Alpha particle transport due to Alfven wave excitation

An overview of progress in linear and nonlinear theory of Alven wave instabilities driven by energetic particles (such as alpha particles in fusion plasma experiments) is presented. Linear analysis shows that a rich spectrum of Alfven waves can be excited in usual tokamaks during low-shear discharges, or in low-aspect-ratio spherical tokamaks. These waves do not readily disappear with increasing plasma beta as previously expected. A nonlinear theory of resonant wave-particle interaction is also described, where collisional effects are included. An interpretation of TFTR experimental data is suggested on the basis of this analysis. This interaction is studied in the case where there is no resonance overlap (single-mode theory), as well as in the regime of mode overlap. Mode-amplitude bursting scenarios are described in both cases, and a surprisingly large saturation level is found near the instability threshold. The theory also describes unstable waves that do nor exist in the absence of energetic particles. A frequency 'chirping' explosive solution is found for such waves (including fishbone oscillations). It is also shown that mode frequency sweeping is a possible method for the conversion of fast-ion free energy to wave energy even in linearly stable systems.