Liquid metal vapour shielding in linear plasma devices

T.W. Morgan1, G.G. van Eden1, P. Rindt2, V. Kvon1, D. U. B. Aussems1, M. A. van den Berg1, K. Bystrov1,N.J. Lopes Cardozo2 and M. C. M. van de Sanden1
1Dutch Institute for Fundamental Energy Research- DIFFER, Eindhoven, The Netherlands

2Eindhoven University of Technology, The Netherlands
LM VS in linear plasma devices

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Going from ITER to DEMO involves large
jumps in several parameters
Property

ITERDEMO1Pulse length~400 s~7200 s

Duty cycle<2%60-70%

Neutron load

0.05 dpa/yr

1-9 dpa/yr

Exhaust power

150 MW

500 MW

Divertor area

~4 m2

~6 m2

Radiated power

80%

97%

Resilience to neutrons and power
excursions becomes more important

Courtesy G. Matthews
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Limiting factors for W in DEMOHigh heat/particles

Thermal shock/fatigue Big ELMs/VDEs/disruptions

ErosionFor 5 mm W
lifetime ~2 years1Cracking (smallELM-like loading)2
Progressive
deterioration3

Melting- irreversible
damageRunaway failure?

1Maissonier NF 2007
2Linke NF 2011
3Loewenhoff FED 2012

LM VS in linear plasma devices

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Limiting factors for W in DEMO
Neutrons

High heat/particles

Thermal shock/fatigue Big ELMs/VDEs/disruptions

ErosionFor 5 mm W
lifetime ~2 years1

Cracking (smallELM-like loading)2
Progressive
deterioration3

1Maissonier NF 2007
2Linke NF 2011
3Loewenhoff FED 2012

LM VS in linear plasma devices

Melting- irreversible
damageRunaway failure?

Transmutation, H+He creation, defects
Smaller operational temperature window
Increased brittleness
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Capillary porous structures (CPSs) create conduction basedstabilized PFCs to contain liquid metals
Replace solid surface with liquid
MHD forces (jxB) destabilize liquids intokamaks (droplets)
Use surface tension/capillary refilling
Replace top region with this combinedmaterial

Evtikhin JNM 1999
Thin CPS layer

W monoblock

Capillary supplyto surface

Coolant pipe
Coolant

LM reservoirLM VS in linear plasma devices

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Benefits of liquid metals for DEMONeutrons
Sputtering

Self replenishment
Higher heat fluxes

Thermal shock/fatigue Big ELMs/VDEs/disruptions

No cracking
Lowered stresses
substrate
ELMs possible(?)

Already molten
Vapour protection

Only influences substrate
Separation of PSI from neutron issue
LM VS in linear plasma devices

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Material options of Li, Sn both have strengths and weaknesses
Choices once cost, availability, activation etc. taken into account

LithiumTin

Low Z

Higher Z

High vapour pressure

Lower vapour pressure

High T retention

Lower T retention

Wesson, Tokamaks (2004)
LM VS in linear plasma devices

Allain and Taylor PoP (2012)
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Linear devices have good flexibility and diagnostic access:good place to investigate vapour shielding
Skimmer plates (differentially pumped chambers)

Plasma beam

Water cooled
vacuum vessel

Rotatable/tiltabletarget holder
Plasma source

Diagnostic ports through coil
LM VS in linear plasma devices

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Superconducting magnetic field coil
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Magnum-PSI/Pilot-PSI good utility for LM study
due to DEMO relevant heat/particle loading

ITER/DEMO divertor strikepoint conditions (detached)
LM VS in linear plasma devices

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van Eck FED (2019)9/28

Vapour shielding: additional loss channels for heat flux
(impurity stimulated “detachment”)

LM VS in linear plasma devices

Solid metal:𝑞𝑝𝑙𝑎𝑠𝑚𝑎 = 𝑞𝑐𝑜𝑛𝑑

Liquid metal:𝑞𝑝𝑙𝑎𝑠𝑚𝑎 = 𝑞𝑐𝑜𝑛𝑑 +𝑞𝑒𝑣𝑎𝑝 + 𝑞𝑟𝑎𝑑 + 𝑞𝑚𝑎𝑠𝑠

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Experiment compared performance of Sn CPS withsolid Mo reference targets

Ion species

Te(eV)

ne(1020 m-3)

qref(MW m-2)

H or He

0.4-3.1

0.6-7.0

0.47-22

LM VS in linear plasma devices

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n.b. Deliberately poorly cooled toreach VS temperature regime

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Vapour interaction with plasma decouples input powerfrom surface temperature
Poorly cooled Sn samples exposed to power load series in pilot-PSI

Tsurf at target centre

van Eden PRL 2016
LM VS in linear plasma devices

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Vapour interaction with plasma decouples input powerfrom surface temperature
Poorly cooled Sn samples exposed to power load series in pilot-PSI

Tsurf at target centre

van Eden PRL 2016
LM VS in linear plasma devices

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Vapour interaction with plasma decouples input powerfrom surface temperature
Poorly cooled Sn samples exposed to power load series in pilot-PSI

