lithium at illinois, hefei, china august 20, 2010 nuclear, plasma, and radiological engineering...

Lithium at Illinois, Hefei, ChinaAugust 20, 2010

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

Contact: [email protected]

Lithium Walls: The Ultimate Technology for Fusion

D.N. Ruzic, M. A. Jaworski1, T.K. Gray1, V. Surla,

W. Xu, S. Jung, P. Raman

1current address: Princeton Plasma Physics Laboratory


Outline 2

Introduction

- Why Lithium is so Good – but you all know that already !

Solid/Liquid Lithium Divertor Experiment (SLiDE)

-Experimental Facility

-Results

-Theory

Lithium Molybdenum Infused Trenches (LiMIT)

- How this could work for HT-7 and future devices

Lithium in Divertor Erosion Vapor Shielding Exp. (DEVeX)

Lithium in the Ion-Interaction Experiment (IIAX)

Conclusions


No cold hydrogen returns from wall: Plasma stays hot

Courtesy: PPPL

What Very-Low Recycling Does for Fusion

Standard Case

Lithium Case


Consequences of LithiumIncreased Confinement Time – seen across the world

Higher Temperatures

Suppression of ELMS (for tokamaks)

Control of Density is possible, even with NBI

Lower Z-effective

Less Fuel Dilution (seen on NSTX)

Negative Consequences?Helium pumping? We have shown this! M. Nieto, D.N. Ruzic, W. Olczak, R. Stubbers, “Measurement of Implanted Helium

Particle Transport by a Flowing Liquid Lithium Film”, J. Nucl. Mater., 350 (2006) 101-112.

Power handling? See the rest of this talk!

4


First Evidence? TFTR Supershots (1984)

Courtesy: PPPL


What Can Go Wrong with a Lithium Divertor?

Lithium melts at 180 C and evaporates very quickly above 400 C

So, it will have to be used ultimately as a flowing liquid

It is a liquid conductive metal and therefore subject to MHD effects. After all, fusion devices have large circulating currents and high magnetic fields.

So, careful planning is needed. Maybe it’s MHD effects can be utilized?

It has an extremely low density (half of water) and high surface tension (4 times water) and therefore difficult to deal with. It is also highly corrosive to some materials, such as copper.

So, careful engineering is needed and new materials (for fusion) need to be developed

6


Outline 7

Introduction




-Results

-Theory





Conclusions


CDX-U Results - The Unexpected Happened

QinputSurface tension drives flow

Visible image

Centerstack

Lithium in tray

R. Majeski et al., “Final results from the CDX-U lithium program,” Presentation at 47th Annual Meeting of the Division of Plasma Physics (APS-DPP), Denver, Colorado, October, 2005.

Trying to melt lithium in CDX-U:- 50 MW/m2 heat flux redistributed

from spot heat- No evaporation despite lithium's

tendency to do so (and purpose of e-beam run!)

Why did the lithium melt the entire tray and not evaporate?- First explanation was

thermocapillary phenomena- Temperature dependent surface

tension resulted in flow and strong convection away from hot spot

- If true, will this work in a divertor without over heating the Li ?


A Typical Fusion Heat Flux

J.N. Brooks, et al. J. Nucl. Matl. 337-339 (2005) 1053-1057.

Magnetic configuration concentrates power in “diverted” plasma

-Peak heat flux, steady-state typically 5-20 MW/m2

-Radiant heat flux at solar surface is ~63 MW/m2

-Transients can push the peak higherThermally driven phenomena depend on

heat flux gradients, not just peak values


SLiDE at Illinois - 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


Machine LayoutA sheet electron beam hits an

instrumented tray filled with lithium in a magnetic field.

Future versions will allow the tray to tilt so the angle between the heat flux and the field will be like in a tokamak – almost parallel.

11


Electron Beam Drawings and Simulations

7 keV, 2 A, 16 G 20 keV, 0.75 A, 1 kGThere are four filaments, each 10cm

long, parallel to eachother


Electron BeamDesigned to mimic

divertor heat flux- Actual run parameters

shown in table- Operated at 300W in this

set of experiments – capable of 15kW, which is 35 MW/m2 !

