Alfvénic turbulence in tokamaks: from micro- to meso-scale fluctuations
Seminar Room 1, Newton Institute
The challenge of understanding fast particle collective behaviors in burning plasmas of fusion interest, such as charged fusion products and supra-thermal particles produced by additional heating and current-drive methods, is to develop a predictive capability for describing energetic particle confinement and its link to the dynamic evolution of thermal plasma profiles. This work focuses on aspects of fluctuation induced fast particle transport, involving both micro- and meso-scales. While micro-turbulence induced transport of energetic particles tends to be diffusive and reduced by finite orbit averaging, meso-scale fluctuations exhibit both coherent and incoherent non-linear behaviors, reflecting the nature of convective and diffusive transport events respectively connected with them. Meanwhile, cross-scale couplings in (w,k) space will play a crucial role in reactor relevant plasmas, where they can influence long time-scale behaviors, and will be mediated by zonal structures, e.g. zonal flows, geodesic acoustic modes and structure formation in the particle phase-space. Characteristic aspects of Alfvénic fluctuations in the Alfvén-acoustic frequency range are discussed here, with emphasis on excitation mechanisms due to thermal plasma free energy sources at short wavelength and to wave-particle interactions with the supra-thermal component at the longer scales. The dense spectrum of Alfvénic fluctuations, expected to dominate burning plasmas of fusion interest, poses challenging and largely unexplored issues connected with fast particle transports at all scales. The main focus of this presentation is the convective transport associated with coherent nonlinear wave-particle dynamics, where fundamental roles are played by plasma non-uniformities and peculiar features of toroidal geometries.