Abstract
Stability of high-beta plasmas is studied on discharges from a series of JET experiments on steady-state and hybrid advanced scenarios, with a wide range of safety factor (q) profiles and normalized beta values extending to βN = 4. Bursting and continuous forms of global n = 1 instabilities are encountered that degrade confinement or, in some cases, give rise to disruptions. Mode frequencies are well above the inverse wall time and correspond to plasma rotation at around mid-radius. Stability boundaries in terms of qmin and pressure peaking are examined. For relatively broad pressure profiles the stability limit decreases from βN = 4 at qmin = 1 to βN = 2 at qmin = 3, while at fixed qmin it decreases with increasing pressure peaking. Metastable and unstable regions are identified in the βN–qmin diagram by mode-trigger analysis. Tearing and kink mode structures are found from phase analysis of temperature profile oscillations; for a selection of kink cases, instability conditions and mode structure are compared with ideal stability calculations.
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