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Edge plasma emissivity profile reconstruction by forward modelling of diode bolometer signals at ASDEX Upgrade

At the tokamak ASDEX Upgrade a large amount of heating power is available, allowing significant studies on plasma radiation, particularly useful for the next step generation of fusion reactors as ITER. Newly implemented AXUV diode bolometers, with tangentially oriented lines of sight (LOS) looking through the plasma edge, open the unique opportunity of detailed emissivity profile reconstructions, exploiting the high temporal and spatial resolution of these diodes. The signals of this diagnostic are line integrated over its LOS, therefore, in order to obtain local radiation profiles, a data inversion is needed. In this work a forward model is developed to obtain edge emissivity profiles out of the diode measurements. It is particularly suitable for the present geometry, with parallel LOS and a large overlap among them, which prevent a data inversion with both the tomographic deconvolution and the Abel inversion. The model hence takes into account the geometry of the viewing volume of the camera inside the tokamak, considering aperture and overlap of LOS. The work includes a numerical validation of the model, and studies of the influence of measurement errors to the reconstructions performed. Once tested, it is applied to reconstruct emissivity profile during H-mode discharges. A comparison of the results with a numerical simulation (STRAHL code) and temporal analyses during a plasma shift (Raus scan) prove the presence of systematic errors in the diode measurements.
In order to obtain reliable results, a recalibration of the diode responsivity is performed. This is done first using the plasma itself as radiation source and then performing a recalibration for the visible range with a common lamp. Consequences of the recalibration are investigated and new emissivity profile reconstructions are discussed. As mentioned above, the main advantage of a diode based bolometry system is the high time resolution of the measurements (5s). An application of the model is hence the study of edge-localized-mode (ELM) -induced radiation. A shift of radiation towards the scrape-off layer (SOL) can be observed during the ELM.

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1 INTRODUCTION 1.1 the energy problem The present energy scenario and the forecast for the next future reveal an increasing energy demand [9]. Moreover, the actual global energy supply is based on fossil fuels for 81%, while just for 13% on different forms of renewable energy (biomass, hydroelectric, combustible waste, etc). The rest relies on nuclear fission [5]. Considering the decreasing availability of fossil resources, the development of alternative and renewable energy sources has become a crucial challenge. Through a continuous research, new ways have been explored, solutions have been suggested, and some possible answers have been found first in nuclear fission, then in investing more in "green en- ergy". It seems anyway more logical to imagine a future where the energy demand is sustained by different sources, in order to maintain a stable en- ergetic and economic system. Energy from sun, wind or similar renewable resources cannot sustain the whole energy demand, being bound to partic- ular environmental conditions. Anyway they can be considered a good and environmentally-friendly alternative to the dependence on main energy re- sources as, for instance, the current coal, gas or fission power plants. But now the main challenge is thus to find a new main energy source: hope- fully long lasting, eco-friendly and inherently safe. A promising candidate fulfilling these conditions is fusion. 1.2 a possible solution to the increasing energy demand: fusion Generally speaking, fusion is a nuclear process in which two light nuclei merge to form a heavier element. In order to reach a fusion reaction, two positive charged nuclei have to overcome the mutual Coulomb repulsion. They have hence to be closer than a distance in the order of 10 -15 m [27]. A vast knowledge about fusion comes from stars, which are the oldest and biggest fusion "plants" existing. Studying the sun, the proton-proton fusion chain has been discovered. In this reaction helium is formed out of hydro- gen, releasing an energy of26.7MeV for each reaction. In the sun, this fusion process is possible due to the high core density (~10 31particles m 3 ), sustained by the gravitational force. This is not attainable on earth, since densities in this range cannot be reached. In order to exploit fusion processes on earth, the most feasible reaction is employing two hydrogen isotopes, namely deu- terium and tritium (reaction1). They are used because of their cross section, which is larger compared to other possible reactions (reactions 2,3,4, see figure 1). 2 D+ 3 T! 4 He+ 1 n+17.6MeV (1) 2 D+ 2 D! 3 He+ 1 n+3.27MeV (2) 1

Laurea liv.II (specialistica)

Facoltà: Scienze Matematiche, Fisiche e Naturali

Autore: Pietro Vincenzi Contatta »

Composta da 86 pagine.

 

Questa tesi ha raggiunto 115 click dal 15/10/2013.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.