Page 218 - Fundamentals of Magnetic Thermonuclear Reactor Design

200 Fundamentals of Magnetic Thermonuclear Reactor Design

and heavy hydrocarbon molecules (more than 80%). Elements such as S, Cl, Ca
and Na are also observed. Oxygen is found in the form of metal oxides, water
vapour films and carbon oxides. Atomic Fe, Cr and Ni, the main metal constitu-
ents, are covered by the layer of impurities and admixtures.
After degreasing, the concentration of C atoms is decreased almost two-
fold, and there are traces of S, Ca, Na and Cl. After the treatment cycle comple-
tion, metal atoms prevail in the surface layer, with C concentration down to
∼30%. A small Ca admixture is also found.
Identifying the optimum thermal-vacuum conditioning temperature is of
practical importance. Heating at ∼600°C helps radically reduce the concentra-
tion of C and O atoms. It continues to fall with temperature, but further heating
leads to diffusion processes and emergence of S atoms from the steel sample
depth. Similar temperature dependencies are found for the ion- and electron-
stimulated gas emission coefficients.
Glow discharge cleaning (GDC) techniques using inert gases are believed
to be quite effective. They involve chamber venting with argon at ∼1 Pa and a
potential difference of ∼1 kV. Argon ions bombarding the wall destruct oxide
films, hydrocarbons and other surface inclusions. Ion partial implantation into
the surface and wall sputtering also take place. After the discharge, the diffusion
and desorption of the implanted particles give rise to a gaseous background. The
latter is suppressed by the chamber heating at 350°C and pumping down. The
dynamics of this process is characterised by the following data. Immediately
after the discharge, at an in-chamber temperature of 300°C, the specific rate
2
3
−8
of Ar desorption is close to 10 m Pa/(s·m ). It becomes almost an order
of magnitude lower after a subsequent 10-h retention interval. After cooling
3
the chamber to room temperature, the rate of desorption falls to 1·10 −12 m ·Pa/
2
(s·m ). Argon can be removed by a helium glow discharge.
The larger the ion atomic mass, the higher the GDC speed. This makes Ar
superior to He and N in terms of performance. The use of the heavier Kr and Xe
is only constrained by the cost of these chemical elements.
The radiation dose stopping changes in the surface layer element content is
−2
19
close to 1.6·10 cm . At such a dose Auger electron spectroscopy reveals the
main alloy components and just a small amount of impurity species, namely, C
in the form of carbides, and O. To improve the cleaning grade, around 10% of
oxygen is added to Ar to bind carbon. This brings the radiation threshold dose
−2
18
down to ∼1 × 10 cm .
When performing a GDC, one may observe – even with a pre-heated cham-
ber – pressure spikes indicating a stimulated gas desorption from high-bond-
energy states. For this reason, it is desirable that a GDC is combined with a
heating to ∼300°C. The desorbed gases’ main component is a CO and mass
spectroscopy also reveals the presence of H , CO and CH .
2
4
2
Importantly, a stainless-steel ion treatment makes steel resistant to atmo-
spheric effects. The concentration of impurity species on a post-treatment sur-
face is only slightly above initial level, even after several exposures to air.