Note: Descriptions are shown in the official language in which they were submitted.
10ti~()13
Ihis invention relates, in general, to atomic beam apparatus, and,
more particularly, to atomic ~eam 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 electro~agnetic radiation in such a
manner that when the frequency of the applied electromag~etic radiation is
at the resonance frequencya5sociated 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 pre-
cise 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 in an atomic beam tube, the particular
resonance of interest is that of the transition between two hy~erfine levels
resulting from the interaction between the nuclear magnetic dipole and the
spin magnetic dipole of the valence electron. ~nly 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 states. Thus, 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 used in the atomic beam tube of
the present invention occurs between the ~F=4, mF=0) and ~F=3, 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 related to the component of this total angular momentum which is in
1068013
the direction of the applied external ma~netic field.
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. All of the atoms of the F=4 state, -~
except those for which mF=-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 ~4J-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 mF~0. This
small magnetic field also serves to establish the spatial orientation of the
selected cesium atoms and~ therefore, the required direction of the microwave
magnetic field.
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.
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After leaving this energy transfer regionJ the beam is acted on by
a second state-selecting magnet, similar to the A-magnet, producing a strong
inhomogeneous field. Ilere the atoms of all the F=3 groups (and also those
of the (4,-4) sublevel) are discarded. Thc only undiscarded atoms are those
of the (4,0) sublevel, which exist at this point only because ofthe 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-
trometer 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 frequency of
the oscillator/multiplier which excites the RF 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 frequncy standards, mechanical alignment of
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 tube being approxi-
mately .001" for effective tube operation. The precise alignment must bepreserved 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 outer vacuum-
tight envelope in an effort to meet the often-conflicting requirements of
rigidity against mechanical shock or vibration, and flexibility to accommodate
to differential expansion disturbance forces in the presence of ~hermal gradi-
ents resulting from bakeout in tube processing and ambient temperatures in
normal tube operation. A further limitation in prior art tubes is that these
3Q structure measures typically result in relatively large and heavy tubes.
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10f~1~()13
character-istics th~t are mo~t undcsirable for certain important applications
such as in air or space craft.
Some prior art cesium tubes have been constructed using two sep-
arate envelopes. The first is an inner mounting channel to which khe oper-
ative components are secured to provide mechanical stability and thermal
isolation; this inner envelope is suspended within an outer vacuum envelope.
Since differential movement between the two envelopes must be allowed for,
such a compound strueture adds complexity to the manufacturing process. This
design also results in a relatively weak mechanical structure.
This invention relates to 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 means for producing a weak generally homogeneous magnetic field
in said radio frequency transition section a second state selector downstream
from said radio frequency transition section for selecting a further portion
of said beam eomprising those beam particles that have undergone a said
resonance transition and detecting means responsive to said partieles in
said further portion of said beam, ineluding a mass speetrometer, that
improvement wherein said apparatus provides a pair of generally horseshoe
shaped permanent magnets oriented to provide two gaps spaeed about 180
apart, a first said gap being downstream of said radio frequency transition
section in the path of said beam, and a second said gap being downstream of
the first, a first pole piece assembly within said first gap and driven by
said permanent magnets, and a second pole piece assembly within said second
gap and driven by said permanent magnets, whereby said first pole piece assem-
bly provides said second state selector, and said second pole piece assembly
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provides said mass spectrometer in s~id detecting means , the magnetic cir-
cuits of said second state selector and said mass spectrometer being in
series.
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 described. me 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 tubes. These novel features make possible a tube, both
more compatible with typical operating environments than conventional de-
vices, and lighter in weight (9 lbs. against the 16 pounds of a typical
prior-art tube).
me design of the present invention eliminates the need for expen- -
sive and complex internal support structures while providing a beam tube
of simple modular design that maintains beam alignment and is highly resis-
tant to external mechanical disturbances such as shock and vibration. At
the same time, the design of the present invention provides excellent ther-
mal isolation for the thermally sensitive components.
me atomic beam tube of the present invention provides a single
structure that serves both as vacuum envelope and as structural member for
the operative components. This envelope is composed of a heavy and relatively
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rigi-l framc and a relatively thin and flexiblc cover sealed to the frame. The
operative elements of tlle tube are secured to the frame; this provides fixed
alignment of these elemellts. The flexible cover accommodates itself readily
to externally cause~ mecllallical distortions without transmitting them to the
frame or to tlle operative elements. ~`he sealed unit acts as a vacuum envelope.
