The total amount of heating power coupled to the plasma Ptot and the energy confinement time are determining parameters for realizing the plasma conditions suitable for the reactor. We recall that the ignition condition can be expressed by the following condition on the triple fusion product :

NT = Ptot2/3 Vol = 3N2T2Vol/Ptot > (NT)ignition (1)

with T ~= 15keV

where = E/Ptot is the energy confinement time, E = 3NT Vol for an isothermal plasma with Ti = Te = T and a plasma volume Vol; N is the plasma density. The value T ~= 15 keV corresponds to the minimum value of (NT)ignition as a function T (see Fig. 1). In the present discussion for the sake of simplicity, we neglect density and temperature profile factors. The heating power in most of the present experiments is given by Ptot = POH + Padd where POH is the ohmic power and Padd is the additional heating due to neutral beam injection or R.F. heating. At ignition, the additional heating power must come completely from the energetic particles produced by the fusion reactions and we must have Ptot = P if we neglect the residual POH and the plasma losses by Bremsstrahlung (PBr [is proportional to] N2T1/2).