Probe Measurements in the X-point Region at ASDEX Upgrade

Plasma fluctuations in the divertor region just below the X-point at ASDEX Upgrade tokamak are investigated using Langmuir probes measurements. These probes are mounted on the X-point manipulator, which provides unique access to measurements of the ion-saturation current through three consecutive regions, the low-field side scrape-off layer, the private flux region and the high-field side scrape-off layer.

This enables a detailed analysis, which are provided in two parts. In the first part, a general fluctuation study is presented, in which regions of low fluctuations are identified. The second part focuses on the characteristics of the fluctuations in the three different regions around the X-point and differentiate the universal features to the fluctuations at the outboard mid-plane.

For the first part, spectral analysis methods reveal the presence of a region of low fluctuations not only in the low-field side scrape-off layer, but also in the high-field side of the private flux region of which the latter has not been reported previously. The two ‘quiescent’ regions are observed both in helium and deuterium plasmas and appear to be generally independent of the auxiliary heating source in use.

For the second part, the application of the filtered Poisson process (FPP) model shows that rather than the universal conditional averaged wave-form of a steep front and trailing wake found at the outboard mid-plane of a wide range of toroidal fusion experimental devices, the X-point fluctuations display a more symmetric wave-form on the low-field side scrape-off layer.

Moreover, it is found that the methods used for the conditional averaged wave-form and power spectral density in the FPP model are able to fit the X-point data successfully when considered individually, but not in a unified manner as generally observed at the outboard mid-plane. The discrepancy shows that the X-point region cannot in general be modeled as a series of uncorrelated pulses satisfying the Poisson distribution.

This also shows that the fluctuations born at the mid-plane are not surviving and dominant in the divertor region, where local instabilities may also contribute to the fluctuations.