PLASMA
In the literature: open
Can helium ash be exhausted fast enough in a reactor
In plain words
Each fusion reaction leaves a helium nucleus behind, and if helium accumulates it dilutes the fuel and stops the burn. It must be removed faster than a certain rate, and it is not certain reactors can do so.
Precise statement
Steady burn requires the ratio $\rho = \tau_{\mathrm{He}}*/\tau_E$ of effective helium particle confinement time to energy confinement time below about 5 to 10. Predict $\rho$ for ITER and a reactor from core helium transport, edge and divertor helium enrichment and pumping. An answer is $\rho$ with uncertainty from validated core-edge-divertor modeling.
What would settle it
Helium transport and exhaust measurements in ITER D-T plasmas, or validated coupled modeling.