Burn threshold for fusion plasmas with helium accumulation

The ignition and burn criteria for fusion plasmas are re-evaluated, taking into account the ratio of α particle confinement time τα to energy confinement time τ. Due to helium ash build-up, the burn threshold is raised substantially compared to the usual ignition criterion. No steady state burn is possible when τα/τ>15, and even small concentration of impurities can substantially reduce this upper limit, resulting in stringent requirements on radial transport, recycling and pumping of helium ash. H-mode plasmas, which may exhibit high values of τα, are undesirable for steady state burning, since large τα implies low fueling rate and hence low fusion power yield. (author) 3 refs., 3 figs