Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
a ~6~6~ 7
This invention relates, in general, to atomic beam apparatus, and,
more particularly, to atomic beam tubes which utilize magnetic hyperfine
resonance 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 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 hyperfine 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, correspondlng 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, m~=0) and (F=3, mF=O) 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
....
~ ~ .
~(3 6~
the direction of the applied external magnetic field.
To cause a transition from one state to the other, an amount of
energy E equal to the difference in energy of orien~ation 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 select-
ing 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, ~nd 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)
.~ 20 are retained in the beam, while the others are discarded. The undiscarded ` `
~1: . . .
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 w2ak uniform C-field to assure the separation
in energy of the mF = states from the nearby states for which mF ~ This ;~
~i~ small magnetic field also serves to establish the spa~ial 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
31 ~ to an oscillating externally generated field of approximately the resonance
~requency required to cause transitions from the (3,0) ~o the (~,0) sublevel.
i ~ ~
.,
. ~ :
-2-
.. . . ... .. .. . ~ . . .. . .
~6~8~7
After leaving this energy transfer region, the beam ls 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 (~, -4) sublevel) are discarded. The only undiscarded atoms are those
of the (4,0) sublavel, ~hich 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 R~ 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 frequency 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 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 bet~een the inner
ætructural assembly of tube elements and either an inner or an outer vacuum-
tight envelope in an effort to meet the often-conflicting requirements o~
rigidity against mechanical shock or vibra~ion, and fle~ibility to accommodate
to ùifferential expansion disturbance forces in the presence of thermal grad~-
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
30~ struc~ure measures typically result in relatively large and heavy tubes, -
:,
_3_ ~
'' ;~ '' '
~ ~CI66~3~7
characteristics that are most undesirable for certain important applications
such as in air or space cra~t.
Some pri3r art cesium tubes have been constructed using two
separate envelopes. The first is an inner mounting channel to which the
operative components are secured to provide mechanical stability and thermal
isolationj this inner envelope is suspended within an outer vacuum envelope.
Since differential movement between the two envelopes must be allowed ~or,
such a compound structure adds complexity to the manu~acturing process.
This design also results in a relatively weak mechanical structure.
This invention relates in a molecular beam tube apparatus includ-
ing a source for providing a directed beam of molecular particles, a first
state selector for selecting a portion of said particles in said beam, an
ion pump for maintaining vacuum within said apparatus, 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 the radio ~requency
transition section, a second state selector downstream ~rom said radio
frequency transition section for selecting a further portion of said beam
comprising those beam particles that have undergone said resonance transi- -
tions, and detecting means responsive to said particles in said further por- -
tion, that improvement wherein said apparatus provides a generally C-shaped
permanent magnet having a curved inner surface and having an axis parallel
wlth said directed beam, said permanent magnet providing a gap and further
including two opposed reentrant portions extending inwardly o~ said inner
surface, a first pair of pole pieces placed in the field of said permanent
; ~ magnet within said gap and providing a de~lecting field, and a second pair
of pole pieces placed between said reentrant portions, said ion pump being
located bètween said second pair of pole pieces, whereby said ~irst pair of
pole pieces is driven by said permanent magnet to provide said first state
selector, and sald second pair of pole pleces and said permanent magnet drive
said ion pump ln parallel with said first state selector.
The present invention integrates the inner assembly and the vacuum
_ 4 _
.:
Y , ,,
1~668~L7
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 sub-assembly 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. The invention also includes novel features
providing good thermal isolation, smaller and more efficient magnetic
structures, smoother transition between strong and weak magnekic fields, and
means to feed in RF energy with less perturbation of the C-magnetic field
than in prior art tubes. These novel feat~res make possible a tube, both
more compatible with typical operating environments than conventional
devices, and lighter in weight (9 lbs. agains-t the 16 lbs. of a typical
prior-art tube).
The design of the present invention eliminates the need for
expensive 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 distrubances 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 -
structure that serves both as vacuum envelope and as structural member for
the operative components. This envelope is composed o~ a heavy and relatively
~ '
. ~, .
"ll, . .
:i '
,.
l . : .
,"1~ . '' ' ' '.
]~
! - 4a - ~
.: ::.: :.:
:i ~ .... .
1~:)6~8~'7
' .
rigid frame and a relatively thin ancl flexible cover sea:Led 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 e*ternally 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 mini-
mum 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 connec~ing structure
that 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
extensi~e 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 proYided in three main modular subassemblies, secured to the frame
by a total of lo 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 relatively
weak (of the order of .060 gauss) and as uniform as possible. Discontinuities
in the C-fïeld are particularly likely to occur in the regions at which the
?~ ~
~ beam~enters and leaves the C-region, and can cause spontaneous transitions
,
(MaJorana transitions3 in the~atomic beam which may distort the performance
~;; of the tube. The present invention provides a C-field winding of ncvel 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
29 as compact, light weight, and simple aa~possible. The particular designs of
-5-
~L~6~8~
.
the A and B magnets in the present invention reali3e such contruction and are
particularly adapted to the modular assembly previously described.
