EUROfusion Assessment of Alternative Divertor Solutions ... · EUROfusion assessment of alternative...

EUROfusion Assessment of Alternative Divertor Solutions for DEMO

Holger Reimerdes For the WPDTT1 project In close collaboration with WPPMI, WPPFC and WPDTT2

EUROfusion Assessment of Alternative Divertor Solutions for DEMO H. Reimerdes1, L. Aho-Mantila2, R. Albanese3, R. Ambrosino3, W. Arter4,

S. Brezinsek5, H. Bufferand6, G. Calabro7, G. Ciraolo6, D. Coster8, H. Fernandes9, J. Harrison4, I. Kaldre10, K. Lackner8, O. Lielausis10,

J. Loureiro9, T. Lunt8, G. Mazzitelli7, S. McIntosh4, F. Militello4, T. Morgan11, N. Pelekasis12, G. Pelka13, V. Pericoli7, V. Philipps5, F. Subba14, F. Tabares15,

B. Viola7, R. Wenninger8, R. Zagorski13 and H. Zohm8

1EPFL-SPC, Lausanne, Switzerland, 2VTT, Finland, 3Università di Napoli, Italy, 4CCFE, Culham, UK, 5Forschungszentrum Jülich, Germany, 6CEA, St. Paul-lez-Durance, France, 7ENEA-

Frascati, Italy, 8Max-Planck-Institut für Plasma Physik, Garching, Germany, 9Universidade de Lisboa, IST, Portugal, 10University of Latvia, Latvia, 11DIFFER, Nieweign, The Netherlands,

12University of Thessaly, Volos, Greece, 13IPPLM, Warsaw, Poland, 14Politecnico di Torino, Italy, 15Ciemat, Madrid, Spain

H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 |

Pursue development of alternative divertor solutions as risk mitigation European roadmap for fusion energy •  Identified heat exhaust as the main challenge towards the

realisation of magnetic confinement fusion

•  Assess and develop alternative divertor solutions in case the baseline solution will not extrapolate to DEMO -  ITER being the ultimate test of the baseline solution

•  Extrapolation of alternatives to DEMO based on today’s “proof-of-principle” experiments and modelling considered too large

•  Consider a dedicated Divertor Tokamak Test (DTT) facility to develop alternative(s) to sufficient maturity for deployment in DEMO


Outline

EUROfusion assessment of alternative divertor solutions for DEMO §  Strategy and baseline

§  Assessment of alternative divertor configurations

§  Assessment of liquid metal PFCs


Assessment strategy

•  Focus on solution with high potential to reach sufficient technological maturity in time for a DEMO to start operation in the 2040s

•  Compare potential “benefits” and “costs” of alternatives with the baseline DEMO solution Ø Produce a short-list of the most promising alternatives


Compare alternatives with the European baseline divertor solution for DEMO •  Conventional DEMO scenario with Pelec = 500 MW (R = 8.8 m,

Bt = 5.8 T, IP = 20 MA) [Wenninger, Kemp, private communication]

-  Single null divertor (SND) -  Tungsten (W) targets


Pheat,eff = 300 MW

2014 DEMO1 (A=3.1)





•  Target requirements -  Te,t < 5 eV to avoid W sputtering -  q⊥,t < 5-10 MW/m2 to avoid target

damage

[You, et al., EFPW (2014)]

Example.: Water cooled ITER-like W monoblock






damage •  Assumptions

-  Power decay length λq,u~ ρpi ~ 3 mm -  Min. grazing angle 1.5 Deg ➜

Aw = 2 m2/target


Example.: Water cooled ITER-like W monoblock

Total radiated fraction

Power to outer target (MW) *

q⊥,outer,max (MW/m2)

Prad/Pheat=90% 20 10

Prad/Pheat=95% 10 5

*Assume a 1:2 in:out asymmetry






damage •  Assumptions

-  Power decay length λq,u~ ρpi ~ 3 mm -  Min. grazing angle 1.5 Deg ➜

Aw = 2 m2/target

Ø Radiate 90-95% of the heating power

Ø Reduce SOL pressure to reduce power deposition due to recombination processes ➜ (partial) particle detachment

Total radiated fraction

Power to outer target (MW) *

q⊥,outer,max (MW/m2)

Prad/Pheat=90% 20 10

Prad/Pheat=95% 10 5

*Assume a 1:2 in:out asymmetry

Extrapolation of baseline solution to DEMO is uncertain Main sources of uncertainty are