The operative elements of the tube are secured to the heavy frame at a minimum
o~ locations, and the connection 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
that is designed to provide a relatively long thermal path to the environment.
~ t 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
e~tensive machining which is both time-consuming and expensive, involving
high labor costs. In the cesium 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 of 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
as compact, light weight, and simple as possible. The particular designs
of the A and B magnets in the present invention realize such construction and
are particularly adapted to the modular assembly previuosly described.
.
013
It is typical in thc assen~bly and processing of molecular beam tubes
to confine the source of the molccular 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 enargy is applied to a heating coil to cause
expansion in a member mechanically linked to a rupturing element. A more
sohpisticated prior art method is to discharge an external capacitor through
electrical conducting paths into the tube, so arranged that a vaporizing 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 ~he beam tube of addi-
tional parts that are used only for this one operation; in particular, means
must be provided to transmit electrical energy through the vacuum envelope
which compllcates the construction of the tube.
The present invention provides a novel ampoule structure and novel
means for opening the ampoule that require no additional part; in particular,
no additional electrical or mechanical feeds through the vacuum envelope are
required.
Other objects, features, and advantages will appear from the
following description of a preferred embodiment of the invention, taken
together with attached drawings thereof, in which:
Figure l is a schematic view of the principal beam-forming and
detecting elements of the tube;
Figure 2 is a perspective view of the elements of Figure l;
Figure 3 isan exploded view of the components ofthe oven and ampoule;
Figure 4 is a cross section of the ampoule;
Figure 5 is a view of the assembled oven;
Figure 6 is a view of the oven with reflector and support structure;
Figure 7 is a ~eeman energy diagram for cesium 133 in the ground
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106~013
electronic statc; showing the transition induced in the beam tube of the in-
vention;
Figure 8 is a schematic view of the control circuitry used with th0
cesium beam tube of the invention;
Figure 9 is a perspective view of the first state selector magnet and
ion pump;
Figure 10 is an exploded perspective view of the first state selector
magnet together with shielding and support structure;
Figure 11 and 12 are longitudinal and cross sections respectively
of the first state selector and ion pump;
Figure 13 is a perspective view of the microwave structure and C-
field coil;
Figure 14 is a perspective view of the C-field coil with portions
broken away;
Figure 15 is a plan view of the unfolded C-field coil;
Figure 16 is a cross section of the assembled C-field coil at a beam
aperture;
Figure 17 is a detail of the conductors of the C-field coil at a beam
aperture;
Figure 18 is an exploded view of the magnetic shield package and con-
tents;
Figure l9 is a cross section of the outer envelope and contents near
the center;
Figure 20 is a perspective view of the B-field magnet and the detect-
or;
Fi~ure 21 shows the elements of Figure 20 with support structure;
Figure 22 and 23 are a plan view and a rear elevation view of the
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.
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Ceneral
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Referring to the drawings, and particularly to Figures 1 and 2, the
basic beam-forming and detecting elements of the cesium tube 11 of the 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-f~ield) 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)
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
remaining 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 multiplier 18.
The electron multiplier 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 have 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,
which is applied to a crystal oscillator 262. The frequency output of ~he
crystal oscillator ~typically 5 megahertz) is controlled by the processed
signal 261 from the cesium beam tube, and then multiplied in the frequency
multiplier chain 26~ and applied ~co tube 11, at the precise resonance frequency
30 Ctypically 9192 mHz~. Multiplier chain ~64 and the controlled oscillator
262 from the microwave generator 266. The usable output signal is derived
from controlled oscillator 262 at 268.
(J13
~ummary_of ~lodular Components
The elements t~lat have been described and shown in Figure 8 are in
general terms old and well-known in the art. The cesium tube of the invention
provides three modular subassemblies including a cesium ampoule and a first
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
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,
lO as previously described, the oven 10 ~with cesium ampoule) and A-magnet 12
(with ion pump), shown separately in the schematic views of Figures 1 and 2,
are combined in an oven/A-magnet assembly module 240 (Figure 24). The RF
interaction region 14 and C-field, shown unenclosed in Figures 1 and 2, are
contained in magnetic shield package 179 (Figure 24). The B-magnet 16~ hot
wire ionizer 20, mass spectrometer 207 and electron multiplier 18 are packaged
together in a detector assembly module 244 (Figure 24). Referring to Figures
24 and 25, modules 240 and 244 and magnetic shield package 179 are essentially
independent of one another and constitute the subassembly units within the
outer package of the beam tube, and are assembled thereto by means of 10
20 screws, as will be described.
