C. Honoré

Scaling laws of density fluctuations at high-k on Tore Supra

Anomalous transport in tokamaks is generally attributed to turbulent fluctuations. Since a large variety of modes are potentially unstable, a wide range of short-scale fluctuations should be measured, with wavenumbers from kρi ~ 0.1 to kρi 1. In the Tore Supra tokamak, a light scattering experiment has made possible fluctuation measurements in the medium- and high-k domains where a transition in the k-spectrum is observed: the fluctuation level decreases much faster than usual observations, typically with a power law S(k) ≡ k−6. A scan of the ion Larmor radius shows that the transition wavenumber scales with ρi around kρi ~ 1.5. This transition indicates that a characteristic length scale should be involved to describe the fluctuation nonlinear dynamics in this range. The resulting very low level of fluctuations at high-k does not support a strong effect of turbulence driven by the electron temperature gradient. For this gyroradius scan, the characteristics of turbulence also exhibit a good matching with predictions from gyro-Bohm scaling: the typical scale length of turbulence scales with the ion Larmor radius, the typical timescales with a/cs; the turbulence level also scales with ρi, according to the mixing length rule.

Doppler backscattering system for measuring fluctuations and their perpendicular velocity on Tore Supra

Backscattering of a microwave beam launched in oblique incidence makes possible measurement of density fluctuations close to the cut-off with a selected wave number k⊥=−2ki, where ki is the beam wave vector at the reflection layer. On the system installed on Tore Supra, the incidence of the Gaussian beam is controlled thanks to a tiltable monostatic antenna. The microwave part of the system is based on a fluctuation reflectometer scheme with heterodyne detection, and the choice of a V band (50–75 GHz) microwave source and O mode polarization is appropriate for typical enhanced plasma regimes (n0=3–7×1019 m−3). Both the scattering wave number k⊥ and the scattering localization r/a can be changed during a shot, owing to the steppable probing frequency and the motorized antenna (tilt angle 0–10°). The wave-number range k⊥ is 4–15 cm−1, with a wave-number resolution around 2 cm−1, and the localization r/a∼0.3–0.85. The Doppler effect also provides the perpendicular velocity profile for the same position ra...

Recent results on turbulence and MHD activity achieved by reflectometry

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 Alfven eigenmodes (toroidal Alfven eigenmodes, Alfven cascades and possibly beta Alfven 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.

Fluctuation spectra and velocity profile from Doppler backscattering on Tore Supra

Backscattering of a microwave beam close to the cut-off allows for measurement of density fluctuations at a specified wave-number, selected by the scattering geometry , where ki is the beam wave-number at the reflection layer. On the Doppler reflectometry system installed on Tore Supra, both the scattering wave-number k⊥ and the scattering localization (r/a) can be changed during the shot owing to the steppable probing frequency and the motorized antenna. Operating in O mode, the spatial and wave-number ranges depend essentially on density profile, typically probing 0.5 < r/a < 0.95 and 2 < k < 15 cm−1. Wave number spectra are similar to those obtained with conventional scattering systems. The perpendicular fluctuation velocity in the laboratory frame is obtained from the Doppler shift of the frequency spectrum Δω = k⊥v⊥. It is dominated by the plasma Er × B velocity. In the core, the latter is mainly due to the projection of the toroidal velocity, as this is shown by comparison with measurements by charge exchange recombination spectroscopy. In the set of analysed Tore Supra ohmic and ICRH plasmas, the observed rotation is consistent with a poloidal velocity in the electron diamagnetic direction and/or a toroidal velocity in the counter current direction. The detailed structure of the velocity profile, at the edge and in different plasma regimes, allows us then to get information on the radial electric field distribution. The dynamics of the fluctuation velocity can be studied from the time frequency analysis of the signal, for investigating intermittent behaviour and transient regimes.

Quasi-optical Gaussian beam tracing to evaluate Doppler backscattering conditions

Microwave beam backscattering near the cut-off layer appears to be the most interesting diagnostic to observe density fluctuation time evolution for a given localization in the plasma and at a defined wave vector. It also provides perpendicular plasma velocity. Scattering only occurs when the Bragg selection rule is fulfilled, i.e. when the scattering wave vector is almost perpendicular to the magnetic field. In order to evaluate these scattering conditions, ray tracing is required. 3D geometry is necessary to evaluate the angle between the magnetic field and the wave vector at the reflection. The ripple effect on the iso-index layer curve cannot be neglected. Scattering localization and wave vector resolution can be approached if single ray tracing is replaced with quasi-optical beam tracing. Optical propagation is still considered in the WKB approximation but the beam is described as multiple connected rays. The beam radial expansion due to diffraction is well described. This approach allows one to compute beam parameters for all data acquisitions (50 triggers per shot) and all shots (40 shots per day) during the following night on a recent personal computer with MatLab©.

Edge turbulence during ergodic divertor operation in Tore Supra

We report new measurements of turbulence during ergodic divertor (ED) operation. At low density, some de-correlation of the turbulence is observed with a decrease of the long timescale structures. It is shown that the typical time involved is compatible with a de-correlation mechanism through radial separation of the B field lines by the ED, with an associated parallel length of the order of the distance between two modules of the ED. This observation reinforces the conclusion drawn in [1] and based on computer simulations. The situation changes when the density is increased: the turbulence level is found to increase. At the highest density, the structure of the turbulent signal is modified and the bursty behaviour suppressed by the ED at low density reappears. These observations lead to the conclusion that the turbulence measured at high density is not sensitive to the ED stabilization effect. This indicates that it could be carried by the ions.

Global full-wave simulation of the Tore-Supra doppler reflectometer

The Tore-Supra doppler reflectometer (TSDR) aims at measuring the poloidal wave number spectrum (k spectrum) of the density fluctuations. To evaluate the performance and the sensitivity of the TSDR, simulations were made using a global full-wave code. The TSDR modeling with a finite-difference time-domain scheme and the required constraints of the simulation are presented, namely the account of the real size of the system: Distance horn plasma and plasma size. Particular attention is given to the role of the radiation pattern both on the TSDR spatial resolution and the wave number detection.