first number F is related to the magnitude of the total angular momentum of the atom (electronic plus nuclear) while the second number mF is ~elated to the component of this total angular momentum which is in : "" ' .:
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the direction of the applied external magnetic Eield.
To cause a transition from one state to the other, an amount of energy E equal to the difference in energy of orientation must be either given -to or taken from the atom. Since all cesium atoms are identical, E is the same for every atom. The frequency f of the electromagnetic energy re-quired to cause a change of state is given by the equation E=hf, where h is Planck's constant. For cesium, the magnitude of f :is approximately 9,192.631770 megacycles.
A conventional cesium atomic beam apparatus provides a source from which cesium evaporates through a collimator which forms the vapor into a narrow beam and directs it through the beam tube.
This collimated beam of atoms is acted upon by a first state sel-ecting magnet or "A" magnet, which provides a strongly inhomogeneous magnetic field. The direction of the force experienced by a cesium atom in such a field depends on the state of the atom. In this field, the energy states F=3 and F=4 are split up into sublevels. ~11 of the atoms of the F=~ state, .~ . . .
except those for which mp- -4, are deflected in one direction, and all other atoms are deflected in the other direction. In the apparatus of the present invention, the F=3 group ~together with the atoms of the ~4, -~) sublevel) are retained in the beam, while the others are discarded. The undiscarded atoms include those of the ~3,0) sublevel.
Upon emergence from the A-field, those atoms enter a central region where they are subjected to a weak uniform C-field to assure the separation ~ ;
in energy of the mF_0 states from the nearby states for which mFtO. This t small magnetic field also serves to es~ablish the spatial orientation of ~he t~ selected cesium atoms and~ therefore, the required direction of the micrcwave t magnetic field.
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i ` While in this uniform weak field region, the cesium beam is subjected to an oscillating externally generated field of approximately the resonance frequency required to cause transitions from the ~3,0) to the ~4,0~ sublevel. ~ ~-1~166i!~
After leaving this energy transfer region, the bearn is acted on by a second state-selecting magnet, similar to the A-magnet~ producing a strong inhomogeneous field. Here the atoms of all the F-3 groups ~and also those of the ~ ) sublevel) are discarded. The only undiscarded atoms are those of the ~,0) sublevel, which exist at this point only because of the induced transition described above. These atoms are allowed to proceed toward a detector of any suitable type~ preferably of the hot-wire ionizer mass spec-tometer type.
The magnitude of the detector current, which is critically dependent upon the closeness to resonance of the applied RF frequency, is used after suitable amplification to drive a servo system to control the requency of -~ the oscillator/multiplier which excites the ~F cavity.
Cesium beam tubes as hitherto constructed have been expensive and difficult to make. To provide a cesium beam tube suitable for use in the usual applications of atomic frequency standards, mechanical alignment of '. t components is critical, and shifts in the alignment can destroy the functional .. . .
~- frequency standard. The tube elements that have been described must be as-sembled and supported in place with a high degree of precision, alignment requirements relative to the beam deflection axis of the itube being approxi-mately .001" for effective tube operation. The precise alignment must be preserved under conditions of mechanical vibration and shock, and of a range of temperature variations typical of practical applications of the tube.
Prior art tubes have employed complicated mounting means between the inner structural assembly of tube elements and either an inner or an ou~er vacuum-tight envelope in an effort to meet ~he often-conflicting requirements of rig1dity against mechanlcal shock or vibration, and flexibility to accommodate -*o differential expansion disturbance forces in the presence of thermal gradi-ents resulting from bake out in tube processing and ambient temperatures in normal tube operation. A further limitation in prior art tubes is that these structure measures typically result in relatively large and heavy tubes, ~: :
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66~1l8 characteristics that are most undesirable for certain important applications ; such as in air or space craft.
Some prior art cesium tubes have been constr~lcted using two separ-ate envelopes. The first is an inner mounting channel to which the operative components are secured to provide mechanical stability and thermal isolation;
this inner envelope is suspended within an outer vacuum envelope. Since dif-ferential movement between the two envelopes must be allowed for, such a com-pound structure adds complexity to the manufacturing processO This design also results in a relatively weak mechanical structure.
This invention relates to a molecular beam tube apparatus including a source for providing a directed beam of molecular particles, a first state selector for selecting a portion of said particles in said beam, a radio fre-quency transition section downstream from said first state selector for caus-ing resonance transitions of some of said selected beam particles, C-field means for producing a weak generally homogeneous magnetic field transverse to said beam in the radio frequency transition section~ a second state selector downstream from said radio frequency transition section for selecting a further portion of said beam comprising those beam particles that have under-gone said resonance transitions, and detecting means responsive to said particles in said further portion, that improvement wherein said C-field means comprises a conductor including a plurality of equally spaced helical turns forming a generally closed loop lying generally in a beam plane including the path of said directed beam, said loop including two end sections transverse to said beam path and parallel to one another, each said end section including a beam aperture having ~irst and second opposed edges interrupting said conductor -in at least two adjacent said helical turns to provide two internal ends ad-jacent~each of said opposed beam aperture edges, at least two conducting jumpers adjacent a said beam aperture, each said jumper having a first jumpeT end --connected to a said helical turn internal end adjacent said first aperture edge, and a second jumper end connected to the said internal end of the same .
~6~il~ll5 said helical turn adjacent said opposed second edge~ whereby said conductor provides a continuous conducting path through said helical turns and said jumpers around said beam apertures, and said magnetic field adjacent a said aperture has greater strength than at other portions of said loop.
The present invention integrates the inner assembly and the vacuum envelope into a single structure, thereby eliminating the need for support elements between the two. It further provides for a modular assembly in which three subassembly units are assembled to the main structural member (which is also a portion of the vacuum envelope) by means of 10 machine screws, as will be describedO The invention also includes novel features providing good thermal isolation, smaller and more efficient magnetic structures, smoother transition between strong and weak magnetic fields, and means to feed in RF
energy with less perturbation of the C-magnetic field than in prior art tubesO
These novel features make possible a tube, both more compatible with typical operating environments than conventional devices, and ligther in weight ~9 lbs.
against the 16 lbs. of a ~ypical prior-art tube).
The design of the present invention eliminates the need for expen-~i sive and complex internal support structures while providing a beam tube of simple modular design that maintains beam alignment and is highly resistant to external mechanical disturbances such as shock and vibration. At the same ` time, the design of the present invention provides excellent thermal isolation for the thermally sensitive components.
The atomic beam tube of the present invention provides a single `i structure that serves both as vacuum envelope and as structural member for ~ the operative ~mponents. This envelope is composed of a heavy and relatively - .:.
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rigid frame and a relatively thin ancl Elexlble cover sealed to the frame. The operative elements of the tube are secured to the frame; this provides fixed alignment of these elements. The flexible cover accommodates itself readily to externally caused mechanical distortions without transmitting them to the frame or to the operative elements. The sealed unit acts as a vacuum envelope.
The operative elements of the tube are secured to the heavy frame at a minimum of locations~ and the connections have low thermal conductivity, in order to isolate the operative elements thermally from the environment. For example, the oven structure is secured to the frame through a connecting structure :~
tha~ is designed to provide a relatively long thermal path to the environment.
It is industry practice to disassemble such tubes when they are no longer operable (generally because the cesium getters are saturated) in order to salvage reuseable components. To disassemble prior art tubes has :
required extensive machining which is both time-consuming and expensive, in-volving high labor costs. In the cesi~ beam tube of the present invention, :~
the operative parts are provided in three main modular subassemblies, secured to the frame by a total of 10 screws, for quick and simple disassembly and ~:
reuse of the modular portions.
The operation of the cesium beam tube, as has been described, re-quires that the A and B magnets provide very strong fields (of the order of 10 kilogauss), while the C-field in the region between them must be re-latively weak ~of the order of .060 gauss) and as uniform as possible. Dis-continuities in the C-field are particularly likely to occur in the regions at which the beam enters and leaves the C-region, and can cause spontaneous ~ transitions (~ajorana transitions) in the atomic beam which may distort the performance o~ the tube. The present invention provides a C-field winding of~novel design that generates a C-field of superior uniformity at the beam ,; . .
apertures.
In general, it is desirable to provide a cesium beam tube that is .
~ 3~ as compact, light weight, and simple as possible. The particular designs , , '` ~: , ~66~3~8 of the A and B magnets in the present invention realize such construction and are particularly adapted to the modular assembl~ previously described.
It is typical in the assembly and processing of molecular beam tubes to confille the source of the molecular beam material in a sealed ampoule during the bakeout and exhaust part of the processing cycle, and as a final stage, while the tube is still being pumped, but after bakeout has been com-pleted, to open the ampoule. Any gases released in the opening process can then be pumped prior to the final sealing off of the tube.
