II: The role of hydrogen chemistry in present experiments and in ITER edge plasmas. D. Reiter

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1 II: The role of hydrogen chemistry in present experiments and in ITER edge plasmas D. Reiter Institut für Plasmaphysik, FZ-Jülich, Trilateral Euregio Cluster Atomic and Molecular Data for Fusion Energy research, Aug.-Sept. 2006, ICTP, Trieste

2 Main focus of coordinated European edge code model validation is on JET JET Furnace chamber: Ø 8.5 m 2.5 m high 3.4 T 7 MA 1 min

3 Motivation ITER divertor will have much ASDEX: n e ~10 20 m -3 larger L n then all existing L~0.5 m tokamaks. It is necessary to take into account λ n-n ~k 10 cm effects which have been safely λ m-i <~10 cm ignored in divertor modelling for most present devices ITER: n e ~10 21 m -3 One such effect is neutral-neutral L~2 m collisions (viscous effects in gas) Large regions of dense, low λ n-n ~k 1 cm temperature plasma requires more λ accurate simulation of the m-i <~1 cm molecular reaction kinetics, including molecule-ion collisions

4 Current hypothesis: in the detached state is the divertor dynamics and chemistry is controlled by Collisionality (inv. Knudsen number) Estimate Collisionality : n e R -n e -Divertor Plasma density ( m -3 ) -R- Major Radius (m) Alcator C-Mod (MIT) 10 times smaller than ITER similar shape higher density Factor 11 away Factor 6 away

5 Alcator C-Mod (MIT)

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7 Predictive quality regarding divertor chemistry??? Separate plasma transport by replacing plasma transport modelling with OSM reconstruction

8 Shot: , at 950ms, <n e >= , I P =0.8 MA, B tor =5.4 T OSM reconstruction (Lisgo et al., 2004)

9 D γ (from D, D 2, D +,D 2+ ): Profile matched, but high by factor 2 Calibration? Atomic Data? Plasma reconstruction? Results very sensitive eg. to T e profile

10 H 2 molecule, status in present divertor code More complete models available, still need to be integrated E [ev] 16 compiled 1997 H compiled E,F B C 13.6 ev Resonance! Singlet a v=3 v=2 v=1 v=0 H 2 b v=14 c n=3 n=2 Triplet system Potential Energy (ev) H 2 + H 2 X 2 Σ g + 1 b 3 Σ u + X 1 Σ g + a 3 Σ g + C 1 Π u E,F 1 + B 1 + Σ u Σ g 2 c 3 Π u 3 H + + H H*+H n=4 n=3 H + H 4 Internuclear Distance (A)

11 Critical for Particle Throughput (convection): Neutral Plenum Pressure Experiment: 25 mtorr Calc 2D (2000) 3 mtorr Calc 2D (2003) 27 mtorr (better A&M data, better transport model, better plasma data) Good match to experiment in 2003, with full 2D kinetic detailed recycling model 11

12 Ionizing area (red), recombining area (green) A quite cold edge plasma is sustained by recycling. It detaches from the divertor targets (plasma pressure drop) 12

13 Additional leakage pathways: 2D 3D (see later)

14 3D Neutral Gas, A&M and PSI Modelling 3D divertor structures (toroidal gap and gussets, bypass and poloidal gap) strong toroidal variations in the divertor neutral pressure

15 Ionization by electron impact on neutral gas

16 Radiation transfer: opacity of Ly-lines (though completely elementary, has long remained unnoticed in divertor modelling) hν+h(1) H*, H*+e H+ 2e (additional path for ionization in dense, low T e divertors)

17 Calc Calc. 2004

18 Neutral Pressure Exp: 25 mtorr Calc 2D (2000) 3 mtorr Calc 2D (2003) 27 mtorr (better A&M data, better Plasma data) However Ly-opacity: 17 mtorr 3D: 11 mtorr Model validation in the presence of many free parameters: include ALL edge physics that we are sure must be operative even while our capability to confirm these directly remains limited

19 Example 2: Hydrogenic ionisation-recombination balance The full database must contain a large number (~ several 100) of individual processes Fortunately: very different timescales are involved (numerically stiff problem): an underlying reduced model exists. This is often referred to as: Collisional radiative models (Astrophysics) Multistep ionisation and recombination models (Plasma physics) Condensed case approximations (Semiconductor physics) Lumped species concepts (Atmospheric research) Intrinsic low dimensional manifolds (combustion flames) 19

20 Atomic and Molecular Database for EIRENE: see: ionisation recombination transport balance: atoms conventional multistep model Extensions for Lyman-α radiation transfer (photoexcitation) Extension for (future) photoionisation simulations 20

21 ionisation recombination - transport : molecules (MAR 1 ) + Sawada/ Fujimoto CR-Model (MAD 2 ) (MAR 2 ) 21

22 Molecule spectroscopy (late nineties): identification of important role of molecule reaction kinetics on overall Divertor dynamics U. Fantz et al.

