Localised Neutron Emission at the edge of high density JET Trace Tritium - ELMy H-mode plasmas
I: Basic physics of edge plasmas in magnetic confinement...
Transcript of I: Basic physics of edge plasmas in magnetic confinement...
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I: Basic physics of edge plasmas in magnetic confinement fusion
D. Reiter
Institut für Plasmaphysik, FZ-Jülich, Trilateral Euregio Cluster
Atomic and Molecular Data for Fusion Energy research, Aug.-Sept. 2006, ICTP, Trieste
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The D-T-Reaction
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ITER: Furnace chamber:Ø 15 m 6.8 m high 5.3 T 15 MA 500 MW 8 min
The ITER Challenge
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Physics of hot plasmacore
Atomic/Molecularprocesses,Plasma material interaction
ITER
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Candle, earth
Convection Only Diffusion (no convection)
Candle, microgravity
(only small, dim burn)
Fresh air
Used air
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There is no natural ash removal mechanism from a magnetically confined fusion flame.
Without active flushing the flame is choked by its own ashwithin about 100 s, well shortof the ITER design values.
Helium ash removal is a critical issueTwo basic concepts for boundary plasma engineering
Limiter(material boundary)
Divertor(magnetically defined boundary)
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Electrons, H+ Ions H, H2, …Neutrals
1) Can sufficient particle throughput be maintained? 2) Can, simultaneously, a reactive plasma protect the
chamber from a thermonuclear plasma?
Questions addressed by Divertor Design for ITER
These Talks: • Time scales• The ITER Divertor Code • Hydrogen chemisty in low temperature Divertor Plasmas • One critical issue: tritium retention, hydrocarbon chemistry
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IP
ID
IDID
Magnetic Fusion: how to produce convection ? DIVERTOR
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Plasma flow fieldin ITER Divertor
The ITER DivertorThe ITER Divertor
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ITER, B2-EIRENE simulation, detached, Te field
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ITER, B2-EIRENE simulation, detached, ne field
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ITER, B2-EIRENE simulation, detached, nA field
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ITER, B2-EIRENE simulation, detached, nM field
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Thanks to: R.Schneider, IPP Greifswald
PSI time and spatial scales
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• Pfus ≈ 540-600 MW⇒ He flux⇒ PSOL≈86-120 MW
• ns ≈(2-4)·1019 m-3
• Sinj ≤ 10·1022 s-1
• Spump ≤ 200 Pa·m-3/s• Zeff≤1.6• CHe ≤6%• qpk ≤10 MW/m2
Provide sufficient convection without accumulating Tritiumand with sufficiently long divertor lifetime (availability).
Engineering parameter in modelling: Spuff ~ (1…13)·1022 s-1
!
?
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• ITER Divertor design is based upon “detachment”
• Detachment is a chemically complex plasma state:
“unknown territory” in fusion,
but well known in low temperature plasma physics
- gas-plasma friction,
- recombining plasmas,
- plasma cooling (radiation)
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Large Helical Device (LHD), Toki, Japan
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Te ne
nA nM
3D LHD Plasma Edge Simulation: EMC3-EIRENE
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Trilateral Euregio Cluster
TEC
Inst itut für PlasmaphysikAssoziat ion EURATOM-Forschungszentrum Jülich
Here: restriction to 2D axi-symmetric plasmas
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Joint European Torus (tokamak)
Break-Even reached in JET
16 MW fusion power400 Mill deg C
reliable data basefor extrapolation to ITER
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Furnace chamber: Ø 8.5 m 2.5 m high 3.4 T 7 MA 1 min
Active role of atomic and molecular processescooling the edge plasma, protecting target surfaces from overexposure
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AUG
JET
ITER
Major Radius
TorusAxis
Extrapolation: present experiments ⇒ ITER
Core:plasma similarity:present experimentsare “wind tunnel experiments”for ITER
Edge: influenced by recycling (of neutral particles), gridsCore: not influenced by recycling
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Extrapolation of core plasma confinement to ITER
ITER referencescenario
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AUG
JET
ITER
Major Radius
TorusAxis
Extrapolation: present experiments ⇒ ITER
Core:plasma similarity:present experimentsare “wind tunnel experiments”for ITER
Edge:Computational plasma edge modelling
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World-wide effort to understand (and predict?) Edge Plasma dynamics on the basis of best knownAtomic and Molecular Data
Estimate “Collisionality”: neR-ne-Divertor Plasma density (1020 m-3)
-R- Major Radius (m)
Alcator C-Mod (MIT)10 times smaller than ITERsimilar shapehigher density
Edge plasma: a) provide conductionb) protect exposed target areas
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Alcator C-Mod (MIT)
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Shot: 990429019, at 950ms, <ne>=1.5 1020, IP=0.8 MA, Btor=5.4 T
OSM reconstruction (Lisgo et al., 2004)
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Ionization by electron impact on neutral gas
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However:Additional Ionization
due to Lyman photon trapping
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• Magnetic confinement is now effective enough to contain the main fusion flame, but it is too good for the plasma edge (SOL): very narrow heat-footprints on targets.
• Magnetic Confinement Fusion Reactors must operate at reduced target fluxes and temperatures (“detached regime”).
• n, T upstream (core) fixed by burn criteria, density limit, etc.
• For ITER: Detached regime: decrease particle flux to target for given upstream conditions: self sustained neutral cushion (reactive plasma) controlled by PMI and A&M
• Divertor detachment physics involves a rich complexity of plasma chemistry not otherwise encountered in fusion devices .
CONCLUSIONS - I
RECYCLING
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• Experimental finding:Sheath limited flow ⇒ high recycling ⇒ detachment
• Theoretical hypothesis:This is brought about by power- and flux dissipationdue to a chemically rich self sustained plasma formed near exposed target surfaces,by the recycling process.
