R. Sabot

Radial wave number spectrum of density fluctuations deduced from reflectometry phase signals

A method to extract the radial wave-number spectrum of the density fluctuations from the reflectometry phase signal is proposed and the main points to interpret it are described. The method buildup is based on reflectometry studies devoted to phase or amplitude variations induced by coherent density perturbations. These previous analyses show that all the wave numbers up to the Bragg detection limit kf (fluctuation radial wave number) <2ko (incident wave number in vacuum) contribute to the phase fluctuations. Although the main part comes from the vicinity of the cutoff layer the entire probing zone contributes to the phase fluctuations. The phase fluctuations dependencies in kf and L (length of the density gradient) found in the literature for the O mode have been estimated for the X mode in the Bragg backscattering zone in the case of extensive density perturbations. The assumptions used to establish this diagnostic and its limitations are discussed. The numerical study with a FDTD code in two-dimensiona...

Studies of high frequency hot ion instabilities by means of correlation ECE on Tore Supra

High frequency (> 100 kHz) modes have been observed for the first time in combined LHCD and ICR-heated minority H (D) plasmas on Tore Supra by means of a correlation ECE diagnostic and by fluctuation reflectometers. These modes have been identified as the toroidicity-induced Alfven eigenmodes (TAEs) that are destabilized by fast minority hydrogen ions with energies above 250 keV. In reversed central shear plasmas with non-linear oscillations of the central temperature, evolution of the TAE frequency is linked to the development of the local safety factor. This confirms that the central temperature and the q-profile evolve as a non-linearly coupled system in the so-called O-regime in Tore Supra.

Diagnostic Systems on Tore Supra

Abstract Realizing high-power long-duration discharges puts specific constraints on diagnostics: Their front parts have to withstand important thermal loads, eventually requiring active cooling of critical parts, and drifts in measurements have to be avoided in order to supply reliable measurement during the whole discharge duration. Furthermore, the importance of diagnostics for missions other than physics understanding, such as machine operation or safety control, increases. The diagnostics system of Tore Supra consists of roughly 30 diagnostics, covering a large range of plasma parameters from the core to the edge. They have been designed for long-duration plasma discharges, which can last up to 1000 s. Their inner components have been dimensioned to endure continuous high-radiation fluxes, and most of them have been conceived to give a fair measurement all along the discharges.

Temperature oscillating regimes in Tore Supra diagnosed by MHD activity

This paper describes what we can learn on the regimes of spontaneous electron temperature oscillations discovered in Tore Supra from the analysis of MHD activity. Since the first observations of this oscillating behaviour of plasma equilibrium, and its interpretation as a predator-prey system involving lower hybrid waves power deposition and electron confinement, analysis of MHD modes has confirmed the reality of safety factor profile oscillations. This points towards the importance of rational values of the safety factor in the transition to transport barriers in reversed magnetic shear plasmas.

Energetic particle driven magnetohydrodynamic instabilities during relaxation cycles in Tore Supra

In the Tore Supra tokamak, magnetohydrodynamic instabilities can be destabilized by energetic particles produced by radio-frequency heating systems. This paper reports on experimental observations of two types of energetic particle driven internal modes: the electron fishbone and the beta-induced Alfven eigenmodes (BAEs). The evolutions of these modes in frequency and radial position during core relaxation cycles were determined by their induced electronic temperature and density fluctuations. Electron fishbones were observed in the so-called oscillation regime, while BAEs observations were performed in the sawtooth regime. The results show that the frequency and radial position of these two instabilities vary substantially during the core relaxation cycles. It indicates that an accurate description of these evolutions must consider the equilibrium profile alterations during the core oscillations.

Experimental observation of m/n = 1/1 mode behaviour during sawtooth activity and its manifestations in tokamak plasmas*

To shed some light on the development of the fast m/n = 1/1 precursor to the sawtooth crash and its influence on plasma transport properties in the vicinity of the q = 1 surface, series of dedicated experiments have been conducted on the Tore Supra and TEXTOR tokamaks. It has been concluded that, before a crash, the hot core gets displaced with respect to the magnetic axis, drifts outwards by as much as 8–10u2009cm and may change its shape. Observation of the magnetic reconnection process has been made by means of electron cyclotron emission diagnostics. The heat pulse is seen far outside the inversion radius. The colder plasma develops a magnetic island on the former magnetic axis, after the hot core expulsion. Different kinds of behaviour of the m = 1 precursor before the crash, with respect to the displacement of the hot core and the duration of the oscillating phase, have been observed. An ideal kink model alone cannot be used for explanation; therefore, resistive effects play an important role in the mode development. Possible mechanisms that lead an m = 1 mode to such behaviour, and their links to the change in the central q-profile, are discussed. Results have been discussed in the light of various theoretical models of the sawtooth.

Snake-like phenomena in Tore Supra following pellet injection

`Snakes are observed in Tore Supra after injection of high velocity solid hydrogen or deuterium pellets ablated inside the q=1 surface. They are detected, immediately after the ablation, as oscillations in the line integrated densities of the central interferometer channels. The corresponding oscillations in the soft X-ray signals detach from the noise about 70 ms later. Snakes survive sawtooth crashes, but are nevertheless affected by them. Variations, during the about 500 ms long lifetime, of the snake radius rs, of the rotation frequency and of the rotation direction are discussed, stressing the effects of the sawtooth crashes. In many snakes rs/r1, where r1 refers to the radius of the q=1 surface, is of the order of 0.5. The snake has an m=1, n=1 helicity, pointing out the problem of the interpretation in terms of current profile. Combined simulations of the snake oscillations in both interferometer and soft X-ray signals have indicated that the impurity (carbon) density inside the snake is much larger than that outside it. Since a change of regime seems to appear about 80 ms after the snake formation on the soft X-ray, it seems plausible that impurity (carbon) accumulation takes place at this time. A stability criterion taking into account both impurity and bootstrap effects is presented; the result agrees with the model proposed by Wesson (1995)

Electron fishbones in FTU and Tore Supra tokamaks

This work presents a comparative study of the experimental observation of MHD instabilities identified as electron fishbone-like modes that appear in plasmas with lower hybrid current drive in FTU and Tore Supra tokamaks. Initially, the mode-induced electronic temperature oscillations measured by electron cyclotron emission were used to study the evolutions of the frequency and position of these modes. In FTU, where fishbones with and without bursting behaviour are observed, it was found that the evolutions of the mode position and frequency follow opposite trend in the two regimes. In Tore Supra, where abrupt changes between modes with different mode structures are often observed, it was found that the mode position evolves continuously and the wavenumbers follow an inverse cascade starting from an m/nxa0=xa04/4 mode and finishing in an 1/1 mode. In a second step, the energy of resonant electrons was estimated from the resonant condition of the precession drift frequency. It was found that in Tore Supra the resonant condition does not change during the frequency jumps. The relevance of the correction due to the pitch angle of the resonant electrons and the possible role of the energetic passing particles in the drive of these modes are discussed from the results obtained in both machines.

Edge cooling experiments and non-local transport phenomena in Tore Supra

Edge cooling experiments have been performed in Tore Supra by impurity injection and oblique pellet injection. Non-local transport (NLT) features have been confirmed in both cases in the ohmic regime. The pellet injection experiments have shown the existence of a threshold on the plasma density ne and current Ip for the observation of this effect: ne/Ip1/2?1.0?1016?(m-3?A-1/2). A second threshold on the relative density variation caused by the pellet injection has also been observed: ?ne/ne?0.5. It has been observed that in the L regime during lower hybrid current drive (LHCD) experiments these limits are extended: [ne/Ip1/2]crit = 1.4?1016?(m-3?A-1/2) and [?ne/ne]crit?0.65. Observations in a LHCD scheme with fully non-inductive plasma current seem to indicate that the mechanism governing the NLT is unlikely to be linked to the plasma current profile effects including the spatial redistribution, the magnetic shear, and the low-m MHD modes.

Determination of the safety factor profile in TORE SUPRA from the largest striations observed during pellet ablation

The largest striations in Hα(Dα) light observed across the ablation cloud of injected hydrogen pellets can be explained in most cases by the presence of resonant magnetic surfaces. The authors suggest that it is possible to identify these striations and therefore to obtain a very detailed profile of the safety factor. This q profile exhibits shear plateaus related to rational q values, thus indicating the presence of magnetic turbulence