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

Abstract

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 the first results obtained on TORE SUPRA indeed show the existence of a spatial resolution.

Localized measurement of short wavelength plasma fluctuations with the DIII-D phase contrast imaging diagnostic

“…When a scattering diagnostic is used to diagnose tokamak turbulence, we can exploit the magnetic geometry to gain localized measurements along the viewing chord [19]. This is possible because the component of the magnetic field perpendicular to the viewing chord varies as a function of chord height.…”

Studies of turbulence and transport in Alcator C-Mod H-mode plasmas with phase contrast imaging and comparisons with GYRO

“…Although the PCI diagnostic normally lacks localization along the chords, the vertical variation of the magnetic field pitch angle allows for some localization of short wavelength fluctuations. [19][20][21][22] The method relies on the fact that the magnetic pitch angle Figure 6 shows how the mode propagating direction is inferred by comparing the measured frequency/wavenumber spectra with the different localizing configurations. In Fig.…”

CO 2 laser based two-volume collective scattering instrument for spatially localized turbulence measurements

“…The detected signal is thus sensitive to the magnetic field pitch angle which can be used to obtain spatially localized turbulence measurements, as was first shown at Tore Supra. 15 The detected heterodyne signal can be written as…”

“…The beam width cannot be increased arbitrarily due to both practical limits and to the fact that a beam of width w inherently integrates over a radial region of size w. Using two narrow beams these limitations can be relaxed and the spatial resolution can be further enhanced. Furthermore, because the two-beam technique described here relies on correlation measurements, whereas the wide beam technique 15 uses the signal from a single scattering volume, there are important differences in the operation of the two methods. The wide beam technique relies on the plasma fluctuations being locally perpendicular to the magnetic field lines, while localization with two separated beams relies on the assumption of fluctuation transport along the field lines.…”

“…Variations to the basic collective scattering arrangement have included correlations between multiple scattering directions, 12 and tomographic inversion of the data from multiple beams. 13 In the second category variations in the radial electric field, 14 or changes in the direction of the magnetic field lines with spatial position, combined with the fact that the turbulent fluctuations are primarily confined to a plane perpendicular to the local magnetic field, 15 have been used for obtaining localized measurements.…”