UNDERSTANDING AND PREDICTING ION HEAT TRANSPORT IN TOKAMAKS

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=| 07Ti/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

Increased risk of second malignancy in pancreatic intraductal papillary mucinous tumors: Review of the literature.

AIM: To analyze the available evidence about the risk of extrapancreatic malignancies and pancreatic ductal adenocarcinoma associated to pancreatic intraductal papillary mucinous tumors (IPMNs). METHODS: A systematic search of literature was undertaken using MEDLINE, EMBASE, Cochrane and Web-of-Science libraries. No limitations for year of publication were considered; preference was given to English papers. All references in selected articles were further screened for additional publications. Both clinical series and Literature reviews were selected. For all eligible studies, a standard data extraction form was filled in and the following data were extracted: study design, number of patients, prevalence of pancreatic cancer and extrapancreatic malignancies in IPMN patients and control groups, if available. RESULTS: A total of 805 abstracts were selected and read; 25 articles were considered pertinent and 17 were chosen for the present systematic review. Eleven monocentric series, 1 multicentric series, 1 case-control study, 1 population-based study and 3 case report were included. A total of 2881 patients were globally analyzed as study group, and the incidence of pancreatic cancer and/or extrapancreatic malignancies ranged from 5% to 52%, with a mean of 28.71%. When a control group was analyzed (6 papers), the same incidence was as low as 9.4%. CONCLUSION: The available Literature is unanimous in claiming IPMNs to be strongly associated with pancreatic and extrapancreatic malignancies. The consequences in IPMNs management are herein discussed

The impact of Participatory Budgeting on health and wellbeing:A scoping review of evaluations

Background: Participatory budgeting (PB), citizens deliberating among themselves and with officials to decide how to allocate funds for public goods, has been increasingly implemented across Europe and worldwide. While PB is recommended as good practice by the World Bank and the United Nations, with potential to improve health and wellbeing, it is unclear what evaluations have been conducted on the impact of PB on health and wellbeing. Methods: For this scoping review, we searched 21 databases with no restrictions on publication date or language. The search term ‘participatory budget’ was used as the relevant global label for the intervention of interest. Studies were included if they reported original analysis of health, social, political, or economic and budgetary outcomes of PB. We examined the study design, analysis, outcomes and location of included articles. Findings are reported narratively. Results: From 1458 identified references, 37 studies were included. The majority of evaluations (n = 24) were of PB in South America, seven were in Europe. Most evaluations were case studies (n = 23) conducting ethnography and surveys, focussing on political outcomes such as participation in PB or impacts on political activities. All of the quantitative observational studies analysing population level data, except one in Russia, were conducted in South America. Conclusion: Despite increasing interest in PB, evaluations applying robust methods to analyse health and wellbeing outcomes are scarce, particularly beyond Brazil. Therefore, implementation of PB schemes should be accompanied by rigorous qualitative and quantitative evaluation to identify impacts and the processes by which they are realised

High current and low q95 scenario studies for FAST in the view of ITER and DEMO

The Fusion Advanced Study Torus (FAST) has been proposed as a possible European satellite, in view of ITER and DEMO, in order to: a) explore plasma wall interaction in reactor relevant conditions b) test tools and scenarios for safe and reliable tokamak operation up to the border of stability c) address fusion plasmas with a significant population of fast particles. A new FAST scenario has been designed focusing on low-q operation, at plasma current IP=10 MA, toroidal field BT=8.5T, with a q95=2.3 that would correspond to IP=20 MA in ITER. The flat-top of the discharge can last a couple of seconds (i.e. half the diffusive resistive time and twice the energy confinement time), and is limited by the heating of the toroidal field coils. A preliminary evaluation of the end-of-pulse temperatures and of the electromagnetic forces acting on the central solenoid pack and poloidal field coils has been performed. Moreover, a VDE plasma disruption has been simulated and the maximum total vertical force applied on the vacuum vessel has been estimated

Progress and verification of DTT ICRF antenna simulation using COMSOL

In this paper we present the extension of a full-wave FEM model (COMSOL®+MATLAB®) - initially developed to compute the electromagnetic field in presence of the anisotropic inhomogeneous plasma of the Electron Cyclotron Resonance Ion Sources (ECRISs) [1] – to the Ion Cyclotron Range of Frequency (ICRF). The model - based on the full non-uniform dielectric tensor in "cold plasma" approximation - has been employed to study antenna geometries of increasing complexity. Various antenna types have been analyzed, starting from single flat strap up to the two straps of an antenna option considered for the Divertor Tokamak Test facility (DTT) [2]. The results have been compared, cross-checked and validated with a simpler COMSOL-based tool [3] and with the TOPICA code [4]

The tunable resonant IC antenna concept and its design for DTT experiment

The intrinsic poor loading of Ion Cyclotron (IC) plasma-facing antennas makes the use of Tuning and Matching Systems (TMSs) a necessity. The antenna plus TMS is a resonant system; in the TMS and access lines high voltages (tens of kV) must be accounted for in the unavoidable unmatched part of the feeding lines. In this work, we propose and test an innovative type of IC launcher; it is based on achieving resonance of the self-standing antenna, i.e. without the TMS. A mechanical full-metal tuning mechanism is described and demonstrated to allow wide-band operation. A systematic analysis of possible antenna topologies has led to identifying a structure that can allow good impedance matching along with compliance with maximum electric field constraints. Most of the design is carried out using a simplified plasma and a commercial analysis tool and then validated with a realistic plasma using TOPICA code