In English

Synthetic synchrotron diagnostics for runaways in tokamaks

Mathias Hoppe
Göteborg : Chalmers tekniska högskola, 2017. 80 s.
[Examensarbete på avancerad nivå]

In tokamaks, highly relativistic electrons known as runaway electrons may be created, for example during disruptions when instabilities rapidly grow and cause operation to abruptly terminate. If control of the runaway beam is lost, the electrons can collide with the device wall and inflict severe damage. Therefore, it is of great importance to understand the processes that give rise to and govern the behaviour of these particles. One of the best ways to learn about the phase-space distribution of runaway electrons in experiments is to measure the synchrotron radiation emitted by them, often in the visible or infrared wavelength ranges, by making camera images or measuring the spectrum. Synchrotron radiation is emitted almost entirely in the forward direction of the electron, contrary to how light is usually emitted by most other light sources, and because of this electrons must be moving towards the observer in order to be seen. As a result, most electrons are invisible to the observer most of the time, and the synchrotron image does not reveal the full runaway beam, rather showing an abstract spot of light that can take on many different shapes. The sharp beaming in the forward direction of synchrotron radiation however puts an extra constraint on the image which allows the full velocity vector of the runaways to be inferred from the image. In this thesis the numerical tool SOFT (for Synchrotron-detecting Orbit Following Toolkit), along with the theory on which it builds, is presented. With SOFT, synchrotron images from runaway populations that are arbitrarily distributed in phase-space can be simulated, taking various kinds of geometric effects (magnetic field geometry, camera placement, viewing direction etc.) into account. The effects on the image due to isolated variations in energy, pitch angle, minor radius and camera location are investigated and analysed. Synchrotron images simulated with analytical avalanche distributions of runaways are interpreted in relation to, and compared with, synchrotron images from mono-energetic runaway populations. All parameters are found to have distinct effects on the synchrotron spot, and it is shown that the synchrotron images due to full distributions of runaway electrons can partly be understood as dominated by particles of a particular energy and pitch angle

Nyckelord: fusion, runaway, synchrotron, guiding-center, SOFT



Publikationen registrerades 2017-05-19. Den ändrades senast 2017-11-29

CPL ID: 249436

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