Cristina Rea

3D effects on the RFX-mod boundary

In present fusion research a strong effort is devoted to the studies of non-axisymmetric magnetic perturbations and consequent field ergodization on the external region of the plasma. On this topic interesting results can be drawn from the helical configuration observed in high-current regimes in reversed field pinches (RFPs) where the small edge helical ripple is sufficient to modulate the plasma?wall interaction and the plasma kinetic properties. This paper presents the most recent experimental results and physical interpretation of the phenomena observed in the edge region of the RFX-mod RFP device. Experimental observations indicate that plasma pressure and floating potential are spatially modulated according to the helical deformation. Helical flow is observed at the edge as a consequence of an ambipolar electric field. Emphasis will be devoted to the determination of the actual phase relation between magnetic perturbation and velocity perturbation. Evidence of the influence of the helical ripple on turbulence properties at the edge is also reported.

Magnetic perturbations as a viable tool for edge turbulence modification

A complete description of the effects of magnetic perturbation on the edge region of RFX-mod is here reported. The flexibility of the RFX-mod device [1] allows for the operation of the machine both as a reversed field pinch (RFP, with maximum current 2?MA) and as a low-current, circular ohmic tokamak (Ip,max?=?0.15?MA). The present paper summarizes the most recent results obtained in both configurations with either spontaneous or induced edge radial magnetic perturbation. Emphasis will be devoted to the experimental characterization of the edge flow, focusing on the phase relation between flow and perturbed magnetic field. These informations are provided for natural and stimulated helical discharges in RFPs, and for tokamak safely operated, thanks to the unique RFX-mod MHD control system, in a wide range of edge safety factor 1.9???q(a)???3.4 with externally imposed helical boundary. For the first time a detailed comparison between this phenomenology in tokamaks and RFPs will be presented, providing experimental measurement of the streamline of E?? B flow around the magnetic perturbation and of the density modulation which exhibits the same periodicity of the perturbation. Strong new indication of the modification of the small scale turbulence in presence of magnetic perturbation is reported: this modification is deeply connected to the variation of turbulence induced particle transport.

Turbulent electromagnetic filaments in actively modulated toroidal plasma edge

Filament or blob structures have been observed in all magnetic configurations with very similar features despite the difference in the magnetic geometry, and are believed to play an important role in convecting particles and energy towards the wall. Despite their different generation mechanism, turbulent structures and edge-localized mode (ELM) filaments share some common physical features. The electromagnetic effects on filament structures deserve particular interest, among others reasons for the implication they could have for ELM, related for instance to their dynamics in the transition region between closed and open field lines or to the possibility, at high beta regimes, of causing line bending which could enhance the interaction of blobs with the first wall.A direct characterization of the effects of active modification of the edge topology on EM turbulent filament structures is presented, comparing reversed field pinch and tokamak configurations. Measurements are obtained in the RFX-mod device, which allows operation in both configurations and with different equilibria. The RFX-mod experiment versatility is exploited also from the point of view of the active control of the edge magnetic topology, equipped with an advanced system for edge boundary feedback control. Three different case studies of actively controlled magnetic perturbations are shown, focusing on the filament interaction with local magnetic islands. High-frequency fluctuations, characterizing electrostatic and magnetic filament features, and the associated transport coefficients have been observed to be strongly affected by the island proximity and topology.

Exploratory Machine Learning Studies for Disruption Prediction Using Large Databases on DIII-D

Abstract Using data-driven methodology, we exploit the time series of relevant plasma parameters for a large set of disrupted and non-disrupted discharges from the DIII-D tokamak with the objective of developing a disruption classification algorithm. We focus on a subset of disruption predictors, most of which are dimensionless and/or machine-independent parameters such as the plasma internal inductance and the Greenwald density fraction , coming from both plasma diagnostics and equilibrium reconstructions. The utilization of dimensionless indicators will facilitate a more direct comparison between different tokamak devices. In order to eventually develop a robust disruption warning algorithm, we leverage Machine Learning techniques, and in particular, we choose the Random Forests algorithm to explore the DIII-D database. We show the results coming from both binary (disrupted/non-disrupted) and multiclass classification problems. In the latter, the time dependency is introduced through the definition of class labels on the basis of the elapsed time before the disruption (i.e., ‘far from a disruption’, ‘within 350 ms of disruption’, etc.). Depending on the formulation of the problem, overall disruption prediction accuracy up to 90% is demonstrated, approaching 97% when identifying a stable and a disruptive phase for disrupted discharges. The performances of the different Random Forest classifiers are discussed in terms of accuracy, by showing the percentages of successfully detected samples, together with the false positive and false negative rates.

A Customer Relationship Management Case Study Based on Banking Data

This work aims to show a product recommender construction approach within the banking industry. Such a model costruction should respect several methodological and business constraints. In particular, analysis’ outcome should be a model which must be easily interpretable when shown to business people. We start from a Customer Relationship Management data set collected in Banking industry. Formerly, data is prepared by managing missing values and keeping only the most relevant variables. Latterly, we apply some algorithms and evaluate them using diagnostic tools.

Comparative studies of electrostatic turbulence induced transport in presence of resonant magnetic perturbations in RFX-mod

Three-dimensional non-axisymmetric magnetic fields are purposely applied to toroidally symmetric fusion plasmas in order to modify transport of heat and particle fluxes to the plasma-facing components (PFCs), and specifically to control and suppress edge localized modes (ELMs) in H-mode tokamak plasmas. This paper presents a comparative study between two magnetic configurations available in the RFX-mod device, namely the original reversed-field pinch configuration and the more recent low-field circular ohmic tokamak. The paper presents a detailed analysis of the modulation of electrostatic turbulence and induced transport in the presence of an externally applied magnetic perturbation, as measured in both configurations. A detailed study of the underlying magnetic topology in both configurations is given through the field line tracing code FLiT. The electrostatic turbulence induced flux is observed to be modulated by the underlying topology, with an enhancement close to the O-point and a reduction at the X-point of the induced magnetic island. The transport modification are discussed in terms of relevant frequency and wavenumber spectrum.