• Experimental tests:numerical experiments with integrated computational plasma edge models
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B2B2--EIRENEEIRENE--computational grids for JET simulationscomputational grids for JET simulations
JET, 1994, MARK-I Divertor JET, 1998, MARK-II Divertor
Experimental findings
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Trilateral Euregio Cluster
TEC
Inst itut für PlasmaphysikAssoziat ion EURATOM-Forschungszentrum Jülich
JET, MARK-I, density ramp-up
-ohmic-no imp. injection-simply: D2-puff
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Upstream:Midplane
X-point
Downstream:Target
Stretching out a flux-tube: M ⇒ X ⇒ D
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Linear, sheath limited regime (convection)( …. 1985)
D X M
Upstream:Midplane
X-point
Downstream:Target
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ALT-II
inner bumperlimiter/divertor
TEXTOR
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DHD2
Plasma Data: ne (r), Te(r), Ti (r) : directly from experiment
TEXTOR
see lecture: Ongena
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Tore Supra
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Linear, sheath limited regime: Tore-Supra, TEXTOR
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Conduction limited (high recycling):dilution by multiple recycling(1985-1995, ITER CDA)
M/CoreXD
• Lower Midplane temperature (higher density), reduced convection (near target re-ionisation) ⇒ parallel temperature gradients: low Te, high ne near the target D
• Non-linear regime: TD ~ nM-2, nD ~ nM
3 and flux ΓD ~ nM2
Upstream:Midplane
X-point
Downstream:Target
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Trilateral Euregio Cluster
TEC
Inst itut für PlasmaphysikAssoziat ion EURATOM-Forschungszentrum Jülich
Trapping of neutral particles in the Divertor: high recycling and detachment regime
Particle Simulation: PMI, A&M Visible light from ASDEX-U Divertor
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Weak (partial) detachment(1995 …., ITER EDA)
Strong (complete) detachment• below 5 eV strong momentum dissipation through
self sustained A&M processes (CX)
•Below 1.5 eV additional reduction of plasma flux by volume recombination (virtual target, neutral cushion).Escape of neutrals to the sides followed by ionisationin hotter plasma (6-7 eV) further upstream
M/Core
M/Core
ΓD ~ nM2 dependence is broken
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Degree of detachment (DOD)Measured and extrapolated ion fluxesto inner and outer divertors, density ramp
JET, (ohmic), DETACHMENT
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Princeton QED device(gaseous Divertor concept simulator)
Scaling of calorimeter signals with gas pressure
Hsu et al., PRL 49, 1001 (1982): QEDSchmitz et al., J.Nucl.Mat. 196-198, (1992): PISCESOhno et al., PRL 81, 818 (1998): NAGDIS
Schematic
Key difference: here: Pgas given. In a fusion device the neutral cushion must be self sustained by recycling process.This issue will be addressed in linear MAGNUM device (FOM)
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Trilateral Euregio Cluster
TEC
Inst itut für PlasmaphysikAssoziat ion EURATOM-Forschungszentrum Jülich
Integrated numerical experimentsplasma edge codes must integrate:• plasma physics, • plasma material interaction (PMI)• atomic and molecular processes (A&M)
Basic processes
Theoretical model
Computational model
plasma PMI, A&M
Coulomb ???
Braginskii,Turbulence ?
???
!
Boltzmann eq.!Monte CarloCodes !
Less certain Less
cer
tain
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• Steady state divertor performance for D-He-C plasma.
B2. 2D, time-dependent set of Braginskiiequations. Finite-volume scheme on quasi-orthogonal grid, multi-fluid,Grad 21-Moments closure.EIRENE. Monte-Carlo neutral transport (see www.eirene.de). Calculates sources for B2 (SN, Smu||, Si,e
E), based on 3D kinetic gas transport, plasma surface interaction and atomic and molecular plasma chemistry
”B2-EIRENE”, special issue in: Contrib. Plasma Physics, 46, No 1-2, 2006
Design of the ITER divertor is guided by large scale code simulations
(numerical book-keeping for all terms already known)
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High Intensity Discharge Lamps
CDM-75 WShop-LightingMaterial:PCA
D2-36 WAutomotive
Material:Quartz
B2B2--EIRENEEIRENE
4 m
m4 m
FIDAPFIDAP--EIRENEEIRENE
Radiation transfer module: verification and validation using HID lamps
ITERITER
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PPFR: average neutral pressure in Private Flux Region
• ITER divertor engineering parameter: • target heat flux vs. divertor gas pressure
▬ 1996 (ITER physics basis1999)
▬ 2003, neutral - neutral collisions
▬ ….+ molecular kinetics (D2(v)+D+, MAR)
▬ 2005, + photon opacity
Consequences for ITER design (B2-EIRENE): shift towards higher divertor gas pressure to maintain a given peak heat flux (Kotov et al., CPP, July 2006)
„Dome“ removal now seriouslyconsidered
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V. Kotov, D. Reiter
• Principal effect: higher density (and somewhat lower temperature) in front of the targets
▬ITER, 100 MW SOL Power, Full Carbon WallIn red: NO photon opacity, In blue: WITH photon opacity
Temperature and density along Inner Target
The effect of photon opacity (Lyα: 95%, Lyβ: 70%)
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Code verification:• the code is around 15 years, • used by a large number of people, • applied to a large number of different experiments• and it is completely open source and transparent
(publicly exposed) from the very beginning
Code validation:• matching experiments is a (trivial) minium requirement
given the sufficiently many free model parameters• separate unknowns from knowns,
by confronting sub-models with dedicated experimental data(Next lecture)