Intensity interferometry for measurement of electron temperature fluctuations in fusion plasmas

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Abstract
Transport-relevant fluctuations of the electron temperature can in principle be measured by radiometry of the electron cyclotron emission (ECE), but they might be buried completely in natural fluctuations of the ECE due to the thermal nature of this radiation. The spatial coherence properties of thermal radiation can be utilized in correlation experiments to overcome the principal difficulties and to make temperature fluctuations accessible to measurement. On the basis of these considerations an intensity interferometer was built and installed on the Wendelstein 7-AS stellarator. The interferometer consists of two identical but independent multichannel heterodyne radiometers viewing the same emitting plasma volume along crossed lines of sight. The angle between the sightlines is chosen sufficiently large to decorrelate the natural fluctuations of the radiation field. A quasithermal microwave radiation source is used to test the intensity interferometer and to simulate the correlation experiment. Measurements of the spatial coherence and the statistical properties are in agreement with theoretical predictions.