This item is only available as the following downloads:
Subject Category: PHYSICS
UNITED STATES ATOMIC ENERGY COMMISSION
HIGH TEMPERATURE ION SOURCE
AND THERMOHM DEVELOPMENT FOR
STABLE ISOTOPE PRODUCTION
P. E. Wilkinson
G. D. Whitman
\ -US DEPOSIT OR
December 5, 1950
Oak Ridge National Laboratory
Oak Ridge, Tennessee
Technical Information Service. Oak Ridge, Tennessee
Date Declassified: January 6, 1956.
This report has been reproduced directly from the best
Issuance of this document does not constitute authority
for declassification of classified material of the same or
similar content and title by the same authors.
Printed in USA, Price 20 cents. Available from the
Office of Technical Services, Department of Commerce, Wash-
ington 25, D. C.
GPO B22451 1
This report was prepared asa scientific account of Govern-
ment-sponsored work. Neither the United States, nor the Com-
mission, nor any person acting on behalf of the Commission
makes any warranty or representation, express or implied, with
respect to the accuracy, completeness, or usefulness of the in-
formation contained in this report, or that the use of any infor-
mation, apparatus, method, or process disclosed in this report
may not infringe privatelyowned rights. The Commission assumes
no liability with respect to the use of,or from damages resulting
from the use of, any information, apparatus, method, or process
disclosed in this report.
HIGH TEMPERATURE ION SOURCE AND THERMOHM
DEVELOPMENT FOR STABLE ISOTOPE PRODUCTION
P. E. Wilkinson and G. D. Whitman
December 5, 1950
ISOTOPE PRODUCTION AND DEVELOPMENT SECTION
C. E. Normand, Supervisor
ISOTOPE RESEARCH AND PRODUCTION DIVISION
C. P. Keim, Director
This report describes the development
of an ion source unit and thermohm for opera-
tion in the range of 650 1000lC.
OAK RIDGE NATIONAL LABORATORY
CARBIDE AND CARBON CHEMICALS DIVISION
UNION CARBIDE AND CARBON CORPORATION
Oak Ridge, Tennessee
Contract No. W-7405-eng-26
HIGH TEMPERATURE ION SOURCE AND THERMOHM
DEVELOPMENT FOR STABLE ISOTOPE PRODUCTION
For several years, the Stable Isotopes Division has received requests
from various sources for the isotopes of the rare earths and other elements
for which no charge materials were available which would operate in modified
Beta ion source units. Due to the lack of suitable ion sources for use in
processing these materials, none of these isotopes had been made available.
A survey of the literature indicated that the rare earth halides have
sufficient vapor pressure for calutron charge materials in a range from
8000C to 10000C. This is considerably above any temperature obtainable
using standard Beta ion sources; consequently a new source has been designed.
This report covers the design and development of a source to operate with
charge bottle temperatures from 6500C to 10000C.
The first attempt to run a rare earth charge material was made using
a source in which the charge material (CeCI3) was heated by electron bombard-
ment. This ion source was unsuccessful due to poor temperature regulation.
A second attempt was made using M14 type source units obtained from the
Beta Development Group. These units had two arc chambers and were designed
for uranium tetrafluoride (UF4) charge material with a temperature range extend-
ing up to about 9000C. Since only one arc is used in stable isotope opera-
tions these sources required minor modifications for operation with cerium.
Operation with the reworked M14's, although much better than previous
rare earth runs, was still unsatisfactory. The major difficulties were:
1. Charge bottle leakage which resulted in heater shorts.
2. Difficult maintenance of the heaters due to fracturing of
the molybdenum elements.
3. The&mohm (resistance thermometer) failures at the elevated
4. Charge condensation in the vapor manifold and arc chamber.
In an attempt to obtain a more satisfactory source, the M14 unit was
redesigned. Separate oven and arc chamber heaters were provided so that
the arc region and the vapor manifold temperatures could be kept suffi-
ciently high to prevent charge vapor condensation. The heaters consisted
of molybdenum wire cast in alundum cement (RA-162). The length and size
of the molybdenum elements were calculated to utilize the full output of
the largest.power supplies available, i.e., 3600 watts on the oven supply
and 1800 watts on the arc chamber heater supply. Since operating temper-
atures up to 12000 were desired, a special thermohm was required; further-
more, provisions to permit thermocouple temperature regulation if the new
thermohm proved unsatisfactory were desirable.
