The optimal conditions under which confined plasmas can reach ignition are
identified, referring in particular to the parameters of the Ignitor machin
e. The key importance of the radial profiles of the particle density, of th
e associated plasma pressure and of the thermal energy diffusivity, is show
n. Peaked density profiles, such as those obtained in the Alcator A and C e
xperiments (at about the same central density and magnetic field as those i
n 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 dens
ity profiles it involves. The roles of collective modes and their interacti
on with both high and low energy alpha particle populations are assessed. F
or the modes generating sawtooth oscillations and involving magnetic reconn
ection the stabilizing effect of 'shoulder' q(psi) profiles is pointed out.