Magnetically-guided liquid
metal divertor (MAGLIMD)
with resilience to disruption
and ELMs
Michiya Shimada
michiya.shimada@gmail.com
10th Anniversary QUEST Symposium
20 July 2018

Table of contents
1. Motivation
2. Power and particle control
3. Start-up and shutdown
4. ELMs
5. Disruption
6. Summary

20 July 2018 QUEST Symposium2

Background-1

Coreplasma

First
wall

Bt
Ipdivertor

Goals of fusion research:• Power-generating core plasma• High density (high Ip)• Good confinement (high Ip)• High purity (low impurity density)• Stability• Hard to avoid disruption and ELMs
• Mitigation of consequences
Plasma- • Steady state (current drive (CD))
facing• CD poses limits in Ip and density
compo• Particle control is a keynents• PFC compatible with the core(PFC)• Limits in impurity ingression• Controlled surface temperature• Radiative cooling (impurity)• No melting or evaporation• Forgiving of transient heat load
• Continuous wall conditioning• Long life
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Function of divertor

The divertor "diverts" the power and particles coming out fromthe core plasma to a volume separated from the core. The
divertor handles significant portion of the power and particles.

inCore plasmaout⇒divertorfusion

fuel (DT)ash (helium)

fuel (DT)impurities
auxiliary
heating

impurities
heat
20 July 2018 QUEST Symposium

4Background-2• Power handling is a major issue in a fusion reactor• Much more serious in DEMO than in ITER because:• x 3-6 more power but a similar device size
• Concerns on tungsten:• DBTT~400°C becomes higher with neutron irradiation and hydrogen
implantation (cracking?)• Disruption control, particularly runaway electron suppression, is a crucial andunresolved issue.
• Under the heat load of unmitigated disruption and ELMs, tungsten targets
would melt and the rough surface after resolidification would deteriorate heat
handling capability.
• Disruption prediction requiring learning process is difficult to implement, since
failure of disruption prediction during the learning process would lead to
unacceptable consequences; furthermore, ingress of first-wall debris isdifficult to predict.
• Strongly mobilized Liquid metal divertor could provide a solution to some or
all of the issues above
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A tokamak with MAGLIMD(Magnetically-guided liquid metal divertor)

symmetry axis

poloidalcross-section

toroidal direction

coreplasma
Bt
Ipliquid
metal

Inlet
(inboard)

outlet(outboard)

outletInlet(inboard) (outboard)

outletInlet(inboard) (outboard)Bird’s eye view of MAGLIMD and flow pattern

20 July 2018 QUEST Symposium

7Magnetically-guided liquid metal divertor (MAGLIMD)
Ip
Bt

LM flow near surface

LM flow //B

LM flow //B

inlet

The inside walls ofthe LM container andthe ducts are electricallyinsulated to reduce the MHD
drag. The flow rate makes acontrol parameter.

outlet20 July 2018 QUEST Symposium

EMPConditioner 8

EMP: electromagnetic pump

Toroidal uniformity

liquid metal inlet/outlet tubes

In a fusion reactor, where the divertor configuration isfixed and the field line in the LM divertor forms a grazing
angle to the surface, inlet/outlet openings can be arranged
in such away that there appears no toroidal gap on the LM
surface despite the openings installed only discretely in thetoroidal direction.
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LM flow rate required to remove heat
To remove power P(W) e.g. with liquid tin with mass density ρ(kg/m3),
specific heat C(J/kg/deg), flux f(m3/s), temperature of supplied tin
Tin(degree C), temperature of exhaust tin Tout(degree C),
P = ρCf (Tout - Tin)
We estimate the LM flux required to remove heat:𝑃f = 𝜌𝐶(𝑇 − 𝑇 )𝑜𝑢𝑡𝑖𝑛e.g. With P = 400 MW, ρ = 7 x 103 kg/m3, C = 228.4 J/kg/deg,
Tout = 400 °C, Tin = 300 °C: f = 2.5 m3/sWith an effective surface area of ~10 m2 (50 m(toroidal) x 0.2m(poloidal)), and the pitch of the field line θ of 0.05, the parallel flow
velocity v// ~5 m/s.