- Typical q0 in NSTX is

~10MW/m2

- Typical dq/dx in NSTX is ~100 MW/m2-m . We matched this number.

Line source with gaussian profile


Tray System OverviewCoolant supplied from

building sources

- Water at 35 psi

- Compressed air at 80 psi

- Steady-state cooling with liquid lithium temperatures and input power range of 50W – 1500W (with stainless steel tray)

28 thermocouples within tray

- 14 positions for heat flux modeling

- 4 external TCs for coolant calorimetry


Data Acquisition and ControlThermocouple data acquired

via computer system

- LABJACK USB data modules and amplifiers digitize analog signals. LabVIEW VI monitors temperatures in real time

- Control of magnets and eBeam voltage via computer system (filaments on manual)

Post-run analysis handled semi-autonomously

- Analysis program written to reference and analyze data sets

- Output summarizes and formats for easy plotting and further analysis


Camera SystemMirror used to view lithium

surface- Electron beam occludes

direct view- Mirror requires periodic

cleaning from lithium evaporation

Two cameras used- Low quality webcam for

simple monitoring- High quality, high-definition

camera used for velocity measurement movies

- Both utilize mirror illumination provided by electron beam filaments


17


Thermocapillary Flow, Basics

QinputSurface tension drives flow

Other surface tension driven flows

- “Tears” or “legs” in wine glass (concentration gradients from alcohol evaporation)

- Soap boats with detergent droplet (concentration gradient)

Thermocapillary

- “Thermo” meaning temperature

- “Capillary” meaning related to surface tension

- Temperature gradient on surface of a liquid creates surface tension gradients

- Surface tension gradients result in flow generation

Long history of study from mid-19th century


Thermoelectric MHD, BasicsThermoelectric effect

- Causes thermocouple junction voltages

- Thermoelectric power present in most materials

- Electromotive force generated by temperature gradients

- Requires different material (or TE power) to provide current return path and generate current

Replace one material with a liquid in magnetic field

- TE current and B-field generate Lorentz foce


Theory: TCMHD and TEMHD● Thermocapillary and TEMHD produce different

flow patterns● TC forces act parallel to surface gradients in surface

tension – force vectors radiate away from heat stripe (induce poloidal flow)

● TEMHD forces are produced in the bulk lithium due to gradients at the lithium-steel interface

● TE current the same process as produces voltages in thermocouple junctions

● Cross-product of JTEMHD and B results in azimuthal flow

● Beam generated forces (JxB) also result in an azimuthal flow (current converges into impact point) – sense of rotation is opposite of TEMHD

● Increased field damps both flows● TCMHD is damped by B-2

● TEMHD is damped as B-1 at high fields (due to thermoelectric propulsive force)

Thermocapillary force vectors.

TEMHD current diagram.


21


Is it really TEMHD? Qualitative Tests

Magnetic field reversal- Flow direction reverses upon reversal of

field- Flow is consistent and steady in swirl

Flow direction consistent with TEMHD source- Mirror system reverses apparent sense

of rotation- Magnets measured to determine

direction- E-beam and TE have opposite rotation

sensesAddition of insulator halts any

swirling flow- Quartz slides added between tray and

lithium- No flow observed at all in these cases


Another test: “Spin Down”● Test based on the time required

for the lithium to come to a stop● Viscous damping brings fluid to a

rest without additional forces to maintain flow (seconds)

● Magnetic fields enhance the destruction of kinetic energy and spin down faster (confirmed with a mercury test)

● If thermoelectric currents exist, these will decay at the thermal time constant of the lithium-tray system (minutes)

● Maintaining the magnetic field will sustain the flow, as opposed to damping it

● Turning magnetic field back on after a viscous spin-down should induce motion once more

Test procedure:

1.Obtain steady thermal conditions and lithium flow

2.Shut off beam and magnetic field and measure spin-down time

3.Return system to steady thermal conditions and lithium flow

4.Shut off beam but maintain magnetic field – measure spin-down time

5.Repeat step 2, but turn magnetic field back on after flow comes to rest and look for spin startup