A number of methods have been used in the prior art for opening the ampoule. One such method is to provide means whereby a member of the ampoule is ruptured when electrical energy is applied to a heating coil to cause expansion in a member mechanically linked to a rupturing element. A more sophisticated prior art method is to discharge an external capacitor through electrical conducting paths into the tube, so arranged that a vaporiæing arc is created at a member of the ampoule which is ruptured by the heat of the ~ arc. Both of these methods require the inclusion in the beam tube of additional ; parts that are used only for this one operation; in particular~ means must be provided to transmi~ electrical energy through the vacuum envelope, which complicates the construction of the tube.
The present invention provides a novel ampoule structure and novel means for opening the ampoule that -require no additional parts; in particular, no additional electrical or mechanical ~eeds through the vacuum envelope are ; required.
Other objects, features, and advantages will appear from the follow-ing description of a preferred embodiment of the invention, taken together wlth attached drawings thereof, in which:
Figure 1 is a schematic view of the principal beam-forming and detect-:
ing elements of the tube;
Figure 2 is a perspective view of the elements of Figure l;
Figure 3 is an exploded view of the components of the oven and ampoule;
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Fig, 4 is a cross section of thc ampol~c;
Fig, S is a ~iew of thc assembled o~en;
Fig, 6 is a view of the oven with reM ector and support structurc;
Fig. 7 is a Zeeman energy diagr~n for cesium 133 in thc ground electronic state; showing the transition induced in the beam tube of the in-vention;
Fig. 8 is a schematic view of the control circuitry used with the cesi~n beam tube of the invention;
Fig. 9 is a perspective view of the first state selector magnet and ion pump;
Fig, 10 is an exploded perspective view of the first state selector ., .
magnet together with shielding and support structure;
Fig. 11 and 12 are longitudinal and cross sections respectively of 1 the first state selector and ion pump;
Fig. 13 is a perspective view of the microwave structure and C-field rcoil;
Fig, 14 is a perspective view of the C_~ield coil with portlons broken away;
Fig, 15 is a plan view of the unfolded C-field coil;
~1l 20 Fig. 16 is a cross section of the assembled C_field Goil at a beam aperture;
Fig, 17 is a detail of the conductors of the C_field coil at a beam erture; ~ -Fig, 18 is an exploded view of the magnetic shield package and con-tents;
Fig, 19 is a cross section of the outcr envelope and con~ents near the center;
.~ ~Fig. 20 is a pcrspectiYc viewOf the B-ficld magnet and the detcctor;
Fig, 21 shows thc elements of Fig, 20 wi~h support structurc;
Fig, 22 and 23 are a plan vicw and a rear elevation view of the , : - ' .. . .
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1~615~3 B-field magnet and the detector;
Figure 24 is an exploded view of the outer packaging and connections and the modular units; and Figure 25 is a longitudinal view partly in section of the assembled ; units of Figure 24.
General Referring to the drawings, and particuiarly to Figures 1 and 2, the basic beam-forming and detecting elements of the cesium tube 11 of t~e in-vention are shown schematically and in perspective. A source of atomic par-ticles includes an oven 10 which evaporates liquid cesium and emits ~through a collimator) a beam of neutral cesium atoms which are statistically distri-buted between two stable energy states, as previously described. The first state selector or A magnet 12 splits these energy states into sublevels and selects the atoms in the F=3 states (together with those in the ~4,-4) sub-level) and deflects the remaining atoms so that they no longer form part of the beam. The beam of selected atoms then passes through the RF interaction ; section 14; in this region a weak homogeneous magnetic field ~C-field) is supplied by the winding 22. Microwave energy is supplied at the resonance frequency to induce transitions of some of the beam atoms from the ~3,0) 20~ state to the ~4,0) state ~Figure 7). The beam atoms in the ~4,0) state are ~ ;~
then~selected by the second state selector or B magnet 16, the atoms in the ~;
remalning states being deflected out of the beam. The cesium atoms selected by the B magnet strike the hot-wire ionizer 20, and an electron is stripped ~ ~ . -:. ...
from each cesium atom, causing the re-emission of cesium ions, which are accelerated through a~mass ;spectrometer 207 into the electron multlplieT 18.
The electron multipllèr provides an output current proportional to the number ; of atoms arriving at the hot wire 20, that is, proportional to the number.
~ of atoms that ha~e been raised to the second state in the microwave cavity. ; ~
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As shown in Figure 8, the output of the atomic beam tube 11 is fed to control electronics 260 which produce a suitable error output signal 261, :
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~9668~Ll3 .:, which is applied to a crystal oscillator 262. Thc frcquellcy output of thc crystal oscillator (typically 5 megahcrtz) is controllcd b~ the proccssed signal 261 from the cesium beam tubc, and then ~ultiplied in the frequency multiplier chain 264 and applied to tube 117 at the precise resonance frcquency (typically 9192 mHz), Multiplier chain 264 and the controlled oscillator 262 from the microwave generator 266, The usable output signal is derived from controlled oscillator 262 at 268, S~unmar~ of ~lodular Components The elements that have been described and shown in Fig. 8 are in general terms old and well-known in the art, The cesium tube of the invention provides ~hree modular subassemblies including a cesium ampoule and a first 'J state selector magnet in combination with the ion pump, a second state selec_ ~ . tor magnet in combination with the mass spectrometer~ and a C-field winding -ii and microwave structure, all of novel design, as well as a novel outer pack-~¦ age for the entire tube.
To provide the advantages of the modular assembly of the invention, as previously described, the oven 10 (~rith cesium ampoule) and-A_magnet 12 ~with ion pump~ sho~n separately in the schematic views of Figs, 1 and 2, are combined in~an oven/A-magnet assembly module 240 (Fig, 24). The P~ inter-action region 14 and C_field, shown unenclosed in Figs, 1 and 2, are contained ~ --j in magnetic shield package 179 (Fig, 24), The B~magnet 16, hot wire ioni~er .
I 20, mass spectrometer 207 and electron multiplier ]8 are packaged together ,I; in a detector assemb]y module 244~(Fig, 24), Referring to Figs. 24 and 25, -1~ ~modùles 240 ana 244 and magnetic shield package 17~ are essentially independent ', .
i~ ~ of one another and constitute the subassembly units iYithin the outcr pac'l~age .j .
~of the beam tube, and are assembled thercto b~ means of 10 screws9 as wil~
bc described, 1~ The details of each of thesc modular components are dcscribed belo-~.
Ovcn~.~ ma~net modulo_ oven and ~npoule The structure of the novel o~en_~mp~u1eassembly 10 of the invention~
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constitutin~ a source for providing a beam of c~siwn particlcs~ is sho~m in detail in Figs. 3-6, The assembly 10 includes collimatin~ means 42, not dcs_ cribed, and ovcn means including a reservior 29 containing a~t ampoule 27, The ampol~e 27 includes a t~n walleA (0,015") ~cnerally cylindrical shell 30 and a top 37 including a fill tube 38, Top 37 and cylinder 30 together fonm an enclosure, The end of shell 30 opposite to top 37 provides an opening 49, A
cup shaped base 34 is sealed into shell opening 49 by an eutectic metal 32 designed to fail mechanically at a temperature of approximately 600C, An example of such an eutectic metal is an alloy of 45% copper and 55~ inditlm, A weak spring 35 is compressed betlreen base 32 and top 37, `~ After the enclosure has been filled ~rith liquid cesium, fill tube,~ .
38 is closed by pinching and heliarc welding, A wire screen mesh 36 having high thermal conductivity surrounds ampoule 27 within reservoir 29, The mesh 36 serves both as a heat transfer element and as a retaining and support eIement for the ampoule, Ampoule 27 is supported within reservoir 29, A copper outer c~linder 28 of reservoir 29 includes an annular recess 40 at its lower portion, A
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wélding ~daptor 39 havingalo~ter flange 41 is bra~ed to recess ~0 of outer cylinder 28. An ampoule support member 43 includes an in~erted cup por~ion 44 and three spaced supports 45, Inverted cup portion 44 of member 43 is heliarc welded at 46 (Fig. 4) to the inner surface of weld mg adaptor flan~e l to seal *le lo~er ~nd~of reservoir 29. This creates an enclosed reservoir space 51 surro~nding base 34 and commu0icating with mesh 36. ~npoule 27 is seated m support member 43 with ampoule baso 34 within spaced supports 45.
Two tantaltun heaters 9~ and 92, rctained in a ccramic support struc-ture 88, are inserted into collimator asse~mbly 42 throu~h quartz tubes 80 and 82,~ Thc ampo~e is opened3 after balceout of thc bcam tube3 by mcans of thcse ~ -heators, which heat the ampoule to 600C, at which tempcraturc the cutectic 30 se.~ fails, Thc combination of the vapor pressure of the C~SiUnl within ampoule -;
"' l~G6818 ~7 and the forcc of comprcssed we~c spring 35 exerts a stress grcater th~
the working strcss of the metal of se~l 32 and pu~ 5 base 34 out of shell 30, thereby relcasing thc cesi~un in the ampoule. Weak sprin6 35 prevents the base from settling back into place, resealing thc ampoule.
In later operation of the tube, tantalum heaters 90 and 92 are used to wann the entire oven assenhly 10 to the operating temperature, typic-ally about 90C. At this temperature the liquid cesi~un in reservoir space 51 slowly ~apori~es and diffuses from the mesh 36 to collimating means 42.
