Ash and burn control in ITER-type tokamak

Ash control in the long-pulse tokamaks such as the ITER is addressed from the point of view of sawtooth pumping and fishbone excitation. An analytic model for predicting the influence of sawtooth and edge pumping on the helium contamination has been developed. One of its uses is to compute the ash density level for a given fusion power versus helium pumping fraction. The change of ash concentration due to sawtooth disruptions is also evaluated. The results show that the helium contamination can be changed drastically if pumping is strong enough. The results also show that adding sawteeth to help remove the ash from the center of the plasma becomes useful when the temperature profiles are parabolic or flatter. The controlled excitation of fishbone oscillations can directly help to control the ash accumulation by ejecting the alphas as they slow down to low energies. Estimates of the fishbone instability threshold rule out low frequency fishbones.<>