Movie of spin-down test“spinDown.mov”


24


Quantitative Analysis: VelocityPurpose: Bring together

thermal and velocity measurements

Velocity measurements based on video analysis- Particles measured on a frame-

by-frame basis- Multiple measurements made

over course of particle visibility- Radial distance also measured

Velocity and radius used to determine most likely velocity at r = 1cm- u(r) ~ r0.5 based on Davidson

swirling flow theory- Consistent with SLiDE data


Heat Flux CalculationTime series data reduced

- Mean taken- Standard error calculated by

standard formulasFourier model applied to

calculate heat flux- Scaling factor applied to account

for tray warping- Radially symmetric pattern

observed- Most obvious in “off-center” sensor

sets- Radially symmetric pattern

observed in all cases run- Some TC pairs eliminated due to

tray damage directly underneath beam strike


Quantitative Prediction: TEMHD● Moderate Hartmann number

regime – TEMHD and MHD braking in equilibrium

● Dependent on thermoelectric power of the metal pair, P

● Temperature gradient along the interface determines flow velocity (for Ha > 1)

● Mean current density due to TE currents depends on geometry and conductivities (C variable)

● Velocity prediction can be converted to Reynolds number using depth as the characteristic length scale

1D current driven flow in a semi-infinite domain.


TEMHD Solution 1: Semi-Infinite DomainSolution for free surface

Identical to Shercliff, 1979 channel flow

Lithium-Iron example: P = 20e-6[V/K], h=5[mm], dT/dy = 1000[K/m]

Pre-factor = 8.4[m/s]

Solution depends on several factors

Hartmann again present

“C” is ratio of liquid/wall impedances

“Pre-factor” and velocity function {...}


Bödewadt-Hartmann FlowBödewadt flow

- Rotating fluid over stationary disk

- Variation of Karman flow (rotating disk)

- Use Karman similarity variables to analyze

Bödewadt-Hartmann flow

- Rotating flow with a magnetic field

- Non-dimensionalized system of equations results

Elsässer No. - Balance of MHD to Coriolis force


Approximate Solutions for Velocity Profile

Distance from wall

Make use of Davidson, 2002 approximate solutions

Linearized equations about core flow solution

Compares well with direct numerical integration

Aids further analysis


Quantitative Assessment --- Consistent with Data

Theory of TEMHD driven swirling flow predicts radial velocities -- agrees with data ! No free parameters

Range of Hartmann number covers peaking area of swirling flow theory

Range of Elsässer number covers MHD and Coriolis dominated flows

Torque balance method works ! TEMHD fits observables.


Spin Down Time Constant – Also ConsistentSpin down time requires

thermal gradients to sustain currents

Thermal time constant of the system can be estimated- Simple thermal resistance

model applied- Measurements from the

system used to make calculation

- 78 seconds is theoretical thermal time constant

Observed spin down time is equivalent to 2-3 thermal time constants – and that is what is observed.


Quantitative Prediction: Thermocapillary MHDConsider a semi-infinite

domain

- Free-surface at y=h

- Magnetic field B

- Surface subject to constant temperature gradient b

Two cases

- No return flow (dP/dx = 0)

- Return flow (dP/dx related to height of the fluid)

Surface tension boundary condition

- Surface tension gradient results in viscous shear at surface


Which one does theory say is dominant?Ratio of Semi-Infinite Solutions

Ratio of TEMHD to TCMHD velocity:

- Ratio = 1 indicates equal effectiveness

- Ratio > 1 indicates TEMHD dominance

- Ratio depends on material parameters capture by dimensionless number, ς, the “Jaworski” number.

- Also depends on container geometry captured in F(Ha) function

In Lithium-steel system, TEMHD dominates for Ha > 1


Ratio of TEMHD to TC in SLiDEAll quantitative data cases show

evidence of swirling flow.

- TEMHD indirectly shown for Ha>1.4 by temperature

- TEMHD directly shown for Ha>17

However, TC was capable of being seen in an oscillatory flow behavior.

