Efeitos da polarização eletrostática de eletrodos na periferia de tokamaks têm sido investigados em pequenos tokamaks e mesmo em alguns tokamaks de grande porte. Em geral as experiências são realizadas em condições em que bifurcação do campo elétrico radial é obtida, processo este identificado como modo H de polarização. No Tokamak TCABR, as experiências indicam que o confinamento aumenta para tensões aplicadas até +300 volts, atingindo um máximo de duas vezes o tempo de confinamento do modo L, mas sem bifurcação. Indícios de bifurcação foram notados com +400 V de polarização, mas a descarga termina devido à excitação da atividade MHD, ainda sob investigação. No presente trabalho, a pesquisa é aprofundada com a utilização de uma sonda de Langmuir com 18 pinos dispostos em duas fileiras sob a forma de um ancinho (rake probe) o que permite a medição da temperatura, densidade e flutuação de potencial ao longo do raio menor na periferia do Tokamak. A resolução temporal desse sistema é de cerca de 0,5 ms, para a temperatura, e 5 microssegundos para densidade e potencial flutuante do plasma. Outra sonda eletrostática com 5-pinos na mesma posição radial, mas em diferentes posições poloidal e toroidal foi usada para medições de turbulência e transporte de partículas. Os efeitos da polarização foram investigados e indicam que os níveis de turbulência e transporte começam a diminuir entre +150 e +200 V e para +300 V chegam a atingir uma quase supressão. Nesse mesmo intervalo de tensão a densidade começa a aumentar e para +300 V chega a ser um fator de aproximadamente 2. Quanto ao perfil de temperatura a variação é pouco significativa, mas as incertezas das medidas são maiores. Esses dados são compatíveis com a criação de uma barreira de transporte na região entre o eletrodo em r = 17 cm e o limitador em a = 18 cm. Além disso, o campo elétrico radial mostra forte cisalhamento nessa região. Tomando o início da subida do potencial flutuante como origem de uma escala de tempo, o atraso temporal do início da subida da densidade de elétrons e o atraso do início do decréscimo do transporte de partículas foram medidos. Os resultados são 50 microssegundos para a densidade de elétrons e 60 microssegundos para o transporte de partículas. A questão dos limiares de potência é discutida no texto. Os dados desta experiência indicam que o campo elétrico radial desempenha o papel principal para a melhoria do confinamento. / Electrode biasing effects in the periphery of tokamaks have been investigated in small tokamaks and in a few large tokamaks. Usually the experiments are performed in conditions were bifurcation of the radial electric field is achieved, identified as biased H mode. In the tokamak TCABR, the biasing experiments indicate that the confinement increases for applied voltages up to +300 Volts, reaching a maximum of twice the confinement time of the L mode, but without bifurcation. Indications of bifurcation were detected with +400 V biasing, but the discharge terminates due to excitation of MHD activity, still under investigation. In the present work, the research is improved with the use of a rake shaped 18-pins Langmuir probe, allowing the measurement of electron temperature, density and potential fluctuations along the minor radius near the edge of the tokamak with time resolution of about 0.5 ms, for the temperature and 5 microseconds for density and floating potential. Another electrostatic probe with 5-pins all in the same radial position, but in different poloidal and toroidal position was used for turbulence and particle transport measurements. The effects of biasing indicate that the levels of turbulence and transport begin to decrease between +150 and +200 V and for +300 V reaches an almost suppression. In that same voltage interval the density, begin to increase and for 300 V reach a factor of about 2. The temperature profile does not change significantly but the uncertainty of the measurements is larger. These data are compatible with the creation of a barrier between the position of the electrode at r = 17 cm and the limiter of the tokamak at a = 18 cm. In addition, the radial electric field data show strong shear in this region. Taking the start of the increasing of the fluctuation potential as the origin of a time scale, the temporal delay of the start of the edge electron density increase and transport decrease were measured. The results are 50 microseconds for the electron density and 60 microseconds for the particle transport. The power threshold for the confinement improvement is discussed in the text. The data obtained in this experiment confirm that the shear of the electric field has the stronger role for the confinement improvement with biasing in TCABR tokamak.

