the case for a liquid lithium-surface divertor · 2017. 6. 20. · components of fusion energy...
TRANSCRIPT
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Nuclear, Plasma, and Radiological Engineering
Center for Plasma-Material Interactions
Contact: [email protected]
The Case for a Liquid Lithium-Surface Divertor
David N. Ruzic, Jean Paul Allain,
Davide Curreli and Daniel Andruczyk
June 20, 2017
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1. Description of the Technology
Flowing molten lithium surfaces are to be used as the plasma-facing components of fusion energy devices
Most likely this would be at the divertor plate, but could also be considered for the first wall.
There are a number of technologies which could be used to do this. They include the
2
I. Lyublinski, et. al., JNM 463 (2015) 1156
J.S. Hu et al Nucl. Fusion
56 (2016) 046011
LiMIT and combinations/variations.CPS, FLiLi,
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Liquid Metal Infused Trenches (LiMIT) 3
Seebeck Effect creates thermoelectric current at junction between liquid lithium and solid trenches when a thermal gradient is present
A transverse magnetic field is applied, which generates a JxB force, propelling the liquid through the trenches
D. N. Ruzic, W. Xu, D. Andruczyk and M. A Jaworski,
“Lithium-Metal Infused Trenches (LiMIT) for Heat Removal
in Fusion Devices”, Nuc. Fusion, 52 (2011) 102002
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Horizontal Flow
Utilizes TEMHD drive for propulsion of liquid lithium through series of trenches
Vertical Flow Sustained flow demonstrated at arbitrary angle
from horizontal (0 to 180 degrees)
Flow can be in any direction. Use what the plasma gives you 4
Lithium wells up the back and flows
out the top
This side is the top
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2. Application of the Technology
A molten surface is self healing. In the event of ELMs, Disruptions, or unforeseen events, the liquid metal can be reintroduced, and the surface contours restored. This is not the case for W or other solids
Lithium is the lowest Z possible, and tolerance is very high
Low-recycling regimes lead to better economics
5
Damage on tungsten PFCs (Photo: Egbert Wessel, Julich
Research Centre)
S. Krasheninnikov, et. al, Physcis
of Plasmas 10 (2003) 1678
Max Planck, IPP Garching,
http://www.ipp.mpg.de/16535/einfuehrung
a) Increase and control of T
at wall
b) Confinement enhancement
c) Full volume used for power
generation
d) Plasma more stable
e) Beta of 20% due to
flattened current density
profile
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No cold hydrogen returns from wall: Plasma stays hot
Courtesy: L. Zakharov PPPL
What Very-Low Recycling Does for Fusion
Standard Case
Lithium Case – Radius Needed is 1/3 so Volume (and therefore cost) of
Fusion Power is Reduced by a Factor of ~27
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3. Expected Performance of the Technology – Critical Variablesa) Trenches should be barely overfilled and present a clean, fresh, conformal surface to the plasma
b) Hydrogenic species once absorbed need to be removed form the molten and returned to the fusion device
c) Lifetime of components must be high and the system deemed safe
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Magnum PSI
Shot Parameters:
• Argon(5.8 Pam3s-1)
• 200 A Source Current
• Mag Field Setting 3
• Target Tilt 750
• Z Position -260 mm
• B = 0.0882 T
• Q = .223 MW/m2
• Shot Number 65
To Neutral Beam
To Divertor
From
Divertor
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d) Flow rate should be high enough to keep the surface from too rapid of evaporation. 8
ΔT ~ 220C inside lithium after
10 seconds of plasma shot
Li surface temperature
increase of ~220C after 10
seconds of discharge
Higher T at the edge
where the flow slows
down
Conduction not convection is
dominant heat transfer mode. With
thin enough system, and redesigned
trench geometry, temperature limits
can be met at 10MW/m2 heat flux
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4. Design Variables – How to Optimize 9
a) Geometry of the structure containing the flow
The physics required for TEMHD-driven flow can be modeled with the following system of equations in COMSOL Multiphysics
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Can even design to eliminate droplet emission
Experiments determined which size trenches were unstable to being hit by ELMS and why droplets are sometimes expelled
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P. Fiflis, M. Christenson,
M. Szott, K.
Kalathiparambil, D.N.