- TEMHD flow distributes heat and smooths out the gradient along the Li – steel interface

- With no gradient there, only the surface temperature gradient exists, therefore TCMHD until interface gradient builds up again

Jaworski Number was always greater than 1 !


Can you see TCMHD in the lulls?

If conditions are right, when the TEMHD flow stops, TCMHD “Maragoni-effect” flow can be seen (motion on surface away from heated stripe due to surface temperature gradient causing a surface tension gradient)

36

If conditions are right, when the TEMHD flow stops, TCMHD “Maragoni-effect” flow can be seen (motion on surface away from heated stripe due to surface temperature gradient causing a surface tension gradient)

TCflowExample.mov


37


Outline 38

Introduction




-Results

-Theory





Conclusions


Apparatus for Seebeck Coefficient

HEATER

T T + T

V

Lithium Extrusion

39How Powerful is the ThermoElectric Effect?

Apparatus for Seebeck

Coefficient Measurement


Big! Use it with what holds the Lithium

Like a thermocouple, a voltage is created at a junction of two metals dependent on the temperature.[1]

A current will flow based on that voltage difference:

where σ is the conductivity, and ΔS is the difference in Seebeck coefficients.[2]

40

Lithium

MolybdenumTSj

j

j

There is a large difference in S between Li and most other metals and it increases with temperature.[3]

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)

Seebeck Coefficient of LiBlack: our measurements Lithium


Does it work? Yes! SLiDE ResultsAs I showed previously, TEMHD is real and

moves Lithium at significant velocities.

41

Theoretical vs experimental velocities :

[4] M.A. Jaworski, et al. Phys. Rev. Lett. 104, 094503 (2010)


Poloidal limiter

Toroidal limiter

Belt limiter

The Idea: “LiMIT” Design

Left is a cross-section of HT-7 showing the toroidal limiter.

Right is the LiMIT concept: molybdenum tiles with radial trenches containing lithium. The trenches run in the radial (polodial) direction such that they lie primarily perpendicular to the torroidal magnetic field.

42

HT-7 Cross-section:

Plasma primary heat-flux location


Lithium Flow in the Trenches is Self-Pumping

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 when “crossed” by the torroidal magnetic field, will create a radial force on the Li driving it along the slot. This flow will transfer the heat from the strike point to other portions of the torroidal limiter. The bulk of the Mo plate could be actively cooled for a long-pulsed device or passively cooled for something like NSTX. Under the plate the Li flows back naturally.

43

The top surface of the Li is hotter than the surface that is deeper. Therefore there is a VERTICAL temperature gradient Hot

Heat flux

Cooling channelsInlet

Outlet

Passive Li replenishment

Li flow BJ

F


Thermoelectric Driven Flow Calculation

Look just at one channel. All channels are equivalent

The width of Li (w) is 1mm and the width of the Mo (t) trench is 1mm. The depth of Li (h) is 5mm. The length of the Li (L) is 100mm. The magnetic field is 2T. The heat flux on top surface is

The unit for heat flux is MW/m2 and the unit for x is m. (based on the formal HT-7 heat flux data)

44

Heat flux: divertor strike point width, 1cm

tw

h Grad T TE current Flow

L

B

Z direction

This represents the divertor heat flux maximum value for the calculations here, but the concept should work for even higher values..

0.07x0.03 when 6

0.1x0.07 and 0.03x0 when 3/2605.03/400 x


Heat flux on the limiter (along radius direction)

The heat flux figure is from F. Gao et al. Fusion Engineering and Design 83 (2008) 1–5

45

0.00 0.02 0.04 0.06 0.08 0.100

2

4

6

8

Hea

t flux

(M

W/m

2)

Distance along the width of limiter (m)

Top heat flux profile

0.07x0.03 when 6

0.1x0.07 and 0.03x0 when 3/2605.03/400 x

6 MW/m2 on our geometrical segment is 960 W


Heat load on LiMIT: What Velocity is Needed?46

To see if a flowing surface can remove that much power with a lower temperature rise, we can use the following equation [5] :

Here the first term is the heat conduction term and the second is the heat convection which transfers the heat along the trench. ΔT+ is the temperature difference across the depth of Li and ΔTll is the temperature difference between the inlet and outlet of the Li trench. When using the HT-7 heat flux data, and both temperature differences

are assumed to be 200K, the velocity needed is u=35.2cm/s. With this flow the temperature rise would only be 200K.

