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UKAEA-CCFE-PR(23)1582023
We propose a resistivity-beta driving mechanism aimed at explaining the appearance of free boundary long wavelength global instabilities in high-beta diverted tokamaks. These perturbations resemble very closely the RWM (resistive wall mode) phenomenon. Performing a proper toroidal analysis, we show that the magnetohydrodynamic stability is worsened…
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UKAEA-CCFE-PR(23)1202023
External kink modes, believed to be the drive of the beta-limiting Resistive Wall Mode, are strongly mitigated by the presence of a separatrix. We thus propose a novel mechanism explaining the appearance of long-wavelength global instabilities in free boundary high-beta diverted tokamaks, retrieving the experimental observables within a physical…
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UKAEA-CCFE-PR(22)052022
The analysis of quiescent discharges exhibiting edge harmonic magnetoydrodynamic activity in the JET-Carbon wall machine is presented. It is observed that the quiescent phase with multiple-n harmonic oscillations is sustained until a threshold in pedestal electron density and collisionality is crossed. The macroscopic pedestal parameters associated…
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UKAEA-CCFE-PR(21)812021
The impact of plasma shaping through magnetic well modifications on the stability of resistive ballooning modes in tokamaks is analysed, also including finite diamagnetic flows. Various limiting cases of the dispersion relation, obtained by matching the averaged ballooning equation across the ideal and resistive layers, are analysed. It is found th…
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UKAEA-CCFE-CP(21)082021
The pedestal plays an important role in determining the confinement in tokamak H-mode plasmas. However, the steep pressure gradients in this transport barrier also lead to edge localized modes (ELMs) [1]. There is good understanding of the pedestal in type I ELM regimes [2], however, type I ELMs are known to damage plasma facing components and f…
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UKAEA-CCFE-PR(21)372021
This work presents an extension of exfernal mode theory where the effects of edge magnetic shear and plasma separatrix are investigated., from which a set of three coupled differential equations describing the dispersion relation are derived. To correctly assess the effect of edge shear on exfernal modes, higher order corrections need to be retaine…
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UKAEA-CCFE-PR(20)1182020
In this work the mechanisms that can lead to disruptive MHD instabilities in the termination phase of plasma pulses on JET are investigated. It is shown that the broadening or the shrinking of the current density profile, as a consequence of a core hollowing or a edge cooling of the electron temperature profile, can lead to the destabilization o…
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UKAEA-CCFE-PR(20)772020
Anisotropy and some limiting toroidal flow effects on the stability of non resonant ideal magnetohydrodynamic modes in hybrid shaped tokamak plasmas are investigated within the ideal MHD infernal mode framework. Such effects are found to alter the plasma magnetic well/hill, which can be interpreted as imparing the average curvature, and the stre…
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UKAEA-CCFE-PR(20)762020
In tokamak operation the control of dangerous MHD instabilities, possibly in r-t scenarios, must rely on prompt robust diagnostics of the state and stability of the system. The set of magnetic signals measured on the outside of the plasma boundary, based on the Zakharov-Shafranov, Shkarowsky, Wootton (ZSSW) current moments has been always used f…
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