Runaway electrons constitute one of the primary threats to future tokamak fusion reactors such as ITER. Successful prevention and mitigation of runaways relies on the development of theoretical models which ac-
curately describe the dynamics of runaway electrons, and these models must in turn be validated in experiments. Experimental validation of models is however often made difficult by the fact that the diagnostic signals obtained in experiments only depend indirectly on the particle dynamics. In this thesis we present a synthetic diagnostic model, implemented in the Synchrotron-detecting Orbit Following Toolkit (Soft),
that bridges this divide between theory and experiment. The synthetic diagnostic calculates the bremsstrahlung and synchrotron radiation diagnostic signals corresponding to a given runaway electron population, which can be directly compared to camera images and radiation spectra obtained in experiments. Bremsstrahlung and synchrotron radiation from runaway electrons are particularly sensitive to the runaway dynamics, and, as we show in this thesis, they provide insight into the runaway energy, direction of motion and position.
This thesis focuses on the so-called geometric effects observed in the detected radiation when magnetic field inhomogeneities and detector properties are taken into account, something which previous studies have
neglected. We characterize the dependence of the observed radiation on the magnetic field geometry, detector properties and runaway parameters, and explain how geometric effects limit the otherwise monotonous growth of the diagnostic response function with the runaway pitch angle.
The synthetic diagnostic model is applied to experiments in the Alcator C-Mod and DIII-D tokamaks, and is used to validate kinetic theory predictions of the electron distribution function. It is found that the
kinetic model agrees well in certain scenarios, and fails in others. In the scenarios where it fails, the synthetic diagnostic model suggests that a mechanism causing a larger spread in pitch angle may be missing from
the kinetic model.
PJ, lecture hall,
01 February, 2019, 10:00
01 February, 2019, 12:00