Transcript In-Beam and Out-of-Beam Graphite Target Tests for the

GRAPHITE SUBLIMATION TESTS with
Inert Gas Mitigation
C. C. Tsai, T. A. Gabriel, J. R. Haines, and D. A. Rasmussen
Oak Ridge National Laboratory
2nd International High Power Targetry Workshop
October 10 , 2005
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
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ACKNOWLEDGEMENT
 Thanks to :
- T.J. McManamy, R. H. Goulding, and A. Fadnek for
valuable advice on design and construction of
test apparatus
- D.O. Sparks for implementing the electronic and
power supply for the test stand
- S.C. Forrester for technical support to vacuum
system and test set up
- D. E. Schechter for advice on cover gas technique
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U. S. DEPARTMENT OF ENERGY
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ORNL Proposed Passively Cooled Graphite
Target for Neutrino Factories
Graphite Rod Target
(15 mm diameter, 800 mm long)
Water-cooled stainless
steel support tube
Graphite
support spikes
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Motivation for Graphite Sublimation Tests
 A radiatively cooled graphite target was proposed
as a candidate for a neutrino factory target
 Radiation cooled design is very robust mechanically, but loss
of material by sublimation will limit the power
 We proposed the use of Helium Cover Gas to
greatly reduce the net erosion rate
 Net erosion of graphite is limited by near surface interactions
between sublimated carbon and He (mean-free-path ~ 1
μm), which leads to redeposition of sublimated graphite
 Graphite sublimation tests are being conducted
in an attempt to better establish limits for a
vacuum environment and validate the use of
helium to suppress sublimation
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Graphite Surface Temperature (K)
Surface Temperatures Exceed 2000K for
Radiatively Cooled Graphite Targets
3000
2500
2000
1500
1000
Average Value for
1.5 MW Facility
500
0
0
200
400
600
800
Volumtric Heating Rate (MW/m3 )
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Test data and theoretical predictions of
graphite coupon sublimation
1.00E+04
Sublimation Rate (mg/h)
1.00E+03
Ref. 3
Ref. 4
The Industrial Graphite Engineering
Handbook, Union Carbide
Corporation, 1969.
Darken, L.S. and Gurry, R.W.,
Physical Chemistry of Metals,
McGraw-Hill, 1953
1.00E+02
1.00E+01
1.00E+00
1.00E-01
Theoretical - Ref. 4
Theoretical - Ref. 3
1.00E-02
Measured
1.00E-03
1.00E-04
1500
2000
2500
3000
Peak Temperature (K)
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Target Lifetime of One Month Is
Possible at Power Levels of 1.2 MW
100
Ersoion Lifetime
(months)
10
1
-
0.1
Graphite rod target
Initial diameter of 15 mm
End-of life at 13 mm diameter
Peak-to-average heat deposition = 2
Saturation pressure data from Ref. 4
0.01
0
0.5
1
1.5
Proton Beam Power (MW)
2
• This lifetime is based on data from previous tests under vacuum conditions
• Hopefully, net erosion rate can be reduced/lifetime extended with He cover gas
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Status of graphite sublimation tests


Vacuum tests on small coupons successfully conducted, but we
experienced arcing damage with helium gas
New tests have been conducted with target rod prototype
 Joule heating at power densities equivalent to beam deposition
 Requires high current (~ 1 kA)
 Use existing facility in ORNL Fusion Energy Division
 Test specimen
 15 mm diameter, 300 mm long
 Clamped at both ends for current feed
Graphite Foil
 Includes water-cooled panel coil shroud and extensive heat shields
1mm Thick
Rod specimen that will be used for new tests
19 mm
25.4 mm
38 mm
63.5 mm
Small coupons used in previous tests
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Graphite Rod Sublimation
Test Device
8" O.D. Flange
20 Hole s on
7" Diam . B.C.
Pyrom e te r
18" O.D.
21" Long
12 Tappe d-Hole s
for 3/8" Scr e w s
at 14" B.C.
Wate r Coole d
Cu Fe e dthrough
Gr aphite Rod
Elbow s
for
Pum ping
Gauge
Wate r
Cooling
Line s
Tur bo
Pum p
Valve
Valve
Me chanical Pum p
Gate
Valve
Vapor Trap
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Graphite Rod Erosion Test Assembly Side View
Pyrometer
8" Flanges
12.5" Opeing Port
18" Cu Rod
Feedthroughs
12" Graphite Rod
18" OD Chamber
Bellow Sections
Water cooled
Weld A Sw agelok
Ultra-Torr Adapter
SS-1-UT-A-4BT
For a 1/16" TC
2.625"
Wate r Coole d 6.25" D
& 14" L SS Coil Pane l
He at Shie lds
Support
18" Copper Rod
Gr aphite
Te r m ianl
1.5"
12" Graphite Rod
Squire-tubing w ater cooled Cu Rod
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Overall Assembly
Ta Heat Shield
14" Long S.S. Coil Panel
0.5" X 1" viewing slot
Gr aphite He at Shie ld
6.25" Diam.
1" Cu Rod
5/8" Gr aphite rod
14"
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Heat loss through Cu current feedlines is
acceptable - Sublimation rate > 70% of theoretical
Frad =
3000.0
Fraction of
Theoretcal Erosion
6.06E01
- 320 MW/m3 (corresponds to 1 MW
beam with factor of 2 peak-to-average)
- 15 mm diameter
- 300 mm long
- 0.8 kA
72.91
%
2000.0
Frad =
1500.0
Fraction of Theoretcal
Erosion =
6.06E-01
1.20
1000.0
500.0
0.0
0
0.02
0.04
0.06
Erosion Rate (mm/month)
Temperature (K)
2500.0
72.91
%
1.00
0.80
0.60
0.08
0.1
0.12
0.14
0.16
Dstance from cooled Cu feedthrough (m)
0.40
Expected mass loss = 650 mg/day
0.20
0.00
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Dstance from cooled Cu feedthrough (m)
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U. S. DEPARTMENT OF ENERGY
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New sublimation Test Apparatus
Was Commissioned in Dec ‘04
Voltage
Resistance (milli-ohm)
3000
Peak Graphite Temperature (K)
18
16
14
12
10
8
6
4
2
2500
2000
1500
1000
500
0
0
0
500
1000
Heating Current (A)
1500
0
500
1000
Heating Current (A)
1500
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TEST PROCEDURE
 Heat up a new graphite rod (GR) under vacuum
 Measure Vh / Ih (or resistance) and temperature of GR and
use the recorded data to guide subsequent tests
 Measure initial GR mass in an electronic balance after
removing the GR from 1-bar He test chamber
 Bake GR at 2000 K for 3 hours under vacuum
 Conduct GR sublimation test at desired temperature (e.g.
2350 K) for a long period of time (5 to 20 hours) under
VACUUM or 1-bar HELIUM (Flowing or Static)
 Wait overnight for GR to cool down and fill the test chamber
with 1-bar helium
 Remove and measure GR mass to estimate sublimation
mass loss rate
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Significant Test Results






