Page 217 - Fundamentals of Magnetic Thermonuclear Reactor Design

Vacuum and Tritium System Chapter | 6 199

profiles may be suitable. Copper-indium bimetallic pads may be effective
for reducing contact resistance. A 10–15 µm indium coating is applied to a
pad made of copper wire or copper plate. The coated pad is heated at tem-
peratures up to 200°C to achieve a contact resistance that allows a hermetic
sealing of the connection and the elastomeric sealant.
l To create an important assembled structure that is expected to be rarely dis-
mantled, it is reasonable to use semiremovable welded antenna connectors.

6.6.4 Wall Cleaning and Conditioning
Cleaning of vacuumed parts is a routine vacuum engineering technological pro-
cedure. Meanwhile, the conditioning of plasma containment vessels designed
for electrical physics applications is a specific stage of the vacuuming process.
Wall conditioning is particularly important for fusion devices and colliders. Its
purpose is to clean the plasma-facing wall surface to almost atomic purity to
eliminate stimulated desorption of gases.
Heating as a conventional adsorbate removing technique is insufficient for
the MFR given the mentioned plasma effect on the wall. Particles bombarding
the wall bring tens and hundreds of electron volts of energy, which is much
greater than even the bond energy of chemisorbed atoms (around 10 eV), to
say nothing about that of adsorbed particles (fractions of electron volt). Fluxes
hitting the wall break the sorbate’s bonding with the crystal lattice and destruct
the oxide and hydrocarbon films causing the oxidation of carbon, sulphur and
other impurity species in the metal and the generation of volatile compounds.
Meanwhile, even a lengthy heat conditioning only affects the adsorbed layers
and is unable to prevent stimulated desorption.
The difference in the nature of thermal sorption-induced and radiation-
stimulated flows is graphically illustrated by the results of measurement of the
residual gas partial composition. In a heated-up stainless-steel chamber, the
residual gas contains H (∼80%), CO (∼10%), CH (∼5%) and CO . An elec-
4
2
2
tromagnetic loading causes a sharp increase in carbon-containing species, and a
mass spectrography assay reveals H (∼30%), CO (∼40%), CO (∼25%) and
2
2
CH . The desorption flow absolute values increase by more than three orders of
4
magnitude. Obviously, the vacuum chamber cleaning and conditioning process
should combine conventional mechanical, physical and chemical pre-treatment
methods with thermal training and ion or radiation treatment of the surface.
A pre-treatment generally includes the removal of mechanical inclusions
and oily films; degreasing in vaporous trichloroethylene, perchloroethylene,
freon and other solvents; use of ultrasonic cleaning in alkaline solutions; wash-
ing in distilled water and drying in clean air at 150°C. This procedure is stan-
dard for austenitic stainless steels, oxide dispersion strengthened nickel-based
alloys, and titanium and copper alloys. Its effectiveness is illustrated by the
Auger electron spectroscopy of austenitic stainless steels. Pre-treatment steel
surface layers contain carbon in the form of free atoms, different hydrocarbons