The power Pdrive required to drive the LM flow f against the gravitation
force is given by:
Pdrive =ρghf ~ 2 MW
for g = 9.8 m/s2 (gravitation) and h = 10 m (height of the divertor LM
surface measured from the EMP). This power is negligible compared withthe power the LM divertor will handle.
20 July 2018 QUEST Symposium

10With all the insulated walls contacting LM, theremaining MHD drag stems from the jtoroidal x Bpforce (jtoroidal is driven by the toroidal component ofthe v x B EM force)
The work done by the MHD drag W candragbe estimated like:Wdrag = jtoroidal x Bp 2δ= σθvꞱB x θB 2δ = σvꞱθ2B2 2δ

Bt
δv//

∆vp

θ =Bp/B, if we assume vꞱ~ v// θ,
Wdrag = σ v// θ3B2 2δ

δv//

The work done by the centrifugal
force Wcf can be made stronger
than Wdrag
Wcf = ρ v// 2/R Δ3

7x10 x5x0.2/8.5Wcf
ρv//Δ/R
Wdrag = σθ3B2 2δ ~ 2x106 x0.053 x62 x2x0.02 ~ 2

Particle control and wall conditioning
400

Intensity (count)

50A

70A

2000

90A

300
Temperature

Bias = -100V

200

100
50

050

250

150

D
Ga-I
In-I
Sn-I

0

350

100

150

Temperature (oC)D-recycling from GaInSn

0200

Time (s)Hirooka, Fusion Eng. Design 117 (2017) 140

hydrogen removable with a flow rate of 2.5 m3/s
0.47ｘ10-4 H/Sn x 2.5 m3/s x 7 x 103 kg/m3 /(0.119 kg/mol) x 6 x1023/mol /(5.3 x 1020/Pam3)= 7.8 x 103 Pam3/s（Particle exhaust rate in a reactor:100-200 Pam3/s)

In JET-ILW experiments, wall conditioning was done every 200 shots（GDC） (Douai (2013))⇒Steady state operation requires continuous wall conditioning ⇒MAGLIMD
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Start-up and shutdown (1)Limiter configuration

j Bp > ρg, j ~ σE
ρg
𝑬>
σBp

Ip
jxB
ILM

At the discharge start-up with limiter configuration,
LM would be ejected if j x B force exceeds thegravitation.

𝟕×𝟏𝟎𝟑 ·𝟗.𝟖
𝑬>= 0.17 V/m
𝟔𝟐×𝟏𝟎 ·0.2

Bp

Bp ~ 0.2 T is assumed. At the discharge start-up,
the minimum toroidal electric field is estimated tobe ~0.3 V/m. At the start-up with limiter
configuration, the divertor might have to be emptyof LM. LM should be supplied into the divertor
after the toroidal E field decreases below this level.
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Start-up and shutdown (2)divertor configuration

At the discharge start-up with divertor
configuration, current will be induced in LM in thesame direction as the plasma current, but LM
would not be ejected toward the core.
The induced LM current and the poloidal fieldfrom it would not disturb plasma operation.

Ip

𝑰𝑳𝑴 ~

Bp

Bp
ILMσ𝑽𝒍· 𝐴𝐿𝑀
𝟐π𝑹
𝟐×𝟏𝟎𝟔 ·𝟏
𝑰𝑳𝑴 ~· 0.1 ~ 4 kA
𝟐π·𝟖.𝟓Note that a typical poloidal coil current of areactor is 10 MA-turn. One-turn loop voltage of 1
V and a poloidal cross section of the LM tray of
0.1 m2 (e.g. 0.5 m wide and 0.2 m deep) are
assumed.
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Start-up and shutdown (3)Limiter configuration

At the discharge shutdown withlimiter configuration, current isinduced in the LM in the direction
opposite to the plasma current.
LM will not be ejected.