Collimator 42 is functionally equivalent to a bundle of small tubes so oriented that a directed beam of cesium atoms emerges. Construction of collimating means is well known to the art, and will not be detailed here The oven support structure is designed to provide thermal isolation ~rom outside the beam tube. Since the oven operates in a vacuum, there is no heat loss from conve~tion; the major loss is by radiation, l~th some loss by conduction. m e oven support structure is therefore constructed of mater-ial of poor thermal conductivity such as staililess steel and includes ear portions 100 and 102 for securing oven 10 to the A ~nagnet assembly, as wiI1 be described, Additionally~ 0.003~l Kapton shims 99 between the ear portions o~ the support structure and the A_magnet assem~l~ further discourage thermal conduction. A radiation shield 104 of highly polished alu~inùm surrounds the major portion of the oven, and preserlts radiation heat loss from the oven~
An oven of the design described required less than two watts for operatioll.
OvenJA-mlagnet modulc: _-magnet and ion pume Refcrring now to Figs 9 through 12~ a pex~lancnt magnet driver 111 is sh2red by the first state selector magnet (~ magnct) 1~ ~Id the ion pump 110. The ion pump perfo~ns the well-kno~n function of ren~oving undesired .
gasses and maintaining tube vacllum during opcration. Permancllt magnet 111 is generally of a typical "C7' shape, but 1~th a novel r~entrant imler surface shape that gives it the distinguishing capability of providing proper fields for both sclection and ion pumping The axis of magnet 111 is parallel ~ith ' .
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the beam.
"Dipole con~iguration" soft iron pole pieces 112 and 114, of a well-known design, are secured in the gap of "C" shaped permanent magnet 111, and provide the inhomogeneous deflecting field of first state selector 12.
Reentrant extensions 108 and 109 of permanent magnet 111 extend in-wardly to~ard one another, and in conjunction with a second pair of short cylindrical pole pieces 116 and 118 provide the field for the ion pump 110, located between pieces 116 and 118. The ion pump is of any suitable design and is well known.
Permanent magnet 111 provides in effect two permanent magnet cir-cuits in parallel to drive both the "A" state selector 12 and the ion pump 110. The magnetic driver is designed to provide approximately 10 K gauss in the state selector circuit while providing approximately 1000 gauss for the ion pump. The compact arrangement of this combination permits the atomic beam tube assembiy to be smaller, lighter, and less expensive than those hitherto constructedJ and is also especially adapted to the modular design of the present beam tube apparatus.
A magnetic shield 132 covers approximately the upper half of the outer surface of magnet 111 and additionally on one end is interposed between ; 20 the magnet and the C-field/microwave structure module 179 (Figure 24). Shield 132 provides aperture 138 for the passage of the atomic beam from the A-magnet ~! 12 to module 179. The structure of shield 132 further provides field control l for the attenuation of the 10 K gauss deflecting field of the A-magnet down -:
s~ to the 0.060 gauss C-field in the RF transition region 14.
A mounting plate 128 is secured ~o the upstream side of permanent I~ magnet 111, and provides brackets 134,and 136. Magnetic shield 132~ stainless ;3 steel spacers 113, magnet 111~ and another pair of stainless steel spacers ~i 117 all are ~astened together by a pair of machine screws 115 passing through 3~ clearance holes in each and threading into tapped holes in mounting plate 128.
~ 30 ~ Oven 10 ~Figure 6) is secured by its support structure ear portions ~ .
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, ~6i~L8 110 and 102 to brackets 134 and 13G, .~ts thcsc brackets are OpCII ill construc-tion, rather than solid, they providc a relatively long thermal path for the conduction of heat from the oven through the brackets to the e~entual pOill~
o~ contact with the outer framc of the beam tube, Shims 99 of 0,003" Kapton are interposed bet~een ears 100 and 102 and brackets 134 and 136 an.d provide further thermal insulation, Oven 10 and A_magnet 12 with ion pump 110 from theo~en/A_magnet module 240 (Fig, 24).
C-field/~icrowave ~tructure module Referring again to Figs, 1, 2 and 4, the C-field and RF ~radio .
frequency) transition section 14, including magnetic shields to be described, are packaged together as a second module 179, As previously described in connection with Fi.g, 2, the cesium atsms that are selected by the A_magnet 12 form a beam that must next pass through RF transition section 14, In this region a weak homogeneous magnetic ~ield ~' (C-field) of approxI~ately ,06 gauss directed transverse to the beam path is , proYided by a single-layer printed circuit solenoid 22 of novel design, The construction and mounting supports of this solenold will be described by re-ference to ~igs. 13 through 19, R'eferring first to Fig, 15, the conductors of solenoid 22 are etched .
by well-kno~n printed circuit techniques from a thin copper layer bonded to a base 152 of polgim1dc material appro~imately 0.002 inch thick, The general shape of the base material 152 and a pattern of eight uni~ol~y-spaced COIl-:
: ductors 150-1 ~hrough 15Q_8 is shol~l in Fig, 15. Fyelet holes 307 are pro-: `vided at each end of the conductors 150. This printed circuit solenoid pro-vidos thin, wido, and closely spacod conductors of ~e~y unifol~n cross scctionalarea and constant ~onductivity, The pri.ntcd circuit solcnoid is assembled into a generally rectangu-; :lar loop as shown parti.cularly in Fig. I~, with thc cyeleted ends of conduc-: 30 tors~l50 offset one conductor in registry so that the complctcd conducting ~: ~
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path will form a one~ yer sp.iral ~rindin~ of equall.y spacccl hel.ical turns Electrical colmection at each of thc offset, but othcrlriso registcrcd, ends of conductors 150 is made by soldering using indium washers (not sho~in) cmd secured b~ rivets 308 inserted through the eyelet holes ~lectrical connection to the solenoid is made by wire leads soldered to eyeletted pads 304 and 306 at the end of each of the outside turns.
The closed loop includes two end sections 140 and 142 that are trans- :
verse to the beam path and parallel to one another Since the assembled ~ :
.~ solenoid winding must lie generally in the plane of the cesium beam, apertures 270 and 271 are provided in end sections 140 and 142 of such a size as to interr~apt conductors 150-4 and 150-5.
Aperture 2?0 in base layer 152 has tlYo.opposed edges 144 (Fig 15) ~ .
that interrupt the two adjacent inner strips lS0 4 and 150-5 of continuous .
conductor 150, to provide four internal ends 122 of strips 150_4 and 150-5 adjacent the aperture edges Ends 122 are eyeletted To provide a continuous ! ~ :
current path, it is necessary to bridge the aperture by conneeting the internal ~
conductor ends. In addition, it is necessary to maintain uniformity of the C_field at the beam apertures insofar as is possible, to avoid field discon- ~.
tinuities causing undesired transitions, as previously explained ~ In the present invention, two patches~318 of prillted circuit material slmilar to-that described are provided to bridge the gaps and maintain Ul~-formity of the C_field, each having an aperture 319. ~o eyelctted conducting .:
jumpers 166 and l68 are bondcd to base layor 320, and angle around aperture ~.
319. Referring particularly to Figs 14 and 17, a:patch 318 is assembled to : ..
the~-~ndin~ by soldering to rivets 182 passing through the eyelets of the ~ -jumpers and of internal ends 122. This construction maintains thc continuous current path through the entire conductor 150 at thc beam apcrtures. J~pers 166 and 168 lead the current around each aperture 270 and 271, effcctively : doublill~:the magnetizing force at tho cdges of thc apertures and tendi.ng to :.
30 : mai.ntain a near uniform distribution of the C-ficld across the aperturcs.
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6~3~L8 This structure providcs an exceedingly close approximatiorL to the ideal of a uniformly distributed current sheet Electrical insulation around the solenoid is provlded by polyimide strips 184 and 186 (Fig. 14) made to the sclme shape as print~d circuit base 152, one being placed on either side of base picce 152.
Inner Magnetic Shield Package The assembled C~field winding 22, comprising the three layers and o patches as described, is mounted on the inner surface of i~mer magnetic ¦ shield 154 (Fig l5 ) and inner shield base plate 156 and is ~eid in place ~` 10 by ri~ets passing through the shield material, the outer margins of the solenoid assembly of base material 152 and insulating strips 184 and 186, and aluminum plates 282 of which representative ones are shown in Fig. 18 The assembly at the a~ertu~elocations 270 and 2?1 is made ~rith aluminum plates 280 that provide apertures to register with apertures 270 and 2~1 -~, - A flop eoil 192 tFigs 2 and 18) is mounted on one of the central i aluminum plates 282 and supported from m ner magnetic shield 154 so that it ~ is eoaxial to the beam axis. This eoil is used in a manner well known to ;;, the prior art t~ introduce a 20 ~hz, electrical signal for the adjustment of he C-field solenoid eurren~, and will not be described further.
