Fast ions and turbulent particle transport in tokamaks
The presence of highly energetic (fast) ions, be it fusion born alpha particles or ions accelerated by auxiliary heating schemes, can affect the heating and transport of all particles in the plasma through two main mechanisms. First, fast ions may excite fast ion driven instabilities. One such example is the toroidal Alfvén eigenmode (TAE) that can be excited by super Alfvénic ions as they slow down due to friction-like collisions with the background plasma and eventually hit the wave-particle resonances. In addition, the fast ions may influence the turbulent transport, driven by temperature and density gradients in the background plasma. The turbulent particle transport, together with particle sources, determines the peaking of the density profile in fusion devices which in turn affect the fusion power generated.
In this thesis we employ a simple one-dimensional electrostatic ``bump-on-tail'' model to qualitatively describe fast ion driven TAEs whose signals exhibit frequency sweeping. The frequency sweeping is tied to the formation and evolution of phase space structures known as holes and clumps in the non-thermal fast particle distribution. We have also made realistic simulations of particle transport driven by Ion Temperature Gradient (ITG) modes in JET plasmas heated by neutral beam injection (NBI). We study the turbulent transport using fluid, gyro-fluid and gyrokinetic simulations. Focus is on particle transport and the effect of the NBI generated fast ions on the density peaking and, finally, the influence of the NBI particle source on the density peaking.
Fusion plasma physics