MATERIAL RELATED ASPECTS OF DENSITY CONTROL IN JET PLASMA DISCHARGES

A study of density control in JET has revealed that dynamic retention in the beryllium surface of the first wall is an important factor in controlling recycling. A self-consistent global particle balance model that simulates the recycling in JET has been devised. The assumption is that particle transport is effectively diffusive in the materials encompassing the plasma and particle release is by recombination to molecules. The effect of neutral particle screening in the plasma edge is also globally taken into account. Comparison with experimental results of discharges of normal duration (20 s) shows good agreement. Discrepancies appear for 60 s long discharges, suggesting changes of material properties.