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Abstract
Two aspects of the interaction between energetic particles and sawtooth oscillations have been studied via numerical simulations.
The stabilizing effect of neutral beam heated ions on the sawtooth instabilities is investigated with the variational dispersion relation analysis. The background plasma is described by magnetohydrodynamics whereas the energetic ions are described by gyro-kinetic equations. Realistic plasma equilibria are provided via an interface with an MHD equilibrium analysis code which takes the TRANSP data as input. The distribution of the neutral beam heated ions is represented by Monte Carlo simulation based on TRANSP data. The bounce-averaged terms in the dispersion relation are evaluated by following the particle trajectories using the Hamiltonian guiding center equations. It was found that the neutral beam heated energetic ions do not provide the necessary contribution for stabilizing the sawtooth mode. Other factors which might also play a role in determining the stability of the plasma are also discussed.
Alpha particle redistribution by sawtooth oscillations is investigated via the Hamiltonian guiding center approach. The effect of finite orbit width is included in this analysis. A model of time evolution of the Kadomtsev-type sawtooth is constructed. Realistic plasma equilibria of TFTR discharges were used. The alpha distribution is simulated using Monte Carlo method. The fact that the presence of more than one poloidal harmonic in the nonlinear stage of the sawtooth crash is necessary to cause significant broadening of the alpha density profile indicates the important role the stochasticity of magnetic field lines plays in this process. Detailed comparisons with observations from recent TFTR Deuterium-Tritium experiments were performed. Our simulation results are in reasonable agreement with $\alpha$-CHERS and show a broadening of alpha particles similar to that seen in PCX measurements.