The details of each of these modular components are described below.
Oven/A-magnet module: oven and ampoule
The structure of the novel oven-ampoule assembly 10 of the invent-
ion, constituting a source for providing a beam of cesium particles, is shown
in detail in Figures 3-6. The assembly 10 includes collimating means 42, not
described, and oven means including a reservoir 29 containing an ampoule 27.
The amp~ule 27 includes a thin walled (0.015") generally cylindrical shell
30 and a top 37 including a fill tube 38. Top 37 and cylinder 30 together -~-
form 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
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e~ample of such an cutectic metal is an alloy of 45% copper and 55% indium.
A weak spring 35 is compressed between base 32 and top 37.
After the enclosure has been filled with liquid cesium, fill tube
38 is closed by pinching and heliarc welding.
A wire screen mesh 36 having thermal conductivity surrounds ampoule
27 within reservoir 29. The mesh 36 serves both as a heat transfer element
and as a retaining and support element for the ampoule.
Amouple 27 is supported within reservoir 29. A copper outer cylin-
der 28 of reservoir 29 includes an annular recess 40 at its lower portion.
A welding adaptor 39 having a lower flange 41 is brazed to recess 40 of outer
cylinder 28. An ampoule support member 43 includes an inverted cup portion
44 and three spaced supports 45. Inverted cup portion 44 of member 43 is
heliarc welded at 46 (Figure 4) to the inner surface of welding adaptor flange
41 to seal the lower end of reservoir 29. This creates an enclosed reservoir
space 51 surrounding base 34 and communicating with mesh 36. Ampoule 27 is
seated in support member 43 with ampoule base 34 within spaced supports 45.
Two tantalum heaters 90 and 92, retained in a ceramic support struc-
ture 88, are inserted into collimator assembly 42 through quartz tubes 80
and 82. The ampoule is opened, after bakeout of the beam tube, by means of
these beaters, which heat the ampoule to 600C, at which temperature the
eutectic seal fails. The combination of the vapor pressure of the cesium
within ampoule 27 and the force of compressed weak spring 35 exerts a stress
greater than the working stress of the metal seal 32 and pushes base 34 out
of shell 30, thereby releasing the cesium in the ampoule. Weak spring 35
prevents the base from settling back into place resealing the ampoule.
In later operation of the tube, tantalum heaters 90 and 92 are
used to warm the entire oven assembly 10 to the operating temperature, typic-
ally about 90C. At this temperature the liquid cesium in reservoir space
51 slowly vaporizes and diffuses from the mesh 36 to collimating means 42.
Collimator 42 is functionally equivalent to a bundle of small tubes so ori-
ented that a d~rected beam of cesium atoms emerges. Construction of colli-
mating means is well known to the art, and will not be detailed here.
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106~ 3
Ihe oven support structure is designed to provide thermal isolation
from outside the beam tube. Since the oven operates in a vacuum, there is
no heat loss from convection5 the major loss is by radiation, with some loss
by conduction. The oven support structure is therefore constructed of mater-
ial of poor thermal conductivity such as stainless steel and includes ear
portions 100 and 102 for securing oven 10 to the A-magnet assembly, as will
be described. Additionally, 0.003" Kapton shims 99 between the ear portions
of the support structure and the A-magnet assembly further discourage thermal
conduction. A radiation shield 104 of highly polished aluminum surrounds
lG the major portion of the oven, and presents radiation heat loss from the oven.
An oven of the design described required less than two watts for operation.
Oven/A-magnet module: A-magnet and ion pump
Referring now to Figures 9 through 12, a permanent magnet driver 111
is shared by the first state selector magnet ~A magnet) 12 and the ion pump
110. The ion pump performs the well-known function of removing undesired
gasses and maintaining tube vacuum during operation. Permanent magnet 111
is generally of a typical "C" shape, but with a novel reentrant inner surface
shape that gives it the distinguishing capability of providing proper fields ~ `
for both selection and ion pumping. The axis of magnet 111 is parallel with
the beam.