•  Power decay length λq and wetted area Aw

•  Confinement and its compatibility with high core radiation

•  Stability of detachment under high heat fluxes

•  Ability to sufficiently suppress transients


Alternative divertor concepts can help in two ways


Increase divertor energy and momentum losses In-situ repair of damaged surface

and/or Decrease peak heat and particle flux onto the divertor target

Increase exhaust capabilities of divertor targets

Liquid metal target armour

Distribute power over a greater surface

Easier access to detachment/wider operating regime

More robust/stable detachment

Alternative magnetic divertor configurations

Greater tolerance for transients

Thinner armour/convection/evaporation for higher heat

handling capability

Outline





Considered alternative configurations

•  X divertor (XD) [M. Kotschenreuther, et al., Phys. Plasmas 14 (2007) 72502]

-  Increase pol. flux expansion to flare flux surfaces towards target

•  Super-X divertor (SXD) [P.M. Valanju, et al., Phys. Plasmas 16 (2009) 056110]

-  Increase major radius of target (only way to increase Aw at constant γ without invoking plasma physics)

-  Combine with XD and flare flux surfaces

•  Snowflake divertor (SFD) [D.D. Ryutov, Phys. Plasmas 14 (2007) 064502]

-  Second order null point expands flux near null point -  In practice always two nearby x-points with SF+ or SF- topology


Considered alternative configurations


Divertor concept* Key characteristic Geometry Effect on power exhaust

X divertor (XD) [Kotschenreuther, et al., Phys. Plasmas 14 (2007) 72502]

Increase pol. flux expansion to flare flux surfaces towards target

Flaring •  Stabilise location of detachment front (?)

Longer connection length/larger SOL volume

•  Cooler targets ease access to detachment •  Higher volumetric losses ease access to

detachment

Lower target tilt •  Recycling neutrals reflect upstream for easier detachment (?)

Super-X divertor (SXD) [Valanju, et al., Phys. Plasmas 16 (2009) 056110]

Increase major radius of target(s)

Increase wetted area •  Lower peak heat flux

Decrease q|| •  Cooler targets ease access to detachment

Introduce gradient in q|| •  Stabilise location of radiation front (?)

Can be combined with an increase of pol. flux expansion

See XD

Snowflake divertor (SFD) [Ryutov, Phys. Plasmas 14 (2007) 064502]

Second order null point - in practice always two nearby x-points with SF+ or SF- topology

Converging flux surfaces towards target

•  Stabilise location of detachment front (?), albeit close/within confined plasma (distinguish SF+ and SF-)

Longer connection length/larger SOL volume

•  Cooler targets ease access to detachment •  Higher volumetric losses ease access to

detachment

Large low field region and larger shear

•  May affect turbulent transport (?) •  May broaden/narrow the SOL (?)

*Not necessarily first incarnation

Evaluate feasibility of alternative configurations and compare costs with baseline


Constraints •  Core plasma parameters: R, Bt, IP, κ95, δ95 •  Current density < 12.5 MA/m2

•  Magnetic field < 12.5 T •  Vertical forces

-  Single PF coil Fz,PF < 450 MN -  CS stack Fz,CS < 300 MN -  CS separation Fz,CS,sep < 350 MN

•  Grazing angle of B-field at target γt =1.5 Deg. – tilt target to close divertor

Costs •  Total PF coil current (weighted with radius) ➜ Cost of coils •  TF volume/plasma volume ➜ Higher costs for TF coils •  Increase of poloidal angle subtended by the divertor ➜ Need for a

higher tritium breeding fraction

The XD configuration

•  Evaluate only as solution for the outer leg

•  Use (need?) internal coils to obtain flux flaring -  Net-zero current in internal coils

allows for modular design

•  Note -  Difficulty to increase flaring above

fx,t/fx,min ~1.5 (because of γ) -  fx,min located in immediate

proximity of the target

Ø Evaluate use of external coils only


The SXD configuration


•  Use (need?) in vessel coil to bend leg

•  Note -  Outer target can be located

behind the neutron shielding -  No significant increase in

connection length


Work in progress

The SXD configuration


•  Use (need?) in vessel coil to bend leg

•  Note -  Outer target can be located

behind the neutron shielding -  No significant increase in

connection length

Ø  Consider 2nd SXD option with poloidal flux expansion -  Increases connection length -  Adds in-vessel coils

