Study of Visible Plasma Radiation by High Resolution Spectroscopy at Additional Plasma Heating by Neutral Beams Injection on the COMPASS Tokamak
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Czech Technical University in Prague Faculty of Nuclear Sciences and Physical Engineering Obor: Fyzikální inženýrství Zaměření: Fyzika a technika termojaderné fúze Study of visible plasma radiation by high resolution spectroscopy at additional plasma heating by neutral beams injection on the COMPASS tokamak DIPLOMA THESIS Autor: Bc. Tomáš Odstrčil Supervisor: RNDr. Jan Stöckel CSc. Year: 2012 zadání prohlášení Acknowledgement I would like to thank my supervisor Dr. Jan Stöckel CSc. for providing a support for my work and a useful comments on this thesis. Moreover I am grateful to members of the COMPASS team for their patient during my in situ calibrations of the spectrometer. And finally I thank also Michal Odstrčil for a thorough proofread of this thesis. Bc. Tomáš Odstrčil Název práce: Studium viditelného záření plazmatu s vysokým spektrálním rozlišením při dodatečném ohřevu plazmatu výkonovými neutrálními svazky na tokamaku COMPASS Autor: Bc. Tomáš Odstrčil Obor: Fyzikální inženýrství Druh práce: Diplomová práce Vedoucí práce: RNDr. Jan Stöckel CSc. ÚFP AV ČR v.v.i. Konzultant: — Abstrakt: Tato práce se zabývá měřením a vyhodnocením viditelného záření nečistot z okraje plaz- matu pro měření teploty a rotace. K měření byl použit spektrometr s vysokou disperzí který byl pečlivě nastaven pro dosažení požadovaného rozlišení . V rámci teoretické části jsou shrnuty nezbytné základy teorie záření plazmatu a důkladně prozkoumána teorie rotace plazmatu. V experimentální části je detailně popsán proces kalibrace spektrometru a následně shrnuto a diskutováno chování plazmatu na základě dat z tokamaku COMPASS. Klíčová slova: Poloidální rychlost plazmatu, Dopplerův posuv, Pasivní spek- troskopie, COMPASS, spektrometer s vysokou disperzí Title: Study of visible plasma radiation by high resolution spectroscopy at additional plasma heating by neutral beams injection on the COMPASS tokamak Author: Bc. Tomáš Odstrčil Abstract: This work deal with the measurement of visible light radiation of the impurities at the plasma edge for estimation of temperature and plasma rotation. The measurement was realized by a high dispersion spectrometer which was patiently focused in order to achieve the desired parameters. In the theoretical part of this theses, theory of plasma radiation was summarized and the theory of the plasma rotation was thoroughly investigated. In the experimental part of this thesis, the focusing proses was described in detail and the observed plasma behavior based on the data from COMPASS tokamak was discussed. Key words: Poloidal plasma velocity, Doppler shift, passive spectroscopy, COM- PASS, high dispersion spectrometer Contents 1 Introduction 9 1.1 Thermonuclear Fusion . .9 1.1.1 What is fusion? . .9 1.1.2 Conditions for Fusion . 10 1.1.3 Magnetics Confinement . 12 2 Tokamak COMPASS 15 2.1 History of the COMPASS tokamak . 15 2.2 The scientist program for the COMPASS . 16 2.3 COMPASS diagnostics . 16 2.3.1 Magnetics diagnostics . 17 2.3.2 Probe diagnostics . 18 2.3.3 Interferometer . 18 2.3.4 Doppler reflectometer . 18 2.3.5 Thompson scattering system . 18 2.3.6 Multi range tomography system . 18 2.3.7 Spectrometers . 19 2.3.8 High resolution spectrometer for plasma rotation measurement . 19 2.4 Neutral Beam Injectors . 20 3 Plasma radiation in tokamaks 22 3.1 Radiative processes . 22 3.1.1 Line radiation . 23 3.1.2 Bremsstrahlung . 24 3.1.3 Recombination radiation . 24 3.2 Collisional processes . 25 6 3.3 Plasma models . 26 3.3.1 TE (Thermodynamic Equilibrium) model . 26 3.3.2 LTE (Local Thermodynamic Equilibrium) model . 27 3.3.3 Coronal model . 27 3.3.4 CR (Collisional Radiative) model . 28 3.4 Line broadening mechanisms . 29 3.4.1 Natural broadening . 30 3.4.2 Doppler broadening . 30 3.4.3 Stark broadening . 30 3.4.4 Zeeman splitting and Motion Stark effect . 31 3.4.5 Instrumental broadening . 32 3.5 Applications in tokamak plasma . 32 4 Plasma Rotation 34 4.1 Momentum balance equation . 35 4.2 Momentum transport . 37 4.2.1 Flux surface average . 38 4.2.2 Momentum sources and sinks . 39 4.3 Neoclassical rotation . 40 4.3.1 Poloidal velocity dumping . 41 4.3.2 Impurities rotation velocity . 42 4.4 Radial electric field . 43 4.5 MHD rotation . 43 5 High dispersion spectrometer 45 5.1 Experimental setup . 45 5.1.1 Description . 46 5.1.2 Detector . 47 5.1.3 Optical path . 48 5.2 Focusing of the spectrometer in dispersion plane . 48 5.2.1 Focusing as a mathematical problem . 50 5.2.2 Optimization algorithms . 50 5.2.3 Focusing on the position of the CIII triplet . 52 5.3 Focusing of the spectrometer in the sagittal plane . 53 7 5.4 Instrumental function . 54 5.4.1 The definition . 54 5.4.2 Measurement of the instrumental function . 56 5.5 Wavelength calibration . 