PRELIMINARY HEATER TESTS
Since heaters of the proposed type had not been used previously, a
series of tests were made to determine their characteristics. The various
heaters were tested in a vacuum by surrounding them with heat shielding,
supplying measured quantities of power to them, and measuring the temper-
ature at the first heat shield with a thermocouple. All of the preliminary
tests were made to determine the maximum temperature which could be attained
at various power input levels. At the same time, resistances of the heaters
at these various temperatures were measured and compared to resistances per-
mitting maximum utilization of available power supplies. None of the
heaters failed during these tests, and temperatures in excess of 15000C
were attained as evidenced by the melting of the several layers of stain-
less steel heat shielding nearest the heaters.
HIGH TEMPERATURE THERMOBM DEVELOPMENT
With temperatures of this order available, it was evident that a
special thermohm would be required; thus tests proceeded simultaneously on
both heater and thermohm development in order to determine the operating
characteristics of the thermohm and to locate it properly with respect to
the heater elements and charge bottle.
Since the primary reason for failure of the standard thermohm at
elevated temperature is exfoliation of the insulating mica, the first need
in the new thermohm was for an insulating material which would be stable at
temperatures up to 12000C. A second requirement was for a case and insu-
lator which would be stable in vacuum at the high temperatures anticipated
since poisoning of the platinum resistance element had to be prevented if
reproducible results were to be obtained.
Certain physical dimensions were also restricted in order to retain
as many of the original design features of the unit as possible.
The insulating material first considered was pure fused A1203, but
fabrication difficulties prevented its use. A magnesium oxide compound,
magnerite, was tried and found unsatisfactory. The final thermohm design
consisted of a .003" platinum wire wound non-inductively on a MgO spool in
a double 3/0-20 thread. The spool was enclosed in a cylindrical case of
purified graphite having a wall thickness of 1/32." This assembly was
approximately 1/2" in length and was inserted in the back of the graphite
oven housing so that the end of the thermohm case was exposed to the charge
bottle. A MgO coupling was used between the graphite case and stainless
support tube, thereby reducing heat transfer by conduction and causing the
thermohm to approach more closely the bottle temperature. Figure 1 shows
the thermohm assembly.
This type of resistance thermometer has been used at temperatures of
10000C without failure and is presently used with a special micromax
reworked to regulate oven temperatures of that order. Figure 2 is a per-
formance curve of the high temperature thermohm and indicates that its
response is virtually linear over the desired range.
Before much work was done on heater development, upper limits of temper
ature were set at approximately 1200 C for the oven and about 12250 for the
arc chamber region. No difficulty was experienced in attaining the desired
zzm Z z
Pg t ma 0
200 400 600 800 1000 1200
TEMPERATURE IN DEGREES CENTIGRADE
PERFORMANCE CURVE OF HIGH TEMPERATURE THERMOHM
oven temperature with the original design, but considerable work was neces-
sary to attain the desired temperature in the are chamber.
The first difficulty encountered with the unit as originally redesigned
resulted from excessive heat loss by radiation from the arc chamber system.
The arc chamber heaters were enclosed in a graphite system which had about
60 square inches of radiating surface, and since graphite is a relatively
good heat conductor, this entire surface approached the temperature of the
charge vapor manifold. Under this condition maximum temperatures of only 700
to 8000C were attained with 1800 watts total heater power. This situation
was partially remedied after considerable development work by separating and
heat shielding the arc chamber system from the remainder of the source front.
This change reduced the radiating area by a factor of about 5.
At the same time, difficulty was experienced with heater life. The
original arc region heaters were rated-not at 900 watts each, but at 240
watts each-and these failed rapidly. The size and length of the molybdenum
wire was recalculated, and a new set of heaters fabricated according to the
altered specifications. These heaters also failed; but the cause of failure
appeared to be the existence of a "hot spot" caused by the proximity of the
oven heater, poor heat conductivity of the alundum, and overlapped heat
shielding at the center of the anti-drain heater rather than to overloading.
Various heater design changes failed to remedy the situation; thus, a structual
change was made in the arc system. The heated region was modified to surround
the heater on three sides with graphite, thereby facilitating heat transfer
from the heater to the arc system. Further heat shielding was added around
the assembly; and as a result, temperatures greater than 10000C were obtained
Tests were initiated to determine the heater life in the redesigned
structure. Heater failure occurred in about 20 hours (max.) at 1800 watts
of power input and arc region temperatures of 900 to O1100C. Examination
of the heaters after failure revealed that the molybdenum elements were
operating at temperatures in excess of 20000C and were being vaporized. At
the same 1~me, the alundum cement-was fusing in the region of the elements;
and, due to the consequent reduction in cement volume and loss of intimate
contact between the alundum and molybdenum wire, the heat transfer from the
wire element to the alundum was reduced. Thus, a further increase in the
wire temperature was required in order to maintain the desired temperature,
These difficulties led to the abandonment of the molybdenum wire-in-ceramic
type of arc region heater.