23 H2(X) -> H2 + (repulsive) -> H + H + H2(X) -> H2*(repulsive) -> H + H* 35 Potential Energy (ev) H 2 + H 2 X 2 Σ g + b 3 Σ u + a 3 Σ g + B 1 Σ u + c 3 Π u C 1 Π u E,F 1 + Σ g H + + H n=4 n=3 H + H x H 2 (v) v=0 v=1 v=2 v=3 v=4 0 0 X 1 Σ g Internuclear Distance (A) H atom energy (ev) 12 14

24 Calculation of H atom velocity H2(X)->H2(b)->H + H Potential Energy (ev) b 3 Σ u + X 1 Σ g + H 2 x H 2 (v) v=0 v=1 v=2 v=3 v= Internuclear Distance (A) H atom energy (ev) 5 6

25 Sensitivity to surface produced vibr. excitation 1-D Monte-Carlo Model of Neutral-Particle Transport (K. Sawada) H 2 (v) population (cm -3 ) H 2 v= T e =2.0eV n e =10 14 cm -3 n H2 =1.0 cm -3 Distance from wall (cm) 3 573K at wall 4 H 2 (v) population (cm -3 ) H 2 v= T e =2.0eV n e =10 14 cm -3 n H2 =1.0 cm K at wall Distance from wall (cm) All H 2 from wall: v=0 All H 2 from wall: v=4 3 4

26 Trilateral Euregio Cluster ASDEX-UPDRADE (IPP Garching) TEC Institut für Plasmaphysik Assoziation EURATOM-Forschungszentrum Jülich 26

27 Trilateral Euregio Cluster Trapping of neutral particles in the Divertor: high recycling and detachment regime TEC Particle Simulation: PMI, A&M Visible light from ASDEX-U Divertor Institut für Plasmaphysik Assoziation EURATOM-Forschungszentrum Jülich 27

28 Trilateral Euregio Cluster TEC Institut für Plasmaphysik Assoziation EURATOM-Forschungszentrum Jülich

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31 Molecular spectroscopy to verify or disproof model predictions

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33 Trilateral Euregio Cluster Self sustained neutral cushion? B2-EIRENE simulation: MAR or MAD? TEC P + H 2 (v) H + H 2+, e + H 2 + H + H* H + H (MAR) P + H 2 (v) H + H 2+, e + H 2 + H + H* H + H + + e (MAD) H 2 density field in Divertor: Neutral gas cushion is strongly reduced by H 2 + p ion conversion only H 2 (v=0) H 2 (v) included Institut für Plasmaphysik Assoziation EURATOM-Forschungszentrum Jülich 33

34 Trilateral Euregio Cluster Self sustained neutral cushion? B2-EIRENE simulation: MAR or MAD? TEC Pressure drop at inner target disappears: re-attachment due to MAD H 2 (v=0) H 2 (v) Institut für Plasmaphysik Assoziation EURATOM-Forschungszentrum Jülich 34

35 Trilateral Euregio Cluster Self sustained neutral cushion? B2-EIRENE simulation: MAR or MAD? TEC Electron temperature field in Divertor H 2 (v=0) H 2 (v) Institut für Plasmaphysik Assoziation EURATOM-Forschungszentrum Jülich 35

36 Trilateral Euregio Cluster Self sustained neutral cushion? B2-EIRENE simulation: MAR or MAD? TEC Changed flow pattern: flow reversal (right) (dominant force (friction) changes sign H 2 (v=0) H 2 (v) Institut für Plasmaphysik Assoziation EURATOM-Forschungszentrum Jülich 36

37 The detached divertor state due to plasma chemistry is a very robust finding from plasma codes. However: Molecule chemistry can change the overall divertor state drastically The initial detached state (1) (no mol. vibr. chemistry) can be recovered from (2) (with mol.vibr.) by either: increasing the upstream density (3) or assuming different (anomalous) cross field transport , 2 2 Outer midplane density profile Outer target density profile 37

38 CONCLUSIONS Magnetic Confinement Fusion Reactors must operate at reduced target fluxes and temperatures ( detached regime ). The plasma regime in the Divertor (near target region) is then that of (high electron density) technical gas discharges / fluorescent lamps / RF discharges. Divertor detachment physics involves a rich complexity of plasma chemistry not otherwise encountered in fusion devices. The role of vibrationally excited molecules on overall Divertor dynamics seems accepted. The relative role of surface processes (vs. volume processes) to establish this vibrational excitation seems less clear.