plasma facing components meeting 4-6 august 2010, los angeles, ca nuclear, plasma, and radiological...

Plasma Facing Components Meeting4-6 August 2010, Los Angeles, CA

Nuclear, Plasma, and Radiological EngineeringCenter for Plasma-Material Interactions

Contact: [email protected]

Lithium Related Experiments at University of Illinois

D. N. Ruzic, D. Burns, S. Jung, P. Raman, M. Tung, W. Xu, V. Surla


Presentation Outline2

SLiDE (Solid/Liquid Lithium Divertor Experiment)

– Recent Results

– Summary and Future Plans

IIAX (Ion-Interaction Experiment)

– Recent Results

– Summary and Future Plans

Supporting Experiments

– Seebeck Coefficient Experiments

– Electrostatic Lithium Dropper

Summary


SLiDE System Overview

E-beam source

Current density profile

Tray10cm

10cm

10cm

25cm

Solid/Liquid Lithium Divertor Experiment (SLiDE)– Produces temperature

gradients with an electron beam

– Creates magnetic field with external magnet system (these tests at normal incidence)

– Measures temperature distribution in tray containing lithium

– Active cooling for steady-state operation

– Camera system monitors surface velocity

Designed, constructed and operated for this work


Solid/Liquid Lithium Divertor Experiment (SLiDE)

Lithium is melted in a square stainless steel (SS) tray (10cm*10cm) by a linear electron beam with a Gaussian distribution.

The beam hit the top surface of the Li in a normal direction. The acceleration voltage is 15kV and the beam current ranges from 0 to 100mA. This leads to a input power of 0 to 1.5kW.

Magnetic field is the same direction with the beam. It can reach as high as 800G.

Top surface is monitored by digital camera.

4


E-beam and thermocouple array

The shape of the beam is affected by the magnetic field. Normally, the width is 1cm and the length is 7cm.

14 pairs of thermocouples embedded in the tray to gain the temperature distribution at the bottom of the Li based on the 1D heat conduction assumption.

5


Thermo-Electric MHD driven swirling flow6

Left figure shows the thermoelectric effect. Thermoelectric effect requires different material (or TE power), close current return path and temperature gradient.

In the experiment the center of the pool of Li is heated by the beam creating a temperature gradient along the interface of Li and stainless steel tray. This generates the thermoelectric current in the bulk of Li. Under the effect of the magnetic field the Lorentz force will drive the Li to swirl.


Experiment ObservationSwirling flow observed. (TEMHD)

The Thermo-Capillary MHD force (Marangoni Effect) would produce flow away from the heat stripe on both sides

If the magnetic field is turned off, the swirling flow will stop in seconds.

If the beam is shut off while the magnetic field is kept on, the thermoelectric effect will maintain the swirling flow for minutes.

Many confirming experiments done….(field restart, insulating tray, etc.

7

Spin down test


Ratio of TEMHD to TC in SLiDEAll data cases show evidence

of swirling flow– TEMHD indirectly shown for

Ha>1.4 by temperature– TEMHD directly shown for

Ha>17– TC capable of being seen

Ratio of TEMHD to TC velocity– Ratio = 1 indicates equal

effectiveness– Ratio > 1 indicates TEMHD

dominanceAll data consistent with this

formulation

M. Jaworski, T.K. Gray, M. Antonelli, J.J. Kim, C.Y. Lau, M.B. Lee, M. J. Neumann, D. N. Ruzic, “Thermoelectric Magnetohydrodynamic Stirring of Liquid Metals”, Phys. Rev. Let., March 2010

“the Jaworski #”


Mike Jaworski’s Thesis Results

Appling the torque balance through the body of Li, an equation is built to describe the relation between the peak angular velocity of the Li and the temperature gradient between the interface of Li and stainless steel tray.

The peak angular velocity changes at different Hartmann numbers (Ha=hB(σ/ρν)^1/2) and different C (C=hσ/tσW).

The peak swirling velocity at the top surface is measured and compared to the theory -- they match!