It is typical in the assembly and processing of molecular beam tubes
to confine the source of the molecular beam material in a sealed ampoule
during the bc~keout 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 vaporizing arc
is created at a member of the ampoule which is ruptured by th~ heat of the
arc. ~oth 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 transmit electrical energy through the vacuum envelope, which com-
plicates 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 feeds through the vacuum envelope are
: ~
reqaired.
~ Other objects, features~ and advantages will appear from the follow-
: m g description of a preferred embodiment of the invention, taken together
with attached drawings thereof, ln which:~
; Figure 1 is a schematic view of the principal beam-forming and
detecting elements of the tube;
29 Figure 2 is a perspecti~e view of the elements of Figure l;
~:
-6- ~ ~
'~' . ' :
;6~il~'7
Figure 3 is an exploded view of tlle components of the 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 Zeeman energy diagram for cesium 133 in the ground
electronic state; showing the transition induced in the beam tube of the in-
vention;
Figure 8 is a schematic view of the control circuitry used with the
cesium beam tube of the invention;
Figure 9 is a perspec~ive view of the first state selector magnet
. and ion pump;
~ Figure 10 is an exploded perspective view of the first state selec- :
`l~ tor magnet together with shielding and support structure;
. ~ .
`.~ Figures 11 and 12 are longitudinal and cross sections respectively
,; , .
of the first state selector and ion pump; :
Figure 13 is a perspectivs view of the microwave structure and C-
~r field coil;
Figure 14 is a perspective view of the C-field coil with portions :;
20 broken away3 .
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
'i, . . . . .
i~ beam aperture;
Figure 17 is a detail of the conductors of the G-field coil at a
beam aperture3
igure 18 is an e~ploded view of the magnetic shield package and
~ contents~
`~ Figure 19 is a cross section of the outer enYelope and contents
`: 29 near the center; . :
: 7
1~6~7
Figure 20 is a perspective view of the ~-field magnet and the
detector;
Figure 21 shows the elements of Figure 20 with support structure;
Figures 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.
Gen ral
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. me first
;~ state selector or A magnet 12 splits these enrgy states into sublevels and
selects the atoms in the F=3 states (together with those in the ~4,-4~ sub-
level) and defleots the remaining atoms so that they no longer form part of
the beam. The beam of selected atoms then passes through the ~F interaction
section 14; in this region a weak homogeneous magnetic field (C-field) is
supplled by the winding ~2. Microwave energy is supplied at the resonance
frequency to lnduce 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 -~
`br the B magnet strlke the hot wire ioni3er 20, and an electron is stripped
: : :
; from each cesium atom, causing the re-emission of cesium ions, which are
~;~ 29 accelerated through a mass spe~trometer 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 ra~sed 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 the
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 264 and applied to tube 11, at the precise resonance frequency
~typically 9192 mHz). Multiplier chain 2Ç4 and the controlled oscillator 262
from the microwave generator 2Ç6. The usable output signal is derived from
~ controlled oscillator 262 at 268.
î Summary of Modular Components
, .
;i The elements that have been described and shown in Figure 8 are in
, general terms old and well-known in the art. The cesium ~ube of the invention
~;~ provides three modular subassemblies including a cesium ampoule and a first ~
;~ state selector magn~t in combination with the ion pump, a second state selec- ~ -
i~ ~ tor magnet in combination with the mass spectrometer, and a C-field ~nding
:. .:
,; ~ -
`~ and microwave structure, all of novel design, as well as a novel outer pack-
~ 20 age for the entire tube. ;
j, To provide the advantages of the modular assembly of the invention,
:1
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 oomblned in an o~en/A~magnet assembly module Z40 ~Figure 24). The RF
~ interactlon region 14 and C-field, shown unenclosed in Figures 1 and 2, are
`~ ~ contained ln~magnetic shield package 179 (Figure 24). Th0 B-magnet lÇ~ hot
wi~re ioniz~er 2Q, mass spectrometer 207 and eIectron multiplier 18 are paclc-
aged together in a detector assembly module 244 ~Figure 24). Referring to
~9 Figures 24 and 25, modules 240 and 244 and magnetic shield package 179 are
' :
_9_
i 1a3668:~
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 screws, as will be described.
The details of each of these modular components are described below.
Oven/A-ma~net module: oven and ampoule
The structure of the novel oven-ampoule assembly 10 of the invention,
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 des-
cribed~ and oven means including a reservaDr 29 containing an ampoule 27. me
ampoule 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 600 C. An
example of such an eutectic metal is an alloy of 45 % copper and 55 % indium.
