Fundamentals of collective scattering experiments

Aspects of collective scattering of mm and sub-mm radiation on density fluctuations in laboratory plasmas have been described. The fluctuation level at a value of the density wave vector k has been expressed in terms of observable quantities, the k-resolution and the scattering volume which in turn have been expressed in terms of beam parameters. This yielded an expression for the fluctuations at k in terms of observable quantities and beam parameters valid for microwave scattering. The total fluctuation level has been found from a summation of the fluctuation levels at different k. This has been done for the cases of spherical isotropy and of isotropy is the poloidal plane. Expressions have been derived for the minimum source power needed in scattering experiments. It has been done for a one-channel detection system without transmission losses and for a multi-channel system with transmission losses. The derivations have been extended, to include the effect of the presence of plasma radiation. All derivations and calculations have been performed for heterodyne detection and for balanced mixing systems. It was found that the values of the minimum required source power are comparable for both methods of detection. Furthermore the minimum source power increases with decreasing wavelength and depending on the applied wavelength. Plasma radiation can considerably effect the minimum source power needed. For tokamaks with B = 3T the source wavelength must be smaller than 1 mm because of the electron cyclotron emission. Thereforemolecular lasers are recommended for a source. A homodyne one-channel balanced mixing system used on TORTUR has been treated. It was found that with the available 65 mW klystron a fluctuation level of 10-3 can be observed. This is a value considerably smaller than the fluctuations expected from the mixing length argument in turbulent transport theories. 17 refs.; 14 figs.; 144 schemes; 7 tabs