9


Newer Results: IR camera system10

The camera Inframetrics 760 is utilized to measure the surface temperature of the liquid lithium.• Bandpass: 3~12 um• Accuracy: +/- 2 C or +/- 2%• Field of view: 15 degree vertical and 20

degree horizontal• Temperature range: -20 C to 400 C

(normal) or 20 C to 1500 C (extended)• Scan rate: 60 Hz• Horizontal resolution: 1.8 mRad, 194

IFOVs/line, 256 Pixels/line

DT-3152 card and software SandIR from Sandia National Lab. Temperature profile can be calculated in the program based on the emissivity map and output as an origin file.

Acknowledgements:

1. Dr. DennisYouchinson (SNL) for the SandIR software

2. Dr. Richard Majeski (PPPL) for the ZnSe window


IR calibration and measurement

The “emissivity” calibration:

Heat the Li in the chamber to a certain temperature (over 180 C, measured by embedded thermocouple)

Adjust the emissivity value in the control program SandIR, let the measured temperature from the program match the thermocouple value.

Repeat the measurement at different temperature values.

The emissivity is 0.045± 0.005

11

The IR measurement:

Chose a clean surface and draw a emissivity map

The reflection of the beam can be identified.

Chose a path to measure the temperature distribution along the radius direction. Try to avoid the pass-by impurity layer which looks like a flowing hot spot.


Top surface temperature

The temperature distribution of the top surface is measured at different magnetic field and different beam power. The thickness of Li is 1cm.

Left result is measured at 300W input power while the right result is measured at 400 G. The increase of magnetic field can affect the swirling speed and focus the beam shape. Corresponding to the different input power, the temperature difference across the radius direction is 92.3 0C (150 W), 112.1 0C (300 W) and 167 0C (450 W).

The temperature of the surface that is directly heated by the beam is not significantly higher than other region.

12

0 1 2 3 4

200

250

300

350

400

Su

rfac

e T

emp

erat

ure

Pro

file

(C

)

Distance from the Center of Li (cm)

134.6 G 265.1 G 399.2 G 544.6 G

0 1 2 3 4

200

250

300

350

400

450

Su

rface T

em

pera

ture

Pro

file

(C

)

Distance from the Center of Li (cm)

150W 300W 450W


Compared with theoretical result

The surface temperature distribution cannot be calculated analytically. A numerical method is necessary.

The temperature distribution at the interface can be gained through the embedded thermocouple arrays. (300W input power, 265G magnetic field, 1cm Li case)

Based on the theory, the velocity field can be calculated.

Since the top heat flux is known, the temperature distribution through the bulk of Li can be calculated through the 3D steady state heat transfer equation. FLUENT is utilized to calculate the result and the surface temperature distribution is chosen to compare with the IR measurement.

The model result, 13% less in value, shows a similar shape with the IR measurement.

13

Top Surface Temperature Bottom Temperature Velocity Field

Top surface temperature comparison


Different thickness of Li

Results of 1cm thick Li and 1.5cm thick Li are compared. For this group of data, magnetic field is 270G and input beam power is 300W.

The temperature results at the interface are almost the same while the flowing velocity of 1 cm Li seems a little bit higher than that of 1.5 cm Li. Thermoelectric effect mainly acts like a force in a thin layer between Li and stainless steel tray. For a thinner layer, it is easier to drive.

14

0 1 2 3 4 5 6 70

50

100

150

200

250

300

350

400

Tem

pera

ture

(C

)

Distance to the center (cm)

Interface T of 1cm Li Upper thermocouple set of 1cm Li Interface T of 1.5cm Li Upper thermocouple set of 1.5cm Li

0 2 4 6

5

10

15

20

25

Surface velocity of 1cm Li Surface velocity of 1.5cm Li

Ve

loci

ty (

cm/s

)



Different thickness of LiAlthough the temperature at the

interface does not change too much, the surface temperature changes a lot.

The temperature profile is higher for 1.5 cm Li. In the swirling flow, the secondary flow in r-z plane is very small compared to the swirling which leads to a low heat transfer rate along the vertical direction.

The slower surface flowing velocity brings up a higher temperature gradient. The center temperature is over 400 0C.

Due to the different surface impurity condition, the temperature measurement for 1.5 cm Li may be higher than the real value.

