ABSTRACT
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.
ABSTRACT
Dust particles have been observed with Thomson scattering systems on several tokamaks. We present here the first evidence of dust particles observed by the new high resolution Thomson scattering system on JET. The system consists of filter spectrometers that analyze the Thomson scattering spectrum from 670 to 1050 nm in four spectral channels. The laser source is a 5 J Q-switched Nd:YAG laser. Without a spectral channel at the laser wavelength, only dust particles that emit broadband light could be detected; these particles have been observed on JET after disruptions. The timing of their emission is clearly different from that expected for a Thomson scattering pulse. The light pulse from dust happens after the peak of the laser light and has a long tail.
ABSTRACT
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.
ABSTRACT
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.
