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DEC 30 1984
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CONF-641110-2
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CONVERSION OF A CALUTRON INTO A 180° SECTOR
SEPARATOR WITH 0.8 INHOMOGENEITY*
t e chimisan
T. W. Whitehead, Jr.
W. K. Dagenhart
W. A. Bell, Jr.
L. 0. Love
Seneste inimitabilmen
'Oek Ridge National Laboratory
Oak Ridge, Tennessee
love this sondern Aviation
The electromagnetic plant at Oak Ridge, Tennessee, was built in
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the early 1940's for the chief purpose of separating the isotopes of
uranium. During 1944-45 more than 1000 separators, which are called
calutrons and bear a resemblance to the Dempster mass spectograph,
were in operation, but by the clo:a of WW II the gaseous diffusion
process, also at Oak Ridge, had proven to be a far more efficient method
for uranium separations and the EM process at Y-12 was phased out. In
the latter part of 1946, the AEC, recognizing the value of the EM
process for separating the isotopes of most elements, directed that a
program of stable isotope separations was to be carried out by ORNL
using the 4 machines which had originally served as a pilot plant for
the entire separations process. Since that time the availibility of
enriched stable isotopes has stimulated a steadily increasing demand
for more and more isotopes at higher purities to be used in many areas
of physical and medical research. Very frequently an enriched stable
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*Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation.
figures not available
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1sotope is used as the basic building block for providing a high purity
radioisotope. For example, since 1960 more than 16 separator years have
been expended to enrich 46Ca--an isotope whiich, when converted by neutron
capture to 47ca, has considerable medical value for the early identifica-
tion of bone cancer.
In the early part of 1960, because of the inability of the four
separators to meet the demand for enriched material, 30 more separators
were reactivated and since that time they have been in continuous
operation to satisfy the requests for enriched isotopes. The first slide
shows the separations facility located in one of the original uranium
separations buildings at Oak Ridge. Originally this track, as it is
called, was not subdivided as you see it here. The entire track weighs
about 3000 tons, and these cross members each represent about 100 tons
of low carbon steel which has been used to subdivide the system magnetically.
This subdivision permits the simultaneous separation of a different ..
element in each of the four sections and gives the process a high degree
of flexibility. For example, calcium separations can be carried out in
one section while tungsten, which requires more than twice the calcium
field strength, can be separated in the next section.
The next slide shows the control room for the separators. Each
separator has an independent electrical supply which provides a high
stability ion accelerating voltage to impell the ion into the magnetic
field, heater power supplies to vaporize charge materials, a high ampere
current supply to heat a tantalum fllament to provide electrons for
ionization and meters to monitor ion currents to the collector.
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The next slide is a picture of a calutron ion source. This section
where the arc 18 formed by accelerating electrons from a heated tantalum
filament here at the end throụgh charge material vapor, 18 operated at
35,000 volts. The potential difference between the filament and the arc
chamber 18 usually about 100 v and 1t 18 adjustable. Also, the are
current is adjustable, usually being between 2 and 4 amps, and 18
automatically maintained at a set value by & regulɛtor which raises or
lowers the filament temperature thus controlling the electron supply.
The ions formed in the arc are extracted from a slit in a direction
perpendicular to the magnetic field. A second electrode at about 20,000
volts negative is used here to aid the extraction and with the third
electrode here provides an electric lens which simulates a line object.
The actual width of the ion source exit slit may be 3/16" to produce large
ion currents of up to 150 ma, hut the effect of electrode system provides
the magnetic field with an apparent width which is much less than this.
The next slide schematically shows the location of the calutron
ion source and collector. The tank walls are made of low-carbon steel
and with non-magnetic top and bottom sections form part of the vacuum
system. At the rear of each tank there is a manifold with two 20" oil
diffusion pumps backed by mechanical pumps which evacuate the system to
operating pressures of 2 x 10-5 torr. Linear magnetic shims are used
in this region on each side of the tank to improve the focus of the ion
beams, and without these shims the calutron would lose most of its
effectiveness as a high current separator. In a completely uniform
magnetic field, the beam divergence customarily accepted by the calutron
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would result in an image width at the collector of about 1/2" which in
most separations would not be of any practical value. The chief advantage
of the linear shims as compared with other methods of correcting the
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field is that they allow the simultaneous operation of several ion
sources stacked parallel with the shims. This feature was used in
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uranium separations when as many as four arcs were operated in the
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sama system. However, .xperience with multi-arc operation since then
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has not been very favorable due to overlapping of the mass spectra and
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beam plasma instabilities. Consequently, most separations have been
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made using only one arc.