The assumed heat flux means the 1mm Li slot needs to handle 960W heat load.

To put this in the proper perspective, the temperature rise over the 5 second shot of an uncooled one-half inch thick (1.2cm) Mo plate the same size as our imagined Li/Mo trench/finger can be calculated using as follows: E = 960W*5s = 4800 J. V = (0.2cm)(1.2cm)(10cm) = 2.4 cm3 Cv=2.57 J cm-3 K-1 for Mo. Therefore the temperature rise for a passive plate is 778K.

TVcE v


What Velocity is Generated by TEMHD ? 47

The Seebeck coefficient for Mo is about 13μV/K (at 400C). The value for Li reported in the literature and measured ourselves is 43μV/K (at 400Cfor a difference of 30μV/K

The current and velocity can be calculated with these two equations

[1]

[1]

Here P is the difference of Seebeck coefficient (thermoelectric power) between two material. Ha is the Hartmann number . C is a parameter coming from the conservation of current:

The velocity with a higher thermoelectric power is 59.6 cm/s. This is large enough to carry away the heat and prevent the Li from getting more than 200C hotter than when it came in.

dz

dTP

CuB

C

Cjs

1

1

1

)tanh(

)tanh(**

HaCHa

HaHa

dz

dT

B

Pu

wBHa

Mo

Li

t

w

C


Further Analysis: 3D – FLUENT Calculation

What will really happen to the temperature of the Li when the strike point heat flux hits LiMIT? Is the 200K gradient assumption reasonable? A simple 3D heat transfer model is run with FLUENT.

FLUENT has convection flow and 3D heat transfer included.

Boundary Conditions: The Li flows through the slot between two Mo plate and flow back below the Li slot. The inlet velocity is set to be 60 cm/s. The initial temperature is 470K. The top heat flux is that on HT-7. The bottom temperature is set to be 470K as a constant. Will we remain under 670K as the one-D calculation indicated?

48

Mo plate

6MW/m2 heat fluxLi channel

Back flow of Li


Answer: Yes !!!!

Left figure is the temperature distribution of the top surface of Li slot and right figure is the temperature change along the center line of the top surface.

The temperature difference between the inlet and the outlet is about 200K– great! The highest temperature is 691K (418C). There will be evaporation there – which gives additional cooling not included in the model. Some evaporation (radiative cooling) is acceptable and even desired.

49

0.00 0.02 0.04 0.06 0.08 0.10450

500

550

600

650

700

750

Tem

pera

ture

(K

)

Position along the trench (m)

Temperature along the center line of the top surface


Does the Radial Temp. Gradient Cause Ejection?

One concern about using free surface Li is the ejection problem. The temperature gradient along the Li flowing direction will generate a thermoelectric current along the same direction and the Lorentz force may could eject the Li into the plasma. Fortunately the primary gradient pushes the Li downward.

On the inboard side, the force is upward. Similar to the capillary porous system (CPS) [6] which effectively has very narrow channels, LiMIT’s trench design has very narrow slots (1 mm) to utilize the capillary force to hold the Li in place.

50

Li

Mo

Heat flux

Grad T TE current

Lorentz Force

B

0.00 0.02 0.04 0.06 0.08 0.10450

500

550

600

650

700

750

Tem

pera

ture

(K

)

Position along the trench (m)

Temperature along the center line of the top surface

F

FJ

J


Does it Work? Capillary Force Balance 51

The thermoelectric current parallel to the Li flowing direction is [1]. Here the temperature difference is also assumed to be 200K and the temperature gradient dT/dy=2000K/m. Under these conditions jTEMHD=1.78*105A/m2. So total current along the Li trench is 0.89A. But only about 2cm Li will provide a upward force and the force from the TEMHD is 0.035N.

