Advances in the FTU collective Thomson scattering system.

The new collective Thomson scattering diagnostic installed on the Frascati Tokamak Upgrade device started its first operations in 2014. The ongoing experiments investigate the presence of signals synchronous with rotating tearing mode islands, possibly due to parametric decay processes, and phenomena affecting electron cyclotron beam absorption or scattering measurements. The radiometric system, diagnostic layout, and data acquisition system were improved accordingly. The present status and near-term developments of the diagnostic are presented.

Mutual interaction of Faraday rotation and Cotton­Mouton phase shift in JET polarimetric measurements.

The paper presents a study of Faraday rotation (FR) angle and Cotton­Mouton (CM) phase shift measurements to determine their mutual interaction and the validity of the linear models presently used in equilibrium codes. Comparison between time traces of measurements and model calculations leads to the result that only an exact numerical solution of Stokes equations can reproduce in all the experimental data. As a consequence, approximated linear models can be applied only in a limited range of plasma parameters. In general, the nonlinear coupling between FR and CM is important for the evaluation of polarimetry parameters.

A new calibration code for the JET polarimeter.

An equivalent model of JET polarimeter is presented, which overcomes the drawbacks of previous versions of the fitting procedures used to provide calibrated results. First of all the signal processing electronics has been simulated, to confirm that it is still working within the original specifications. Then the effective optical path of both the vertical and lateral chords has been implemented to produce the calibration curves. The principle approach to the model has allowed obtaining a unique procedure which can be applied to any manual calibration and remains constant until the following one. The optical model of the chords is then applied to derive the plasma measurements. The results are in good agreement with the estimates of the most advanced full wave propagation code available and have been benchmarked with other diagnostics. The devised procedure has proved to work properly also for the most recent campaigns and high current experiments.

Investigating the possibility of a monitoring fast ion diagnostic for ITER.

In burning plasma fusion devices, fast ion transport plays a central role in the performances of the machines. Moreover the losses of energetic particles might cause severe damages on plasma facing components. Therefore real time measurements of fast ion transport would provide valuable information for safe and reliable plasma operations. In this paper, we examine the feasibility of a monitoring system based on active charge exchange recombination spectroscopy making use of the 0.5 MeV/amu ITER heating neutral beams for detecting fast (4)He(+2) (alphas) particles in ITER plasmas. There are two time scales relevant to fast ion dynamics: the first is the slowing down time of the distribution function which is of the order of 1 s, and the second is the time scale of burstlike transport events such as collective Alfven mode excitations, which--for typical ITER plasma parameters--can be as low as 0.2-1 ms. To detect such fast events a broadband high-throughput spectrometer is needed, while for the reconstruction of the alpha velocity distribution function a higher resolution spectrometer and longer integration time are necessary. To monitor a spatial redistribution of fast particles due to the propagation of the instability, it is proposed to use a limited number of spatial channels, looking at the charge exchange He II spectra induced by the heating beams, whose energy matches the slowing down energies of fast particles. The proposal is to share the motional stark effect periscope on equatorial port 3 [A. Malaquias et al., Rev. Sci. Instrum. 75, 3393 (2004)] adding additional fibers and suitable instruments. A signal to noise ratio of 5 could be achieved with a spatial resolution of a/15 and a time resolution of 5 ms, in a broad spectral band of 100 A, corresponding to the spectral broadening of the line emitted by alpha particles with energies DeltaE < or = 1.5 MeV. Fast H and D ion populations created by heating neutral beam or ion cyclotron resonance heating are expected to produce significantly lower charge exchange signal levels and can only be monitored on substantially longer time scales as it is expected because of the strong energy difference with respect to the heating neutral beam and the consequently low charge exchange cross sections.

Evolution of the millimeter-wave collective Thomson scattering system of the high-field tokamak Frascati Tokamak Upgrade.

We first describe the improved receiving system of the diagnostic experiment of millimeter-wave collective Thomson scattering being run on the Frascati Tokamak Upgrade (FTU), and then discuss some peculiar problems and new operating procedures related to the investigation of strong anomalous spectra of nonthermal origin, many-orders-of-magnitude stronger than the ion thermal feature merged in them, systematically observed in the experimentation, and finally ascribed to a perturbation of the gyrotron that generates the probing beam. Arguments in favor of a more general valence of the solutions actuated for the specific case of FTU are finally given.

Thomson-scattering diagnostic on the Frascati tokamak upgrade.

The Frascati tokamak upgrade Thomson-scattering system is used for the measurement of electron-temperature and electron-density spatial profiles along the vertical diameter of the tokamak at 19 spatial points up to 10 times in a single plasma discharge, with a spatial resolution that ranges from 2 cm in the central region to 4 cm in the plasma edge. The radiation source is a Nd:YLF laser that operates at 1053 nm, with a divergence of 0.4 mrad full angle, and is capable of delivering a burst of 10 pulses with energies of 4.5 J/pulse; the interpulse time can be regulated from 20 to 100 ms. The scattered radiation is collected by two objectives: the first looks at the plasma center, and the second at the plasma edge. Bundles of optical fibers in the focal plane of the objectives carry the scattered light from the tokamak hall to a set of 19 interference-filter polychromators, whose transmission is 70%, and the rejection of the stray light at the laser wavelength is 1/10(7). The detectors are avalanche photodiodes ith a noise-equivalent power of the order of 10(-13) W/(Hz)(½) at 1053 nm. The spectral calibration of the polychromators is presented. The absolute calibration of the scattering system for the electron-density measurement has been carried out by the use of Raman scattering on hydrogen and deuterium. Examples of the results of the temporal evolution of T(e) and n(e) spatial profiles are presented for ohmic plasma heating, lower-hybrid current drive, and a pellet-injection experiment. The electron-temperature and electron-density profiles measured through Thomson scattering are compared with the temperatures measured through the use of electron-cyclotron emission and the density profiles obtained from the interferometer data.

Evidence for multistability of light solitons in SF(6) absorption measurements.

We report experimental evidence of the multistability of reshaped pulses that are observed when a 1.5-nsec CO(2)-laser pulse interacts with a pure SF(6) absorber. The data demonstrate that the dependence of the reshaped pulse delay on peak power is not a single-valued function under various experimental conditions.