Kink instabilities in high-beta JET advanced scenarios

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 q-profiles and a range of normalized beta extending to β N = 4. Bursting and continuous forms of global n = 1 instabilities limit the achievable β N or degrade confinement. Stability boundaries in terms of q min and pressure peaking are determined. For relatively broad pressure profiles the limit decreases from β N = 4 at q min = 1 to β N = 2 at q min = 3, while at fixed q min it decreases with increasing pressure peaking. Triggering mechanisms and the internal structure of continuous n = 1 instabilities are analysed. A new form of instability that grows on typical resistive timescales but has internal kink structure is identified.

Tearing mode formation induced by internal crash events at differentβN

“…In some aspects, the situation is similar to advanced tokamak regimes, in which ideal resistive wall modes convert into tearing modes with the same helicity. Such observations were reported by Buratti from JET tokamak [14].…”

RFX-mod: A multi-configuration fusion facility for three-dimensional physics studies

“…This is observed both in RFPs, 9 where the helical equilibrium tends to be more robust as the plasma current increases, 10 and in tokamaks, from early observations of density snakes 11 to recent results on persistent n ¼ 1 modes in advanced scenarios. 12 Spontaneous helical states can be interpreted both in RFP and tokamak as helical equilibrium states, as confirmed by simulations with 3D equilibrium codes initially developed for stellarators 13,14 and 3D nonlinear magnetohydrodynamic (MHD) codes. 15,16 3D equilibrium modeling, in particular, has recently predicted that the ITER hybrid scenario may have a helically symmetric core.…”

Large ELM-like events triggered by core MHD in JET advanced tokamak plasmas: impact on plasmas profiles, plasma-facing components and heating systems

“…T rad increases in the region of small optical thickness (τ op <<1) beyond the sepatatrix [7]. A fishbone mode is excited initially [8] in the vicinity of the ion ITB base at R = 3.55m [8]. The phase of the T rad perturbation is a continuous function of R and it corresponds to the kink like mode (in contrast to the tearing like mode, where the phase experiences a π-shift inside the zone where the mode is localised).…”