S. Soldatov

Influence of the dynamic ergodic divertor on transport properties in TEXTOR

Experiments to investigate transport properties under the influence of the dynamic ergodic divertor (DED) on TEXTOR are discussed. Relativistic runaway electrons are applied for studying transport properties of ergodization such as enhanced runaway loss. The ergodization causes an enhanced loss rate; this loss is higher for low relativistic electrons than for highly relativistic ones, in good agreement with particle orbit mapping. Edge transport can be controlled by the DED perturbation: in limiter H-mode plasmas ELM-like particle and heat bursts associated with the formation of enhanced edge pressure gradients are mitigated in the 6/2 configuration on the expense of a reduced pedestal height. Finally, the plasma is driven back to L-mode under the influence of the magnetic perturbation. In the 3/1 configuration the onset of tearing modes limits the possibility to affect edge transport. A mode of spontaneous density built-up has been found for the TEXTOR-DED as well. This mode is in particular strong for an inward shifted plasma; the built-up has a resonant character with respect to q(a). Langmuir probe measurements with two probe arrays show a strong influence of the magnetic ergodization on both the edge plasma equilibrium and fluctuation parameters. In particular, in the ergodic zone the turbulence properties and turbulence-driven flux are profoundly modified.

Reflectometry measurements during operation of the dynamic ergodic divertor at TEXTOR

The O-mode correlation reflectometer at TEXTOR is used for the investigation of the propagation and the properties of the turbulence at the plasma edge. The multi-horn antenna set-up allows the investigation of the correlation properties of the turbulence and its rotation. Turbulence and plasma rotation for different plasma conditions are found to be in agreement and based on this comparison the radial electric field is estimated. In addition the density fluctuation level is estimated from the standard deviation of the phase of the reflected signal.The influence of a magnetic perturbation field, generated by the dynamic ergodic divertor (DED), on the turbulence and transport properties is studied and compared with plasmas without such a field perturbation. The strength and radial range of the perturbation field can be widely varied. Together with tangential neutral beam injection in co- and counter-current directions, the turbulent transport is investigated. The combination of neutral beam injection and the DED enables the modification of the plasma rotation profile. It can lead either to the generation of a locked mode or the formation of a transport barrier.

Estimates of the local parameters of plasma density fluctuations by reflectometry measurements

The question is considered of how to estimate the parameters of local plasma density fluctuations from reflectometry measurements made by probing the plasma with an extraordinary electromagnetic wave. In the geometrical-optics approximation, a formula is derived that relates the fluctuation amplitude of the phase of the reflected signal to the amplitude of local plasma density fluctuations and the range of its applicability is considered. The spectral sensitivity of reflectometry measurements in a reflection region of finite dimensions to poloidal perturbations with wavenumbers k⊥ ⪝ k0 is estimated by the phase-screen method, and the expressions obtained are compared with the results of numerical simulations. Based on the relationships derived, an algorithm is proposed for recovering the amplitude of the local plasma density fluctuations from the fluctuations in the reflected reflectometer signal. The results obtained are compared with the results of the full-wave simulations of the reflection of microwaves from a turbulent plasma. Finally, an example is given of how to recover the data on the amplitude of the local plasma density fluctuations in the T-10 tokamak plasma.

Investigation of the possibility of using correlation reflectometry to determine the parameters of small-scale turbulence in the core of a tokamak plasma

Results are reported from investigating the possibility of using reflectometry to determine the parameters of small-scale turbulence in the core of a tokamak plasma. In particular, the extent to which the reflectometric measurements are local is estimated. Experimental data from reflectometric measurements in the gradient zone, as well as the cross spectra and radial correlation functions, are presented. A turbulence model is constructed that yields spectral, statistical, and correlation turbulent plasma properties close to the measured ones. The model is used to simulate the propagation of electromagnetic radiation in a turbulent plasma.

Direct comparison of turbulence measurements by Langmuir probe technique and by reflectometry

A comparative analysis of the experimental results from two diagnostic methods—reflectometry and Langmuir probe technique—is carried out, and advantages and drawbacks of these methods are considered; in particular, a comparison is made between their spectral characteristics. The problems arising in interpreting the experimental reflectometric results are thoroughly discussed. To resolve these problems, a stochastic turbulence model with statistical, spectral, and correlation turbulent plasma properties close to the measured ones is constructed. The model is used to simulate the propagation of an electromagnetic wave in a model turbulent plasma. A comparison of the numerical results with the experimental data shows a high spatial locality of reflectometric measurements.