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Study of instabilities in tokamak plasmas using radiation diagnostics
Imríšek, Martin ; Tichý, Milan (advisor) ; Igochine, Valentin (referee) ; Jardin, Axel (referee)
Title: Study of instabilities in tokamak plasmas using radiation diagnostics Author: Martin Imríšek Department: Department of Surface and Plasma Science Supervisor: Prof. RNDr. Milan Tichý, DrSc. Consultants: Mgr. Vladimír Weinzettl, PhD. RNDr. Jan Mlynář, PhD. Abstract: The thesis focuses on the characterisation of sawtooth instability and its effect on selected edge plasma processes at COMPASS. It is shown that saw- tooth crash precedes the vast majority of L-H transitions, lower occurrence of ELMs and transitions to ELM-free H-mode. The sawtooth instability also af- fects H-L transitions, as their increased occurrence is observed in the middle of the sawtooth cycle. In the thesis, optimised tomography of the electromagnetic emission at COMPASS and JET is also applied to study instabilities in tokamak plasmas. The performance of fast tomographic methods with potential for real- time plasma control purposes is also studied. As the magnetic field configuration plays a vital role in plasma stability and tomographic reconstructions, the thesis also includes simulations of magnetic fields. These simulations were applied to optimise the design of coils for COMPASS-U. Keywords: tokamak, tomography, sawtooth instability, electromagnetic emission iii
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RE beam generation in MGI disruptions on COMPASS.
Ficker, Ondřej ; Mlynář, Jan ; Macúšová, Eva ; Vlainić, M. ; Weinzettl, Vladimír ; Urban, Jakub ; Čeřovský, Jaroslav ; Farník, Michal ; Markovič, Tomáš ; Papřok, Richard ; Vondráček, Petr ; Imríšek, Martin ; Tomeš, Matěj ; Havlíček, Josef ; Varju, Jozef ; Varavin, Mykyta ; Bogár, Ondrej ; Havránek, Aleš ; Gospodarczyk, M. ; Rabinski, M. ; Jakubowski, M. ; Malinovski, K. ; Zebrowski, J. ; Plyusnin, V. ; Papp, G. ; Pánek, Radomír ; Hron, Martin
In the experiments with the massive gas injection (MGI) triggered disruptions in the COMPASS tokamak, post-disruptive runaway electron (RE) beams are generated occasionally. In this contribution, a relatively large set of discharges from the recent years is analysed in order to improve our understanding of the processes behind the beam generation. It seems that despite the low toroidal field applied in the discharges (typically 1.15 T) the magnetic perturbations may be small enough to allow the acceleration and confinement of the electrons. The influence of the different components and different sections of the frequency spectrum of the magnetic perturbations on the beam generation and RE beam current is examined. Fast visible camera images are also analysed. The relevance of COMPASS runaway electron experiments for large machines is discussed. Moreover, the potential of less common diagnostic tools (Cherenkov detector, vertical ECE, ...) to clarify the role of the pre-disruption fast electron population and the influence of prompt runaway electron losses in the current quench is discussed using the recent test measurements.
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Global Power Balance in Non-Stationary Discharge Phases in the COMPASS Tokamak
Havlíček, Josef ; Imríšek, Martin ; Kovařík, Karel ; Weinzettl, Vladimír
The global power balance between different input power and sink/loss channels is important for understanding of tokamak physics, particularly for various scaling laws. The power balance is measured easily during stationary discharge phases because the terms for both magnetic field energy and plasma thermal energy build-up can be neglected. However, many important events in tokamak plasmas occur during non-stationary phases of the discharge. This article describes, quantifies and discusses the terms of the global power balance, including time dependent parts, and exemplifies them on the typical ohmic discharge of the COMPASS tokamak.
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Observation of Sawtooth Oscillations in the COMPASS Tokamak
Imríšek, Martin ; Havlíček, Josef ; Weinzettl, Vladimír ; Mlynář, Jan
The sawtooth instability in tokamak plasmas results in periodic relaxations of the core plasma density and temperature. The physics of sawtooth is still not fully understood. It is predicted that fusion-born alpha particles will lead to long sawteeth. However, longer sawteeth can seed other instabilities which cause further degradation of plasma confinement. This paper provides brief introduction into sawtooth physics and observations of sawtooth instability in COMPASS. Furthermore, evidence of triggering transition to high confinement regime by sawtooth crash is presented
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