UNDERSTANDING AND PREDICTING ION HEAT TRANSPORT IN TOKAMAKS
Creators: name: BAIOCCHI, BENEDETTA
Publisher: name: Universit� degli Studi di Milano
Year: 2012

description: 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 quantitatively significant for changes of the ion stiffness, and comparable with the ones due to changes of ion threshold and Ti pedestal height, when they are varied in an experimentally realistic range. In ITER, the calculated fusion power, which is what really matters for a fusion device, is as much affected by variations of ion stiffness as by changes of ion threshold and Ti pedestal height. This work gives the indication that all the three investigated parameters influence comparably the core performances in present and future machines. In particular the quantitative level of ion stiffness, which is a parameter not much considered until now, and assumed or predicted very high in most existing models, can be a useful knob to act upon in order to optimize the scenario performance and must be taken into account for an accurate predictive modelling of future machines. Third, a prediction work for the foreseen scenarios of FAST has been carried out, using a mixture of first-principle models and experiment driven considerations. The results obtained in the two previous steps have led to the conviction that predictive modelling of future devices cannot neglect including toroidal rotation profiles and their effects on transport, which is not common practice in tokamak simulation work. Both H-modes and fully non-inductive AT scenarios have been simulated, predicting profiles of current, ion and electron temperature, density and toroidal rotation. Various heating options have been explored. The simulations have provided a set of FAST scenarios in which fast particle and burning plasma studies can be performed, reaching values of thermal and fast particle energy contents well in line with the needs for exciting meso-scale fluctuations with the same characteristics of those expected in reactor relevant conditions. Fourth, linear gyro-kinetic simulations have been carried out to check the validity of simplified threshold formulae used in simulations in the high toroidal field and high density FAST plasmas. Very good agreement was found between the analytical threshold approximation and the GKW simulations with adiabatic electrons, whilst the threshold with kinetic electrons is slightly lower. The discrepancy is anyway small enough to justify the use of the threshold analytic approximation for FAST simulations, taking into account the other sources of uncertainty linked to various other modelling approximations and to empirical extrapolations from experimental data of existing machines.