Data di Pubblicazione:
2012
Citazione:
UNDERSTANDING AND PREDICTING ION HEAT TRANSPORT IN TOKAMAKS / B. Baiocchi ; tutor: R. Pozzoli ; co-tutor: P. Mantica ; coordinatore: M. Bersanelli. Universita' degli Studi di Milano, 2012 Feb 17. 24. ciclo, Anno Accademico 2011. [10.13130/baiocchi-benedetta_phd2012-02-17].
Abstract:
One of the most attractive options to satisfy the continuously growing world energy demand is controlled thermonuclear fusion. The scientific and technological work for achieving it has been significantly boosted after the recent decision to build the international tokamak ITER (International Thermonuclear Experimental Reactor). Amongst the physical problems still open, understanding and controlling heat transport is of primary importance for the optimization of the operational scenarios of ITER. Given the complexity of plasma transport processes, a full theoretical understanding of the experimental observations and validated numerical models for the simulation of a complete tokamak discharge are not yet available. Work in this field is therefore actively ongoing, with a view to increasing integration between theoretical developments, experimental results and numerical predictions. This is the context in which the present thesis work takes place. It has long been known that the high measured levels of heat transport in tokamaks are due to turbulent phenomena, in particular the so-called drift waves. The ion heat transport, on which is focused this thesis, is carried by ion temperature gradient (ITG) modes, that are destabilized when a threshold value of the inverse ion temperature gradient length (1/LTi=|∇Ti/Ti|) is exceeded. Above threshold, the ion heat flux is a strongly increasing function of 1/LTi, which prevents the Ti profiles from departing significantly from threshold, a property known as profile stiffness. The main target of ion heat transport studies is to find ways to suppress or mitigate ITG modes, namely by increasing the threshold or reducing the stiffness level, in order to be able to achieve high core Ti values without having to rely on too high edge Ti values, which would raise plasma-wall interaction issues. Sophisticated ion heat transport experiments carried out at the JET tokamak have recently indicated that a strong reduction of ion stiffness takes place in presence of low magnetic shear and high toroidal rotation. This mechanism has been proposed as the key ingredient to explain the improved core ion confinement observed in Hybrid scenarios or Advanced Tokamak (AT) scenarios with Internal Transport Barriers, two regimes that are considered for ITER operations beyond the standard inductive H-mode regime.
This thesis work starts from the above mentioned JET results and from the already developed theoretical models and existing numerical codes, and includes four main items of work, with the purpose of integrating experimental analysis and theory-based numerical modelling of JET experiments, in order to reach predictive capabilities for the future tokamak FAST, a device proposed by the Italian Fusion Association as a possible ITER satellite. First, new experiments have been carried out in JET and analyzed in detail, in order to assess if the cause for ion stiffness reduction is the rotation value or the rotational shear. The data analysis has given as result that it is the absolute value of the rotational shear the key factor for ion stiffness mitigation. This gives the indication for ITER that the necessary condition for reducing the ion stiffness and access improved core confinement regimes is to induce some rotational shear, which may be easier than achieving high absolute values of rotation.
Second, a numerical study has been carried out in JET and ITER plasmas to quantify the impact of ion threshold and stiffness on global confinement and fusion power compared to the effect of edge Ti value. This work has the aim of evaluating if threshold and particularly stiffness are indeed two useful control tools for scenario performance optimization. The variation of global confinement has been found quantitative
Tipologia IRIS:
Tesi di dottorato
Keywords:
plasma physics ; tokamak ; ion heat transport ; ITG ; threshold ; stiffness ; scenarios prediction ; FAST ; JET ; ITER ; gyro-kinetic simulation
Elenco autori:
B. Baiocchi
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