The sides of inner magnetic shield 1$4 (Fig 18), paralleling ~ ~le beam path, provide magnetic end eaps for solenoid 22. The resulting field '~ aeross the plane of solenoid 22 thereby approximates t~le classical ~niform field of an infinitely long solenoid ~th ~lux lînes nol~al to the c~sium beam path Inner magnet shield 15~ in combination with spaced outer magnetic shield 1S7 effectlvely attenuates the strong magnetic fields prodllced by the A and B magnets and also shields the RF transition region from external mag-netic perturbations, ~,~ Microwave radiation ~ ~ Referring particularly to Figs 1, 2 and 18, microwave radla~ion `~ 3~ is supplied ~thin RF interaction section 14 by ~ravegl~de structure 190~ ~-hich : :
, : - 15 --~6~ 8 is of the standard "Ramsey" type and well known in the art. It w:ill not be described here.
In prior art atomic beam tubes, constructed with separate mcchanical protective and vacuum isolation envelopes, differential motions between the two envelopes have made it necessary to provide flexible connection means -between the microwave structure and the exterior of the tube, capable oE
accommodating to such motions. Such flexible means requires a relatively large aperture, typically two inches in diameter, in the magnetic shield struc-ture to accommodate the connection. Such a large aperture introduces per-10 turbations in the magnetic C-field due to leakage effects, which must in turn be compensated for~ for example by providing ex~ra "baffling means" as in United States Patent No. 3,670~171 (Lacey et al) issued June 13, 1972.
In the present invention, the combination of mechanical support and vacuum isolation envelope into a single structure eliminates such differential motions. The inlet arm of microwave structure 190 can therefore be intimately , brazed to the lower surface of inner shield base plate 156. This construction ayoids the need for a large aperture through the magnetic shield; a relatively small aperture 194, about 1" x 1/2", is prot,ided in base plate 156 (Figure 18). ~ -~Such a small aperture introduces only relatively small perturbations into the 20 C-fieid, eliminating the need for "baffling" or other compensating structure~
and this structure is therefore advantageous.
Outer magnetic shield packa~e Referring particularly to Figures 18 and 19, inner magnetic shield package is contained within an outer magnetic shield 157 and outer base plate 159. Apertures 167 and 169 are provided for the cesium beam. The entire unit -of outer and inner magnetic shield packages, with the contained RF transition section~ forms the C-fleld/microwave structure module 179 (Figure 24).
Second state selector fB-ma~netltdetector module Referring now ~o Figures 20-23, permanent magnets 198 and 199~ each 30 generally of horseshoe form, are secured to a detector table 196, ~md lie in : ' . - ' .
66~
a horizontal plane containing the beam axis. Magnets 198 and 19~ are assembled to provide two gaps spaced about 180 apar-t, one gap being downstream of R~
` transition section 1~ Oll the beam axis and the other slightly offset therefrom ; and downstream of the first. Soft iron po]e pieces 200 and 201, whose con-igurations are identical to those of the A-magnet pole pieces, are provided in the first gap between permanent magnets 198 and 199, on the beam axis.
Pole pieces 200 and 201 are driven by magnets 198 and 199, and act as the second state selector (or B-magnet) 16. A second pole piece assembly 204 is provided in the second gap between permanent magnet pieces 198 and 199, slightly offset laterally ~rom the beam axis and downstream from the first .~ ~
;; gap; pole piece assembly 204 is driven by permanent magnets 198 and 199 to function as a mass spectrometer 207. Thus the second state selector and :~
the mass spectrometer are driven in series by a single pair of permanent , magnet pieces 198 and 199. This combination contributes to making the cesium .'Z
, beam tube of the present invention smaller and lighter than prior art atomic ~,~ beam tubes. - `
~Detector table 196 is provided with three mounting tabs to which is secured a hot wire ionizer assembly 21 including hot wire 20. An electron multiplier and shield assembly 18 is secured beneath detector table 196, and aperture 203 is provided in table 196, corresponding with an aperture 205 in the electron multiplier shield. The B-magnet 169 mass spectrometer Z07, "
~' hot wire io~izer assembly 21 and electron multiplier assembly 18 together il . .
. make up B-magnet/detecter module 244 (Figure 24).
`! The beam of cesium atoms that emerges from the RF transition sec-tion 14 (Figure 2) contains certain atoms that have undergone a transition i~ and other atoms to be discarded. The atoms selected by second state 'i selector or B-magnet 16 strike the hot wire 20, which is of a standard type -? and will not be further described. Ho~ wire 20 strips an electron from each neutral cesium atom that strikes it, and re-emits a positively charged ,~ . . . .
~ 30 cesium ion. The cesium ions are then sorted by mass spectrometer 207 from ~ `
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impurities unavoidably emitted by hot wi:re 20 and are directed into electron multiplier 18, which produces an amplified output proportional to the number o atoms incident upon the first dynode of the multiplier.
:~ Outer package ; .
. Referring particularly to Figures 24 and 25, the outer package of the atomic beam tube of the invention is a single vacuum tight envelope com- .
' posed of a rigid base 210 ~Figure 24), made of 1.8 inch thick stainless steel, --: and a relatively thin and flexible cover 212 made of 1 mm thick stainless , steel. Base 210 provides the necessary ports with vacuum ~ight feed-through ~ 10 connections to power and RP sources, which are standard and will not be des- ~ -i cribed in detail. The three main subassemblies or modules 179, 2~0 and 244, . ..
which have previously been described in detail, are secured to base 210. .
In assembly, oven/A-magnet module 240 is secured to supports 222 and 224 on base 210 by two machine screws ~00. Thus the path for heat con- . .
duction from oven 10 to the exterior environment of the cesium tube extends through open brackets 134 and 136 and supports 222 and 224 to frame 210. ::. -~ This structure provides a rela*ively long thermal path and aids in isolating i; oven 10 from the outside environment.
~1~ The C-field/microwave structure module 179 is secured to four posts 226 by four machine screws 228. B-magnet/detector module 24~ is secured to :
brackets 234 and 236 by four machine screws 237. Detector table 196 and brac-, ': kets 234 and 236 together provide a relatively long thermal path from ionizer 20 to the environment outside the beam tube.
Cover 212 is welded ~o base 210 after the necessary connections have been made *o the feed-through connectors. The tube is then evacuated under high *emperature conditions.
This modular construction of the beam tube, with each module or subassembly individually secured at a minimum of points to the rigid frame of . .
~: *he single envelope s*ructure, provides alignment and support for the modules : 30 whlle simultaneously providing *hermal isolation and mechanical pro*ec*ion . .
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of the components in the mod~Lcs from tho outsidc environment, ~t the same timc, the relativel~ flexible cover accon~nodatcs to thcrm~l and mech~nical stresses induced b~ the welding operatioll; an buter st~lcture entirely of the ~hicker material would not provide this flexibi:Lity, and alignnlent d.if_ f-cultios wo~ld r~s~lt.
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This invention relates, in general, to atomic beam apparatus, and, more particularly, to atomic beam tubes which utilize magnetic hyperfine re-sonance transitions.
Atomic beam tubes are the basic frequency determining elements in extremely stable frequency standards. Fundamentally, an atomic beam frequency standard detects a resonance within a hyperfine state of the atom to obtain a standard frequency. To utilize this resonance, atomic particles, such as cesium atoms, in a beam interact with electromagnetic radiation in such a manner that when the frequency of the applied electromagnetic radiation is at the resonance frequency associated with a change of state in the particular atoms, the atoms in selected atomic states are deflected into a suitable detector. The frequency of the applied radiation is modulated about the precise atomic resonance frequency to produce a signal from the detector cir-cuitry suitable for the servo control of a flywheel oscillator. Control cir-cuitry is thus employed to lock the center frequency of the applied radiation to the atomic resonance line.
When cesium atoms are employed ln an atomic beam tube, the particular resonance of interest is that of the transition between two hyperfi~ne levels resulting from the interaction between the nuclear magnetic dipole and the -spin magnetic dipole of the valence electron. Only two stable configurations of the cesium atom exist in nature, in which the dipoles are either parallel or anti-parallel, corresponding to two allowed quantum sta~es. Thusg in the absence of an external magnetic field, there are two hyperfine energy levels, each of which may be split by an external magnetic field into a number of Zeeman sublevels.
The hyperfine resonance transition ~sèd in the atomic beam tube of the present invention occ~s between the ~(F=4, mF-0) and ~F=39 mF-0) states, where the first number F is related to the magnitude of the total angular momentum of the atom (electronic plus nuclear) while the second number mF is ~elated to the component of this total angular momentum which is in : "" ' .:
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the direction of the applied external magnetic Eield.
To cause a transition from one state to the other, an amount of energy E equal to the difference in energy of orientation must be either given -to or taken from the atom. Since all cesium atoms are identical, E is the same for every atom. The frequency f of the electromagnetic energy re-quired to cause a change of state is given by the equation E=hf, where h is Planck's constant. For cesium, the magnitude of f :is approximately 9,192.631770 megacycles.
A conventional cesium atomic beam apparatus provides a source from which cesium evaporates through a collimator which forms the vapor into a narrow beam and directs it through the beam tube.
