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, the thermal energy diffusivity, the plasma pressure, the function q(y), etc., is demonstrated. Peaked density profiles, such as those obtained in the Alcator A and C experiments (at about the same central density and magnetic field as in Ignitor), characterized by minimal thermal diffusivities and high plasma purity, are shown to be best suited for ignition. The H-mode regime in Ignitor is accessible but not considered a priority because of the typically flat density profiles. The role of collective modes and their interaction with both high and low energy a-particle populations are assessed. For the modes generating sawtooth oscillations and involving magnetic reconnection the stabilizing effect of "shoulder" q(y) profiles is pointed out together with the role of the trapped ion population.