Materials experience with tokamak plasmas

Abstract

The primary effects of using various wall and limiter materials have been in the amount and kind of impurities they introduce into the plasma. The materials employed to date include gold, stainless steel, inconel, glass, alumina and titanium for first walls and carbon, stainless steel, inconel, alumina, silicon carbide, boron carbide, molybdenum, tantalum, titanium and tungsten for limiters and divertors. Limiter surfaces bear the brunt of the plasma bombardment and so typically introduce impurities far out of proportion to their relative size. The ratio of the bombarded limiter area to that of the first wall is of order 1 to 10 3. The effect of first wall materials per se on the plasma is often largely masked by coatings of foreign substances. Carbon and oxygen compounds are the most abundant impurities observed on walls with surface analysis techniques and this correlates well with plasma spectroscopic observations which show carbon and oxygen levels of 1–6% in typical tokamak plasmas. Metals with intermediate atomic numbers such as Fe, Ni, Cr and Ti usually constitute less than 1% of the plasma density. Limiters normally are the principal contributors to the heavy metal contamination observed in many plasmas. Plasma impurities act to reduce the energy stored in the plasma through line, bremsstrahlung and recombination radiationi Heavy metals are particularly bad since the radiated power is a strong function of the atomic number, ~Z 3.7. As a consequence many tokamaks now employ either carbon or stainless steel limiters.