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

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


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)


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


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)


Tokamak vs. Stellarator


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


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


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


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!


Results: Be Self-Sputtering Yields in Low Energy Range

Surfaces: and Energies: 20, 50, 75 and 100 eV 1000 bombardments each

0001 1120


Movies


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?


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.


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)


Erosion and Cratering Caused by Arcs

R. Behrisch: Surface Erosion by Electrical Arcs (1986)


… 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

Documents

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