Search

In experiments on the FT-1 tokamak, the effect is observed of the cross-polarization scattering of microwave radiation by low-frequency plasma microturbulence in the upper hybrid resonance region. The radar diagnostics based on this effect was used to measure the frequencies and wavenumbers of the fluctuations that cause cross-polarization scattering.

The wavenumber-resolved radar backscattering diagnostics in the upper hybrid resonance (UHR) region was used to study low-frequency short-scale turbulence in the FT-1 tokamak. The scattered spectra were measured for different delay times of the scattered signals. It is shown that the width of the spectrum of enhanced scattering by spontaneous fluctuations is proportional to the delay time. Possible mechanisms for the formation of the scattered spectra are proposed and discussed. The results of simulations and additional experiments were used to determine the dominant mechanisms governing the formation of the scattered spectra in the FT-1 tokamak. These mechanisms are related to the effect of multiple small-angle scattering of both the probing wave and the waves backscattered in the UHR region by long-scale density fluctuations and to the Doppler effect caused by the entrainment of short-scale fluctuations by the long-scale turbulent flow.

The propagation of lower hybrid (LH) waves in a tokamak plasma in the presence of an LH resonance surface is studied experimentally with the use of a specially elaborated technique based on the backscattering of the probing microwave radiation in the upper hybrid resonance region. The technique provides resolution in the wave vectors of the scattering density fluctuations. The conditions are determined under which the LH wave propagates in accordance with the predictions of linear theory and is converted into the short-wave-length ion Bernstein mode. The parameter range is found in which the predictions of linear theory fail to hold and the nonlinear effects come into play during LH wave conversion. The radial wavelengths of the LH and ion Bernstein waves are determined.

Experiments at the FT-2 tokamak had demonstrated effective plasma LH heating, which was accounted for by both direct absorption of RF power and plasma transport suppression. The improved core confinement accompanied by Internal Transport Barrier (ITB) formation was observed. The RF pulse switch off is followed by triggering of LH transition and the External Transport Barrier (ETB) formation near the last closed flux surface. The present paper is devoted to a much more detailed study of the radial electric fieldEr behaviour in the region of ITB and ETB and its influence on the tokamak microturbulence in these regions. The new experimental data were obtained by spatial spectroscopic technique using additional pulse helium puffing in hydrogen plasma. Simultaneously microscale plasma oscillations in the frequency band (0.01–2) MHz are observed with local enhanced microwave scattering diagnostics and by x-mode fluctuation reflectometry. Experiments demonstrate that the improved confinement is associated with the modification of microturbulence by the shear of theE×B poloidal velocity. This conclusion is also confirmed by the data obtained by Langmuir probes in the edge plasma.

The upper hybrid resonance (UHR) scattering technique possessing such merits as one-dimensional probing geometry, enhancement of cross section, and fine localization of scattering region is modified in the new diagnostics under development to achieve wave number resolution. The fluctuation wave number is estimated in the new technique from the scattering signal time delay measurements. The feasibility of the scheme is checked in the proof of principal experiment in a tokamak. The time delay of the UHR scattering signal exceeding 10 ns is observed. The small scale low frequency density fluctuations are investigated in the UHR RADAR backscattering experiment. The UHR cross-polarization scattering signal related to small scale magnetic fluctuations is observed. The lower hybrid (LH) wave propagation and both linear and nonlinear wave conversion are investigated. The small wavelength (λ⩽0.02 cm) high number ion Bernstein harmonics, resulting from the linear wave conversion of the LH wave are observed in a tok...

The radar enhanced scattering (RES) diagnostic is shown to be a feasible tool for studying small-scale plasma fluctuations and waves. This method is based on the effect that an RF wave launched into the plasma is considerably slowed down near the hybrid resonance so that the backscattered signal is received with a large time delay. Fairly good spatial and wavenumber resolution of this diagnostic are demonstrated in various laboratory plasmas.

We describe an enhanced scattering diagnostic method based on the scattering of an extraordinary electromagnetic wave close to the upper hybrid resonance. The method has a high sensitivity and spatial resolution and allows observation of the plasma density fluctuations with characteristic scale-lengths much smaller than the wavelength of the probing radiation in vacuum. The theory of enhanced scattering is presented together with experimental results obtained in small laboratory installations and in the research tokamak FT-2. The experiments were aimed at the study of small-scale intrinsic fluctuations in quiescent plasmas, the propagation of lower hybrid waves and the parametric instabilities.