Rollover: confinement degradation during TFTR discharges

The time evolution is described of TFTR neutral beam heated plasmas that often featured a decreasing energy content called `rollover'. Rollover affects the gross plasma parameters such as energy content and fusion power produced, as well as the central plasma parameters and the plasma transport coefficients. Rollover was studied using a database of all the TFTR plasmas produced over a two year period. The plasma behaviour at the time of peak energy content was related to the behaviour at a later quasi-equilibrium time. The main conclusion is that limiter power handling issues were the underlying cause of TFTR rollover. This study re-emphasizes the fact that power handling is an important issue for plasma facing components in tokamaks.