"Dipole configuration" 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 toward one another, and in conjunction with a second pair of short
cylindrical pale 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 100~ gauss for the
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10~i~013
ion pump. rhe compact arrangcmcnt of this combination permits thc atomic beam
tube assembly to be smaller, lighter, and less expensive than those hitherto
constructed, and is also especially adaptcd to the modular design of the pre-
sent 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
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
for the attenuation of the 10 k gauss deflecting field of the A-magnet down
to the 0.060 gauss C-field in the RF transition region 14.
A mounting plate 128 is secured to the upstream side of permanent
magnet 111, and provides brackets 134 and 136. Magnetic shield 132, stainless
steel spacers 113, magnet 111, and another pair of stainless steel spacers
117 all are fastened together by a pair of machine screws 115 passing through
clearnace holes in each and threading into tapped holes in mounting plate 128.
Oven 10(Figure 6) is secured by its support structure ear portions
110 and 102 to brackets 134 and 136. As these brackets are open in construc-
tion, rather than solid, they provide a relatively long thermal path for the
conduction of heat from the oven through the brackets to the eventual point
of contact with the outer frame of the beam tube. Shims 99 of 0.003" Kapton
are interposed between ears 100 and 102 and brackets 134 and 136 and provide
further thermal insulation.
Oven 10 and A-magnet 12 with ion pump 110 from the oven/A-magnet
module 240 (Figure 24).
C-fieldtMicrowave Structure module
Referring again to Figures 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 inconnection with Figure 2, the cesium atoms
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 field
~C-field) of approximately .06 gauss directed transverse to the beam path is
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provided hy a single^layer printed circuit solenoid 22 of novel design. The
construction and mounting supports of this solenoid will be described by re-
ference to Figures 13 through 19.
Referring first to Figure 15, the conductors of solenoid 22 are etch-
ed by well-known printed circuit techniques from a thin copper layer bonded to
a base 152 of polyimide material approximately 0.002 inch thick~ The general
shape of the base material 152 and a pattern of eight uniformly-spaced con-
ductors 150-1 through 150-8 is shown in Figure 15. Eyelet holes 307 are pro-
vided at each end of the conductors 150. This printed circuit solenoid pro-
vides thin, wide, and closely spaced conductors of very uniform cross sectionalarea and constant conductivity.
The printed circuit solenoid is assembled into a generally rectangu-
lar loop as shown particularly in Figure 14, with ~he eyeletted ends of conduc-
tors 150 offset one conductor in registry so that the co~pleted conducting -
path will form a one-layer spiral winding of equally spaced helical turns.
Electrical connection at each of the offset, but otherwise registered, ends
of conductors 150 is made by soldering using indium washers ~not shown) and
secured by rivets 308 inserted through the eyelet holes. Electrical connection
to the solenoid is made by wire leads soldered to eyeletted pads 304 and 306 ~i
2Q 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 si~e as to -
interrupt conductors 150-4 and 150-5.
Aperture 270 in base layer 152 has two opposed edges 144 (Figure 15)
that interrupt the two adjacent inner strips 150-4 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 connecting 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-
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tinuities causing undesired transitions, as previously explained.
In the present invention, two patches 318 of printed circuit material
similar to that described are provided to bridge the gaps and maintain uni-
formity of the C-field, each having an aperture 319. Two eyeletted conducting
jumpers 166 and 168 are bonded to base layer 320, and angle around aperture
319. Referring particularly to Figures 14 and 17J a patch 318 is assembled to
the winding by soldering to rivets 182 passing through the eyelets of the
jumpers and of internal ends 122. This construction maintains the continuous
current path through the entire conductor 150 at the beam apertures. Jumpers
10 166 and 168 lead the current around each aperture 270 and 271, effectively
doubling the magnetizing force at the edges of the apertures and tending to
maintain a near uniform distribution of the C-field across the apertures.
This structure provides an exceedingly close approximati~n to the ideal of a
uniformly distributed current sheet.
Electrical insulation around the solenoid is provided by polyimide
strips 184 and 186 (Figure 14) made to the same shape as printed circuit base
152, one being placed on either side of base piece 152.
Inner Magnetic Shield Package
The assembled C-field winding 22~ comprising the three layers and
two patches as described, is mounted on the inner surface of inner magnetic
shield 154 (Figure 15) and inner shield base plate 156 and is held in place
by rivets 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 Figure 18. The
assembly at the aperture locations 270 and 271 is made with aluminum plates
280 that provide apertures to register with apertures 270 and 271.
