graphite sublimation tests with inert gas …...• argon cover gas works as well as helium cover...

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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY

GRAPHITE SUBLIMATION TESTS withInert Gas Mitigation

C. C. Tsai, T. A. Gabriel, J. R. Haines, and D. A. RasmussenOak Ridge National Laboratory

2nd International High Power Targetry Workshop

October 10 , 2005

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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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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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Surface Temperatures Exceed 2000K for Radiatively Cooled Graphite Targets

0

500

1000

1500

2000

2500

3000

0 200 400 600 800

Volumtric Heating Rate (MW/m3)

Gra

phite

Sur

face

Tem

pera

ture

(K)

Average Value for 1.5 MW Facility

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Test data and theoretical predictions of graphite coupon sublimation

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1500 2000 2500 3000

Peak Temperature (K)

Subl

imat

ion

Rat

e (m

g/h)

Theoretical - Ref. 4Theoretical - Ref. 3Measured

Ref. 3 The Industrial Graphite Engineering Handbook, Union Carbide Corporation, 1969.Ref. 4 Darken, L.S. and Gurry, R.W., Physical Chemistry of Metals, McGraw-Hill, 1953

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Target Lifetime of One Month Is Possible at Power Levels of 1.2 MW

- 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

0.1

1

10

100

0 0.5 1 1.5 2Proton Beam Power (MW)

Erso

ion

Life

time

(mon

ths)

• 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

− Includes water-cooled panel coil shroud and extensive heat shields

38 mm

19 mm

63.5 mm

25.4 mm

1mm Thick

Rod specimen that will be used for new tests

Small coupons used in previous tests

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18" O.D. 21" Long

12 Tapped-Holes for 3/8" Screws at 14" B.C.

8" O.D. Flange 20 Holes on 7" Diam. B.C.

Graphite Rod Sublimation Test Device

Graphite Rod

Water Cooled Cu Feedthrough

Pyrometer

Turbo Pump

Elbows for

Pumping

Mechanical Pump

Gate ValveWater

Cooling Lines

Gauge

Vapor Trap

ValveValve

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Graphite Rod Erosion Test Assembly Side View

Water Cooled 6.25" D & 14" L SS Coil Panel

Heat Shields

Bellow Sections

18" Cu Rod Feedthroughs

12" Graphite Rod

18" Copper Rod

Squire-tubing water cooled Cu Rod

Graphite Termianl

12" Graphite Rod

2.625"

Pyrometer

1.5"

Support

18" OD Chamber

12.5" Opeing Port

8" Flanges

Weld A Swagelok Ultra-Torr Adapter SS-1-UT-A-4BT For a 1/16" TC

Water cooled

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14" Long S.S. Coil Panel

6.25" Diam.

14"

Ta Heat Shield

Graphite Heat Shield

5/8" Graphite rod1" Cu Rod

0.5" X 1" viewing slot

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Heat loss through Cu current feedlines is acceptable - Sublimation rate > 70% of theoretical

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

Dstance from cooled Cu feedthrough (m)

Tem

pera

ture

(K)

6.06E-01

Frad =72.91

%

Fraction of Theoretcal Erosion

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16Dstance from cooled Cu feedthrough (m)

Eros

ion

Rat

e (m

m/m

onth

)6.06E-01Frad = Fraction of Theoretcal

Erosion = 72.91

- 320 MW/m3 (corresponds to 1 MW beam with factor of 2 peak-to-average)

- 15 mm diameter- 300 mm long- 0.8 kA

Expected mass loss = 650 mg/day

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New sublimation Test Apparatus Was Commissioned in Dec ‘04

0

2

4

6

8

10

12

14

16

18

0 500 1000 1500Heating Current (A)

VoltageResistance (milli-ohm)

0

500

1000

1500

2000

2500

3000

0 500 1000 1500Heating Current (A)

Peak

Gra

phite

Tem

pera

ture

(K)

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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• 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 Arand 2.56 mg/h for He.

• The following observed features could be due to gas impurity1. The mass loss rate in the static helium cover gas lower than that in the flowing

helium cover gas. 2. 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

0.0001

0.001

0.01

0.1

1

10

100

1000

10000

100000

0 500 1000 1500 2000 2500 3000 3500GR Temperature, K

Subl

imat

ion

Rat

e, m

g/h

Theoretical Curve

Test Data in Vacuum

Test Data in Helium

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Graphite Rod Sublimation Test Parameters

Mass Loss Rate Test Duration Vh Ih Rh Cover Gas(mg/h) (h) (V) (A) (milli-ohm) (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

graphite sublimation tests with inert gas …...• argon cover gas works as well as helium cover...

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