In a deuterium-tritium plasma the reaction D + T gives He-4 (3.5 MeV) + n (14.1 MeV), 17.59 MeV in total. The volumetric fusion power is P_f = n_D n_T <sigma v> E_f with n_D = n_T = (1 - d) n / 2. The reactivity <sigma v>(T) uses the Bosch-Hale parametrisation (valid 0.2 to 100 keV). Only the alpha particles (one fifth of the energy) stay in the plasma; the neutrons leave and are used in the blanket.

Power balance: heating P_alpha + P_ext equals losses 3 n T / tau_E + P_brems, where bremsstrahlung is 5.35e-37 n^2 sqrt(T) W/m3 (T in keV, Z = 1). The gain is Q = P_f / P_ext. Setting P_ext = 0 gives the ignition condition n tau_E = 3 T / (E_alpha <sigma v> / 4 - C sqrt(T)), whose minimum triple product is about 3e21 keV s/m3 near 14 keV (the Lawson criterion).

Simplifications: uniform profiles, equal ion and electron temperatures, a single confinement time, no synchrotron radiation, no helium ash accumulation, no stability limits (the 3D page covers qualitative plasma instability). This is a teaching model, not a reactor design tool.