Dottorato in ENERGIA E AMBIENTE

Titolo della tesi: Prediction and analysis of JET fusion performance based on reduced first principle transport models

Plasma operating conditions in Tokamaks can be estimated using empirical scaling laws. Usually, the design of new machines deals with the technological limits of the components and the desired operating conditions by means of plasma engineering (0D) parameters. However, plasma performance depends on local plasma parameters (1D) and on the operating scenario. Plasma scenarios can be assessed and optimised to achieve the desired operating conditions. Energy and particle transport in fusion plasmas is one of the main actors in determining the evolution of a plasma scenario both in present experiments and in future reactors. The Joint European Torus (JET) experiment has operated in deuterium (D) and tritium (T) main ion plasma composition in 1997 (DTE1) and in 2021 (DTE2). The most important differences between the two experimental campaigns are related to the differences in the plasma facing components, carbon (C) in DTE1 and Be/W in DTE2, the increased additional heating power and the presence of improved diagnostics, especially in the plasma edge which is determinant in the global plasma performance. After DTE1 the high levels of T retention in the C-wall have been considered unacceptable for a reactor leading to a change in the design of the International Tokamak Experimental Reactor (ITER) with the substitution of the C-wall with a metallic wall. DTE2 campaign at JET aimed at studying D-T plasmas in the closest conditions to ITER operations. On the contrary of DTE1 the recent campaign focused on the stationarity of the performance and on addressing ITER-relevant aspects such as α-particles physics, plasma wall interactions and plasma heating schemes. In preparation to D-T operations, a wide experimental and modelling activity has been performed at JET in order to optimise the plasma scenarios. In this thesis the development of the JET baseline scenario will be described. The baseline scenario is a high confinement mode (H-mode) plasma, characterized by the presence of Edge Localized Modes (ELMs), where the confinement relies on high plasma current. In ITER D-T operations, the baseline scenario is envisaged to achieve a gain factor, defined as the ratio between the fusion power and the input power, Q = Pfus/Pin ≈ 10. In this work, starting from the results achieved at JET in D plasmas, through integrated modelling, the extrapolations in D-T main ion plasma composition will be shown. To do so, reduced first principles transport models have been used under different assumptions, and in a wide range of plasma operating conditions. QuaLiKiz and TGLF transport models have been validated in reference D plasmas, and their extrapolation capability with different plasma parameters has been tested performing blind predictions. The results of the predictive modelling will compared with the experimental data and analysed in order to address the sensitivity of the plasma scenario to the experimental boundary conditions. The QuaLiKiz transport model has also been validated against the experimental results produced at JET in DTE1. Before the beginning of the DTE2 campaign, an estimate of the particle sources required to sustain a 50-50 D-T baseline plasma has been obtained. The results of the predictive simulations presented in this work helped JET control team to prepare the baseline fuelling scheme. This contribution boosted JET D-T operations iv v without spending experimental time, neutron and T budget in optimising the fuelling control. The results of the predictive modelling performed in preparation to DTE2 are presented and discussed. The sensitivity of the predictions to plasma parameters such as current, toroidal magnetic field, pedestal confinement and impurity content are analysed together with the sensitivity to the available amount of auxiliary heating power. We will present and discuss the experimental results obtained in DTE2 by the baseline scenario. In the last part of this thesis, the implications of the modelling assumptions performed on D pulses will be compared with the assumptions done on D-T discharges with the experimental boundary conditions. The key parameters needed for reliable predictions of future experiments will be discussed both in D and D-T main ion plasma composition. The estimate of particle sources obtained before the DTE2 campaign has been adjusted to reproduce the experimental conditions, leading to an estimate of the different fuelling channels and an evaluation of the wall sources.