Ip
Bp×ILMjxB
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Start-up and shutdown (4)divertor configuration

Ip
Bp

Bp×

ILM

At the discharge shutdown withdivertor configuration, current
will be induced in LM in thedirection opposite to theplasma current. LM would notbe ejected toward the core if
the loop voltage does notexceed the following value.
𝑽𝒍j Bp < ρg, j ~ σE ~ σ
𝟐π𝑹
𝟐π𝑹ρg
𝑽𝒍 <σBp𝑽𝒍 <𝟐π·𝟖.𝟓 ·𝟕×𝟏𝟎𝟑 ·𝟗.𝟖
𝟐×𝟏𝟎𝟔 ·0.2

20 July 2018 QUEST Symposium= 9.2 V16

Edge localized mode (ELM)ASDEX

ELM can be triggered at the edge pedestal region, with a steep gradient of density,
temperature and pressure, releasing a large amount of energy, which causes a largetransient heat load on the divertor
Wesson Tokamaks 17
20 July 2018 QUEST SymposiumElectrically separating the inner and outer channels
could make MAGLIMD resilient to ELMs
The original scheme might bevulnerable to splashes due to jx B force associated with ELMs
j ×Bjsol(ELM)Bt
Inlet(inboard)

outlet(outboard)

inlet

outletoutletinlet

Rayleigh-Taylor and Kelvin-Helmholtz instabilities
D = -ρg + (j × B)·n - ɣk2 + ppk
D>0: unstable, D<0: stable. The first term: gravitation force. The second: j x B force(vertical component). The third: surface tension. The last: KH driving term. pp is plasmapressure at the sheath. k is the wavenumber perpendicular to B (most unstable).

Li is stable for RT instabilities at ~103/m, requiring capillary pore structure(CPS)with sub-mm mesh. CPS makes convective transport extremely difficult.

High mass density of Sn makes it much more stable. Separation of the twodivertor channels and ELM mitigation make Sn surface stable.
20 July 2018 QUEST Symposium

19Prompt Redeposition
The prompt redeposition of W has aparticularly large effect in ITER ELMs
because of the high plasma density (>1 x1021 m-3) and high electron temperature(>100 eV) near the divertor targets(Chankin (2014))
The formula for the fraction of nonredeposition (Dux (2011)):𝑝2
𝑓𝑛𝑜𝑛−𝑟𝑒𝑑𝑒𝑝 =1+𝑝2

𝑝 = 𝜏𝑖𝑜𝑛 𝜔𝑔𝑦𝑟𝑜 = λ𝑖𝑜𝑛 /𝜌𝑊𝑚𝑎𝑥 +At the ELM condition, p <<1, fnon-redep ~ p2 <<1. The electric field in the magnetic presheath (MPS) prevents the W ions from entering the main plasma beyond MPS (Chankin(2014)).
For the case of Sn at the ELM condition:1 x 1021 m-3 and 100 eV, p ~ 0.01:
Almost complete prompt redeposition of tin, similar to W, is expected.
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Disruption (toroidal current at CQ)
divertor configuration

Ip
Bp
jLM

Bp
jLM

At the current quench (CQ) of adisruption
withdivertorconfiguration, current will be
induced in LM in the same
direction as the plasma current.
LM would not be ejected
toward the core but the coreplasma would be attracted
toward the divertor (benignVDE), which will eventually
result in limiter configuration(next slide).

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Disruption (toroidal current at CQ)

Ip

Bp

VDE will eventually lead tolimiter configuration, then the jx B force due to the toroidal
current induced in LM and thepoloidal field will eject the LM
into the core (automatic
disruption mitigation). The j x Bforce during the current quench
would be much stronger than
the gravitation force.

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Disruption (VDE)

Ip

Since the vertical speed of
VDE is slow (~0.5 s), thelevel of LM can beheightened so that the
top of the dome will beprotected.
The level of the liquid metal
surface can be increased at a
rate of:dh/dt ~ f/(2πRw) ~ 2.5/(2π·8.5·0.5) ~ 0.1 m/s
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Disruption (halo current)
The j x B force due to halo current and the toroidal
field does not eject the LM toward the core.

Ip
Bt

Ip
Bt I
halojhalojhalojhalojxB
20 July 2018 QUEST Symposium

24automatic disruption mitigator
A toroidally continuous tube, installed at thelower midplane with its top open, is filled withLM. At the current quench (CQ) of a disruption,current will be induced in LM in the directionof the plasma current.