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The next slide shows an ion collector used in the separation of
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calcium isotopes. Part of the defining plate has been cut away to show
the collector pockets. 40ca is by far the most abundant isotope and
during normal operations about 120 ma 18 collected in this large water-
cooled pocket. 46ca, which is collected in this pocket, has a natural
abundance of 3/1000 of one percent and it takes about 80 hours to collect
1 mg of 50% material. This is an enhancement factor of 35,000 over the
naturally occuring concentration. The next slide shows the focused
1on beam entering the collector pocket. This is 120 milliamps of 40ca
and if it were allowed to go into the 46ca collector pocket for 1 second
each day, the 46ca assay would drop from 50% to 43%.
The calutron has been a reliable and flexible system for the separation
of moderate quantities of isotopes. However, it does have some disadvantages.
The linear shims provide a reasonably good image but there is room for
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improvement. One of the most serious disadvantages is the systems limited
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mass dispersive power, which, although it gave an adequate separation of
3/20" between 2350 and 238U, provides a rather cramped 0.115" between
206pb and 207P0. The difficulties involved in stopping and holding
35%v ion beams 1/10" apurt are obvious. To improve this condition the
possibility of converting a calutron to a sector separator with a high
field inhomogeneity have been carefully studied.
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The term inhomogeneous magnetic field is being used to lable
system which has circular symmetry and a radially decreasing field
strength. The next slide shows a cross section of a set of circular iron
shims that would be used to produce a field of this type. This is the
region that would be used by the ion beam. As you can see from this
force diagram, an ion which deviates from the midplane is restored by
these force components. If the field obeys this approximate midplane
relationship B B. () osasi the centrifugal force on the lon
will decrease more rapidly than the field strength and the orb:it will
be stable about the r. position. An ion source can be imagined here
at r which ejects ions of a constant energy directed into the plane of
the diagram. Tons of different isotopes will have different momenta,
and they will be in equilibrium at larger or smaller radii if they are
heavier or lighter. These ions will oscillate about the radial equilibrium
positions as they travel from the source to the collector and the amplitude
of this oscillation will depend on the constant n--large values of n
resulting in large amplitudes. For certain values of n such as 0.5 and
0.8 the frequency of radial oscillation 18 an integer multiple of the
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oscillation back and forth in this direction and this is known as a
double-focusing lens. The significance of the amplitude of the
radial oscillation is that a n = 0.8 separator will produce five times
more separation between the beams of isotopes than a calutron.
The next slide shows the 0.8 sector separator that can be made by
converting a calutron to an inhomogeneous field system. The source and
receiver are now outside the main magnet which is one of the distinguishing
features of sector separators. The beam enters here, travels on approximately
a 24" radius (the same as a calutron), and 18 collected here almost
7 ft. in front of the main magnet. The theoretical angular acceptance
shown here is $2.50 and 18 considerably less than the calutron angular
acceptance. This reduced beam acceptance 18 related to the high field
inhomogeneity and it is the price that must be paid for high mass dispersion.
The next slide shows a comparison between the amount of separation
in a calutron and the separation that will be available in a 0.8 sector
separator. This increase in separation will greatly facilitate the
construction of collectors and the enrichment of a particular isotope
districte inste scommes
the
the Greate
should be dramatically improved--particularly in the heavy mass range.
One of the major obstacles to be overcome in developing the 0.8
sector is the achievement of a precisely shaped magnetic field. This
international and initiation
necessary precision is a result of the high dispersion of the instrument
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and deviations from an Ideal field shape cause considerable image defocusing
at the collector. Unfortunately, due to the effect of the fringing
fleld at the magnet boundaries, it is not possible to obtain a complete
theoretical solution before the machine 18 built. The fringing field
of a calutron was measured and the results were extrapolated to the
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0.8 sector by means of a computer program which calculated trajectories
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in the field representation. The results shown on an earlier slide
which located the source and receiver were based on this calculation.
This is not an absolutely correct picture of the 0.8 sector due to the
approximations made in the fringing field region, but the overall appearance
of the machine will not change much and the field 'shape used in this
calculation represents the best point from waich to begin the development
of this system.
The next slide shows how a computer program is used to calculate
jhe exact pole piece shape necessary to produce the required magnetic
field. Since the iron will have a high permeability over the range of
field strengths used, the shim surface may be considered as analogous
to an electrostatic equipotential without much loss of accuracy. The
midplane is also an equipotential since symmetry has been assumed and
all field lines are normal to this plane. The computer makes potential
assignments in this net and then adjusts these values until the Laplace
of the potential is zero. This potential mapping permits the calculation
of the midplane field strength and the shim boundary may be changed until
the desired field is obtained.
It is anticipated that about two years will be required to fully
develop this system and that an extensive series of magnetic field
measurements and reworking of the shim contour will be necessary in
order to realize the full potential of the separator. However, when the
development is complete, the resulting improvement in the purity of the
isotopic sample should more than justify the effort. It is felt that
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this machine will be able to meet the demands for small quantities of
extremely pure isotopes in the high mass range that cannot be supplied ·
by tire calutron in a single separation.
--wis.ba msinstruiter
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DATE FILMED
2 / 25 65






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