Optimal regimes for ignition and the Ignitor experiment

The optimal conditions under which confined plasmas can reach ignition are identified, referring in particular to the parameters of the Ignitor machine. The key importance of the radial profiles of the particle density, of the associated plasma pressure and of the thermal energy diffusivity, is shown. Peaked density profiles, such as those obtained in the Alcator A and C experiments (at about the same central density and magnetic field as those in Ignitor), characterized by minimal thermal diffusivities and high plasma purity, are best suited for ignition. The H mode regime can be accessed in Ignitor but is not considered a priority because of the typically flat density profiles it involves. The roles of collective modes and their interaction with both high and low energy alpha particle populations are assessed. For the modes generating sawtooth oscillations and involving magnetic reconnection the stabilizing effect of `shoulder' q(ψ) profiles is pointed out.