Abstract
Disruptions in tokamaks are very often associated with the slowing down of magneto-hydrodynamic (MHD) instabilities and their subsequent locking to the wall. To improve the understanding of the chain of events ending with a disruption, a statistically robust and physically based criterion has been devised to track the slowing down of modes with toroidal mode numbers n = 1 and mostly poloidal mode number m = 2, providing an alternative and earlier detection tool compared to simple threshold based indicators. A database of 370 discharges of axially symmetric divertor experiment—upgrade (AUG) has been studied and results compared with other indicators used in real time. The estimator is based on a weighted average value of the fast Fourier transform of the perturbed radial n = 1 magnetic field, caused by the rotation of the modes. The use of a carrier sinusoidal wave helps alleviating the spurious influence of non-sinusoidal magnetic perturbations induced by other instabilities like Edge localized modes (ELMs). The indicator constitutes a good candidate for further studies including machine learning approaches for mitigation and avoidance since, by deploying it systematically to evaluate the time instance for the expected locking, multi-machine databases can be populated. Furthermore, it can be thought as a contribution to a wider approach to dynamically tracking the chain of events leading to disruptions.
Highlights
The most promising and studied reactor configuration aimed at producing electricity from nuclear fusion is the Tokamak, in which specific mixtures of gases are confined in a toroidal shaped chamber by a magnetic field having two main components: a toroidal and a poloidal one [1]
The main effect of the added sinusoidal function is to lock the average frequency of rotation of the mode (6) to a reference value, ν, when no mode can be observed or when it rotates at a frequency follows the averaged frequency evaluated by pulse the fast Fourier transform (FFT)
An estimator based on the FFT of the integrated radial perturbation of the magnetic field at axially symmetric divertor experiment—upgrade (AUG) has been described to track the slowing down of n = 1 instabilities, using only 10kHz signals of four saddle coils
Summary
The most promising and studied reactor configuration aimed at producing electricity from nuclear fusion is the Tokamak, in which specific mixtures of gases (hydrogen and its isotopes) are confined in a toroidal shaped chamber by a magnetic field having two main components: a toroidal and a poloidal one [1]. The magnetically confined plasma is a challenging system, both under a physical and an engineering point of view, since it requires achieving high performances and stability at the same time The latter, being the primary matter of concern for industrial applications, is often compromised by the so called plasma instabilities, whose occurrence can affect the performances and even cause the premature termination of the pulses; the study of their zoology in a plasma for fusion applications represents a wide branch of the existing literature [1,2,3,4].
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