High temperature ion source and thermohm development for stable isotope production

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Title:
High temperature ion source and thermohm development for stable isotope production
Physical Description:
17 p. : ill. ; 27 cm.
Language:
English
Creator:
Wilkinson, P. E
Whitman, G. D
Oak Ridge National Laboratory
U.S. Atomic Energy Commission
Publisher:
United States Atomic Energy Commission, Technical Information Service
Available from the Office of Technical Services, Dept. of Commerce
Place of Publication:
Oak Ridge, Tenn
Washington, D.C
Publication Date:

Subjects

Subjects / Keywords:
Ion sources   ( lcsh )
Isotope separation   ( lcsh )
Resistance thermometers   ( lcsh )
Genre:
federal government publication   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
This report describes the development of an ion source unit and thermohm for operation in the range of 650-1000°C.
Statement of Responsibility:
P.E. Wilkinson and G.D. Whitman.
General Note:
Cover title.
General Note:
"December 5, 1950."
General Note:
"Contract No. W-7405-eng-26."
General Note:
"Y-705"--Cover.
General Note:
Work performed at Oak Ridge National Laboratory, Y-12 Area."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 005254782
oclc - 728098311
System ID:
AA00012267:00001


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Y-705

Subject Category: PHYSICS




UNITED STATES ATOMIC ENERGY COMMISSION



HIGH TEMPERATURE ION SOURCE
AND THERMOHM DEVELOPMENT FOR
STABLE ISOTOPE PRODUCTION


By


P. E. Wilkinson
G. D. Whitman


I =




\ -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
available copy.

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




ABSTRACT

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
Y-12 AREA
CARBIDE AND CARBON CHEMICALS DIVISION
UNION CARBIDE AND CARBON CORPORATION



Oak Ridge, Tennessee


Contract No. W-7405-eng-26




4.




HIGH TEMPERATURE ION SOURCE AND THERMOHM
DEVELOPMENT FOR STABLE ISOTOPE PRODUCTION


INTRODUCTION


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.


PREVIOUS EXPERIENCE

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

temperatures.

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




6.


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.


HEATER DEVELOPMENT

Before much work was done on heater development, upper limits of temper
0 o
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











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200 400 600 800 1000 1200
TEMPERATURE IN DEGREES CENTIGRADE


PERFORMANCE CURVE OF HIGH TEMPERATURE THERMOHM


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10.




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




* 11.


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




12.


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

graphite.

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.






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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

control difficulties.

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

micromax.

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.


CONCLUSIONS


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.


ACKNOWLEDGMENTS

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

report.






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