Page 204 - Fundamentals of Magnetic Thermonuclear Reactor Design

186 Fundamentals of Magnetic Thermonuclear Reactor Design

FIGURE 6.1 Frequency of unipolar microarcs emergence at different surface preparation
methods. (1) Stainless at time of shipment, (2) stainless steel after seasoning in oil, (3) polished
stainless steel, (4) polished stainless steel after seasoning in oil, (5) stainless steel after polish-
ing and blackening, (6) stainless steel after polishing with corundum powder, (7) aluminium at
time of shipment, (8) aluminium after degreasing, (9) aluminium after seasoning in oil, (10) pol-
ished aluminium, (11) aluminium after polishing and seasoning in oil, (12) gold after degreasing,
(13) gold after seasoning in oil, (14) gold after seasoning in oil and heating at 500°C, (15) gold
after degreasing and treatment with corundum powder.

The introduction of oxygen and carbon-containing impurities into the plasma
is associated with processes taking place on the FW surface under plasma irra-
diation. Materials usually contain oxygen, carbon and hydrogen. The dissocia-
tion energy of oxides present there is close to 5 eV, while most of the particles
bombarding the wall have a much greater energy.
The bombardment leads to oxide dissociation, with oxygen breaking away
to immediately oxidise hydrogen and carbon and form volatile compounds that
get into the plasma. This process extends both to the surface area and to the
near-surface layer, accessible to fast electrons. Their absorption disturbs the
oxygen atom bonds with neighbouring lattice nodes and increases the density
of defects. This facilitates the increase in the oxygen mass diffusivity. The gen-
eration of oxygen-containing impurities involves large masses of metal and can
only be retarded by lowering the dissolved oxygen concentration, material high-
temperature degasification in vacuum and cleaning of the wall surface prior to
the operating cycle.