Tokamak Research Paper

CHAPTER 3:

ROLE OF DUST IN TOKAMAK

I. INTRODUCTION

It is well known that small particles exist in magnetic confinement fusion devices.1 If, for instance, the bottom area of a tokamak is cleaned after some time of operation, fine grain material is readily collected. Wiping vertical surfaces with a clean cloth leaves black stains indicating dust. The consequences of dust for the complex system of a fusion device and its thermonuclear plasma are just beginning to be addressed.2–4 One may distinguish between aspects affecting safety of a thermonuclear fusion reactor and those related to plasma operation and plasma performance. Work on the first aspect has received a significant impact from the ITER ~Ref. 5! project ~International

They are then released from the limiter by plasma–surface interactions. Arcing occurs mostly during the start-up phase of the plasma or at disruptions. Electric fields are strongest during these phases leading to a local breakdown of the plasma sheath. Coagulation of particles from the oversaturated vapor phase during arcing may be another possible mechanism for formation of the metal spheres (see below).

The failure mode of carbon material for repeated exposure to low powers is a loosening of the material structure by propagating cracks initiated by large stress and strain. Finally the ejection of grains may occur. In the dust collected from TEXTOR graphite grains were easily distinguished from flakes by their facetted appearance. They have an average diameter of typically 5–20 mm. For higher power fluxes

(ELMs, disruptions) the evaporation or sublimation of material is dominant. The vapor has a high density close to the surface and the particles have a short mean free path. Coagulation processes from supersaturated vapor leads to the formation of small particles16,17 with average diameters of several

10–100 nm. This mechanism was verified in a laboratory experiment in which different carbon materials were repeatedly exposed to power loads of 0.5–1.5 GWm-2 using a high power electron

flaking of redeposited films;

b. agglomeration from supersaturated vapor;

c. coagulation of metal atoms on hot carbon surfaces; and possibly

d. growth of dust in the cold edge of fusion plasmas, e.g., in a detached divertor.

The fluence dependence of some mechanisms makes them particularly important for long pulse and steady state devices.

Dust may become a safety hazard due to its high chemical reactivity and due to the incorporated tritium. Dust may also affect operation of fusion devices and the plasma performance by various effects. The radioactive decay may lead to charging of the dust grain. This may allow the dust removal out of a vessel by an ‘‘electrostatic dust sweeper.’’ When gas is filled into the torus the formation of a nuclear induced plasma may occur.

It is thus important to investigate the interaction between the dust and the fusion plasma. Practically no experimental data are available on this subject to date. Bringing together the experience from plasma–surface interactions in fusion devices, the experience from the operation of fusion devices and the knowledge from the field of dusty plasma research may hopefully lead to synergisms for solving some of these important