The diploma thesis is focused on the study of low-temperature plasma by methods of computational physics, namely particle modelling. The solution focuses on the time- effective execution of simulations, which is concerned, among the other things, with the use of parallelization techniques aimed at maximize shortening of the calculation time. The Langmuir probe was chosen as the most common representative of the probe diagnostics. The main goal of this work was to observe plasma interaction with the embedded solid. Current research on plasma modelling intensively explores dynamic phenomena such as wave propagation or turbulence and instability. For this reason, much of the work is devoted to the dynamics in electropositive and electronegative plasma.

The ball-pen probe is a relatively new diagnostic method, that has been designed for direct measurement of plasma potential in magnetized plasmas. Nowadays, it is routinely used at several high-temperature plasma devices in Europe and it has been tested also in conditions of low-temperature plasma, which are substantially different from that of high-temperature plasma. The measurements performed so far showed, that ball-pen probe could be used also in low-temperature plasma. However, more measurements were needed to prove the applicability of ball-pen probe. Therefore, the main aim of this work is to compare the method of ball-pen probe with simultaneous measurements using Langmuir and emissive probe. Measurements were performed in DC discharge of cylindrical magnetron at various discharge conditions and radial positions of probes. Powered by TCPDF (www.tcpdf.org)

The aim of the thesis is to examine plasma-wall interaction using computer modeling. Tokamak- relevant plasma conditions are simulated using the particle-in-cell model family SPICE working in three or two dimensions. SPICE model was upgraded with a parallel Poisson equation solver and a heat equation solver module. Plasma simulation aimed at synthetic Langmuir probe measurements were performed. First set considered a flush-mounted probe and the effect of variable magnetic field angle was studied with aim to compare existing probe data evaluation techniques and assess their operational space, in which the plasma parameters estimation via fit to the current-voltage characteristic is accurate. Second simulation set studied a protruding probe pin. Effective collecting area of such probe was investigated with intentions of density measurement collection. This area was found to be influenced by a combination of two factors. First, the density dampening inside the magnetic pre-sheath of the probe head, and the second, the extension of the area caused by Larmor rotation. A comparison with experimental results obtained at COMPASS tokamak was was performed, confirming these results. Keywords Langmuir probe, simulation, particle-in-cell, tokamak, Poisson equation, COMPASS 1

The subject of the master thesis is the extension of the measurement of plasma para- meters by the Langmuir probe in a system with a planar magnetron and a hollow cathode operating in pulse mode. The main tasks are to modify the measuring circuit to increase the maximum probe current and to put the USB oscilloscope into operation for data collection with higher resolution and higher sampling rate. Furthermore, the function of the entire device will be verified using test circuits and also by measuring the probe characteristics in discharges in a system with a magnetron and a hollow cathode in both continuous and pulse mode. 1