This collimated beam of atoms is acted upon by a first state sel-ecting magnet or "A" magnet, which provides a strongly inhomogeneous magnetic field. The direction of the force experienced by a cesium atom in such a field depends on the state of the atom. In this field, the energy states F=3 and F=4 are split up into sublevels. ~11 of the atoms of the F=~ state, .~ . . .
except those for which mp- -4, are deflected in one direction, and all other atoms are deflected in the other direction. In the apparatus of the present invention, the F=3 group ~together with the atoms of the ~4, -~) sublevel) are retained in the beam, while the others are discarded. The undiscarded atoms include those of the ~3,0) sublevel.
Upon emergence from the A-field, those atoms enter a central region where they are subjected to a weak uniform C-field to assure the separation ~ ;
in energy of the mF_0 states from the nearby states for which mFtO. This t small magnetic field also serves to es~ablish the spatial orientation of ~he t~ selected cesium atoms and~ therefore, the required direction of the micrcwave t magnetic field.
!
i ` While in this uniform weak field region, the cesium beam is subjected to an oscillating externally generated field of approximately the resonance frequency required to cause transitions from the ~3,0) to the ~4,0~ sublevel. ~ ~-1~166i!~
After leaving this energy transfer region, the bearn is acted on by a second state-selecting magnet, similar to the A-magnet~ producing a strong inhomogeneous field. Here the atoms of all the F-3 groups ~and also those of the ~ ) sublevel) are discarded. The only undiscarded atoms are those of the ~,0) sublevel, which exist at this point only because of the induced transition described above. These atoms are allowed to proceed toward a detector of any suitable type~ preferably of the hot-wire ionizer mass spec-tometer type.
The magnitude of the detector current, which is critically dependent upon the closeness to resonance of the applied RF frequency, is used after suitable amplification to drive a servo system to control the requency of -~ the oscillator/multiplier which excites the ~F cavity.
Cesium beam tubes as hitherto constructed have been expensive and difficult to make. To provide a cesium beam tube suitable for use in the usual applications of atomic frequency standards, mechanical alignment of '. t components is critical, and shifts in the alignment can destroy the functional .. . .
~- frequency standard. The tube elements that have been described must be as-sembled and supported in place with a high degree of precision, alignment requirements relative to the beam deflection axis of the itube being approxi-mately .001" for effective tube operation. The precise alignment must be preserved under conditions of mechanical vibration and shock, and of a range of temperature variations typical of practical applications of the tube.
Prior art tubes have employed complicated mounting means between the inner structural assembly of tube elements and either an inner or an ou~er vacuum-tight envelope in an effort to meet ~he often-conflicting requirements of rig1dity against mechanlcal shock or vibration, and flexibility to accommodate -*o differential expansion disturbance forces in the presence of thermal gradi-ents resulting from bake out in tube processing and ambient temperatures in normal tube operation. A further limitation in prior art tubes is that these structure measures typically result in relatively large and heavy tubes, ~: :
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66~1l8 characteristics that are most undesirable for certain important applications ; such as in air or space craft.
Some prior art cesium tubes have been constr~lcted using two separ-ate envelopes. The first is an inner mounting channel to which the operative components are secured to provide mechanical stability and thermal isolation;
this inner envelope is suspended within an outer vacuum envelope. Since dif-ferential movement between the two envelopes must be allowed for, such a com-pound structure adds complexity to the manufacturing processO This design also results in a relatively weak mechanical structure.
This invention relates to a molecular beam tube apparatus including a source for providing a directed beam of molecular particles, a first state selector for selecting a portion of said particles in said beam, a radio fre-quency transition section downstream from said first state selector for caus-ing resonance transitions of some of said selected beam particles, C-field means for producing a weak generally homogeneous magnetic field transverse to said beam in the radio frequency transition section~ a second state selector downstream from said radio frequency transition section for selecting a further portion of said beam comprising those beam particles that have under-gone said resonance transitions, and detecting means responsive to said particles in said further portion, that improvement wherein said C-field means comprises a conductor including a plurality of equally spaced helical turns forming a generally closed loop lying generally in a beam plane including the path of said directed beam, said loop including two end sections transverse to said beam path and parallel to one another, each said end section including a beam aperture having ~irst and second opposed edges interrupting said conductor -in at least two adjacent said helical turns to provide two internal ends ad-jacent~each of said opposed beam aperture edges, at least two conducting jumpers adjacent a said beam aperture, each said jumper having a first jumpeT end --connected to a said helical turn internal end adjacent said first aperture edge, and a second jumper end connected to the said internal end of the same .
~6~il~ll5 said helical turn adjacent said opposed second edge~ whereby said conductor provides a continuous conducting path through said helical turns and said jumpers around said beam apertures, and said magnetic field adjacent a said aperture has greater strength than at other portions of said loop.
The present invention integrates the inner assembly and the vacuum envelope into a single structure, thereby eliminating the need for support elements between the two. It further provides for a modular assembly in which three subassembly units are assembled to the main structural member (which is also a portion of the vacuum envelope) by means of 10 machine screws, as will be describedO The invention also includes novel features providing good thermal isolation, smaller and more efficient magnetic structures, smoother transition between strong and weak magnetic fields, and means to feed in RF
energy with less perturbation of the C-magnetic field than in prior art tubesO
These novel features make possible a tube, both more compatible with typical operating environments than conventional devices, and ligther in weight ~9 lbs.
against the 16 lbs. of a ~ypical prior-art tube).
The design of the present invention eliminates the need for expen-~i sive and complex internal support structures while providing a beam tube of simple modular design that maintains beam alignment and is highly resistant to external mechanical disturbances such as shock and vibration. At the same ` time, the design of the present invention provides excellent thermal isolation for the thermally sensitive components.
The atomic beam tube of the present invention provides a single `i structure that serves both as vacuum envelope and as structural member for ~ the operative ~mponents. This envelope is composed of a heavy and relatively - .:.
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rigid frame and a relatively thin ancl Elexlble cover sealed to the frame. The operative elements of the tube are secured to the frame; this provides fixed alignment of these elements. The flexible cover accommodates itself readily to externally caused mechanical distortions without transmitting them to the frame or to the operative elements. The sealed unit acts as a vacuum envelope.
The operative elements of the tube are secured to the heavy frame at a minimum of locations~ and the connections have low thermal conductivity, in order to isolate the operative elements thermally from the environment. For example, the oven structure is secured to the frame through a connecting structure :~
tha~ is designed to provide a relatively long thermal path to the environment.
It is industry practice to disassemble such tubes when they are no longer operable (generally because the cesium getters are saturated) in order to salvage reuseable components. To disassemble prior art tubes has :
required extensive machining which is both time-consuming and expensive, in-volving high labor costs. In the cesi~ beam tube of the present invention, :~
the operative parts are provided in three main modular subassemblies, secured to the frame by a total of 10 screws, for quick and simple disassembly and ~:
reuse of the modular portions.
The operation of the cesium beam tube, as has been described, re-quires that the A and B magnets provide very strong fields (of the order of 10 kilogauss), while the C-field in the region between them must be re-latively weak ~of the order of .060 gauss) and as uniform as possible. Dis-continuities in the C-field are particularly likely to occur in the regions at which the beam enters and leaves the C-region, and can cause spontaneous ~ transitions (~ajorana transitions) in the atomic beam which may distort the performance o~ the tube. The present invention provides a C-field winding of~novel design that generates a C-field of superior uniformity at the beam ,; . .
apertures.
In general, it is desirable to provide a cesium beam tube that is .
~ 3~ as compact, light weight, and simple as possible. The particular designs , , '` ~: , ~66~3~8 of the A and B magnets in the present invention realize such construction and are particularly adapted to the modular assembl~ previously described.
It is typical in the assembly and processing of molecular beam tubes to confille the source of the molecular beam material in a sealed ampoule during the bakeout and exhaust part of the processing cycle, and as a final stage, while the tube is still being pumped, but after bakeout has been com-pleted, to open the ampoule. Any gases released in the opening process can then be pumped prior to the final sealing off of the tube.
A number of methods have been used in the prior art for opening the ampoule. One such method is to provide means whereby a member of the ampoule is ruptured when electrical energy is applied to a heating coil to cause expansion in a member mechanically linked to a rupturing element. A more sophisticated prior art method is to discharge an external capacitor through electrical conducting paths into the tube, so arranged that a vaporiæing arc is created at a member of the ampoule which is ruptured by the heat of the ~ arc. Both of these methods require the inclusion in the beam tube of additional ; parts that are used only for this one operation; in particular~ means must be provided to transmi~ electrical energy through the vacuum envelope, which complicates the construction of the tube.
The present invention provides a novel ampoule structure and novel means for opening the ampoule that -require no additional parts; in particular, no additional electrical or mechanical ~eeds through the vacuum envelope are ; required.
