Heating, Confinement and Extrapolation to Reactors

The total amount of heating power coupled to the plasma P_tot 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 = P_tot²/3 Vol = 3N²T²Vol/P_tot > (NT)_ignition (1)

with T ~= 15keV

where = E/P_tot is the energy confinement time, E = 3NT Vol for an isothermal plasma with T_i = T_e = 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 P_tot = P_OH + P_add where P_OH is the ohmic power and P_add 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 P_tot = P if we neglect the residual P_OH and the plasma losses by Bremsstrahlung (P_Br [is proportional to] N²T^1/2).