Impurity Control and Its Impact Upon Start-up and Transformer Recharging in NET

Control of the release of impurities and their subsequent ingress and exhaust from tokamak plasmas has been the subject of intensive studies aimed at both the prediction of reactor burn condition and the interpretation of results from present experiments. In contrast, control concepts which are specific to current-initiation, current ramp-up and RF heating during start-up of a reactor such as NET have to date received but little attention. The requirements for impurity control during the burn phase can be summarised as follows. Firstly, the necessity to dissipate about 80 MW of non-radiated plasma power without incurring either an excessive release of impurities from the plasma collection surfaces or an unacceptable buildup of impurities within the hot reacting core of the D/T plasma. Secondly, the ability to pump neutral helium gas at the rate of about 2 × 1020 atoms/s in such a manner that the helium ash concentration within the reacting plasma does not exceed ~ 10%. The level of impurity contamination should neither degrade energy confinement (which implies a maximum concentration of 10−3 to 10−4 respectively of medium to high atomic number impurities) nor cause excessive contribution to the plasma β (which implies a typical concentration of light impurities of ~ 10−2). During start-up, the concentration of impurities is not likely to be critical for current-initiation but, especially in the case of high atomic number elements, it becomes significant during the ramp-up and heating phases and it will have a powerful effect upon the ignition margin.