Recent results on turbulence and MHD activity achieved by reflectometry

Abstract

Over the last years, owing to hardware progress and the development of new methods, reflectometry has become a common diagnostic on plasma fusion devices. This paper presents some results obtained with reflectometry on transport, turbulence and magnetohydrodynamic (MHD). The emphasis is put on some new results from Tore-Supra. Combining the density profile and fluctuation measurement, it was shown on Tore-Supra that the particle pinch inside the q = 1 surface is close to the neoclassical value in ohmic plasma, while the observed small diffusion is in agreement with a very low level of density fluctuations inside the q = 1 surface. In β scaling experiments, no change in the fluctuation levels was found on Tore-Supra, in agreement with the observation of weak confinement degradation with increasing β. Zonal flows have been detected by Doppler reflectometry in ASDEX-U and with correlation reflectometry in T-10. On Tore-Supra, a fast decrease in the density fluctuation level at high poloidal wavenumbers was measured with Doppler reflectometry, suggesting a minor role of electron temperature gradient driven modes. Various forms of Alfvén eigenmodes (toroidal Alfvén eigenmodes, Alfvén cascades and possibly beta Alfvén eigenmodes) have been detected with reflectometry in TFTR, JET and Tore-Supra. The density fluctuations induced by the mode were found to be higher on the high-field side.

Core turbulence behavior moving from ion-temperature-gradient regime towards trapped-electron-mode regime in the ASDEX Upgrade tokamak and comparison with gyrokinetic simulation

“…As the turbulence level increases, the Doppler reflectometry response eventually becomes non-linear and can even saturate [93]. It has been observed on many experiments that the radial turbulence profiles are low (ñ/n < 1%) in the core and several percent in the edge [80,94]. In agreement with these results, the radial turbulence level profile in Fig.…”

Fully electromagnetic nonlinear gyrokinetic equations for tokamak edge turbulence

“…It has been almost universally observed 17,18,59 that the relative density fluctuation amplitude δn n 0 increases from the core to the edge monotonically in tokamak plasmas when there are no transport barriers. In the core, the level is often less 9 than one percent, 8,11,13 while towards the LCFS, it typically reaches ∼ 10 −1 .…”

Localization of MHD and fast particle modes using reflectometry in ASDEX Upgrade

“…The mode exhibited a core localized peak towards the high-field-side (HFS), however, no detailed comparisons with theoretical models were made [13].…”