H-mode pedestal characteristics and MHD stability of the edge plasma in Alcator C-Mod

Under most operational conditions of Alcator C-Mod, the dominant type of H-mode is the steady state enhanced D α (EDA) mode, characterized by good energy confinement, continuously degraded impurity confinement and absence of regular edge localized modes (ELMs). In this regime, a quasicoherent (QC) electromagnetic mode ( k θ ~5 cm -1 , f ~100 kHz) is observed, localized in the region of the density pedestal. Experimental evidence suggests that the mode is responsible for enhancement of particle transport. It is shown experimentally that the QC mode can exist in a well defined region in edge temperature-safety factor space, favouring high edge q values ( q 95 ⩾3.5) and requiring moderate pedestal temperature T e ped ⩽ T c ( T c ~400 eV). As edge temperature and pressure gradient increase, the QC mode is replaced by broadband low frequency fluctuations ( f <50 kHz). Small grassy ELMs are observed in these discharges. Analysis of ideal ballooning stability of the C-Mod edge shows that the edge pressure gradient is not limited by infinite n ideal ballooning mode if edge bootstrap current, even reduced by high edge collisionality, is taken into account. Linear stability analysis of coupled ideal peeling/ballooning medium n modes, driven by combination of edge pressure and current gradients, shows that the modes become marginally unstable at C-Mod edge in the range of pressure gradients where the grassy ELMs are observed (∇ P ⩾1.2×10 7 Pa Wb -1 rad). This result is consistent with a model of the ELMs as intermediate n peeling/ballooning modes. On the other hand, demonstrated stability of the ideal modes in EDA regime, together with the fact that low T e ped is required for its existence, supports theoretical models showing resistive character of the QC mode.