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 | Page 2
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 3
Outline
EUROfusion assessment of alternative divertor solutions for DEMO § Strategy and baseline
§ Assessment of alternative divertor configurations
§ Assessment of liquid metal PFCs
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 4
Outline
EUROfusion assessment of alternative divertor solutions for DEMO § Strategy and baseline
§ Assessment of alternative divertor configurations
§ Assessment of liquid metal PFCs
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 5
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 6
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 7
Pheat,eff = 300 MW
2014 DEMO1 (A=3.1)
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 8
• 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
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 9
• Target requirements - Te,t < 5 eV to avoid W sputtering - q⊥,t < 5-10 MW/m2 to avoid target
damage • Assumptions
- Power decay length λq,u~ ρpi ~ 3 mm - Min. grazing angle 1.5 Deg ➜
Aw = 2 m2/target
[You, et al., EFPW (2014)]
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
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 10
• Target requirements - Te,t < 5 eV to avoid W sputtering - q⊥,t < 5-10 MW/m2 to avoid target
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 11
Alternative divertor concepts can help in two ways
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 12
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
EUROfusion assessment of alternative divertor solutions for DEMO § Strategy and baseline
§ Assessment of alternative divertor configurations
§ Assessment of liquid metal PFCs
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 13
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 14
Considered alternative configurations
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 15
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 16
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 17
The SXD configuration
• Evaluate only as solution for the outer leg
• Use (need?) in vessel coil to bend leg
• Note - Outer target can be located
behind the neutron shielding - No significant increase in
connection length
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 18
Work in progress
The SXD configuration
• Evaluate only as solution for the outer leg
• 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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 19
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 20
XD and SFD are feasible with geometric variations being of order 1
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 21
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 22
Scoping studies identified model capabilities
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 23
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 24
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 25
Outline
EUROfusion assessment of alternative divertor solutions for DEMO § Strategy and baseline
§ Assessment of alternative divertor configurations
§ Assessment of liquid metal PFCs
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 26
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 27
• 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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 28
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 29
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 30
[You, et al., EFPW (2014)]
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)]
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 31
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 32
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 33
Extra slides
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 34
• 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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 35
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 36
Ongoing configuration development
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 37
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
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 38
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)
H. Reimerdes | 1st IAEA TM on Divertor Concepts | Vienna | October 1, 2015 | Page 39