1. CMS HIP Plasma-Wall Interactions – Part I: In Fusion Reactors Helga Timkó Department of Physics University of Helsinki Finland

2. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 20082 Plasma-Wall Interactions – Outline Part I: In Fusion Reactors Materials Science Aspect -Materials for Plasma Facing Components -Beryllium Simulations Arcing in Fusion Reactors Part II: In Linear Colliders Arcing in CLIC Accelerating Components Particle-in-Cell Simulations Future Plans for a Multi-scale Model

3. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 20083 Materials Science Aspect of Plasma-Wall Interactions Plasma particles cause erosion of first wall components Materials considered for plasma facing components: Carbon, graphite (C) -Based on thermal and electrical conductivity properties, -Erosion and irradiation properties, -Plasma discharge probability and costs. Tungsten (W) -High melting point, WC’s are subject to research Beryllium (Be) -Low Z  good mechanical & thermal properties, -Resistance to radiation Problem with C: traps tritium and erosion leads to dust in the plasma  divertor needed – absorbs ashes (α)

4. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 20084 ITER First Wall Materials For ITER, decision has been made Nevertheless, it is important to make predictions & consider other possibilities for DEMO ITER = originally International Thermonuclear Experimental Reactor, meaning ‘direction’, ‘way’ in Latin DEMO = DEMOnstration Power Plant Tokamak = toroidalnaya kamera & magnitaya katushka, i.e., toroidal chamber & magnetic coil

5. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 20085 Some Background to the Research Done Controlled Fusion From the plasma & magnetic side, quite well established already -Remaining: to combine tokamaks & stellarators -Tokamak: current in the plasma, -stellarator: twisted magnetic field Problematic: the materials science side, in: -Plasma-facing components -Sensors, cameras, etc. Important to know for future models (DEMO)  research done in the Accelerator Lab: -Erosion of materials -Radiation damages (in steels)

6. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 20086 Tokamak vs. Stellarator

7. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 20087 The Task … is to simulate D → Be bombardment cascades Motivation: Russell Doerner’s experiments D → Be University of California, San Diego USA; IAEA collaboration Not much data on Be yet, has become interesting only recently; especially not in the low-energy region Method: Molecular Dynamics (MD) simulations What is MD? (cf. PIC) Method for computing the time evolution of particle positions and velocities, with a given potential, in discrete approx. With MD, can simulate the formation of vacancies and interstitials, clusters, etc., i.e., changes in structure MD can be classical as well as quantum mechanical Very important

8. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 20088 Time Scales in MD In MD simulations Timesteps of order ~ fs Can simulate happenings in a time scale ~10-1000 ps With a multi-scale scheme, i.e., combining with other methods, up to ~ ms predictable In ITER, e.g., 10 yrs building time 20 yrs of operation To understand what happens in time scales of 20 yrs we need to understand first the fs scale gain information -on chemical sputtering Ymeasurable -erosion

9. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 20089 The Code Parcas, by Prof. Kai Nordlund Some 100 parameters, very wide range of applications -Amongst others, built-in temperature & pressure control During the years several potential models, possibilities of changing the features & characteristics of the simulation celll etc. were included Versatile: from nanotubes & nanoclusters to reactor materials, http://beam.acclab.helsinki.fi/sim/http://beam.acclab.helsinki.fi/sim/ Potentials usually fitted to existing models Be-Be repulsive potential done and tested Still problems with the Be-D potential…  project not finished yet

10. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 200810 How a Cascade Simulation Looks like Create a simulation cell: HCP for pure Be At about 3000 atoms Set boudary conditions: during cascade, periodic in x & y Relaxing the cell to desired temperature (320 K) First the cell is periodic in all directions, for fixing, want to remove periodicity in z-direction Shifting layers – needed before fixing Fixing the lowest layers in that direction, in which the bombardment will happen (z-dir.) Fixing → to simulate bulk below Cycle: 1. Bombardment (5 ps) + relaxation (2 ps) 2. Shifting the cell randomly In reality, much longer timescales!

11. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 200811 Results: Be Self-Sputtering Yields in Low Energy Range Surfaces: and Energies: 20, 50, 75 and 100 eV 1000 bombardments each

12. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 200812 Movies

13. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 200813 Arcing in Fusion Reactors – Another Example of Plasma-Wall Interactions Since ~ 1970’s problems with arcing Arcs or sparks cause Erosion and impurities in the plasma  Instabilities, or even breakdown & undesirable cooling Presence of contamination enhances arcing! Burkhard Jüttner has done research on arcing until 1990’s Phenomenon known since ancient times, but what do we understand of it?

14. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 200814 Arcing – a Plasma Physical Phenomenon » Continuous plasma discharge between electrodes « Flow of high density plasma High currents also, 1-10A Can be DC or RF discharge Onset of arcing not very well understood at all There can be different triggers, e.g., tips, rough surface The discharge itself is continuous (cf. sparks) Goes on as long as the electric field is maintained Until a certain saturation is reached What stops an arc? We don’t know either.

15. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 200815 The Process of Arcing After onset of arcing, continuous electron and ion plasma flow, from the cathode to the anode (usually in vacuum) Arc spots: centres of plasma outflow Emission types: field and thermal emission (Ohmic heating) Unipolar arcs are also possible B. Jüttner: Cathode Spots of Electric Arcs (2001)

16. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 200816 Erosion and Cratering Caused by Arcs R. Behrisch: Surface Erosion by Electrical Arcs (1986)

17. Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 200817 … Next Week Arcing in CLIC accelerating components What are Particle-in-Cell simulations? How can we model arcs with PIC? Thank You! Bibliography: IPP – Kernfusion, Berichte aus der Forschung B. Jüttner, Cathode Spots of Electric Arcs, J. Phys. D: Appl. Phys. 34 (2001) R103-R123 R. Behrisch, Surface Erosion by Electrical Arcs, (1986), in collection Physics of Plasma- Wall Interactions in Controlled Fusion