Ø  Evaluate simple SXD using external coils only


Work in progress

The SFD configuration

•  Closed divertor relaxed for lower SPs to increase flexibility of configuration

•  Note -  Meeting force constraints is

challenging, but feasible


XD and SFD are feasible with geometric variations being of order 1


SND XD SXD SFD Limit

Costs

Max. Σ |IPF| (MA·∙turns) 160 194 174

Total IPF,internal (MA·∙turns) -‐ 10 -‐

Max. force on single coil Fz,PF (MN) 145 301 439 < 450

Max. CS separaJon force Fz,CS (MN) 130 244 329 < 350

Flux swing for current drive (V⋅S) 330 340 215

VTF/Vplasma 2.9 3.6 3.8

Bene

fits L||,outer (ρu=3mm) (m) 114 146 245

fx,t/fx,min 1 1.43 1

Rt/Rx 1.04 1.14 1.19

Wor

k in

pro

gres

s

Use models with various degrees of sophistication to predict exhaust performance in DEMO

•  Proposed figures of merit -  Density for the onset of detachment -  Impurity concentration for required divertor power loss (e.g. for

q⊥,t,max = 10 MW/m2) -  Max. divertor power loss before loss of stability/convergence

•  Evaluate He pumping capability

•  Evaluate impurity retention in divertor (seeded and sputtered) and effect on global confinement


Scoping studies identified model capabilities


x: works (with limited addiJonal effort), o: works with caveats (e.g. excessive run Jme), -‐: does not work

COREDIV TECXY OSM EMC3 EDGE2D SOLPS SOLEDGE2D

Confi

g. DEMO SND x x x x x x x

DEMO XD x x x x x x x DEMO SXD x x x x x x x DEMO SF+/SF-‐ o/o x/o o/o x/x -‐/-‐ o/-‐ x/(x)

Physics

Private flux region -‐ o x x x x x Target geometry -‐ -‐ x x x x x Dri_s -‐ o o -‐ ? o (o) KineJc neutrals -‐ -‐ o o o o o ImpuriJes x x o o ? x o

-  All models calibrate (diffusive) transport coefficients to estimate of λq,u in the reference SND scenario

-  All models neglect effect of geometry on turbulent transport!

Scoping studies identified model capabilities


x: works (with limited addiJonal effort), o: works with caveats (e.g. excessive run Jme), -‐: does not work

COREDIV TECXY OSM EMC3 EDGE2D SOLPS SOLEDGE2D

Confi

g. DEMO SND x x x x x x x

DEMO XD x x x x x x x DEMO SXD x x x x x x x DEMO SF+/SF-‐ o/o x/o o/o x/x -‐/-‐ o/-‐ x/(x)

Physics

Private flux region -‐ o x x x x x Target geometry -‐ -‐ x x x x x Dri_s -‐ o o -‐ ? o (o) KineJc neutrals -‐ -‐ o o o o o ImpuriJes x x o o ? x o

-  All models calibrate (diffusive) transport coefficients to estimate of λq,u in the reference SND scenario

-  All models neglect effect of geometry on turbulent transport!

➜  Expect first results by the end of 2015

Assessment: Alternative configurations

•  Alternative configurations come with increased costs that can be quantified

•  Potential benefits are less certain, but predictions face similar uncertainties as predictions for the baseline solution -  Ongoing effort aims at estimates relative to the baseline solution


Outline





Liquid metal based concepts

•  Radiation cooling -  Eroded/evaporated liquid increases divertor

radiation (stabilising feedback)

•  Evaporation cooling -  Condensation of evaporated metal spreads heat

over a larger surface area


•  Static liquid: Heat removal via conduction -  Stabilised by a capillary porous system (CPS) [Golubchikov, et al., JNM

(1996)]

•  Moving liquid: Heat removal via advection -  Liquid metal infused trenches (LIMIT): Stabilise by trenches and drive with

thermo-electric currents [Ruzic, et al., NF (2011)]

Vapour shielding

Liquid metal based concepts


LM concept Heat removal Stabilisation Critical issues

Static liquid Conduction Capillary porous system (CPS) [Golubchikov, et al., JNM (1996)]

•  Limited heat removal potential

Moving liquid Advection Trenches [Ruzic, et al., NF (2011)]

•  High MHD forces in reactor relevant magnetic fields oppose movement and limit heat removal capability

Radiation cooling

Radiation e.g. CPS •  Li (required for core compatibility) is a ‘poor’ radiator

Evaporation cooling

Evaporation e.g. CPS •  High net mass flow (1kg Li/sec for 20MW)

•  Compatibility of high Li pressure with He pumping capability?

