Neoclassical islands, beta-limits, error fields and ELMs in reactor scale tokamaks

An assessment is presented of the impact of recent magnetohydrodynamic rese arch results on performance projections for reactor scale tokamaks as exemp lified by the ITER Final Design Report (ITER/FDR) facility. For nominal ELM y H mode operation, the presence and amplitude of neoclassical tearing mode s governs the achievable beta value. Recent work finds that the scaling of beta at which such modes onset agrees well with a polarization drift model, with the consequence that, with reasonable assumptions regarding seed isla nd width, the mode onset beta will be lower in reactor scale tokamaks than in contemporary devices. Confinement degradation by such modes, on the othe r hand, depends on relative saturated island size which is governed princip ally by beta and secondarily by upsilon* effects on bootstrap current densi ty. Relative saturated island size should be comparable in present and reac tor devices. DT ITER demonstration discharges in JET exhibited no confineme nt degradation at the planned ITER operating value of beta(N) = 2.2. Theory indicates that electron cyclotron current drive can either stabilize these modes or appreciably reduce saturated island size. Turning to operation in candidate steady state, reverse shear, high bootstrap fraction configurati ons, wall stabilization of external kink modes is effective while the plasm a is rotating but (so far) rotation has not been maintained. Recent error f ield observations in JET imply an error field size scaling that leads to a projection that the ITER/FDR facility will be somewhat more tolerant to err or fields than thought previously. ICRF experiments on JET and Alcator C-Mo d indicate that plasmas heated by central energetic particles have benign E LMs compared with the usual type 1 ELM of NBI heated discharges.