Kinetic modelling of runaways in plasmas

Doctoral thesis, 2019

The phenomenon of runaway occurs in plasmas in the presence of a strong electric field, when the accelerating force overcomes the collisional friction acting on the charged particles moving through the plasma. Electron runaway is observed in both laboratory and space plasmas, and is of great importance in fusion-energy research, where the energetic electrons can damage the plasma-facing components of fusion reactors.

In this thesis, we present a series of papers which investigate various aspects of runaway dynamics. We advance the kinetic description of electron runaway by deriving and analyzing a fully conservative large-angle collision operator suitable for studying runaway dynamics, and explore its impact on runaway generation and decay. We also present a generalization of the Landau-Fokker-Planck equation to describe screening effects in partially ionized plasmas, providing improved capability of modelling the effect of runaway mitigation schemes in fusion devices.

The emission of synchrotron and bremsstrahlung radiation are important energy-loss mechanisms for relativistic runaway electrons, and they also provide essential diagnostic tools. We demonstrate the need for a stochastic description in order to accurately describe the effect of bremsstrahlung radiation losses on the electron motion. Synchrotron radiation is often emitted at visible and infrared wavelengths in tokamaks, allowing the emission to be readily observed. We have developed a synthetic synchrotron diagnostic tool, SOFT, which provides new insight into how features of the runaway distribution can affect the observed emission patterns.

Finally, we have investigated the runaway dynamics of ions and of positrons which are generated during runaway. The runaway description in these cases differs from regular electron runaway due to the high mass of the ions, and the fact that positrons are created with a large momentum antiparallel to their direction of acceleration.