A weak spring 35 is co~pressed between base 32 and top 37.
After the enclosure has been filled with liquid cesium, fill tube
3~ is closed by pinching and heliarc welding.
A wire screen mesh 36 having high thermal conducti~ity 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.
Ampoule 27 is supported within reservoir 29. A copper outer cylinder
28 of reser-roir 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
29 41 to seal the lower end of reservoir 29. This creates an enclosed reservoir
--10--
~ ~L0668~7
space 51 surrounding base 34 and communicating with mesh 36. ~mpoule 27 is
seated in support member 43 with ampoule base 34 within spaced supports 45.
Two tantalum heaters 90 and 92, retained in a ceramlc support struc~
ture 88, are inserted into collimator assembly 42 through quartz tubes 80 and
82. The ampoule is opened, after bakeout oE the beam tube, by means of these
heaters, which heat the ampoule to 600 C, at which temperature the eutectic
seal fails. The combination of the vapor pressure of the cesium within am~
poule 27 and the force of compressed weak spring 35 exerts a stress greater
than the working stress of the metal of 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 90 C. At this temperature the liquid cesium in reservoir space
`~r 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 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
from outside the beam tube. Since the oven operates in a vacuu~, there is
no heat loss from convection; 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
the major portion of the oven, and presents radiation heat loss from the oven.
29 An oven of the design described required less than two watts for operation.
-11-
"
~66~3~L7
Oven/A-magnet module _- agnet and ion_pum~
Referring now to Figures 9 through 12, a per~lanent magnet dri-ver
;~ 111 is shared by the first state selector magnet ~A magnet) 12 and the ion
pump 110. me ion pump performs the well~known func-tion of removing undesired
gasses and maintaining tube vacuum during operation. Permanent magnet 111 is
generally of a typical "C" shape7 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.
I'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 pole pieces 116 and 118 provide the field for the ion pump 110, ~ `
located between pieces 116 and 118. me ion pump is of any suitable design -;
and is well known.
,; . .
~3,~ 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
lIO. The magnetic driver is designed to provide approximately 10 X gauss in
", .. ..
;1 the state selector circuit while providing approximately 1000 gauss for the
on pump. The compact arrangement of this combination permits the atomic beam
; tube assembly to be smaller, lighter7 and less expensive than those hitherto
constructed, and is also especially adapted to the modular design of the
present beam tube apparatus.
.~
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 1~9 (Figure 24). Shield
29 132 provides aperture 138 for the passage of the atomic beam from the A-magnet
-12-
~1~ fii68~7
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-~agnet down
to the 0.060 gauss C-field in the RF transition region 14.
A mo~mting plate 128 is secured to the upstream side of permanent
magnet 111, and provides braclcets 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
clearance 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 o~en/A-magnet
module 240 (Figure 24).
Referring again to Figures 1, 2 and 4, the C-field and RF (radio
frequency) transitlon section 14, including magnetic shields to be described,
are packaged together as a second module 179.
As previously described in connection 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 homog~neous magnetic
fïeld (C-field) of approximately .06 gauss directed transverse to the beam
path is provided by a single-layer prlnted circuit solenoid 22 of noYel design.
The construction and mounting supports of this solenoid will be described by
reference to Figures 13 through 19.
Referring first to Figure 15, the conductors of solenoid 22 are
- 3
68~7
etched 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 pa.ttern of eight uniformly-
spaced conductors lS0-1 through 150-8 is shown in Figure 15. Eyelet holes
307 are provided at each end of the conductors 150. This printed circuit
solenoid provides thin, wide, and closely spaced conductors of very uniform
cross sectional.area and constant conductivity .
The printed circuit solenoid is assembled into a generally rectangu-
lar loop as shown particularly in Figure 14, wi~h the eyeleted ends of conduc- ~ .
10 tors lS0 offset one conductor in registry so that the completed 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 ~hrough the eyelet holes. Electrical connec- -
. tion to the solenoid is made by wire leads soldered to eyeletted pads 304 .:
and 305 at the end of each of the outside turns.
, , . - .
me closed loop includes two end sections 140 and 142 that are trans- :-
~,
verse to the beam path and parallel to ~ne another. Since the assembled
`~ solenoid ~inding must lie generally in the plane of the cesium beam7 apertures
., .
20 270 and 271 are provided in end sections 140 and 142 of such a size as to
` ~ interrupt conductors 150-4 and 150-5.
~;
~ Aperture 270 in base layer 152 has two opposed edges 144 (Figure
:
lS) 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 proride a continuous
`~ current path, it lS 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-
tinuities causing undesired transitions, as previously explained.
-14- ;
:
.. .... . . . .. .
