The reduction of surface oxides and carbon during discharge cleaning in tokamaks: Some kinetic-mechanistic aspects

Reduction by hydrogen of the residual oxide and carbon present on the inner wall of the tokamak type of nuclear fusion test reactor is found to be an efficient method of ensuring test conditions relatively free from such impurities. The inner wall, which is usually made of stainless steel or Inconel, is exposed to a low pressure electrical discharge through molecular hydrogen within the tokamak vessel which provides the atomic hydrogen necessary for the reduction process. As this discharge cleaning proceeds, the surface oxide and carbon are reduced, with the consequent emission of H₂O, CH₄ and CO as “impurities” from the walls into the chamber.

The analysis of these “impurity” gases during discharge cleaning indicates emission of wall impurity reduction products to be driven by a diffusion-controlled physical process. Evidence for this comes in part from the time-variation characteristics of emission products where a classical t^{-$\text{1}\text{2}$} dependence is found. Comparatively low values of the activation energy for oxide reduction (5.8 kcal mol^-1 or less) confirm this mechanism as the rate-determining step and can be shown to support the same mechanism even in situations where the impurity-time characteristics differ from the classical case. An oxide reduction sequence is presented which is consistent with the kinetic parameters derived from data on discharge cleaning. Kinetic similarities are also shown between the discharge cleaning process in tokamaks and the reduction of iron oxides by hydrogen at high temperature.

Interesting surface problems in this field are focused on by a brief review of representative discharge cleaning data, followed by an analysis and interpretation that approaches the problem from a kinetic-mechanistic viewpoint.