Effects of low-Z impurities during the start-up phase of a large tokamak

The requirements placed on a tokamak Ohmic heating system (i.e. loop voltage) to initiate the plasma become more severe as the size increases because of the current density decrease. During the start-up phase even small concentrations of low-Z impurities can affect the plasma energy balance very substantially and have very important effects on the evolution of the discharge. The start-up phase has been studied using a simple zero-dimensional computer code. Since the dominant energy loss mechanisms during start-up, radiation and ionization, are a volume effect, the zero-dimensional code was adequate to treat this phase. The results of this study which have been applied to TFTR indicate that the plasma evolution is a sensitive function of the applied loop voltage, impurity concentration, initial filling pressure and the manner in which gas is fed into the discharge.