Localized structures of the magnetic field strength depression are often observed in the interplanetarymedium, and they are called ‘magnetic holes’ after the original work of Turner et al. 1977. A numberof observations of similar features have been reported, while the mechanisms of their origin have notfully understood yet. The scale size of their structures varies from several to a few thousand of the protongyro radii, and their characteristic orientations of the magnetic field also vary, and therefore differenttypes of the magnetic holes have been suggested. To date, the magnetic holes are classified into Mirrormode and magnetic decreases (Tsurutani et al. 2011). Despite a large number of papers that report theobservational characteristics of the magnetic holes, many identify the feature using only the magneticfield data. This is due to the scale size of the structure at a large speed of the solar wind medium, thespatial resolution of the plasma instruments is often insufficient while the magnetic field instrument canusually obtain the data in high enough time resolution.The Cassini spacecraft orbited Saturn for almost 17 years and obtained a large amount of data in/near theKronian magnetosphere, where the series of the magnetic depletions have been also observed (Smith et al.1980). The Langmuir Probe (LP) onboard Cassini measures the spacecraft potential and, in turn, measuresthe electron density in in-situ in the outer magnetosphere and solar wind region. This measurement hasbeen done using the LP sweep mode which samples the current-voltage curve of the probe every 10 minin the outer magnetosphere. The LP has also been operated in the continuous mode that measures theprobe current at a fixed bias potential every 16 s allowing to calculate the electron density in a smallerscale that is required for the studies magnetic holes. However, there is no general calibration so far inorder to conduct a statistical study in the outer magnetosphere region. The goal of this project is toinvestigate the possibility to use the LP data for the magnetic hole study, calibrate the LP continuousmode to derive the plasma density near the magnetospheric of Saturn, and investigate the scale size of theplasma density structure in the magnetic holes, i. e. plasma β, the field strength and density.The calibration of the continuous data was done by finding a relation between the current at 11 V, whichis a typical bias voltage of continuous mode, and the spacecraft potential obtained by the LP the sweepmode data. Is is expected that the current at 11V is linearly proportional to the floating potential andtherefore can be used to derive the electron density with the potential and density relationship presentedby Morooka et al. 2009. I found that the spacecraft attitude against the sun has a strong effect on therelation, and derived 11V current-floating potential relationship depending on the different spacecraftattitude.Using the LP continuous data calibration above, I investigated the electron density characteristics aroundthe magnetic hold structure, and confirmed that they are generally in anticorrelation relationship. I estim-ated also the plasma β assuming a constant temperature of 100 eV and investigated their characteristicsfor the different types of magnetic holes (linear and rotational holes) both in the magnetosheath and theholes in the solar wind for the year 2011. For the Cassini dataset during 2011, various different shapeand sizes of magnetic hole events have been found. Most (80%) of the MHs appeared within a groupedstructure, while the rest (20%) are found as isolated type holes in the magnetosheath. Among the isolatedMHs, about 30% had "Gaussian shape" and about 40% had a substructure. The scale size for the electrondensity for the isolated holes were on average 50 s in the solar wind, and 75 s (the rotational holes) and120 s (the linear holes) in the magnetosheath. Therefore, I confirmed that the LP can obtain enough datapoints to resolve the magnetic holes structure in the magnetosheath. The Cassini LP data resolution isalso capable to resolve some of the magnetic hole structure in the solar wind.In summary, I confirmed that the Cassini LP continuous data calibrated in this study is capable toinvestigate the different types of magnetic hole structures. Using this calibrated electron data statisticallyfor the large number of Cassini orbit would helpful to further identify the MHs structures in the solar wind and the magnetosheath that can be a key to understand the generation mechanisms of the magneticholes. / Lokaliserade strukturer med låg magnetfältstyrkan ses ofta i interplanetära mediet och de kallas ’mag-netiska hål’ (MH) (Turner et al. 1977). Trots et antal observationer av sådana strukturer har observeratsär deras generationsmekanism ännu förstådd. Storleken av strukturerna varierar från ett fåtal till någratusen protongyroradier och även deras kännetecknande inriktningar i magnetfältet varierar. På grund avdetta har olika typer av MH förslagits. Idag klassificerar man MH som ’mirror mode’ och magnetiskaminskningar (Tsurutani et al. 2011). Många studier har undersökt de magnetiska hålens egenskaper,men tyvärr oftast baserats endast på magnetfältsdata. Detta kan bero på strukturernas storlek vid en storsolvindshastighet, där plasmainstrumenten oftast inte har tillräckligt hög tidsupplösning för mätningar,medan magnetfältsinstrumenten kan oftast tillhandahålla data i hög tidsupplösning.Cassini-rymdfarkosten kretsade runt Saturnus i nästan 17 år och erhöll stora mängder data i och näraSaturnus magnetosfär. Langmuir-sonden (LP) ombord Cassini mäter rymdfarkostens potential ochdärmed mäter den elektrontätheten i rymden. Instrumentet fungerar som en slags väderstation för rym-dplasma och möjliggör mätningen av fundamentala plasmaparametrar såsom elektrontäthet, jontäthet,elektrontemperatur och jonmassa i en tät plasmaområdet av nära Saturnus. I den yttre magnetosfären därden plasmatätheten är låg, kan LP mäta rymdfarkosts potential och plasmatätheten. Mätningen, så kallade’sweep mode’ kan skaffades var 10:e minuter. LP:en mäter också i ’kontinuerlig mode’ som möjligenkan mäta plasmatätheten i mer frekventa men den behöver allmän kalibrering. I detta projekt undersökerjag möjligheten att använda LP kontinuerlig data för att studera MH, skapa kalibraring funktion för’kontinuerlig mode’ för att uppskatta plasmatätheten i Saturnus magnetosfär, och även att undersökastorleken och karaktär av plasmatäthetenstrukturen i MH.Jag undersökte först relationen mellan LP ström vid 11V och rymdfarkostens potential i sweep mode data.De härledda funktionerna användes vidare för att uppskatta densiteten med användning av relationenmellan rymdfarkostens potential och elektrontätheten (Morooka et al. 2009). Jag upptäckte också attden kontinuerlig mode funktionen är olika beroende på LP sensors läge i förhållande till solen ochrymdfarkosten. Hur Cassini är inriktad har en stor effekt på relationen och därför beskriva jag fyra olikarelationer för olika inriktningsregioner. Med den kontinuerlig mode funktionen jag härlett, undersöktejag struktur av magnetiska hålen som har listats av Tomas Karlsson på KTH. År 2011 innehåller MH medmycket olika former och storlekar. Den mest (80%) MH identifierades som grupp och resten (20%) varsom isolerade MH i magnetosheath. Av dessa isolerande hål har ca. 30% en Gauss-form och nästan 40%av MH verkar ha en understruktur. Genom att jämföra magfältdatan med elektrontätheten bekräftadejag den allmänna antikorrelationen mellan magnetfältstyrkan och elektrontätheten i MH-strukturerna.Dessutom hittar jag en ökning av elektron β som beräknas med en temperatur av 100 eV för linjära ochroterade MH i den magnetosheath samt MH i solvinden under 2011. Storleken av de isolerade magnetiskahålen är i genomsnitt 50 s i solvinden, 75 s (roterade magnetiska hål) och 120 s (linjära magnetiska hålen)i magnetosheath:en. Därför kan Cassini LP ha tillräcklig många datapoäng för att upplösa struktur avMH i magnetosheath. I solvinden kan LP upplösa en del av relativt stora MH.Sammanfattningsvis kan LP:s kontinuerlig kalibreringen från detta projekt användas för att analyserade olika strukturerna och storlekar av MH. Med denna kalibrerade plasmatäthet data är det möjligt attidentifiera olika MH karaktär i statistiskt för det stora antalet Cassini data. Det skulle vara en stor hjälpför att förstå genereringsmekanismerna av de magnetiska hålen.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第18385号 / エネ博第297号 / 新制||エネ||61(附属図書館) / 31243 / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 水内 亨, 教授 前川 孝, 教授 佐野 史道 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