15

0 1 2 3 4100

200

300

400

500

Sur

face

tem

pera

ture

(C

)


Temperature of 1.5cm Li Temperature of 1cm Li

Peak Heat Flux Value in SLiDE for this work: 1 MW/m2

Can do over 10 MW/m2 at higher beam power


Dry-out of the center16

At a certain magnetic field, if the input beam power is too high, the center of the Li will dry out. At the dry out condition, the tray will be exposed and the temperature of the edge will usually increase quickly to a high temperature.

Since the vapor pressure of lithium above 400 0C may be high for tokamak operation, the dry-out is an important problem for application of free surface Li in tokamak.

Dry-out of the center


Dry-out of the center in SLiDE In the experiment, a relation between

the magnetic field and the input beam power is shown. The magnetic field is set to a value and the input power is increased to the point where the dry-out just appears.

The right side in the graph show the conditions at which dry-out happens in SLiDE.

Thinner Li is easier to dry-out although the surface temperature seems to be lower.

Marangoni effect may be coupled in the flow. The unbalanced surface tension tends to drag the Li from the center while swirling flow may hold the lithium from radius flow.

17

200 300 400 500 600 700 8000

200

400

600

800

1000

1200

1400

1.5cm Li 1cm Li

Inp

ut

po

we

r (W

)

Magnetic field (G)

Dry-out region

What this means for a liquid metal divertor concept will be part of our “future work”


Newest Work-Molybdenum as tray material

0.5 mm thick Mo foam on 1mm thick SS plate as bottom plate. The thickness of Li is 1.5 cm, the input power is 300W and the magnetic field is about 400G.

Around 400 0C, Seebeck coefficient of Mo is13 μV/K while that of SS is -3.4 μV/K. Thermoelectric power between Li and Mo is smaller than that of Li and stainless steel which causes a lower TEMHD driven force.

The swirling flow on top of Mo is slower than that on stainless steel even the temperature gradient in Mo/Li case is higher (Future Work).

18

0 1 2 3 4300

350

400

450

500

550

Li/Mo Li/SS

Tem

pera

ture

(C

)

Distance to the center (cm)1.0 1.5 2.0 2.5 3.0

0

5

10

15

20

25

30

Li/Mo Li/SS v=11.8*r^1/2 v=5.3*r^1/2

velo

city

(cm

/s)



Future Work- LIMIT

Concept for heat removal using TEMHD. The Li flows in the slots of the Mo plate powered by the vertical temperature gradient. This vertical temperature gradient generates vertical current which will be affected by the torroidal magnetic field to drive the Li. This flow will transfer the heat from the strike point to other part. The main part of the Mo plate is cooled to get a stable temperature gradient while the back flow of Li also needs cooling.

19

Hot

Heat flux

Cooling channelsInlet

Outlet

Passive Li replenishment

Li flowCool

More details on LIMIT in Ruzic’s talk tomorrow (Friday Morning Session)


Ion-Surface InterAction eXperiment (IIAX)

20


Ion-Surface InterAction eXperiment (IIAX)21

Target holder with a UHV heater

Li evaporator

QCM

Gas IN

Faraday cup

Plasma cup

ExB filter

LiCl holder

• Facility to Study Erosion of PFC materials Past work involved measuring overall erosion rates of plain, and

lithium coated ATJ graphite under light ion bombardment. A new facility was designed and built for chemical erosion

measurements

Li evaporatorwith shutter

Plasma cup

Sample holder with UHV heater


Experimental Set up- Chemical Erosion

Chemical Erosion Measurements• Differential Pumping Arrangement• Residual Gas Analyzer• RF Plasma• Target Heater• Lithium Evaporator

22


Recent Results - Physical Erosion23

K. Ibano, V. Surla, and D.N. Ruzic, “Sputtering and Thermal Evaporation Studies of Lithiated ATJGraphite”, IEEE transactions on Plasma Science, Vol. 38, No.3, March 2010

-1.00E-007

-5.00E-008

0.00E+000

5.00E-008

1.00E-007

1.50E-007

2.00E-007

2.50E-007

3.00E-007

3.50E-007

4.00E-007

23500 24000 24500 25000 25500 2600031392

31394

31396

31398

31400

31402

31404

From X = 25201 to 25501 = -0.00396 Hz/sec;