Other objects, features, and advantages will appear from the follow-ing description of a preferred embodiment of the invention, taken together wlth attached drawings thereof, in which:
Figure 1 is a schematic view of the principal beam-forming and detect-:
ing elements of the tube;
Figure 2 is a perspective view of the elements of Figure l;
Figure 3 is an exploded view of the components of the oven and ampoule;
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Fig, 4 is a cross section of thc ampol~c;
Fig, S is a ~iew of thc assembled o~en;
Fig, 6 is a view of the oven with reM ector and support structurc;
Fig. 7 is a Zeeman energy diagr~n for cesium 133 in thc ground electronic state; showing the transition induced in the beam tube of the in-vention;
Fig. 8 is a schematic view of the control circuitry used with the cesi~n beam tube of the invention;
Fig. 9 is a perspective view of the first state selector magnet and ion pump;
Fig, 10 is an exploded perspective view of the first state selector ., .
magnet together with shielding and support structure;
Fig. 11 and 12 are longitudinal and cross sections respectively of 1 the first state selector and ion pump;
Fig. 13 is a perspective view of the microwave structure and C-field rcoil;
Fig, 14 is a perspective view of the C_~ield coil with portlons broken away;
Fig, 15 is a plan view of the unfolded C-field coil;
~1l 20 Fig. 16 is a cross section of the assembled C_field Goil at a beam aperture;
Fig, 17 is a detail of the conductors of the C_field coil at a beam erture; ~ -Fig, 18 is an exploded view of the magnetic shield package and con-tents;
Fig, 19 is a cross section of the outcr envelope and con~ents near the center;
.~ ~Fig. 20 is a pcrspectiYc viewOf the B-ficld magnet and the detcctor;
Fig, 21 shows thc elements of Fig, 20 wi~h support structurc;
Fig, 22 and 23 are a plan vicw and a rear elevation view of the , : - ' .. . .
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1~615~3 B-field magnet and the detector;
Figure 24 is an exploded view of the outer packaging and connections and the modular units; and Figure 25 is a longitudinal view partly in section of the assembled ; units of Figure 24.
General Referring to the drawings, and particuiarly to Figures 1 and 2, the basic beam-forming and detecting elements of the cesium tube 11 of t~e in-vention are shown schematically and in perspective. A source of atomic par-ticles includes an oven 10 which evaporates liquid cesium and emits ~through a collimator) a beam of neutral cesium atoms which are statistically distri-buted between two stable energy states, as previously described. The first state selector or A magnet 12 splits these energy states into sublevels and selects the atoms in the F=3 states (together with those in the ~4,-4) sub-level) and deflects the remaining atoms so that they no longer form part of the beam. The beam of selected atoms then passes through the RF interaction ; section 14; in this region a weak homogeneous magnetic field ~C-field) is supplied by the winding 22. Microwave energy is supplied at the resonance frequency to induce transitions of some of the beam atoms from the ~3,0) 20~ state to the ~4,0) state ~Figure 7). The beam atoms in the ~4,0) state are ~ ;~
then~selected by the second state selector or B magnet 16, the atoms in the ~;
remalning states being deflected out of the beam. The cesium atoms selected by the B magnet strike the hot-wire ionizer 20, and an electron is stripped ~ ~ . -:. ...
from each cesium atom, causing the re-emission of cesium ions, which are accelerated through a~mass ;spectrometer 207 into the electron multlplieT 18.
The electron multipllèr provides an output current proportional to the number ; of atoms arriving at the hot wire 20, that is, proportional to the number.
~ of atoms that ha~e been raised to the second state in the microwave cavity. ; ~
~: .
As shown in Figure 8, the output of the atomic beam tube 11 is fed to control electronics 260 which produce a suitable error output signal 261, :
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~9668~Ll3 .:, which is applied to a crystal oscillator 262. Thc frcquellcy output of thc crystal oscillator (typically 5 megahcrtz) is controllcd b~ the proccssed signal 261 from the cesium beam tubc, and then ~ultiplied in the frequency multiplier chain 264 and applied to tube 117 at the precise resonance frcquency (typically 9192 mHz), Multiplier chain 264 and the controlled oscillator 262 from the microwave generator 266, The usable output signal is derived from controlled oscillator 262 at 268, S~unmar~ of ~lodular Components The elements that have been described and shown in Fig. 8 are in general terms old and well-known in the art, The cesium tube of the invention provides ~hree modular subassemblies including a cesium ampoule and a first 'J state selector magnet in combination with the ion pump, a second state selec_ ~ . tor magnet in combination with the mass spectrometer~ and a C-field winding -ii and microwave structure, all of novel design, as well as a novel outer pack-~¦ age for the entire tube.
To provide the advantages of the modular assembly of the invention, as previously described, the oven 10 (~rith cesium ampoule) and-A_magnet 12 ~with ion pump~ sho~n separately in the schematic views of Figs, 1 and 2, are combined in~an oven/A-magnet assembly module 240 (Fig, 24). The P~ inter-action region 14 and C_field, shown unenclosed in Figs, 1 and 2, are contained ~ --j in magnetic shield package 179 (Fig, 24), The B~magnet 16, hot wire ioni~er .
I 20, mass spectrometer 207 and electron multiplier ]8 are packaged together ,I; in a detector assemb]y module 244~(Fig, 24), Referring to Figs. 24 and 25, -1~ ~modùles 240 ana 244 and magnetic shield package 17~ are essentially independent ', .
i~ ~ of one another and constitute the subassembly units iYithin the outcr pac'l~age .j .
~of the beam tube, and are assembled thercto b~ means of 10 screws9 as wil~
bc described, 1~ The details of each of thesc modular components are dcscribed belo-~.
Ovcn~.~ ma~net modulo_ oven and ~npoule The structure of the novel o~en_~mp~u1eassembly 10 of the invention~
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constitutin~ a source for providing a beam of c~siwn particlcs~ is sho~m in detail in Figs. 3-6, The assembly 10 includes collimatin~ means 42, not dcs_ cribed, and ovcn means including a reservior 29 containing a~t ampoule 27, The ampol~e 27 includes a t~n walleA (0,015") ~cnerally cylindrical shell 30 and a top 37 including a fill tube 38, Top 37 and cylinder 30 together fonm an enclosure, The end of shell 30 opposite to top 37 provides an opening 49, A
cup shaped base 34 is sealed into shell opening 49 by an eutectic metal 32 designed to fail mechanically at a temperature of approximately 600C, An example of such an eutectic metal is an alloy of 45% copper and 55~ inditlm, A weak spring 35 is compressed betlreen base 32 and top 37, `~ After the enclosure has been filled ~rith liquid cesium, fill tube,~ .
38 is closed by pinching and heliarc welding, A wire screen mesh 36 having high thermal conductivity surrounds ampoule 27 within reservoir 29, The mesh 36 serves both as a heat transfer element and as a retaining and support eIement for the ampoule, Ampoule 27 is supported within reservoir 29, A copper outer c~linder 28 of reservoir 29 includes an annular recess 40 at its lower portion, A
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wélding ~daptor 39 havingalo~ter flange 41 is bra~ed to recess ~0 of outer cylinder 28. An ampoule support member 43 includes an in~erted cup por~ion 44 and three spaced supports 45, Inverted cup portion 44 of member 43 is heliarc welded at 46 (Fig. 4) to the inner surface of weld mg adaptor flan~e l to seal *le lo~er ~nd~of reservoir 29. This creates an enclosed reservoir space 51 surro~nding base 34 and commu0icating with mesh 36. ~npoule 27 is seated m support member 43 with ampoule baso 34 within spaced supports 45.
Two tantaltun heaters 9~ and 92, rctained in a ccramic support struc-ture 88, are inserted into collimator asse~mbly 42 throu~h quartz tubes 80 and 82,~ Thc ampo~e is opened3 after balceout of thc bcam tube3 by mcans of thcse ~ -heators, which heat the ampoule to 600C, at which tempcraturc the cutectic 30 se.~ fails, Thc combination of the vapor pressure of the C~SiUnl within ampoule -;
"' l~G6818 ~7 and the forcc of comprcssed we~c spring 35 exerts a stress grcater th~
the working strcss of the metal of se~l 32 and pu~ 5 base 34 out of shell 30, thereby relcasing thc cesi~un in the ampoule. Weak sprin6 35 prevents the base from settling back into place, resealing thc ampoule.
In later operation of the tube, tantalum heaters 90 and 92 are used to wann the entire oven assenhly 10 to the operating temperature, typic-ally about 90C. At this temperature the liquid cesi~un in reservoir space 51 slowly ~apori~es and diffuses from the mesh 36 to collimating means 42.
Collimator 42 is functionally equivalent to a bundle of small tubes so oriented that a directed beam of cesium atoms emerges. Construction of collimating means is well known to the art, and will not be detailed here The oven support structure is designed to provide thermal isolation ~rom outside the beam tube. Since the oven operates in a vacuum, there is no heat loss from conve~tion; the major loss is by radiation, l~th some loss by conduction. m e oven support structure is therefore constructed of mater-ial of poor thermal conductivity such as staililess steel and includes ear portions 100 and 102 for securing oven 10 to the A ~nagnet assembly, as wiI1 be described, Additionally~ 0.003~l Kapton shims 99 between the ear portions o~ the support structure and the A_magnet assem~l~ further discourage thermal conduction. A radiation shield 104 of highly polished alu~inùm surrounds the major portion of the oven, and preserlts radiation heat loss from the oven~
An oven of the design described required less than two watts for operatioll.
