Resonant interaction of fast particles with Alfvén waves in spherical tokamaks
Creators: name: Lilley, Matthew Keith
Publisher: name: Imperial College London
Year: 2009

description: The Spherical Tokamak (ST) concept has become one of the main avenues in magnetic nuclear fusion research since STs successfully demonstrated plasma operation at [Beta] = 2P[mu]0=B2~1. Next step ST machines aiming at achieving burning plasma conditions in high [Beta] plasmas are being planned, such as the Spherical Tokamak Power Plant (STPP) and the Component Testing Facility (CTF). Instabilities of fast particle-driven Alfven eigenmodes are often observed in present-day STs. Such instabilities, driven by fusion-born alpha particles as well as by fast ions produced with auxiliary heating schemes, in the next step STs may pose a major problem as these instabilities may affect confinement and losses of the fast ions. A theory of compressional Alfven eigenmodes (CAE) with frequencies above the deuterium cyclotron frequency,[omega] &gt; [omega]cD, is developed for plasma parameters of a STPP, and modes in the ion-ion hybrid frequency range, [omega]cT &lt; [omega] &lt; [omega]cD, are also investigated in order to assess the potential of diagnosing the deuterium-tritium (D-T) ratio. For the 1-D character of a STPP equilibrium with [Beta]~1 , a `hollow cylinder' toroidal plasma model is employed for studying CAEs with arbitrary values of the parallel wave-vector k[||] = k[.]B/|B|. The existence of weakly-damped CAEs, free of mode conversion, is shown to be associated with the `well' in the magnetic field profile, B = B (R), that can exist at the magnetic axis. A significant part of this thesis focusses on the experimentally observed effects of resonant wave-particle interaction between Alfven waves and fast particles in the Mega Amp Spherical Tokamak (MAST) device at the Culham Laboratory, UK, and in the LArge Plasma Device (LAPD) in the University of California, Los-Angeles, USA. New robust experimental scenarios for exciting CAEs in the MAST spherical tokamak are developed, and interpretation of the observed CAEs in the frequency range [omega]cD/3 &lt; [omega] &lt; [omega]cD is given in the context of the 1-D ST model and the Doppler shifted cyclotron resonance. The e ciency of the Doppler resonance between co and counter directed fast ions and left and right hand polarised Alfven waves is further assessed experimentally on the LAPD device, with probe ions injected in the presence of Alfv en waves launched by an external antenna. The developed theory of CAEs is then applied to a calculation of the linear kinetic drive of CAEs in the MAST experiments. A model representation of the fast ion distribution function, produced by neutral beam injection (NBI), is used by fitting to the TRANSP Monte-Carlo NBI modelling results. The main free energy sources associated with temperature anisotropy and bump-on-tail are estimated analytically, and the CAE stability boundary is qualitatively assessed. In order to explain the experimentally observed difference between steady-state and pulsating Alfvenic modes, the non-linear theory of fast particle driven modes near marginal stability is extended to include dynamical friction (drag). For the bump-on-tail problem, the drag is shown to always give an explosive amplitude evolution in contrast to diffusion in velocity space in the vicinity of the wave-particle resonance. This is then extended to the case of experimentally observed NBI-driven toroidal Alfven eigenmodes (TAEs) in the MAST machine. The experimentally observed differences between TAEs driven by fast ions produced with ion cyclotron resonance heating (ICRH) and NBI are then interpreted. The problem of drag dominated collisions for modes excited by fusion-born alpha particles in burning plasmas such as a STPP and ITER is underlined.