CMS HIP Plasma-Wall Interactions – Part I: In Fusion Reactors Helga Timkó Department of Physics...
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Transcript of CMS HIP Plasma-Wall Interactions – Part I: In Fusion Reactors Helga Timkó Department of Physics...
CMS
HIP
Plasma-Wall Interactions – Part I: In Fusion Reactors
Helga Timkó
Department of Physics
University of Helsinki
Finland
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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)
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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)
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 6
Tokamak vs. Stellarator
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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!
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 11
Results: Be Self-Sputtering Yields in Low Energy Range
Surfaces: and Energies: 20, 50, 75 and 100 eV 1000 bombardments each
0001 1120
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 12
Movies
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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?
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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.
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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)
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 16
Erosion and Cratering Caused by Arcs
R. Behrisch: Surface Erosion by Electrical Arcs (1986)
Helga Timkó, University of Helsinki Laudatur Seminar, 9th Sept. 2008 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