Ruzic, “Free surface
stability of liquid metal
plasma facing
components”, Nuclear
Fusion, August 2016
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b) Texturing of surfaces can aid or prevent wetting
Using a femtosecond laser creates a nano-texture which prevents wetting. Mirror-polishing the surface makes wetting much easier.
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0
20
40
60
80
100
120
140
160
200 230 260 290 320 350 380 410 440 470
Conta
ct
angle
(°)
Substrate temperature (°)
Phase II results on textured SS Phase I results on textured SS
Phase I results on untextured SSCoherent Femtosecond Laser
200 250 300 3500
20
40
60
80
100
120
Co
nta
ct
an
gle
(d
eg
)
Surface temperature (°C)
161
77.67
roughness (nm)
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c) Alter the liquid metal itself: Sn-Li Eutectic? 12
Sn-Li has been shown to have Li surface segregation of a few atomic layers. Could one get some of the good PFC properties of Li yet have the much lower vapor pressure of Sn?
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d) Design of the delivery system 13
Develop new distribution system
(fully integrated distributor shown
here) to minimize potential for
lithium leakage
Evaporation
Wetting
Heater system tests
for the EAST design
Getting the lithium into the device, how to heat it once it is in the device, and how to engineer each of the sub-systems are all key design variables which can dramatically effect the outcome
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5. Risks and Uncertainties
a) Lithium Absorbs All the Tritium in the World There is a finite supply, and the inventory of a zero-recycling machine could be so large that there would not be enough for even one reactor
b) Lithium is Too Corrosive and Difficult to HandleLithium reacts violently with water, it leaches alloying materials out of steel embrittling it, and it has such a high surface tension that is creeps everywhere
c) Lithium gets Too Hot and Cannot Handle the Heat FluxLithium has an extremely high vapor pressure at 400-450C, and that is too low a temperature limit for a surface which has to tolerate 10-20 MW/m2
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What could possibly go wrong ?
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6. Current Maturity in TRL Levels 15
TRL-3 is completed – independent systems of various types have been shown to work in a variety of locations across the world
TRL-4 has been done in some manners, but the independent parts need to be integrated together with heat and hydrogen removal
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7. Required Development for this Technology 16
a) Determine how quickly D (as a substitute for T) is absorbed by Li and how quickly it can be separated.
Pbase = 1 – 2 x 10-7
Torr
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In-Situ Environmental XPS, RGA and SEM Capabilities 17
TDS spectra tracked a number of species during the heating and cooling processes with and without a hydrogen background
o All samples loaded using the same procedure and heated at a ramp rate of 0.5 K s-1
Magnitudes of the secondary features for the M/q = 2 trend increased substantially in the presence of a hydrogen background. These same features also shifted in temperature
The key temperature value denoted by the primary change in inflection increased by 90 oC more energy required to overcome dissociation activation energy for appreciable hydrogen release
Studies have been done to look at how the
presence of oxygen and water affect the
evolution kinetics of hydrogen from LiH, but
little has been done to look at these same
kinetics in the presence of a hydrogen gas
environment
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Full Lithium Loop in a Tesla-level Steady State Toroidal Plasma
With EAST fields of 1.9-2.5 T near the edge (or
similar fields provided by permanent magnets),
the LiMIT module will require an EM Pump
driving current of 10’s of Amps, depending on
final design parameters – easily feasible
Induction heating is
the preferred heating
method since it is
faster than resistive
heating and takes
advantage of the
metallic properties of
the effluent Li stream
This is the
Key
Experiment
which is
Needed.
Can you
really take
out and put
back the D
fast
enough?