A series of tests were run on a graphite arc chamber electrically
insulated by Lava A from the remainder of the ion source unit and heated by
the passage of current from a filament transformer. Although the graphite
temperature exceeded 10000C, the temperature of the Lava was between 800C
and 9000C. This temperature was considered inadequate as charge condensation
could occur on the Lava forming the wall of the vapor manifold. Consequently
this approach was abandoned.
Development of satisfactory heaters with the same physical dimensions as
the wire-in-ceramic type was next undertaken. Graphite elements were
employed; and various insulating materials and types of heater terminations
were tested. Graphite elements cast on the surface of alundum cement with
welded-on stainless steel terminals were tried; however, fabrication diffi-
culties forced abandonment of this type. A graphite element clamped between
two pieces of "steatite" was tried, but the steatite melted. Finally, graphite
elements supported in Lava A insulators were tested, and after some develop-
ment, were found to be satisfactory.
Results obtained on the molybdenum-in-ceramic type arc region heaters
indicated that the oven heaters would be more satisfactory if constructed
using graphite elements and Lava A insulators. A set of oven heaters of
this type was fabricated, tested and found to be satisfactory. Figure 3
shows an oven and an arc region heater both of which were fabricated from
The tests of the thermal characteristics of the ion source were made in
Tank II XBX using a nitrogen supported arc. The oven heaters were supplied
with a maximum of 1800 watts and a steady power of 1500 watts, and the entire
assembly was surrounded by four layers of stainless steel heat shielding.
The arc chamber heaters were supplied with 1800 watts from a special 4KVA
insulating transformer; and two layers of graphite, two of tantalum, and
four of stainless steel were used as heat shielding around the arc system.
With this arrangement, vapor manifold and arc chamber temperatures in excess
of 15000C were obtained with corresponding oven temperatures from 1GOOC to
12000C. Figure 4 illustrates the arc chamber and heater assembly without
heat shielding. Higher oven temperatures were attainable by adding more
heat shielding; but rapid deterioration of the stainless steel charge
bottle occurred at temperatures in excess of 12000C.
A second series of production tests was run using cerous chloride
(CeCl3) charge material, and difficulties encountered on these runs were:
1. Failure of arc chamber heater lead.
2. "Hunting" of the micromax and consequent oven temperature
3. Charge blow-out during initial pump-down.
4. Short charge bottle life.
The first of these difficulties was caused by the motor action of the
heater lead in the magnetic field when currents from 30 to 36 amperes (1250
to 1800 watts) were supplied to the heaters. This was partially overcome by
placing lock nuts in all possible locations and by using heavy copper leads
hard-soldered to the stainless steel machine screws with a heavy fillet.
Micromax hunting was overcome by placing an extra terminal strip in
the source unit terminal box for use of the thermohm leads only thereby
removing them from the vicinity of the heater leads. This change reduced
markedly the pickup in the thermohm circuit and consequent hunting of the
Charge "blow-out" during pumpdown was severe because of the unrestricted
path between the bottle and arc exit slit. Baffles were inserted in the
bottle thr ats and manifold to reduce this effect; but it was found advis-
able to reduce the initial pump-down rate with a restricted roughing line.
Stainless charge bottles were found to have a limited life (two runs)
and several bottles were fabricated from graphite which proved to be satis-
factory. Figure 5 shows a graphite charge bottle and oven heater assembly.
A source using graphite heating elements supported on Lava A insu-
lators is now available for use with charge materials whose operating
temperatures are in the range from 6500C to 1000C, and a thermohm suit-
able for use in this temperature range has been developed.
The work described in this report was carried out under the direction
of C. P. Keim, Director of the Isotope Research and Production Division, Oak
Ridge National Laboratory, Y-12 Plant, and H. W. Savage of the Isotope
Research and Development Department. Tank tests were under the direction
of L. O. Love.
Special recognition is due F. R. Duncan of the Isotope Research and
Development Department and the Materials Laboratory under the supervision of
P. J. Hagelston.
C. E. Normand and J. R. Patton cooperated in the preparation of this
:r.r .- >. : ., ';:- ~ ~:... s~a~8s~r ..
1PO 224jl 2
UNIVERSITY OF FLORIDA
III iIII IIIIIII IIIII I I 11H IIIII III
3 1262 08909 0285
xml version 1.0 encoding UTF-8
REPORT xmlns http:www.fcla.edudlsmddaitss xmlns:xsi http:www.w3.org2001XMLSchema-instance xsi:schemaLocation http:www.fcla.edudlsmddaitssdaitssReport.xsd
INGEST IEID EFUG4DXUH_QTA6IG INGEST_TIME 2012-10-23T14:46:25Z PACKAGE AA00012267_00001
AGREEMENT_INFO ACCOUNT UF PROJECT UFDC