Impact of zonal flows on turbulent transport in tokamaks
Zonal flows (ZFs) are known to play a prominent role in the self-regulation of turbulence in tokamak plasmas. Non-linear fluid numerical simulations of flux-driven turbulence in the core and in the Scrape-Off Layer (SOL) of tokamak plasmas are presented here, which investigate the dependence of zonal flows on plasma and geometric parameters, and their effect on turbulent transport. Core Ion Temperature Gradient (ITG) turbulence is studied with a three-dimensional global model in which zonal flows are damped by ion-ion collisions. Improved confinement is observed as collisionality is decreased at constant injected heat flux. This is found to result from the non-linear upshift of the effective threshold for ITG instability, mediated by an increase of the ZF shear. It is also shown that the threshold for the turbulence onset, and therefore the effective conductivity, increases with input power. The fluctuation characteristics are analyzed in detail and correlation functions are presented. Interchange turbulence is studied in the SOL assuming flute modes. When the so-called sheath conductivity, which governs parallel losses, is small, turbulent transport is controlled by ZFs: its level strongly increases when ZFs are artificially suppressed. At sufficiently larger conductivity ZFs eventually no longer regulate the turbulence. In this case, the sheath response prevents local interactions in wavenumber space to drive ZFs and it clamps the electric potential to the floating potential.
Bibliographic Reference: An oral paper given at: 20th IAEA Fusion Energy Conference Organised by: International Atomic Energy Agency Held at: Centrtro de Fusao Nuclear, Vilamoura (PT)
Availability: Available from Association EURATOM-CEA, Département de Recherches sur la Fusion Contrôlée, CEA Cadarache, F-13108 St Paul-Lez-Durance, France Tel: (+33) 4 42 25 70 01; Fax: (+33) 4 42 25 64 21 E-mail: email@example.com
Record Number: 200518046 / Last updated on: 2005-04-06
Original language: en
Available languages: en