Plasma-Wall Interactions – Part I: In Fusion Reactors

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

2. 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. 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. 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. 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. Tokamak vs. Stellarator

7. 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. 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 Y measurable • erosion

9. 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/ • 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. 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. Results: Be Self-Sputtering Yields in Low Energy Range • Surfaces: and • Energies: 20, 50, 75 and 100 eV • 1000 bombardments each

12. Movies

13. 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. 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. 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. Erosion and Cratering Caused by Arcs • R. Behrisch: Surface Erosion by Electrical Arcs (1986)

17. … 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