Ip

Bp
jLM

A tube of 1cm (w) x 1cm (h), 50 m long,will hold liquid tin of 35 kg, to be ejectedat ~5m/s, sufficient to quench runawayelectron.
20 July 2018 QUEST Symposium

The resulting j x
B force willeject the LM
toward the core,providingautomatic
disruption
mitigation.

25

Experiments in QUEST (RIAM, Kyushu Univ., baking
temperature up to 500 °C) are being discussed• A tray for LM installed on the bottom part of VV• Inlet and outlet tubes from/to LM conditioner• A liquid tin flow rate of 2.5 litre/s will suffice forthe heat exhaust of 400 kW• The particle exhaust rate up to 8 Pam3/s• Hydrogen solubility is lower with low
temperature, enabling recovery of hydrogenwith cooling of LM• Steady state particle exhaust would be essential
for long pulse operation, which is a goal ofQUEST

Liquid metalconditioner
M. Shimada and K. Hanada “Conceptual Design of Manetically-Guided Liquid Metal
26
Divertor on QUEST” Proc. Plasma Conference (2017) 22P-101.

In the long pulse operation of QUEST, wall
saturation increases particle recycling anddischarge characteristics, which are expected
to improve with active pumping

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Preliminary experiment in QUEST (under discussion)

A LM container will be installed in the QUEST VV. The flow of LM pumped by EMP
will be diagnosed and compared with CFD calculation. The behaviour of LM with CT
injection will also be investigated.

Proof-of-Principle experiment (1)Top viewRQS

PElectricalconnection

An acrylic casing is installed the QUEST vacuum
vessel. A combined magnetic field (toroidal andvertical) is applied. Injection of liquid metal(Galinstan) from P and exhausted from S createsa flow pattern as illustrated. Cross-field flowfrom Q to R would suffer from MHD drag, whichis compensated by a step on the liquid metal
surface. Measurement of velocity and step iscompared with CFD calculation.
Cross-sectional view PQQBP
Elevation viewQRSP

Cross-sectional view RSRSBProof-of-Principle experiment (2)LM volume = 0.1m(w) x 0.2m(l) x 0.05m(h) = 1 litre
Replacement in e.g. 10 s → 0.1 litre/s or 6 litre/min.
For Inlet/outlet tube cross section 4cm2,v(LM velocity) = 25 cm/s
1
1
dynamic pressure = ρv2 ~ · 6.4×103· 0.252 ~ 200 Pa
22

MHD drag = vBθ·σ·Bθ·δ =0.25·0.5·0.1·3×106·0.5·0.1·0.05=94 Pa → 1.4 mm step on LM surface (e.g. larger θ?)
Elevation =0.5 m → static pressure: ρgh = 3.1×104 Pa
Total pressure ~ 3.1×104 Pa

20 July 2018 QUEST Symposium

30Electromagnetic pump on loan to
RIAM from NIFS

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The parameters of PoP exp. are within the capabilityof the electromagnetic pump

Operation point

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32Summary
1. An innovative concept of divertor power and particle control
is proposed and discussed. This new concept could provide a
simple and compact scheme for power and particle control offusion reactors with easy maintenance and high reliability.
2. Centrifugal force is expected to be significant, driving thepoloidal flow in the private region.
3. Electrical separation of the two divertor channels could
enhance resilience to ELMs
4. During current quench in a disruption, toroidal current isinduced in the liquid metal divertor, in the same direction of
the plasma current. The resultant EM force pulls the main
plasma toward the divertor (benign VDE) or splash LM towardthe main plasma. (Automatic disruption mitigation)
5. PoP experiments are being discussed.
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より詳細に検討・議論すべき課題
より発展させるための示唆

原型炉設計への貢献

QUEST使命達成へ貢献

QUESTの超長時間化研究、ダイバータ開発、熱・粒子制御法開発

MAGLIMD実験
原理検証実験（応研、中部大）

MAGLIMDの予備検討

材料開発（東工大）

数値流体力学（応研）NIFSからの電磁ポンプ据付
2018年7月QUEST研究会（嶋田）