OvenJA-mlagnet modulc: _-magnet and ion pume Refcrring now to Figs 9 through 12~ a pex~lancnt magnet driver 111 is sh2red by the first state selector magnet (~ magnct) 1~ ~Id the ion pump 110. The ion pump perfo~ns the well-kno~n function of ren~oving undesired .
gasses and maintaining tube vacllum during opcration. Permancllt magnet 111 is generally of a typical "C7' shape, but 1~th a novel r~entrant imler surface shape that gives it the distinguishing capability of providing proper fields for both sclection and ion pumping The axis of magnet 111 is parallel ~ith ' .
, .. . .. : .. , . .
~L~i6~8~
the beam.
"Dipole con~iguration" soft iron pole pieces 112 and 114, of a well-known design, are secured in the gap of "C" shaped permanent magnet 111, and provide the inhomogeneous deflecting field of first state selector 12.
Reentrant extensions 108 and 109 of permanent magnet 111 extend in-wardly to~ard one another, and in conjunction with a second pair of short cylindrical pole pieces 116 and 118 provide the field for the ion pump 110, located between pieces 116 and 118. The ion pump is of any suitable design and is well known.
Permanent magnet 111 provides in effect two permanent magnet cir-cuits in parallel to drive both the "A" state selector 12 and the ion pump 110. The magnetic driver is designed to provide approximately 10 K gauss in the state selector circuit while providing approximately 1000 gauss for the ion pump. The compact arrangement of this combination permits the atomic beam tube assembiy to be smaller, lighter, and less expensive than those hitherto constructedJ and is also especially adapted to the modular design of the present beam tube apparatus.
A magnetic shield 132 covers approximately the upper half of the outer surface of magnet 111 and additionally on one end is interposed between ; 20 the magnet and the C-field/microwave structure module 179 (Figure 24). Shield 132 provides aperture 138 for the passage of the atomic beam from the A-magnet ~! 12 to module 179. The structure of shield 132 further provides field control l for the attenuation of the 10 K gauss deflecting field of the A-magnet down -:
s~ to the 0.060 gauss C-field in the RF transition region 14.
A mounting plate 128 is secured ~o the upstream side of permanent I~ magnet 111, and provides brackets 134,and 136. Magnetic shield 132~ stainless ;3 steel spacers 113, magnet 111~ and another pair of stainless steel spacers ~i 117 all are ~astened together by a pair of machine screws 115 passing through 3~ clearance holes in each and threading into tapped holes in mounting plate 128.
~ 30 ~ Oven 10 ~Figure 6) is secured by its support structure ear portions ~ .
i~
I - 12 ~
~ .
, ~6i~L8 110 and 102 to brackets 134 and 13G, .~ts thcsc brackets are OpCII ill construc-tion, rather than solid, they providc a relatively long thermal path for the conduction of heat from the oven through the brackets to the e~entual pOill~
o~ contact with the outer framc of the beam tube, Shims 99 of 0,003" Kapton are interposed bet~een ears 100 and 102 and brackets 134 and 136 an.d provide further thermal insulation, Oven 10 and A_magnet 12 with ion pump 110 from theo~en/A_magnet module 240 (Fig, 24).
C-field/~icrowave ~tructure module Referring again to Figs, 1, 2 and 4, the C-field and RF ~radio .
frequency) transition section 14, including magnetic shields to be described, are packaged together as a second module 179, As previously described in connection with Fi.g, 2, the cesium atsms that are selected by the A_magnet 12 form a beam that must next pass through RF transition section 14, In this region a weak homogeneous magnetic ~ield ~' (C-field) of approxI~ately ,06 gauss directed transverse to the beam path is , proYided by a single-layer printed circuit solenoid 22 of novel design, The construction and mounting supports of this solenold will be described by re-ference to ~igs. 13 through 19, R'eferring first to Fig, 15, the conductors of solenoid 22 are etched .
by well-kno~n printed circuit techniques from a thin copper layer bonded to a base 152 of polgim1dc material appro~imately 0.002 inch thick, The general shape of the base material 152 and a pattern of eight uni~ol~y-spaced COIl-:
: ductors 150-1 ~hrough 15Q_8 is shol~l in Fig, 15. Fyelet holes 307 are pro-: `vided at each end of the conductors 150. This printed circuit solenoid pro-vidos thin, wido, and closely spacod conductors of ~e~y unifol~n cross scctionalarea and constant ~onductivity, The pri.ntcd circuit solcnoid is assembled into a generally rectangu-; :lar loop as shown parti.cularly in Fig. I~, with thc cyeleted ends of conduc-: 30 tors~l50 offset one conductor in registry so that the complctcd conducting ~: ~
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~ - 13 _ ~66~
~.
path will form a one~ yer sp.iral ~rindin~ of equall.y spacccl hel.ical turns Electrical colmection at each of thc offset, but othcrlriso registcrcd, ends of conductors 150 is made by soldering using indium washers (not sho~in) cmd secured b~ rivets 308 inserted through the eyelet holes ~lectrical connection to the solenoid is made by wire leads soldered to eyeletted pads 304 and 306 at the end of each of the outside turns.
The closed loop includes two end sections 140 and 142 that are trans- :
verse to the beam path and parallel to one another Since the assembled ~ :
.~ solenoid winding must lie generally in the plane of the cesium beam, apertures 270 and 271 are provided in end sections 140 and 142 of such a size as to interr~apt conductors 150-4 and 150-5.
Aperture 2?0 in base layer 152 has tlYo.opposed edges 144 (Fig 15) ~ .
that interrupt the two adjacent inner strips lS0 4 and 150-5 of continuous .
conductor 150, to provide four internal ends 122 of strips 150_4 and 150-5 adjacent the aperture edges Ends 122 are eyeletted To provide a continuous ! ~ :
current path, it is necessary to bridge the aperture by conneeting the internal ~
conductor ends. In addition, it is necessary to maintain uniformity of the C_field at the beam apertures insofar as is possible, to avoid field discon- ~.
tinuities causing undesired transitions, as previously explained ~ In the present invention, two patches~318 of prillted circuit material slmilar to-that described are provided to bridge the gaps and maintain Ul~-formity of the C_field, each having an aperture 319. ~o eyelctted conducting .:
jumpers 166 and l68 are bondcd to base layor 320, and angle around aperture ~.
319. Referring particularly to Figs 14 and 17, a:patch 318 is assembled to : ..
the~-~ndin~ by soldering to rivets 182 passing through the eyelets of the ~ -jumpers and of internal ends 122. This construction maintains thc continuous current path through the entire conductor 150 at thc beam apcrtures. J~pers 166 and 168 lead the current around each aperture 270 and 271, effcctively : doublill~:the magnetizing force at tho cdges of thc apertures and tendi.ng to :.
30 : mai.ntain a near uniform distribution of the C-ficld across the aperturcs.
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.
6~3~L8 This structure providcs an exceedingly close approximatiorL to the ideal of a uniformly distributed current sheet Electrical insulation around the solenoid is provlded by polyimide strips 184 and 186 (Fig. 14) made to the sclme shape as print~d circuit base 152, one being placed on either side of base picce 152.
Inner Magnetic Shield Package The assembled C~field winding 22, comprising the three layers and o patches as described, is mounted on the inner surface of i~mer magnetic ¦ shield 154 (Fig l5 ) and inner shield base plate 156 and is ~eid in place ~` 10 by ri~ets passing through the shield material, the outer margins of the solenoid assembly of base material 152 and insulating strips 184 and 186, and aluminum plates 282 of which representative ones are shown in Fig. 18 The assembly at the a~ertu~elocations 270 and 2?1 is made ~rith aluminum plates 280 that provide apertures to register with apertures 270 and 2~1 -~, - A flop eoil 192 tFigs 2 and 18) is mounted on one of the central i aluminum plates 282 and supported from m ner magnetic shield 154 so that it ~ is eoaxial to the beam axis. This eoil is used in a manner well known to ;;, the prior art t~ introduce a 20 ~hz, electrical signal for the adjustment of he C-field solenoid eurren~, and will not be described further.
The sides of inner magnetic shield 1$4 (Fig 18), paralleling ~ ~le beam path, provide magnetic end eaps for solenoid 22. The resulting field '~ aeross the plane of solenoid 22 thereby approximates t~le classical ~niform field of an infinitely long solenoid ~th ~lux lînes nol~al to the c~sium beam path Inner magnet shield 15~ in combination with spaced outer magnetic shield 1S7 effectlvely attenuates the strong magnetic fields prodllced by the A and B magnets and also shields the RF transition region from external mag-netic perturbations, ~,~ Microwave radiation ~ ~ Referring particularly to Figs 1, 2 and 18, microwave radla~ion `~ 3~ is supplied ~thin RF interaction section 14 by ~ravegl~de structure 190~ ~-hich : :
, : - 15 --~6~ 8 is of the standard "Ramsey" type and well known in the art. It w:ill not be described here.