From X = 24488 to 25092 = -0.00263 Hz/sec;

From X = 23604 to 24242 = -0.0032 Hz/sec;

200910281

f 1-f2 (

Fre

qu

en

cy d

iffe

ren

ce)

(Hz)

Time (seconds)

K1

97

A C

urre

nt [A

]

Ionization fraction = Y(I)/ Y(T)

where Y (T) is total sputter yield given by Y(T) = Y (A) + Y (I)

Y (A) is the sputter-yield due to atoms and Y(I) is the sputter-yield due to ions

Normally, Y(T) is measured with QCM . By negatively biasing the target, Y(A) is obtained and hence Y(I).

Ionization fraction for a 2000 eV Lithium ion beam sputtering of lithiated graphite is found to be 30% ± 6%.


Methodology- Chemical Erosion

Experiments are conducted for three different configurations (with and without plasma):

•With no target in the main chamber in order to provide a baseline measurement (Contribution from the walls)•With ATJ graphite target in the main chamber to study chemical erosion compounds produced (Walls + Target)•With Li on ATJ graphite to study the effect of lithium treatment on chemical erosion. (Walls + Target)

In addition to the above experimental studies, the effect of temperature on chemical erosion of ATJ graphite is investigated by mouting target on a button heater which is controlled by a temprature controller.

The ability to bias the target allows energy dependent erosion measurements.

24


Results - Chemical Erosion

20 22 24 26 28

5E-7

1E-6

1.5E-6

2E-6

2.5E-6

3E-6

2E-5

4E-5

"mass 28"CD

4

ATJ Graphite Lithiated ATJ Graphite No Target

Par

tial P

ress

ure

(Tor

r)

Mass (amu)

Effect of Temperature

Effect of Bias

0 1000 2000 3000 4000 50001E-7

1E-6

1E-5

1E-4

Target heated gradually

Plasma OFF

Plasma ON without biasing

Plasma ON

Plasma ON with biasing

Partia

l Pre

ssure

in T

orr

Time in Seconds

CD4

C2D

2 (+N

2)

D2

ATJ Graphite

0 200 400 600 800 1000 1200 1400 1600 1800

3E-7

6E-7

9E-7

1.2E-6

1.5E-6

1.8E-62E-4

3E-4

4E-45E-4

Plasma OFF

Plasma ON with biasing

Plasma ON without biasing

Partia

l P

ressure

(Torr

)

Time (Sec)

C2D

2

D2

CD4

Plasma ON

Plasma OFF

mass 28

mass 28

25

• Species of Interest – CD4, Mass 28 (CO/C2D2)

• Temperature enhances chemical erosion

• Li suppresses CD4 but “mass 28” is unaffected

• Effect of bias is seen


Analysis- Chemical Erosion

Where Sj Signal at mass, jMij cracking pattern contribution of species ‘i’ to mass j Pi – partial pressure of species ‘i’

We know Sj, Mij, and we can solve for Pi.

i

iijj PMS

0

)()(

)(2

i

j iiijji

i

P

thatSuch

PMSPfwhere

PfMin

26

Acknowledgements: Dr. J.P. Allain, Dr. M. Neito


Summary and Conclusions27

• For physical sputtering, Lithium is known to sputter as ions, and the ionization fraction of 30%±6% is obtained for Lithium ion beam sputtering at 2000 eV.

• For chemical erosion measurements of carbon, the erosion products with Deuterium plasma are found to be dominated by CD4 and Mass 28. For other products, future work is involved in decoupling radicals to cracking patterns of the RGA.

• Chemical erosion increases with temperature.• Lithium treatments on ATJ graphite have shown to suppress the

chemical erosion of graphite. Although it is qualitative at this point, CD4 erosion is suppressed by a factor of at least two, while Mass 28 remains the same.

• Lithium treatments on ATJ graphite have been shown to be advantageous not only for physical erosion but also for chemical erosion (CD4) and more studies are underway to resolve other species.


