Burn condition, helium particle confinement and exhaust efficiency

The burn condition for D-T plasmas is derived in a form which accounts for the stationary helium concentration determined by the coupling of the fusion power and helium production. The ratio of the global alpha particle confinement timeto the energy confinement time τE is taken as a quantity characterizing the prevailing confinement and exhaust regime. For an ignited and stationary burning D-T plasma this ratio needs to remain sufficiently below 15 or below 10 for typical impurity concentrations. This poses a lower limit on the required helium exhaust efficiency and indicates that operational regimes are needed where the particle confinement time does not diverge too far from the energy confinement time. Applied to the D-3He fusion reaction, the analysis leads to even more stringent limits on the ratio,which must be smaller by a factor of three to four compared to the D-T case. It is found that, for any given temperature (and for otherwise equal exhaust and recycling conditions), in general burning can be achieved at two different values of the fusion parameter n T τE and ash concentration.