57 5.5.1 Differential Doppler spectroscopy line . 57 5.5.2 Reference line . 60 5.5.3 Other ways . 60 5.6 Limits of the achievable accuracy . 63 5.6.1 Statistical uncertainty . 63 5.6.2 Systematic uncertainty . 65 6 Spectrometers control 68 6.1 Ocean Optics spectrometers . 68 6.1.1 Spectrometer Control . 68 6.1.2 Data analyse . 68 6.2 High resolution spectrometer . 70 6.2.1 Andor Camera control . 70 6.2.2 Data extraction . 70 6.2.3 Data analyzing . 70 7 Results 71 7.1 Spectrometer configuration . 71 7.2 Line integrated characteristic of the plasma . 72 7.2.1 Zero momentum . 72 7.2.2 First momentum . 74 7.2.3 Second momentum . 74 7.3 Plasma rotation . 78 7.3.1 Neoclassical prediction . 78 7.3.2 Rotation in ohmically heated plasma . 79 7.3.3 Rotation in NBI heated plasma . 79 7.3.4 Rotation measured on COMPASS in Culham . 83 7.4 Ion temperature . 83 Conclusions 88 8 Chapter 1 Introduction 1.1 Thermonuclear Fusion One of the main tasks for mankind in the next decades will be probably solution of prob- lems with growing consumption of energy. Moreover increasing demands on the stability and greenness of the power sources limit the acceptable solutions. The steep raise of energy consumption can be expected also in the developing countries and it can not be expected that people from the developed countries will be willing to decrease their standards of living. Therefore a new sources of energy must by produced. Almost 80% of the world energy is created by burning of the fossil fuels (coal, natural gas and petrol). However, the fossil fuels have limited reserves and therefore it will soon run out. The next disadvantage of the fissile fuels is production of pollution and so called greenhouse gases. The second most important energy source is nuclear fission. The fission energy is much cleaner compared to the previous one but it suffers from several disadvantages i.e requirements of long term storage for nuclear waste or political pressures that made it almost impossible to build a new fission power plants now. The last important group of power sources are so called it renewable energy sources. However, these sources also suffer from many disadvantages, for example low power density, low reliability and high price. Therefore among other energy sources currently developed, the thermonuclear fusion should have advantage of cheap and geographically evenly distributed fuel, little of the long term radioactive waste and high power density. On the other hand these advantages are balanced by physical, engineering and economical difficulties connected with develop- ing and building of thermonuclear power plant. The thermonuclear fusion energy can be successful only if we will find a way how to produce it cheaply. 1.1.1 What is fusion? The nuclear fusion is defined as a process by which two atomic nuclei are jointed together to create one single heavier nucleus. Energy is produced during the fusion if the fusion product is not heavier than iron (Fig. 1.1) that has highest binding energy. Most of the fusion energy is released in the interior of the stars where hydrogen and also heavier nucleus are fused. But not all fusion reactions are useful for the production of the energy 9 on the Earth. The cross section of the most common proton-proton reaction in the Sun is so low that power density in the sun is less than 300 W/m3 [1]. However, much more promising fusion reaction has been discovered. The highest reaction cross section at the lowest energies has reaction of the deuterium and the tritium (Fig. 1.2) 2 3 4 1 1D +1 T →2 He +0 n. Fusion of deuterium with tritium create helium, freeing a neutron, and release 17.6 MeV of energy. The neutron obtains 4/5 of fusion energy and the α-particle only 1/5 because of energy and momentum conservation laws. High energy neutrons are disadvantage of this reaction, because they damage plasma facing materials (PFM). On the other hand, fusion power is distributed to the deeper layers of the container of the plasma, therefore the surface power density incidenting on the PFM is much lower. Further, tritium is radioactive isotope of hydrogen, beta emitter, with half-live of 12 years. Tritium economy and tritium safety will markedly increase complexity and price of the fusion power plants. Therefore so called advance fuels, where fusion products are only charged particles, can be tested in future. An example of such reaction is D +3 He →4 He + p. This type of the reaction can not be accomplish in the present-day devices because the fusion temperature is more than 10× higher. 9 O16 Fe56 U235 8 12 C U238 He4 7 6 Li7 Li6 5 4 3 H3 He3 2 1 H2 Average binding energyper nucleon(MeV) Average 1 0 H 0 30 60 90 120 150 180 210 240 270 Number of nucleons in nucleus Figure 1.1: Binding energy curve of the most common isotopes 1.1.2 Conditions for Fusion The plasma is a relatively complex physical system which is still not fully understood.