Thomson scattering system on FTU tokamak: Calibration, operation, resultsa)

Abstract

The Frascati tokamak upgrade (FTU) Thomson scattering system is employed for the measurement of the electron temperature and density spatial profiles along the vertical torus diameter in 19 spatial points up to ten times in a single plasma discharge with a spatial resolution ranging from 2 cm in the central region to 4 cm in the plasma edge. The radiation source is a Nd:YLF laser at 1053 nm. The scattered radiation is collected by two objectives: the first looks at the plasma center, 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 1/107. The detectors are avalanche photodiodes with a NEP of the order of 10−13 W/(Hz)1/2 at 1053 nm. The absolute calibration for the electron density measurement has been carried out by Raman scattering on hydrogen and deuterium. Examples of temporal evolution of Te and ne spatial profiles are presented for ohmic plasma heating, lower hybrid current drive, and pellet injection experiment. A comparison between scattering data with interferometer for the density measurement, and ECE for the electron temperature shows agreement between the diagnostics. The system is controlled by two computers: a real-time computer for the laser settings, while the detection system parameters and data acquisition are managed using CAMAC by the data acquisition system (DAS) of FTU.

Polarization of incoherent Thomson scattering for electron temperature measurement

“…dependencies in (60) and (61). At a given T e and θ the minima of both factors as functions of X is reached in the vicinity of X=0.…”

“…Indeed, as seen from (60) and (61), the reduction of the intensity caused by the relativistic terms in the q-factors takes place even for scattering on a single moving electron ( -> q 1 0 ). In the same case of a single moving electron, the scattered radiation remains monochromatic and, therefore, completely polarized with D=0 (see equation (39)).…”

“…After scattering on a single moving electron the matrixŴ yields the scattered radiation Thus, the degree of depolarization is linked not to the depolarization factors q total or q x but to the difference between them. Using the expressions in large brackets in (60) and (61) yields an estimate for this difference to a leading order approximation in b  1…”

Overview of the FTU results

“…It turns out that for temperature below 8 keV the two measurements agree inside the error bar, while for Te TS > 8 keV the two measurements are different. The measurement of Te TS corresponds to the position of the scattering volume in the z = 2 cm over the equatorial plane [29], while the measurement of Te ECE corresponds to the maximum electron temperature measured by Michelson interferometer looking into a line of sight along the equatorial plane [30] with a space resolution of 3 cm. The data related to the pulses 43099, 43100 and 43101 are collected in the plot shown in figure 10(b), where the ratio Te TS /Te ECE is reported versus Te TS .…”

Electron Density Measurement by Raman Calibration of the HuanLiuqi-2A Thomson Scattering System

“…The anti-Stokes rotational Raman scattering from nitrogen [4][5][6][7][8][9], hydrogen [10][11][12][13], deuterium [8,9] and air [14] has been applied to obtain electron density data of Thomson scattering systems on many tokomak devices. With a wavelength shift caused by the rotational Raman scattering, it is easier to undertake density calibrations, even if the strayed laser light is very strong.…”