Tsurf at target centre

Temperaturelocked throughshotTemperature rise
cut off

van Eden PRL 2016
LM VS in linear plasma devices

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Temperature locking when vapour pressure and plasmapressure ~ matches
Equilibrium Tsurf at target centre

Plasma pressure vs vapour pressure

van Eden PRL 2016
LM VS in linear plasma devices

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Overall reduction in power to cooling water of ~one third

Evaporation alone cannot explain energy
loss: relatively high re-deposition ratemeans most εevap returns to surface

van Eden PRL 2016
LM VS in linear plasma devices

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Strong recombination occurs due to lowered Te
Te Sn shots vs Mo shots

Atomic/Molecular processes

van Eden PRL 2016
LM VS in linear plasma devices

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Li targets with a reservoir were used to permit long-timescale tests

Component similar to and designed to test design for NSTX-URindt FED 2016
LM VS in linear plasma devices

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Helium plasma loading to determine power handling
limit in VS conditions

ParameterValueB

1.2 Tqref

8-10 MW m-2

Te~4 eV

ne~4×1020 m-3

Rindt NF (2019)
LM VS in linear plasma devices

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Similar vapour shielding effect observed for Li as for Sn
High heat load can be sustained(9 MW m-2 peak heat load)
Temperature cut off

Sn vapour limit is ~1700 °C
oscillations

Prediction for Li is therefore ~700-900 °CHeat flux (MW m-2)

Prediction well matched by observation

Rindt NF (2019)
LM VS in linear plasma devices

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Analytical description of VS mechanism

Heat conducted through target.

Power dissipated by lost LM.

𝑸𝒑𝒍𝒂𝒔𝒎𝒂 = 𝐐𝐜𝐨𝐧𝐝 (𝑇𝑠𝑢𝑟𝑓 ) + (𝟏 − 𝑹)𝚪𝐞𝐯𝐚𝐩 𝑇𝑠𝑢𝑟𝑓 ∙ (𝛜co𝐨𝐥 + 𝛜𝒆𝒗𝒂𝒑 )𝑅=?

𝛜𝐜𝐨𝐨𝐥 =?

Net lithium loss rate.
Limited by available supply

Rindt NF (2018)
LM VS in linear plasma devices

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Lithium energy dissipation through ionization and radiation canbe estimated as ~ 5 eV
Based on lifetime of Li samples
R~0.9 is measured

Dissipatedenergyperparticle

Taking into account energy
dissipation via ionization andradiation from non-promptly
redeposited Li we get ϵcool ~5 eV
for Magnum-PSI experiments

𝝐𝒄𝒐𝒐𝒍 [𝒆𝑽]𝑻𝒆 [𝒆𝑽]

Goldston NME (2017)
LM VS in linear plasma devices

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A temperature plateau occurs when dissipation
via Li becomes dominant.
Tungsten substratemelting, 3422 oC

Li dissipated regime~800 oC

surfacetemperature

conductiveregime~10 MW/m2

deposited power

LM VS in linear plasma devices

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FEM shows good agreement with experiment
Temperature plateau when
Li dissipation becomessignificant.

Temperature[oC]bare molybdenum

lithium

Time[s]Rindt NF (2018)
LM VS in linear plasma devices

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Benefit for DEMO of vapour shieldingoDecouples incoming heat load from surface
temperature and reduces cooling requirementsoMaximum impurity influx ~fixedoFor Sn adds protection for off-normal events:
adds robustness and is more forgivingoFor Li constant operation could be possible

LM VS in linear plasma devices

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1026
1024

Wide evaporation range

Narrow temperature range

Evaporation flux (m-2 s-1)

Vapour shielding leads to stable power reduction totarget over wide Pinput due to natural negative feedback

1022
1020
1018
1016

0200

400600Operationpoint800

1000 1200

Temperature (°C)

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Steady state operation with a Li divertor in DEMO canhandle very large heat loads
Want to determine potential of Li VS for DEMO
Assume a target with same thermal conductance (C)as ITER monoblocks
For steady state detached conditions tolerable
power density dominated by conduction
For e.g. slow transients ϵcool is larger and can inprinciple handle very large power densities

Rindt NF (2019)
LM VS in linear plasma devices

Requires strong baffling however (vapour box*) tolimit core influx to tolerable levels *Goldston Phys. Scr. (2017)
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An LM-coated surface is also very resilient to transient events

Survival without substrate damage is defined only
by loss of all LM surface layer

LM VS in linear plasma devices

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An LM-coated surface is also very resilient to transient events

Survival without substrate damage is defined only
by loss of all LM surface layer
Divertor can withstand ELMs, VDEs, disruptions
Evaporation temporarily above tolerable load but
should be seen as damage mitigation during loss ofcontrol events and can tolerate small ELMs

LM VS in linear plasma devices

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Main challenge is to engineer closed divertor and ensure efficient andeffective Li extraction

LM VS in linear plasma devices

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Conclusions
plasmaheat

Strong interactions between impurities and plasma
leads to strong cooling and energy loss during highevaporation: vapour shielding
Leads to a natural negative feedback withtemperature locking at pressure balance point
Extrapolation to DEMO indicates high heat loads canbe tolerated with Li if a vapour box divertor can be
successfully engineered

Vapour cushion Liquid metal
Solid wall

LM VS in linear plasma devices

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