Weakly collisional trapped electron mode (TEM) turbulence has a robust inward particle flux component associated with a linearly stable eigenmode that is excited nonlinearly by spectral energy transfer from the unstable TEM mode. The nonlinear mixture of the two eigenmodes achieved in saturation cannot be described by the quasilinear approximation, hence the inward flux component, which combines with the outward quasilinear flux to produce the net flux, is fundamentally nonlinear. The net flux, which remains outward but is significantly reduced by the inward component, depends on the gradients of density and temperature. This dependence, which establishes whether the flux is diffusive, convective, or something else, is sensitive to the details of the saturation. Saturation is calculated asymptotically in an ordered expansion in collisionality and the ratio of density to temperature gradient scale length. Spectral transfer is highly anisotropic and saturation must account for the energy transfer to zonal modes with zero poloidal wavenumber. Even though zonal modes do not contribute directly to the particle flux they change the fluctuation level and gradient scaling of both the unstable and stable eigenmodes. The result is a flux that is neither diffusive nor convective, but is driven by temperature gradient and enhanced by density gradients that are flat or nearly so. Near the instability threshold the inward component is particularly strong.
Nonlinear Inward Particle Flux in Trapped Electron Turbulence / P. W., Terry; Gatto, Renato; D. A., Baver; S., Gupta. - ELETTRONICO. - (2006), p. TH/P2-16. (Intervento presentato al convegno IAEA Fusion Energy Conference 2006 tenutosi a Chengdu, China nel 16 - 21 October).
Nonlinear Inward Particle Flux in Trapped Electron Turbulence.
GATTO, RENATO;
2006
Abstract
Weakly collisional trapped electron mode (TEM) turbulence has a robust inward particle flux component associated with a linearly stable eigenmode that is excited nonlinearly by spectral energy transfer from the unstable TEM mode. The nonlinear mixture of the two eigenmodes achieved in saturation cannot be described by the quasilinear approximation, hence the inward flux component, which combines with the outward quasilinear flux to produce the net flux, is fundamentally nonlinear. The net flux, which remains outward but is significantly reduced by the inward component, depends on the gradients of density and temperature. This dependence, which establishes whether the flux is diffusive, convective, or something else, is sensitive to the details of the saturation. Saturation is calculated asymptotically in an ordered expansion in collisionality and the ratio of density to temperature gradient scale length. Spectral transfer is highly anisotropic and saturation must account for the energy transfer to zonal modes with zero poloidal wavenumber. Even though zonal modes do not contribute directly to the particle flux they change the fluctuation level and gradient scaling of both the unstable and stable eigenmodes. The result is a flux that is neither diffusive nor convective, but is driven by temperature gradient and enhanced by density gradients that are flat or nearly so. Near the instability threshold the inward component is particularly strong.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.