1.
2.
The high temperature oven can be operated continuously under
vacuum or 1-bar inert cover gas such as helium, Argon, etc.
GR Resistance character is similar in vacuum or in cover gas.
The heating power in 1-bar inert cover gas is slightly higher than that
under vacuum
GR mass loss rate (mass loss in mg / test duration in hour) in 1-BAR
HELIUM cover gas is lower than that under VACCUM conditions
Argon cover gas works as well as helium cover gas for mitigating the
GR sublimation erosion; measured mass loss rate is 1.7 mg/h for Ar
and 2.56 mg/h for He.
The following observed features could be due to gas impurity
The mass loss rate in the static helium cover gas lower than that in the flowing
helium cover gas.
The mass loss rate decreases with the increase of the test duration under the
static helium cover gas.
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Theoretical Curve of Graphite Rod with
Test Data
Theoretical Curve, Data in Vacuum, and Data in Helium
100000
Sublimation Rate, mg/h
10000
Theoretical Curve
1000
Test Data in Vacuum
100
Test Data in Helium
10
1
0.1
0.01
0.001
0.0001
0
500
1000
1500
2000
2500
3000
3500
GR Temperature, K
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Graphite Rod Sublimation Test Parameters
Mass Loss Rate Test Duration
(mg/h)
(h)
Vh
(V)
Ih
(A)
Rh
(milli-ohm)
Cover Gas
(Vacuum or He)
5.82
5
18.6
1214
15.32
Vacuum
3.4
5
18.9
1230
15.37
Flowing He
2.56
5
18.9
1230
15.37
Static He
0.78
20
18.9
1230
15.37
Static He
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Static Helium Cover Gas Mitigates Sublimation
of Graphite Rod To 1/30 Of That Under Vacuum
 Average mass loss rate of 2.56 mg /h for a test
duration of 5 hours
 Average mass loss rate of 0.78 mg/h for a test
duration of 20 hours
 If the test conditions are identical for the above 2
cases, the average mass loss rate for the last 15
hours of the 2nd case is 0.19 mg/h
 Ratio of 0.19 mg/h to 5.82 mg/h (mass loss rate
under vacuum) is 1/30
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Effective Sublimation Erosion Rate Measured
Could be 0.025 g/h/m²




Graphite Rod (GR): 305 mm Long & 15.88 mm in Diameter
38 mm long at each end of the GR was mounted to GR terminal.
~38 mm long at each end of GR yields insignificant sublimation.
Effective length of the GR center section contributing to
sublimation erosion is ~153 mm.

Effective surface area of the GR is 76.3 cm².

Effective sublimation erosion rate of the GR, equivalent to a mass
loss rate of 5.82 mg/h under vacuum conditions, is 0.77 g/h/m².

If the test conditions for the 2 static helium cases are identical, the
mass loss rate during the last 15 hours could be 0.19 mg/h or the
effective sublimation erosion rate to be 0.025 g/h/m².
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Concluding Remarks
 Graphite sublimation tests have
demonstrated and proven that 1-bar inert
cover gas could substantially mitigate
graphite sublimation erosion rate
 Gas impurity increased the average
sublimation erosion loss rate
 Additional tests with pure inert cover gas
should yield better data for designing high
power and long life targets
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U. S. DEPARTMENT OF ENERGY
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