668~ `In the present invention, two patches 318 of printed circuit mat-
erial similar to that described are provided to bridge the gaps and maintain
uniformity of the C-field, each ha~ing an aperture 319. Two eyeletted conduct-
ing jumpers 166 and 168 are bonded to base layer 320, and angle around aper-
ture 319. Referring particularly to Figures 14 and 17, a patch 318 is assem-
bled to the winding by soldering to rivets 182 passing through the eyelets of
the jumpers and of internal ends 122. This construction maintains the contin-
uous current path through the entire conductor 150 at the beam apertures.
Jumpers 166 and 168 lead the current around each aperture 270 and 271, effect-
ively 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 approximation 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 one either side of base piece 152.
Inner Ma~nebt Shield Packa~e
The assembled C-~ield 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 h~ld in place
by rivets passing through the shield material, the outer margins of the sole-
noid as~embly of base material 152 and insulating strips 184 and 186~ and
~ alu~inum 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 (Figures 2 and 18) is mounted on one of the c~ntral
aluminum plates 282 and supported from inner magnetic shield 154 so that it
is coa~ial to the beam axis. This coil is used in a man~er well known to the
29 prior art to introduce a 20 khz. electrical signal for the adjustment of the
~15- -
~C16~817
C-field solenoid current, and will not be described further.
The sides of inner magnetic shield 154 (Figure 18), paralleling the
beam path, provide magnetic end caps for solenoid 22. r~he resulting 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.
10 Microwave radiation
Referring particularly to Figures 1, 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.
~r In prior art atomic beam tubes, constructed with separate mechanical
f 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 tube7 capable of
accommodating to such motions. Such flexible means requires a relatively
large aperture, typically two inches in diameter, in the magnetic shield
structure to acco~modate the connection. Such a large aperture introduces
perturbations in the magnetic C-field due to leakage effects, wh~ch must in
turn be compensated for, for example by providing extra "baffling means" as
in United States Patent No. 3,670,171 (Lacey et al) issued June 13~ 197~
In the present invention, the combination of mechanical support and
racuum lsolation en~elope into a single structure eliminates such differen- ;~
tial motions. The inlet arm of microwave structure 190 can therefore be
intimately braæed to the lower surface of inner shield base plate 156. This
zg construction avoids the need for a large aperture through the magnetic shield;
-16- -
~1~66~
a relatively small aperture 194, about 1" x 1/2", is provided in base plate
156 (Figure 18). Such a small aperture introduces only relatively small
perturbations into the C-field, eliminating the need for "baffling" or other
compensating structure, and this structure is therefore advantageous.
Outer m~n ti hield packa~e
Referring particularly to Figures 19 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 transit~on
section, forms the C-field/microwave structure module 179 (Figure 24).
Second state selector_(B-ma~et)/detector module
Referring now to Figures 20-23, permanent magnets 198 and 199, each
generally of horseshoe form, are secured to a detector table 196, and lie in
a hori~ontal 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 therefrom
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
,,
i in the first gap between permanent magnets 198 and 199, on the beam axis.
1 20 Pole pieces 200 and 201 are driven by magnets 198 and 199, and act as the
i! 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 first gap; pole
plece assemb b 204 is dri~en by permanent magnets 198 and 199 to function as
~ a mass spectrometer 207. Thus the second state selector and the mass spec-
,~ trometer 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.
29 Detector table 196 is provided with three mounting tabs to which
-17-
~6~3~7 ~:
is secured a hot wire ioni~er 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 16, mass spectrometer 207,
hot wire ionizer assembly 21 and electron multiplier assembly 18 together make
up B-magnet/detector 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
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 neu~ral
cesium atom that strikes it, and re-emits a positively charged cesium ion.
Th0 cesium ions are then sorted by mass spectrometer 207 from impurities un-
avoidably emitted by hot wire 20 and are directed into electron multiplier
,1 ~
- 18, which produces an amplified output proportional to the number of atoms
1` incident upon the first dynode of the multiplier.
`~ Outer packa~
~? 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-
20 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
1~ . .
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.
, .. ..
i In assembly, oven/A-magnet module 240 is secured to supports 222
and 224 on base 210 by two machine screws 400. ~hus the path for heat con~
duction from oven 10 to the e~terior environment of the cesium tube extends
29 throu~h open brackets 134 and 136 and supports 222 and 224 to frame 2100
-18-
:
' ' ~ '
~ ; ~C16~L7
This structure provides a relatively long thermal path and aids in isola~ing
~ oven 10 from the outside environment.
i 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 ~37. Detector table 196 and brac~
kets 234 and 236 together provide a rela~ively long thermal pa~h 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
high tempera~ure 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
!' the single envelope structure, provides alignment and support for the modules
"I
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 cover 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. ~ -
:~ : .. , ~,
,~:
:
.:
