P. Hennequin

Residual parallel Reynolds stress due to turbulence intensity gradient in tokamak plasmas

A novel mechanism for driving residual stress in tokamak plasmas based on k∥ symmetry breaking by the turbulence intensity gradient is proposed. The physics of this mechanism is explained and its connection to the wave kinetic equation and the wave-momentum flux is described. Applications to the H-mode pedestal in particular to internal transport barriers, are discussed. Also, the effect of heat transport on the momentum flux is discussed.

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.

Localized measurements of turbulence in the TORE SUPRA tokamak

A collective infra-red laser scattering diagnostic has been installed on the TORE SUPRA tokamak for the measurement of plasma density fluctuations. For the range of wavenumbers explored (3-15 cm-1), the scattering angles are very weak ( approximately 1 mrad). Consequently, the scattering signals are averaged along the whole observation chord, resulting in poor longitudinal spatial localization. By virtue of the pitch angle variation of the magnetic field lines in the tokamak, and of the perpendicularity of the turbulence wavevector to these field lines, it has been possible to obtain partial spatial resolution along the direction of the beam. Good agreement between the experimental and theoretical angular resolution of the diagnostic as well as the results of cross-correlation performed on the signals obtained by two simultaneous probing beams also justify this novel concept.

ALTAIR: An infrared laser scattering diagnostic on the TORE SUPRA tokamak

A collective laser light scattering diagnostic ALTAIR (a french acronym for local analysis of anomalous transport using infrared light), using a CO2 laser beam (λ=10.6 μm) has been realized to measure plasma density fluctuations in the TORE SUPRA tokamak. This article describes in detail the optical setup, the signal processing, acquisition, and control systems required for this experiment. As the density fluctuations propagating in tokamaks have small wave numbers and require small scattering angles, such scattering experiments are considered as having no resolution along the beam. However, taking advantage of the pitch angle variation of the magnetic field lines around the magnetic axis along a vertical chord, it has been possible to obtain partial spatial localization of the scattering volume by rotating the direction of the analyzed wave vector in a horizontal plane. Heterodyne detection is used to determine the fluctuations propagation direction. The experiment has been tested on acoustic waves and t...

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...

Complete multi-field characterization of the geodesic acoustic mode in the TCV tokamak

The geodesic acoustic mode (GAM) is a coherently oscillating zonal flow that may regulate turbulence in toroidal plasmas. Uniquely, the complete poloidal and toroidal structure of the magnetic component of the turbulence-driven GAM has been mapped in the TCV tokamak. Radially localized measurements of the fluctuating density, ECE radiative temperature and poloidal flow show that the GAM is a fully coherent, radially propagating wave. These observations are consistent with electrostatic, gyrokinetic simulations.

Turbulence and energy confinement in TORE SUPRA Ohmic discharges

Results on confinement and turbulence from a set of Ohmic discharges in TORE SUPRA are discussed. Attention is focused on the saturation of the energy confinement time and it is emphasized that this saturation can be explained by a saturation of the electron heat diffusivity. The ion behaviour is indeed governed by dilution and equipartition effects. Although the ion heat transport is never neoclassical, there is no enhanced degradation at saturation. This behaviour is confirmed by turbulence measurements using CO2 laser coherent scattering. The level of density fluctuations follows the electron heat diffusivity variations with the average density. Waves propagating in the ion diamagnetic direction are always present in turbulence frequency spectra. Thus, the saturation cannot be explained by the onset of an ion turbulence. The existence of ion turbulence in the edge at all densities cannot be excluded. However, this ion feature in scattering spectra could be explained by a Doppler shift associated with an inversion point of the radial electric field at the edge

WEST Physics Basis

With WEST (Tungsten Environment in Steady State Tokamak) (Bucalossi et al 2014 Fusion Eng. Des. 89 907-12), the Tore Supra facility and team expertise (Dumont et al 2014 Plasma Phys. Control. Fusion 56 075020) is used to pave the way towards ITER divertor procurement and operation. It consists in implementing a divertor configuration and installing ITER-like actively cooled tungsten monoblocks in the Tore Supra tokamak, taking full benefit of its unique long-pulse capability. WEST is a user facility platform, open to all ITER partners. This paper describes the physics basis of WEST: the estimated heat flux on the divertor target, the planned heating schemes, the expected behaviour of the L-H threshold and of the pedestal and the potential W sources. A series of operating scenarios has been modelled, showing that ITER-relevant heat fluxes on the divertor can be achieved in WEST long pulse H-mode plasmas.

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.