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• W2102613360 abstract "This Thesis presents a series of results on the development of advanced magnetic feedback schemes for the active control of magnetohydrodynamic (MHD) instabilities and error elds obtained in two magnetically conned toroidal experiments:the RFX-mod reversed-eld pinch (RFP) in Padova, Italy, and the DIII-D tokamak at General Atomics, San Diego, CA, USA.In the last years, these two devices have explored dierent types of highperformance regimes, also thanks to their sophisticated active control systems.In RFX-mod, high-plasma current experiments, up to 2MA, were performed for the rst time in a RFP. These experiments allowed for the discovery of a new self-organized helical equilibrium with good connement properties [40]. Instead, in DIII-D, steady-state, high-performance tokamak operations are being explored.The scientic programs of these experiments, in particular on error eld and MHD mode control, can give precious contributions to the International Thermonuclear Experimental Reactor (ITER) and to magnetic fusion research in general.The RFP and the tokamak are toroidal devices for the magnetic connement of thermonuclear plasmas. An introduction to thermonuclear fusion, the main requests to exploit fusion as a future energy source, the magnetic connement of the plasma, and the MHD model which describes the plasma behavior in many cases of interest will be given in Chapter 1. The role of magnetic feedback control for the development of advanced operational regimes in RFX-mod and in DIII-D will be also discussed in this Chapter.Chapter 2 describes the two experiments above mentioned and their magnetic feedback control systems. They are in fact equipped with very exible systems devoted to the control of MHD instabilities and error elds. In particular the feedback control strategies that are crucial for the work discussed in this Thesis will be presented.The rst important result of this Thesis is reported in Chapter 3 and regards the optimization of multi-mode control of tearing instabilities in RFX-mod.Tearing modes, which sustain the reversed-eld conguration typical of RFP experiments through a dynamo mechanism, can not be completely suppressed by magnetic feedback control. Nonetheless, it is important to reduce their edge radial magnetic eld amplitude to the lowest possible value, since it produces a deformation of the last closed ux surface, enhancing the plasma-wall interaction. In this work, the control of tearing modes has been optimized by using a non-linear model of the tearing mode dynamics in presence of the multiple-shell layout of RFX-mod and of the magnetic feedback system. This model of tearing modes has been implemented in a code named RFXlocking and previously described in [84]. Given the good match between the model predictions and the experimental mode behavior, the RFXlocking code has been used as a tool to identify a new set of mode control parameters (i.e. feedback gains), which allow to reduce the radial magnetic eld of multiple tearing modes at the plasma edge, maintaing at the same timethe modes into rotation and avoiding coil current saturations. The optimization approach consisted in simulating the mode dynamics varying the feedback gains and identifying the gain set, which fullls the requirements above described. Once the gain set was identied, an extensive experimental campaign was performed on RFX-mod, obtaining satisfactory results in terms of edge radial magnetic eld reduction and also conrming the code predictions.The magnetic feedback optimization performed during this Thesis work concerned not only tearing modes, but also the main magnetic eld errors present in RFX-mod. The presence of poloidal gaps in the RFX-mod wall modies the pattern of eddy-currents induced in it by the vertical magnetic eld during the plasma current ramp-up, thus forming toroidally-localized error elds to which tearing modes are phase-locked. Two advanced feedback control strategies have been applied to correct these error elds: a multi-mode control scheme and adynamic decoupling scheme.Regarding the rst feedback control strategy, a Simulink model of the RFXmod magnetic feedback system has been used to identify the feedback gains, which allow a signicant reduction of the error eld amplitude, avoiding coil current saturations.A dynamic decoupler has also been used to compute oine the feedback currents needed to cancel the error elds. As will be described in Chapter 4, these two techniques have been tested during a dedicated experimental campaign. The best result in terms of error eld reduction has been obtained when both multimode control and the decoupler are used. With error eld correction during the plasma current ramp-up, the phase-locking among tearing modes is no more localized near the poloidal gaps of the wall, thus reducing the plasma-wall interaction at these positions.As mentioned above, the high-current RFX-mod experiments have disclosed a promising physics regime, where the RFP spontaneously evolves towards an Ohmic helical equilibrium. This new magnetic equilibrium is characterized by a single helical magnetic axis and helical magnetic surfaces in the plasma core. This leads to a signicant decrease in the stochastic transport and to the formation of core electron temperature barriers. During the last experimental campaign, it has been demonstrated that a (1;-7) helical equilibrium can be sustained and controlled by applying helical boundary conditions at the plasma edge through magnetic feedback. In this Thesis work, Chapter 5 and Chapter 6 deal with the optimization of the helical boundary conditions used to control the helical equilibrium. The optimization procedure uses control strategies analogous to those described above and adopted to improve the control of tearing modes and error elds. The RFXlocking code has been modied by adding the possibility to apply a helical boundaryconditions. In this way, the mode dynamics has been simulated with this new helical boundary, by varying the feedback gains and the amplitude and phase of the helical magnetic eld perturbations imposed at the plasma edge. A model-basedoptimization approach similar to the one described in Chapter 3 has been adopted here to identify the feedback gains that allow to produce the requested radial eld pattern at the edge with the lowest possible coil current. A partial gain scan has been performed in the experiment and the results conrm the model predictions.The main outcomes of the model-based optimization and an analysis of the effects on the plasma performance of the applied helical boundary conditions are described in Chapter 5.Vacuum eld analyses described in Chapter 6 reveal that, when rotating magnetic eld perturbations are applied through magnetic feedback, as in the case of the helical equilibria above described, error elds are induced by the frequency response of the wall to external magnetic elds varying in time. These error elds, that are mainly introduced by the presence of the toroidal and poloidal gaps in the wall structure, may somehow aect the good connement properties of the helical equilibrium. For this reason, a dynamic decoupler similar to the one used to correct the error elds in the current ramp-up phase of the plasma discharges has been applied. Encouraging results in terms of error eld reduction are obtained.The frequency-response of the wall to any external time-varying magnetic eld has been investigated also in the DIII-D tokamak, in the framework of a collaboration between the RFX-mod and DIII-D teams. In the DIII-D control algorithm, the magnetic feedback measurements are usually real-time compensated for spurious magnetic elds, due for instance to the feedback and axi-symmentric coils.These contributions are calculated from the zero-frequency coupling coecients between each actuator and sensor. In this way the eects of eddy-currents induced in the wall are neglected. The relevance of these frequency-dependent, orAC eects, on RWM and error eld control has been evaluated by analyzing past experiments. The analyses suggested that, if the wall frequency response is not taken into account in the feedback compensation scheme, error elds can be introducedwhen doing magnetic feedback. These can be important especially at high β, where uncorrected error elds can be strongly amplied by the plasma.An AC compensation algorithm has been implemented and tested in real-time in dry-shots and Ohmic plasmas. More tests of this algorithm at high β have been proposed for the next experimental campaign to assess its relevance on plasmaperformance in scenarios where the plasma is less resilient to error elds. The main outcomes of this Thesis work is reported in Chapter 7.Chapter 8 summarizes the main conclusions of this work and describes a series of experiments that could be made both in RFX-mod and DIII-D in the near future, to further develop the studies started with this Thesis." @default.
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• W2102613360 date "2011-01-31" @default.
• W2102613360 modified "2023-09-24" @default.
• W2102613360 title "Improved feedback control of MHD instabilities and errors fields in reversed-field pinch and tokamak" @default.
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