Synchrotron radiation from runaway electrons in plasmas

Licentiate thesis, 2015

Highly relativistic runaway electrons are of great concern in the area of fusion energy research, since their presence in tokamak plasmas have the potential to hinder the successful and stable operation of the device, and potentially cause severe structural damage. In this thesis, runaway electron generation dynamics is investigated using the newly developed efficient computational tool CODE. A particular emphasis is given to the synchrotron radiation emitted by the runaways, as this is an important source of information about their properties. The synchrotron emission spectrum is studied, as well as the effects of radiation back-reaction on the electron distribution and the runaway electron dynamics. Synchrotron emission back-reaction is found to have a significant impact on the runaway distribution, leading to an increase in the critical electric field for runaway generation, as well as the appearance of non-monotonic features in the runaway tail for electric-field strengths above a certain threshold, potentially acting as a source of bump-on-tail instabilities. Both of these effects may contribute to reduce the severity of the runaway problem, although their importance is largest at high temperatures and low densities, likely making the impact in a tokamak disruption scenario limited. It is also found that the previously used approximation of considering only the emission from the most strongly emitting particles when modeling the synchrotron spectrum from runaway distributions can produce highly inaccurate results, and that the use of the full runaway distribution in this context is necessary.