Ø  Focus on LM targets based on capillary porous systems (CPS) -  Solving issues of CPS based solutions is a necessary requirement for

moving liquids

Consider Li and Sn (and their alloy) which face different challenges


Lithium Li Tin Sn Z 3 50

Tmelt (°C) 181 233

Tmax,2 (°C)* 482 1255

Tboil (°C) 1340 3053

Power exhaust potential ~10MW/m2* 25MW/m2*

Core compatibility Good, but fuel dilution might become a problem

Poor, accumulates in the core and strongly radiates

Hydrogenic retention Can be high, but depends on temperature

Low

*[Coenen, et al, PS (2014)]

•  Stationary power exhaust potential of conduction-based systems determined by surface temperature and thickness of components

•  Surface temperature is determined by -  Allowable erosion -  Realisable replenishment rate

Need complete target design to evaluate heat removal capability •  Heat removal capability may be limited by temperature range of

non-armour components of the target



Example.: Temperature of CuCrZr tube limited by - Irradiation hardening at T<200°C - Irradiation creep at T>300°C

Characterisation of material properties

•  Measurements of temperature enhanced sputtering -  Sn CPS in Pilot-PSI [Morgan, et

al., JNM (2015)]

•  Measurements of hydrogenic

retention -  Exposure of Sn to D plasmas in

ISTTOK yields low retention rate (0.068%) [Loureiro, et al., ISLA (2015)]


Pilot-PSI Sn CPS

ISTTOK Sn sample holder

Move towards demonstration of LM power handling capability

•  Test cooled (➜ stationary operation) LM limiters in FTU

•  Cooled Li limiter (CLL): first experiments in 2014/15, but not stationary, CLL currently being improved -  Li CPS with W mesh

•  Cooled Sn limiter (2016) -  Sn CPS, plan to test prototypes in Pilot-PSI

•  Increase FTU discharge length from 1.5 to 5s (4.5s achieved) for stationary exhaust

•  Develop a diverted FTU configuration


See G. Mazzitelli, “Experimental results with the Cooled Lithium Limiter (CLL) on FTU”, this meeting.

Assessment: LM based concepts

•  Need for the development of an integrated solution with the full LM cycle demonstrating control over the LM and hydrogenic inventory -  Replenishment -  LM purging -  LM migration and recovery

•  Need for the development of a target with a reactor compatible structural design -  Materials including coolant -  Dimensions

•  Possibility for game changers -  Possible tolerance to transients -  Possible elimination of tile gaps


Extra slides


•  The plasma temperature at the target has to be smaller than 5eV to avoid erosion (through physical sputtering)

•  The heat flux onto the divertor target q⊥,t has to be smaller than 5 MW/m2 to avoid melting (target technology might increase this value by x2-3)

•  Even with a low target temperature the power density due to the potential energy of the ions would exceed the heat flux limit

➜ Need to reduce pressure at target, i.e. detach the divertor


q||,t = !||,t !kTt +Epot( ) = pt !Tt1 2 +Epot

Tt1 2

"

#$

%

&'

Other alternative configurations

•  Double null divertor

•  X-point target [B. LaBombard, et al., APS (2013)]/tripod [G.Y. Zheng, et al., FED (2014)] (between SF- and XD) -  Also motivated by recent SF- calculations [T. Lunt, et al, EPS (2015)]

•  Negative triangularity tokamak (NTT) [M. Kikuchi, et al., 1st International E-Conference On Energies 2014]

-  Relies heavily on core physics


Ongoing configuration development


XD with external PF coils only SXD with external PF coils only

Modelling

•  Heat removal (incl. transients) -  Heat conduction (e.g. RACLETTE, ANSYS)

•  Wetting and replenishment -  Fluid modelling

•  Edge transport, effect on core performance

-  Transport models (e.g. COREDIV) with appropriate boundary conditions


Dependence of seepage velocity of Li on the capillary radius

[Ben

os, Pelekasis, GRA

CM (2

015)]

Li/SN alloys appear to unite positive characteristics of each component

•  Low vapour pressure

•  Preferential evaporation of Li •  Negligible D retention (indicated by ISTTOK experiments)


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