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If you would like to join the seminar and are not currently affiliated with ANU, please contact Kenneth Duru at kenneth.duru@anu.edu.au.

In fusion energy research, one approach to produce thermonuclear power is to confine a hot plasma using magnetic fields and inject enough input power such that the energy of plasma ions is increased to levels required for fusion reactions to occur. This implies that energetic particles exist abundantly in a magnetic confinement experiment either externally heated or as fusion products e.g. alpha-particles. Interaction of these high-energy particles with weakly damped plasma waves accounts for excitation of these waves known as energetic particle driven instabilities.  The hard non-linear phase of this phenomenon can lead to emergence of chirping waves with long deviations of the frequency in a few milliseconds. In toroidal configurations, these waves can lead to ejection of the particles from the hot core of the plasma and degrade the confinement and the machine performance. Therefore, it is essential to develop theory and simulation models to better understand and control chirping waves in future fusion plasmas, such as ITER.

In 2010, a 1D electrostatic nonperturbative model was developed by Breizman for an adiabatic study of chirping waves as a 1D paradigm of their electromagnetic counterparts. In order to build more realistic models to better understand experimental chirping signals, nonperturbative theory needs to be extended. This requires new theoretical frameworks capable of describing electromagnetic waves and also capturing particle guiding-centre dynamics in magnetic field lines. To achieve this, new mathematical models have been developed in the theory part of this PhD dissertation which will be discussed in this talk. These models resolve the perturbed distribution function of fast particles using an adiabatic approach i.e. convective transport of fast particles in phase space. Finally, to validate and justify this mechanism, self-consistent simulations have been performed using the hybrid plasma model of the MEGA code to illustrate convective transport of energetic particles in tokamak geometry i.e. a toroidal configuration. These numerical calculations will be covered in the simulation part of the talk. In this respect, a novel conservation law for particle dynamics is introduced which remains valid even during frequency chirping. Subsequently, a new phase-space analysis tool is applied to the simulation data which shows formation and evolution of phase space holes and clumps on appropriate sub-slices of phase-space. The new analysis reveals that these coherent phase-space structures carry the corresponding trapped particles in phase space associated with an inward/outward drift of the particles. For a toroidicity-induced Alfven eigenmode, the mechanism of frequency chirping phenomenon has been clarified and the observations of the wave behaviour are consistent with the adiabatic theory.