Density fluctuation measurements with laser scattering

Thesis

This thesis describes the development and application of a new, non-invasive, diagnostic based on far forward scattering(FFS) of electromagnetic radiation for investigating density fluctuations in fusion research plasmas.

The spatial distribution and phase of the oscillations induced on the profile of a Gaussian laser beam transmitted through a disturbed refractive medium can be predicted in terms of the characteristics of the disturbance, which can in turn be deduced from them. The theory of these interactions is presented with various models of perturbations to account for differing experimental conditions including one developed by the author which takes into account the effect of broadening in the K spectrum of the fluctuations.

Two sets of experiments are discussed in this thesis, both using a FFS apparatus based around a CO₂ laser and an array of 12 photoconductive detectors. The first successfully tested the prediction of FFS theory under controlled conditions using a transducer to generate ultrasonic waves in air, and an air jet to produce turbulence. Numerical reconstruction of the scattering structure was achieved under these ideal conditions.

In the second set of experiments, FFS was performed on plasma in the small tokamak TOSCA. Results are presented for density fluctuations with wavenumbers around 2 cm^-1 , a bandwidth of several kHz and a relative fluctuation level of 6%. This level is roughly independent of the variation in the mean density and is close to the mixing length level, indicating strong turbulence. The fluctuations are found to be mainly poloidal and follow a v^-2 power law. The average phase velocities are around 4.5 km/sec and are slightly larger than the electron diamagnetic drift velocity. The waves are tentatively identified with the electron drift mode. Coherent signals due to long wavelength MHD modes are detected. Results from a Langmuir probe corroborate most of these data.

Authors: Côté, A; Côté, Alain

• Publication date: 1989
• DOI: 10.5287/ora-jvxajpn2y
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Electromagnetic waves; Tokamaks; Laser plasmas