Ignition of an overheated, underdense, fusioning tokamak plasma

Methods of igniting an 'overheated' but 'underdense' D-T plasma core with a cold plasma blanket are investigated by using a simple two-zone model with a variety of transport scaling laws, and also using a one-dimensional transport code. The power consumption of neutral-beam injectors required to produce ignition can be reduced if the underdense core plasma is heated to temperatures much higher than the final equilibrium ignition values, followed by fuelling from a cold plasma blanket. The reduction in power consumption varies from a modest saving if energy confinement improves rapidly with increasing density in the reactor regime, to a significant saving if energy confinement improves slowly with increasing density. It is also found that the allowed impurity concentration in the initial hot core can be higher than normally permitted for ignition provided that the blanket is free from impurities.