A flop coil 192 ~Figure 2 and 18) is mountsd on one of the central
aluminum plates 282 and supported from inner magne~ic shield 154 SO that i*
is coaxial to the beam axis. This coil is used in a manner well known to
the prior art to introduce a 20 kHz. electrical signal for the adjustment of
the C-field solenoid current, and will not be described further.
The sides of inner magnetic shield 154 (Figure 18~, paralleling
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the beam path, provide magnetic end caps for solenoid 22. The resuiting field
across the plane of solenoid 22 thereby approximates the classical uniform
field of an infinitely long solenoid with flux lines normal to the cesium beam
path. Inner magnet shield 154 in combination with spaced outer magnetic
shield 157 effectively attenuates the strong magnetic fields produced by the
A and B magnets and also shields the RF transition region from external mag-
netic perturbations.
Microwave radiation
Referring particularly to Figures l, 2 and 18, microwave radiation
is supplied within RF interaction section 14 by waveguide structure 190, which
is of the standard "Ramsey" type and well known in the art. It will not be
described here.
In prior art atomic beam tubes, constructed with separate mechanical
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 of
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-
turbations in the magnetic C-field due to leakage effects, which must in turn
be compensated for, for example by providing extra "baffling means" as in ~ -
United States Patent No. 3,670,171 CLacey 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 const~uction
avoids the need for a large aperture through the magnetic shield; a relatively
small aperture 194, about 1" x 1/2", is provided in base plate 156 ~Fi$ure 18).
Such a small aperture introduces only relatively small perturbations into the -
C-fi~ld, eliminating the need for "baffling" or other compensating structure,
and this structure is therefore advantageous.
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: - ~
10~()13
Outer magnetic shield package
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-field/microwave structure module 179 ~Figure 24).
Second state selector (B-magnet)/detector module
Referring now to Figures 20-23, permanent magnets 198 and 199, each
generally of horseshoe form, are secured to a detector table lg6, and lie in
a horizontal plane containing the beam axis. Magnets 198 and 199 are assembled
to provide two gaps spaced about 180 apart, one gap being downstream of RF
transition section 14 on the beam axis and the other slightly offset therefr0m
and downstream of the first. Soft iron pole pieces 200 and 201~ whose con-
figurations 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 from the beam axis and downstream from the ~irst
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
beam tube of the present invention smaller and lighter than prior art atomic
beam tubes.
De~ector 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 ~able 196, corresponding with an aperture 205
in the electron multiplier shield. The B-magnet 16, mass spectrometer 207,
hot wire ionizer assemb-ly 21 and electron multiplier assembly 18 together
make up B-magnet/detectoT module 244 ~Figure 24).
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- "
.
... - , ~ - . . ..
10~;~013
The beam ~f cesium atoms that emerges from the RF transition sec-
tion 14 (~igure 2) contains certain atoms that have undergone a transition
and other atoms to be discarded. The atoms selected by second state
selector or B-magnet 16 strike the hot wire 20, which is of a standard type
and will not be further described. Hot wire 20 strips an electron from
each neutral cesium atom that strikes it, and re-emits a positively charged
cesium ion. The cesium ions are then sorted by mass spectrometer 207 from
impurities unavoidably emitted by hot wire 20 and are directed into electron
multiplier 18, which produces an amplified output proportional to the number
of 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 tight feed-through
connections to power and RF sources, which are standard and will not be des-
cribed in detail. The three main subassemblies or modules 179, 240 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 400. 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 relatively long thermal path and aids in isolating
oven 10 from the outside environment.
The C-field/microwave structure module 179 is secured to four posts
226 by four machine screws 228. B-magnet/detector module 244 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 to base 210 after the necessary connections have
been made to the feed-through connectors. The tube is then evacuated under
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,: : .................... . ~ , : ~,
.... :,
. : ' ~ - - .
013
high temperaturc condit;ons.
This modular construction of the beam tube, with each module or
subassembly individually secured at a minimum of points to the rigid frame of
the single envelope structure, provides alignment and support for the modules
while simultaneously providing thermal isolation and mechanical protection
of the components in the modules from the outside environment. At the same
time, the relatively flexible oover accommodates to thermal and mechanical
stresses induced by the welding operation; an outer structure entirely of
the thicker material would not provide this flexibility, and alignment dif-
ficulties would result.
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