In prior art atomic beam tubes, constructed with separate mcchanical protective and vacuum isolation envelopes, differential motions between the two envelopes have made it necessary to provide flexible connection means -between the microwave structure and the exterior of the tube, capable oE
accommodating to such motions. Such flexible means requires a relatively large aperture, typically two inches in diameter, in the magnetic shield struc-ture to accommodate the connection. Such a large aperture introduces per-10 turbations in the magnetic C-field due to leakage effects, which must in turn be compensated for~ for example by providing ex~ra "baffling means" as in United States Patent No. 3,670~171 (Lacey et al) issued June 13, 1972.
In the present invention, the combination of mechanical support and vacuum isolation envelope into a single structure eliminates such differential motions. The inlet arm of microwave structure 190 can therefore be intimately , brazed to the lower surface of inner shield base plate 156. This construction ayoids the need for a large aperture through the magnetic shield; a relatively small aperture 194, about 1" x 1/2", is prot,ided in base plate 156 (Figure 18). ~ -~Such a small aperture introduces only relatively small perturbations into the 20 C-fieid, eliminating the need for "baffling" or other compensating structure~
and this structure is therefore advantageous.
Outer magnetic shield packa~e Referring particularly to Figures 18 and 19, inner magnetic shield package is contained within an outer magnetic shield 157 and outer base plate 159. Apertures 167 and 169 are provided for the cesium beam. The entire unit -of outer and inner magnetic shield packages, with the contained RF transition section~ forms the C-fleld/microwave structure module 179 (Figure 24).
Second state selector fB-ma~netltdetector module Referring now ~o Figures 20-23, permanent magnets 198 and 199~ each 30 generally of horseshoe form, are secured to a detector table 196, ~md lie in : ' . - ' .
66~
a horizontal plane containing the beam axis. Magnets 198 and 19~ are assembled to provide two gaps spaced about 180 apar-t, one gap being downstream of R~
` transition section 1~ Oll the beam axis and the other slightly offset therefrom ; and downstream of the first. Soft iron po]e pieces 200 and 201, whose con-igurations are identical to those of the A-magnet pole pieces, are provided in the first gap between permanent magnets 198 and 199, on the beam axis.
Pole pieces 200 and 201 are driven by magnets 198 and 199, and act as the second state selector (or B-magnet) 16. A second pole piece assembly 204 is provided in the second gap between permanent magnet pieces 198 and 199, slightly offset laterally ~rom the beam axis and downstream from the first .~ ~
;; gap; pole piece assembly 204 is driven by permanent magnets 198 and 199 to function as a mass spectrometer 207. Thus the second state selector and :~
the mass spectrometer are driven in series by a single pair of permanent , magnet pieces 198 and 199. This combination contributes to making the cesium .'Z
, beam tube of the present invention smaller and lighter than prior art atomic ~,~ beam tubes. - `
~Detector table 196 is provided with three mounting tabs to which is secured a hot wire ionizer assembly 21 including hot wire 20. An electron multiplier and shield assembly 18 is secured beneath detector table 196, and aperture 203 is provided in table 196, corresponding with an aperture 205 in the electron multiplier shield. The B-magnet 169 mass spectrometer Z07, "
~' hot wire io~izer assembly 21 and electron multiplier assembly 18 together il . .
. make up B-magnet/detecter module 244 (Figure 24).
`! The beam of cesium atoms that emerges from the RF transition sec-tion 14 (Figure 2) contains certain atoms that have undergone a transition i~ and other atoms to be discarded. The atoms selected by second state 'i selector or B-magnet 16 strike the hot wire 20, which is of a standard type -? and will not be further described. Ho~ wire 20 strips an electron from each neutral cesium atom that strikes it, and re-emits a positively charged ,~ . . . .
~ 30 cesium ion. The cesium ions are then sorted by mass spectrometer 207 from ~ `
':
~ - 17 -:
:~ . .. . , . , .. : .. - ~ .. .. . . . .
~668~L~
impurities unavoidably emitted by hot wi:re 20 and are directed into electron multiplier 18, which produces an amplified output proportional to the number o atoms incident upon the first dynode of the multiplier.
:~ Outer package ; .
. Referring particularly to Figures 24 and 25, the outer package of the atomic beam tube of the invention is a single vacuum tight envelope com- .
' posed of a rigid base 210 ~Figure 24), made of 1.8 inch thick stainless steel, --: and a relatively thin and flexible cover 212 made of 1 mm thick stainless , steel. Base 210 provides the necessary ports with vacuum ~ight feed-through ~ 10 connections to power and RP sources, which are standard and will not be des- ~ -i cribed in detail. The three main subassemblies or modules 179, 2~0 and 244, . ..
which have previously been described in detail, are secured to base 210. .
In assembly, oven/A-magnet module 240 is secured to supports 222 and 224 on base 210 by two machine screws ~00. Thus the path for heat con- . .
duction from oven 10 to the exterior environment of the cesium tube extends through open brackets 134 and 136 and supports 222 and 224 to frame 210. ::. -~ This structure provides a rela*ively long thermal path and aids in isolating i; oven 10 from the outside environment.
~1~ The C-field/microwave structure module 179 is secured to four posts 226 by four machine screws 228. B-magnet/detector module 24~ is secured to :
brackets 234 and 236 by four machine screws 237. Detector table 196 and brac-, ': kets 234 and 236 together provide a relatively long thermal path from ionizer 20 to the environment outside the beam tube.
Cover 212 is welded ~o base 210 after the necessary connections have been made *o the feed-through connectors. The tube is then evacuated under high *emperature conditions.
This modular construction of the beam tube, with each module or subassembly individually secured at a minimum of points to the rigid frame of . .
~: *he single envelope s*ructure, provides alignment and support for the modules : 30 whlle simultaneously providing *hermal isolation and mechanical pro*ec*ion . .
~ : ,;'':
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.. .
..
1~)668~
of the components in the mod~Lcs from tho outsidc environment, ~t the same timc, the relativel~ flexible cover accon~nodatcs to thcrm~l and mech~nical stresses induced b~ the welding operatioll; an buter st~lcture entirely of the ~hicker material would not provide this flexibi:Lity, and alignnlent d.if_ f-cultios wo~ld r~s~lt.
' , .
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. .
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~ ,: :
' . ~
Claims
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a molecular beam tube apparatus including a source for providing a directed beam of molecular particles, a first state selector for selecting a portion of said particles in said beam, a radio frequency transition section downstream from said first state selector for causing resonance transitions of some of said selected beam particles, C-field means for producing a weak generally homogeneous magnetic field transverse to said beam in the radio frequency transition section, a second state selector downstream from said radio frequency transition section for selecting a further portion of said beam comprising those beam particles that have undergone said resonance transitions, and detecting means responsive to said particles in said further portion, that improvement wherein said C-field means comprises a conductor including a plurality of equally spaced helical turns forming a generally closed loop lying generally in a beam plane including the path of said directed beam, said loop including two end sections transverse to said beam path and parallel to one another, each said end section including a beam aperture having first and second opposed edges interrupting said conductor in at least two adjacent said helical turns to provide two internal ends adjacent each of said opposed beam aperture edges, at least two conducting jumpers adjacent a said beam aperture, each said jumper having a first jumper end connected to a said helical turn internal end adjacent said first aper-ture edge, and a second jumper end connected to the said internal end of the same said helical turn adjacent said opposed second edge, whereby said conductor provides a continuous conducting path through said helical turns and said jumpers around said beam apertures, and said magnetic field adjacent said aperture has greater strength than at other portions of said loop.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA318,219A CA1066818A (en) | 1974-10-09 | 1978-12-19 | Cesium beam tube |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/513,289 US3967115A (en) | 1974-10-09 | 1974-10-09 | Atomic beam tube |
| CA237,259A CA1056957A (en) | 1974-10-09 | 1975-10-08 | Cesium beam tube |
| CA318,219A CA1066818A (en) | 1974-10-09 | 1978-12-19 | Cesium beam tube |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1066818A true CA1066818A (en) | 1979-11-20 |
Family
ID=27164144
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA318,219A Expired CA1066818A (en) | 1974-10-09 | 1978-12-19 | Cesium beam tube |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1066818A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN117790042A (en) * | 2024-02-27 | 2024-03-29 | 中国科学院国家授时中心 | Optical pumping atomic beam device based on curved four conductors |
-
1978
- 1978-12-19 CA CA318,219A patent/CA1066818A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN117790042A (en) * | 2024-02-27 | 2024-03-29 | 中国科学院国家授时中心 | Optical pumping atomic beam device based on curved four conductors |
| CN117790042B (en) * | 2024-02-27 | 2024-05-28 | 中国科学院国家授时中心 | Optical pumping atomic beam device based on curved four conductors |
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