Future Plans for IIAX

Chemical erosion studies to continue

Proposed thrust: Study of mixed materials (containing lithium)

Electrostatic analyzer (ESA) Surface morphology important

Experimental upgrade – XPS in IIAX (allows in-situ determination of the constituents on the surface)

ESA – (allows determining the species coming out of the surface)

Sputtering of Li/W, Li/Mo

Determine metrics and solutions to retain the activity of Li surface

28



Seebeck Coefficient Measurements

29


Apparatus for Seebeck Coefficient

HEATER

T T + DT

DV

Lithium Extrusion

30


Seebeck Coefficient of Lithium

0 20 40 60 80 100 120 140 160 180 200 2200

10

20

30

40

Experimental Data Bidwell (1924)

Se

eb

eck

Co

effi

cie

nt (

V

/K)

Temperature ( oC)

Typical graph showing the Seebeck Coefficient of Li as a function of temperature

31


Seebeck Coefficient of Lithium

0 50 100 150 200 250

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

Se

eb

eck

Co

effi

cie

nt (V

/K)

Temperature (0C)

Bidwell (1924) measurement Li-7 @ coil position1 Li-7 @ coil position 2 Li-7 @ coil position 3 Data set matched Bidwell

The Seebeck coefficient measurements are very sensitive to the coil position

32


Seebeck Coefficient of Lithium-6

0 50 100 150 200 25002468

101214161820222426283032343638

Se

eb

eck

Co

effi

cie

nt (V

/K)

Temperature (0C)

Li-6 Data 1 Li-6 Data 2 Bidwell for Li-7

33

Acknowledgements: Dr. Dennis Mansfield (PPPL) for providing Li-6


Seebeck Coefficient Measurements

Future Work: • Seebeck coefficient of Sn or other material to verify the apparatus• Seebeck coefficient measurement of lead lithium

0 20 40 60 80 100 120 140 160 180 200 220 2400

4

8

12

16

20

24

28

32

36

Se

ee

be

ck C

oe

ffici

en

t (

V/K

)

Temperature (0C)

Lithium7 Lithium6 Bidwell

34



Lithium Dropper Experiment

35


Electrostatic Lithium Dropper ConceptConcept: A device that can produce a spray of charged lithium droplets (liquid phase), allowing oppositely charged surfaces to be coated evenly with Lithium.

The dropper is a hollow stainless steel tube with a very small orifice at the end.

The dropper is filled with solid lithium and heated until the lithium melts.

As Argon back pressure is applied to the dropper, droplets of molten Lithium will form, with positive charge, and fall from the dropper.

The dropper is given a high positive voltage, and the surrounding chamber is given a low positive voltage. The QCM is grounded.

The positively charged Lithium droplets are attracted to the grounded QCM, and are there collected and measured.

Mass, flowrate, and distance traveled by the Lithium droplets can be measured, determining if a Lithium spray device is feasible.

High Voltage

Argon Back Pressure

Grounded

Charged Lidroplets

HeatedDropper

QCM

Low Voltage

36


Lithium Dropper First Result

Test Run: Dropper Temp: 197oC, Argon Bac k Pressure: 40 PSI, Chamber Pressure: 500 mTorr (No bias)

String of molten Lithium visibly moving through orifice!

Dropper w/ Heating Wire

QCM

Dropper Orifice:Solid cap with 0.003 mm – diameter hole

Future Orifices:Solid cap with multiple rectangular holes: 0.003 mm x 1 mm (Rectangles seperated by 0.1 mm)

(Lithium string length: ~0.5 mm)

37


SummarySeveral Lithium related experiments are ongoing at CPMI

SLiDE

Successful in studying the flow of liquid lithium (experiment+ theory)

Future Direction

Extensive modelling (to include Dry-out)

Test other substrates relevant to NSTX or LTX

LIMIT concept

IIAX

Erosion Measurements (Physical sputtering, Ionization fractions)

Future Direction Chemical erosion measurements

Study of Mixed materials containing lithium

Upgrades for surface composition, Li active surface

Need feedback for the DOE renewal proposal

38

plasma facing components meeting 4-6 august 2010, los angeles, ca nuclear, plasma, and radiological...

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los angeles

magnetic field

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beam current ranges

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