The plasma environment of an active comet provides a unique setting for plasma physics research. The complex interaction of newly created cometary ions with the flowing plasma of the solar wind gives rise to a plethora of plasma physics phenomena, that can be studied over a large range of activity levels as the distance to the sun, and hence the influx of solar energy, varies. In this thesis, we have used measurements of the spacecraft potential by the Rosetta Langmuir probe instrument (LAP) to study the evolution of activity of comet 67P/Churyumov-Gerasimenko as it approached the sun from 3.6 AU in August 2014 to 2.1 AU in March 2015. The measurements are validated by cross-calibration to a fully independent measurement by an electrostatic analyzer, the Ion Composition Analyzer (ICA), also on board Rosetta. The spacecraft was found to be predominantly negatively charged during the time covered by our investigation, driven so by a rather high electron temperature of ~5 eV resulting from the low collision rate between electrons and the tenuous neutral gas. The spacecraft potential exhibited a clear covariation with the neutral density as measured by the ROSINA Comet Pressure Sensor (COPS) on board Rosetta. As the spacecraft potential depends on plasma density and electron temperature, this shows that the neutral gas and the plasma are closely coupled. The neutral density and negative spacecraft potential were higher in the northern hemisphere, which experienced summer conditions during the investigated period due to the nucleus spin axis being tilted toward the sun. In this hemisphere, we found a clear variation of spacecraft potential with comet longitude, exactly as seen for the neutral gas, with coincident peaks in neutral density and spacecraft potential magnitude roughly every 6 h, when sunlit parts of the neck region of the bi- lobed nucleus were in view of the spacecraft. The plasma density was estimated to have increased during the investigated time period by a factor of 8-12 in the northern hemisphere and possibly as much as a factor of 20-44 in the southern hemisphere, due to the combined effects of seasonal changes and decreasing heliocentric distance. The spacecraft potential measurements obtained by LAP generally exhibited good correlation with the estimates from ICA, confirming the accuracy of both of these instruments for measurements of the spacecraft potential. / <p>QC 20200602</p>

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