There are many species of impurities, but one of the most deleterious is oxygen, which originates from the inevitable oxidation process in the materials inside the vacuum vessel. Oxygen release into the plasma can be limited by using a first-wall armour material that has a capacity to "getter" oxygen atoms and strongly bind them in its atomic lattice. This capacity was one of the main reasons that justified using beryllium as the first-wall armour material in ITER; however, an updated understanding of the extensive implications of the use of beryllium has led ITER to
Boronization, a plasma-chemical process that consists in coating the entire plasma-facing surface of a fusion device with a thin boron layer, ''is the racehorse of wall conditioning,'' says Prof. Jörg Winter, who pioneered the technique 35 years ago.
Boron is a brittle and dark metalloid that has applications in the semiconductor and metalworking industries. During construction, ITER has used boron's appetence for neutrons in certain
Boronization is now routine. Here, a 1991 photo of Prof. Winter (right) and colleagues ready for the final rehearsal prior to the first boronization of the US tokamak DIII-D. The implementation of diborane gas as a ''vehicle'' for boron explains the protective gear.
Boronization is routine—since 1991, it has been performed more than 100 times in the US tokamak DIII-D, for instance—but has never been experimented in a fusion device as large as ITER where approximately 700 square metres of first-wall surface need to be coated; there are also specific challenges when operating with deuterium-tritium plasmas. In addition, boronization requires equipping the ITER vacuum vessel with a number of additional glow discharge electrodes, dedicated fuelling lines for diborane, and specific equipment for which space availability must be identified. "The boronization recipe stands," says Prof. Winter—but in ITER it will require a completely new set of utensils.
The ITER Tokamak was to be equipped with seven electrodes for glow discharge cleaning (GDC) using hydrogen, deuterium or helium plasmas. Boronization requires four more in order to obtain a perfectly homogenous boron film on the entire 700 square metre surface of the ITER first wall.