Challenges in edge modeling IPP-Teilinstitut Greifswald, EURATOM Association, Wendelsteinstraße 1, D Greifswald, Germany Outline: 1. Motivation 2. Plasma model 3. Neutral model 4. Turbulence 5. Multi-scale strategies 6. Plasma-wall interaction 7. Remarks on Integrated Modeling Max-Planck-Institut für Plasmaphysik, EURATOM Association Ralf Schneider

Motivation Max-Planck-Institut für Plasmaphysik, EURATOM Association ultimate goal: predictive quality for tokamaks and stellarators (‘integrated modeling’) What are the problems to be solved in edge modeling? Which strategies should be followed? What are the consequences of that?

Motivation Max-Planck-Institut für Plasmaphysik, EURATOM Association edge modeling made considerable progress: yes! but: long list of topics Summary report of the American Divertor and Edge Plasma Theory Working Group, J. Neuhauser: ITER Workshop

Motivation Max-Planck-Institut für Plasmaphysik, EURATOM Association edge modeling made considerable progress: yes! but: long list of topics A. Kukushkin: ITER meeting 1993

Plasma edge physics Max-Planck-Institut für Plasmaphysik, EURATOM Association

drifts and currents: open questions Bruce Scott: Physics of Plasmas 10 (2003) 963 only anomalous viscosity – either Reynolds stress or correction from gyroviscosity – can create anomalous transport anomalous resistivity cannot create anomalous transport or radial electric field, because resistivity is a momentum conserving friction between electrons and ions -> classical equation for the potential (div j = 0) numerical performance: time-step limits for complete Newton solvers (UEDGE) and equation sub-cycling (B2); semi-implicit solvers? (Zagorski et al.; see also talk by A. Kalyentev) Plasma models

Max-Planck-Institut für Plasmaphysik, EURATOM Association Plasma models

Max-Planck-Institut für Plasmaphysik, EURATOM Association Plasma models

Max-Planck-Institut für Plasmaphysik, EURATOM Association Plasma models

Max-Planck-Institut für Plasmaphysik, EURATOM Association kinetic corrections: fluid corrections or coupling with kinetics Plasma models Coupling with kinetic code (BGK): 2 kinetic equations for thermal conductivity and viscosity (Kukushkin, Runov, Igitkhanov 1994)

Max-Planck-Institut für Plasmaphysik, EURATOM Association 3D and ergodic effects (see also talk by A. Kalyentev) Plasma models

Max-Planck-Institut für Plasmaphysik, EURATOM Association Monte Carlo vs. fluid: different level of accuracy and complexity; flux limits necessary for making the fluid model realistic Neutral models D. Coster et al., EPS2005 atomic and molecular data: below 5 eV??

Molecular physics Max-Planck-Institut für Plasmaphysik, EURATOM Association Franck-Condon atoms: low plasma temperature -> mostly molecules reflected from saturated walls

MAR Max-Planck-Institut für Plasmaphysik, EURATOM Association

MAD and MAI Max-Planck-Institut für Plasmaphysik, EURATOM Association

THE central problem Turbulence

Max-Planck-Institut für Plasmaphysik, EURATOM Association fitting of transport coefficients Turbulence

Max-Planck-Institut für Plasmaphysik, EURATOM Association fitting of transport coefficients Turbulence physics-based scaling full coupling

Multi-scales sputtered and backscattered species and fluxes Plasma-wall interaction Molecular dynamics Binary collision approximation Kinetic Monte Carlo Kinetic model Fluid model impinging particle and energy fluxes Max-Planck-Institut für Plasmaphysik, EURATOM Association

Chemical Erosion of carbon by hydrogen produces hydrocarbon species (C x H y ) Dissociation & Recombination's leads to amorphous hydrocarbon layer formation Carbon acts as sponge for hydrogen Tritium is retained by co-deposition with carbon, on the plasma facing sides or on remote areas. Hydrogen G F Counsell, Plasma Sources Sci. Technol. 11 (2002) A80–A85 Hydrocarbon-codeposition Max-Planck-Institut für Plasmaphysik, EURATOM Association

2eV CH 3 onto amorphous hydrocarbon Classical MD Max-Planck-Institut für Plasmaphysik, EURATOM Association MD studies of interaction of hydrocarbons with amorphous carbon: empricial Brenner potential reflection coefficients of hydrocarbons on amorphous hydrocarbon (collaboration with K. Nordlund, Univ. Helsinki, U. v. Toussaint, IPP Garching, D. Naujoks, IPP Greifswald) PhDs (HGF funded), Amit Raj Sharma, Abha Rai Energy (eV) CH x Reflection coefficient CH CH 2 CH 3 CH 4

Max-Planck-Institut für Plasmaphysik, EURATOM Association Multi-scale strategies WWW Uppsala Univ. Sweden Classification: A) Microscale info. local B) Microscale info. global C) Combination of (A) and (B) D) Self-similarity in scales Serial coupling Concurrent “on the fly” coupling Renormalization group Multi-scaling Paradigms:

Multi-scale approach Microscales Molecular Dynamics (MD) Mesoscales Kinetic Monte Carlo (KMC) Macroscales KMC and Monte Carlo Diffusion (MCD) Max-Planck-Institut für Plasmaphysik, EURATOM Association ´Intelligent´ coupling necessary

Meso/macro-scales Strong dependence on void sizes and not void fraction Large variation in observed diffusion coefficients standard graphites highly saturated graphite Max-Planck-Institut für Plasmaphysik, EURATOM Association Diffusion coefficients without knowledge of structure are meaningless

Multi-scale modeling of hydrogen transport in porous graphite inclusion of molecules for re-emission, extension to chemical sputtering (Küppers-Hopf-cycle) (collaboration with M. Warrier, Max-Planck-India-Fellowship) PhD (HGF funded): Abha Rai Experiment: P. Franzen, E. Vietzke, J. Vac. Sci. Technology A12(3), 1994 H-atom release limited by detrapping process, not by diffusion Modeling: results matches very well exp. Hydrogen re-emission Max-Planck-Institut für Plasmaphysik, EURATOM Association Temperature (K) Reemitted Flux (%) USB15 EK96 H2H2 H Re-emitted Flux (%) Temperature (K) H 5% Void H 2 5% Void H 9% Void H 2 9% Void

Max-Planck-Institut für Plasmaphysik, EURATOM Association a VERY personal view! technical remarks: open source standard interfaces benchmarking Integrated modeling physics remarks: hierarchical models needed (upgrading, downgrading) validation (experiment, theory) Combination and coupling of more and more codes will not improve the reliability and predictive quality Depending on the problem and the question one needs very different tools (intelligent interpolation, interpretation, basic physics studies, …)

T. Angot et al., University of Provence, Marseille STM of graphite surfaceSimulation Modelling of Hydrogen bombardement of single crystal Max-Planck-Institut für Plasmaphysik, EURATOM Association surface science and low-temperature plasma physics Model systems needed

Summary Max-Planck-Institut für Plasmaphysik, EURATOM Association Multi-scale physics: combination of methods ´Intelligent´ coupling necessary !?? Hierarchy of models (downgrading, upgrading) Real structure of the material to be included Model systems needed Standards for the modules: open source code, interfaces, benchmarking, model validation