The capillary force is 2ΣL and Σ=0.3N/m [7] at 600K. So the capillary force is about 0.06N.

Aren’t there radial fields and eddy currents that could make things bad? Sure, but there are also a number of mitigating factors:

The capillary force is not effected by the thermoelectic power, P. It P is actually smaller, the forces will balance. Also, if the trenches were narrower instead of 1mm wide, the force would be higher. A coarse mesh could even be used which would have even higher restorative forces.

The leading edges of the Mo fingers could be Mo-sprayed, greatly increasing the surface area and therefore the capillary force. This would easily hold the Li in too.

Also, the part of the Li with the highest ejection force is also the hottest Li and has likely already evaporated anyway!

dy

dTP

CjTEMHD

1

1


Can Also Work with Porous Material

Utilize porous material infused with liquid metal

- Porous material structure creates TE loops with liquid metal

- Small pores create strong capillary forces at free-surface (similar to CPS)

- Pore size can be engineered to optimize TEMHD effect

Primary temperature gradient is due to incident heat flux

- Pumps liquid metal radially

- Passive replenishment system for a liquid surface !


We will test this: Future Work at Illinois 53

The SLiDE experiment at Illinois is being reconfigured to test this concept. We expect to be able to show radial flows of Li along radial trenches in a Mo plate and measure the flow velocity compared to calculations. An electron beam is used to provide the heat flux while the magnet can generate about 800 Gauss magnetic field parallel to the tray surface. The temperature rise of the Li will also be monitored and compared to theory.

Return channels (lithium hydraulic engineering) will also be tested to find a design compatible with tokamak operation. I have ideas on this for HT-7.

E-beam line source

Li TrayMagnet coil

Heat Stripe for SLiDE’s Electron Beam

B


Conclusions for LiMITA Mo trench structure with flowing Li is proposed as a

potential method to absorb the high heat flux on the torroidal limiter. The thermoelectric effect is utilized to drive the Li flowing along the radius trench direction.

The heat transfer ability is estimated based on the 1-D heat transfer model and simulated in the 3-D domain. Both give positive results showing the ability of flowing Li to mitigate the peak heat flux and to transfer the heat without an unacceptable temperature increase.

The ejection problem is analyzed and should be able to be suppressed by the capillary force.

A simple trench structure system is under construction at UIUC to validate this design. Use on NSTX is being contemplated. It works for divertor tokamaks too.

54


Reference

[1] M.A. Jaworski, Ph. D. thesis, University of Illinois, Urbana, IL (2009)

[2] J.A. Shercliff, Thermoelectric magnetohydrodynamics, J. Fluid Mech. 91, 231 (1979)

[3] P. Ioannides, et al. Journal of Physics E 8, 315 (1975)

[4] M.A. Jaworski, et al. Phys. Rev. Lett. 104, 094503 (2010)

[5] M.A. Jaworski, et al. Journal of Nuclear Materials 390–391, 1055–1058(2009)

[6] V. A. Evtikhin, et al., Fusion Engineering and Design 49–50 (2000) 195–199.

[7] M. A. Abdou,et al. On the exploration of innovative concepts for fusion chamber technology: APEX interim report.282 Technical Report UCLA-ENG-99-206, University of California, Los Angeles, November 1999.

55


Outline 56

Introduction




-Results

-Theory





Conclusions


What is Vapor Shielding?What is Vapor Shielding?

-- High density of eroded material at PFM surface

-- Vapor absorbs a significant fraction of incident plasma energy

-- Reradiates absorbed energy

Should:

-- Reduce PFM surface temperature

-- Therefore reduce amount of material eroded

Effect had not been directly verified experimentally, so we built an experiment at Illinois to investigate the phenomenon

A. Hassanein and I. Konkashbaev. J. Nuc. Mat., 313–316 (2003) 664-669.


Divertor Edge Vapor Shielding Exp. (DEVeX)ESP-gun (2004-2006)

- 500 J Capacitor Bank

- 1017 ≤ ne ≤ 5(10)18 m-3

- Te ~ 50 eV- Edep ~ few Joules

Needed More Edep!