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Hybrid Illinois Device for Research and Applications (HIDRA)Stellarator as well as tokamak capabilities
o Vessel splits in half, easier access to install larger components
Magnet configuration
o 40 toroidal coils
o 4 helical coils
o 2 vertical field coils
o 84 ports, 6 sizes accessible
Stellarator operations in Greifswald as WEGA
o R0 = 0.72 m
o r = 0.19 m
o B0 = 0.087 – 0.5 T
o fgyr = 28 GHz, Pgyr = 10 kW cw, 40 kW pulsed
o fmag = 2.54 GHz, Pmag = 6 kW + 20 kW
o ne < 1×1018 m-3
o Te = 5 – 25 eV
o tpulse < 60 min
o Γ = 1×1022 m-2s-1
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b) Devise techniques to safely handle and utilize Li and Sn-Li
Assessment of wetting and lithium transport can be done in laboratory experiments
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A Technology Program is Essential !Stainless steel
2 SS samples
loaded in the
chamber
Ʌripples = 780nm
3.5 cm
1 c
m
Heated stage
Lithium injector
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Sn-Li offers advantages which can be evaluated
Materials Characterization Test Stand (MCATS) includes improved sample stage and liquid metal injector
Manual screw-type injector allows for fine control of droplet deposition
Sample stage allows for smooth horizontal and vertical linear motion, as well as tilting for dynamic wetting angle testing
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LEIS spectra indicating segregation
(80% Sn – 20% Li system)[3].
R. Bastasz, FED 72 (2004) 111-119.
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Sn-Li wetting test show that more research is needed
Tested Sn-Li wetting on stainless steel
Up to temperatures of 400°C, Sn-Li is highly non-wetting
In order to get Sn-Li to wet, we will need to investigate surface treatments, like plasma cleaning, polishing, or evaporative coatings
New results on polished W showed no wetting up to 435°C
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Wetting below 90°
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c) Create and test a flowing lithium system for high heat fluxes
Experiments on EAST are crucial. Results from a World-Class tokamak will show that the technology is possible and that the results push the overall performance in the right directions.
Initial tests in HIDRA will prove the technology and robustness
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EMC-Eirene modeling of HIDRA
plasmas, and equilibriums which
allow full-size placement of EAST
components in the HIDRA edge-
particle-flux.
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DEVeX materials testing device
Coaxial plasma gun with theta-
pinch creates ELM-like heat
loads, depositing around 0.15
MJ m-2 over about 150 𝝁s.
Used for materials testing.
Jung, S et al. Fusion Engineering and Design 89.12
(2014)
Fiflis, P., et al. Nuclear Fusion 56.10 (2016) Christenson, M., Master’s Thesis, UIUC (2015)
Future work will utilize upgraded hardware
to inject plasma as a compact toroid,
increasing the target heat flux
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3-D Modelling of full lithium flows including MHD effects
Gaussian heat flux centered on shallow flow region
Maximum velocity increased to ~60 cm/s
o Goal of 70 cm/s to prevent lithium evaporation
The non-z temperature gradients contribute less to overall flow conditions
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Proof-of-Concept for high-heat flux, “fast” lithium flow across target
Impurity motion tracked to infer velocity
Good agreement between experimental and modeled velocities
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Measured velocity (cm-s-1) Velocity from simulation results
(cm-s-1) at same locationsDeep part of
trenches
Turbulent region Deep part of
trenches
Turbulent region
1.4 4.2 1 - 5 3 - 10
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HIDRA can also be operated as a tokamak to boost heat flux
Toroidal magnetic field B0 = 1.43 T
−Plasma characteristics without RF heating:
Plasma current IP = 45 – 60 kA
Ohmic heating power Pohm = 100 – 130 kW
Peak electron density ne = 1.6×1019 m-3
Peak electron temperature Te = 600 – 900 eV
Peak ion temperature Ti = 150 – 250 eV
Impurity level Zeff = 3 – 6
Energy confinement time τE = 3 – 5 ms
Pulse Duration 50 ms
Tokamak operation from 1975 –1982 used for studying RF and Lower Hybrid heating scenarios
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Conclusions 28
Liquid metals offer a superior alternate approach to the most demanding PFC challenges of fusion. Not only could they withstand high and off-normal heat fluxes without permanent damage, they offer entire new regimes of fusion device operation.
The low-recycling wall, if it can be made to work, could reduce the size and cost of fusion energy devices ten-fold.
To realize this awesome potential, further innovative technology development is required. The US is the present leader in this field. It is imperative to the future of fusion energy that such programs continue and grow. Liquid-lithium surfaces are an innovation which could fulfill fusion’s promise.