Needed a bigger θ-pinch!

Construction began Fall 2006

Operational Winter 2008Plasma!(with some grounding issues)


Details of DEVeX

Segmented θ-coil

Target Chamber-10-7 mTorr base pressure

Li Magnetron Transmission plates-1” thick Al plates-15 Coaxial Transmission lines

Cu Flux Conserver

Coil ID 10 cm Peak Current 300 kA

Divergence Angle ~ 1˚ Rise Time 3.5 μs

Length 30 cm Pulse Length ~ 300 μs

Translation Distance 22 cm H2 Pressure1-5 mTorr

(static)

Capacitor Bank-fired by spark gap incurring lots of loss-60kV capacitors, but used at less than 30kV. Ultimate stored energy is 250kJ. We only used up to 20kJ.


Lithium MagnetronUsed to deposit thin layers of

lithium onto stainless steel (SS) target

< 100 nm thick

30 min deposition @ 400 mA

Allows the removal of surface contamination from lithium before deposition

30 min -- 2 hours

@ 100 -- 200 mA

Comparison between bare SS and lithium coated target


Time Averaged Density of Incident Plasma

Stark broadened Hβ line

Gaussian fit

Hβ line less sensitive to Te

Machine broadening ~ 0.036 nm

Measured widths range from:

Corresponds to

For 20 - 25 kV discharges


Target Region - Te

Triple probe gives a Te of ~70 eV. This is suspect due to certain aspects of the probe.

Triple probe Te data was an over-estimate and not backed up by target temperature rise.

We estimate more in the range of 10’s of eV for the Te and Ti in our plasma burst.


Target Region - ne

20 kV → 3 X 1021 m-3

450 μs pulse duration

Noise spikes, at beginning, from θ-coil ringing

Triple probe density data is consistent with Stark broadening


Thermal response of target plate

TC bead

Top View


Target Surface Temperature


Outline 66

Introduction




-Results

-Theory





Conclusions


Ion Interaction experiment (IIAX)

Colutron Ion Source ( >1014 ions/cm2-sec) for both Gaseous and Metal Species (H+, D+, He+ and Li+)

67

Target holder with a UHV heater

Li evaporator

QCM

Gas IN

Faraday cup

Plasma cup

ExB filter

LiCl holder


Quartz Crystal Microbalance 68

0 5000 10000 15000 20000 25000 30000

5975900

5976000

5976100

5976200

5976300

5976400400oCFrom X = 1712 to 5117 = -0.00228 Hz/sec

Te

mp

era

ture

( oC)

QC

M F

req

ue

ncy

(H

z)

Time

250

300

350

400

450

500

550

600

650

700

Evaporated/Sputtered particles are detected by QCM.

target

1 1 mY

S m I t

: Sputtering Yield [atoms/ion]: sticking coefficient: fraction reaching to QCM: mass of target material: average current: Mass change on QCM crystal

YStargetm

Imt


Results: Li on C has suppressed evaporation 69

200 300 400 5001014

1015

1016

1017

1018

1019

1020

1021

1022

1023

1024

Li/ATJ D-Li/ATJ Li/SS D-Li/SS bulk Li (ref)

Eva

po

ratio

n F

lux

(#/m

^2/s

)

Surface Temperature (oC)

Evaporation fluxes from intercalated Li are suppressed.

D-saturation also suppresses evaporation fluxes.

Evaporation flux of Li

Vapor pressure of Li


Conclusions: Impact on the Fusion CommunityA flowing lithium divertor could be pumped by the very

heat flux it is supposed to remove.

Temperature of Li could be kept below the point where it significantly evaporates

Vapor shielding suggests thin lithium films absorbs large fraction of energy deposited during an ELM or disruption

-- Could “save” the PFCs from large temperatures that would otherwise be encountered possibly damaging the underlaying structure behind the lithium PFC.

Sputtering of Li is suppressed by impurities and by absorbed D.

The future of Lithium is Bright!

lithium at illinois, hefei, china august 20, 2010 nuclear, plasma, and radiological engineering...

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