Resolving the bulk ion region of millimeter-wave collective Thomson scattering spectra at ASDEX Upgrade

Abstract

Collective Thomson scattering (CTS) measurements provide information about the composition and velocity distribution of confined ion populations in fusion plasmas. The bulk ion part of the CTS spectrum is dominated by scattering off fluctuations driven by the motion of thermalized ion populations. It thus contains information about the ion temperature, rotation velocity and plasma composition. To resolve the bulk ion region and access this information, we installed a fast acquisition system capable of sampling rates up to 12.5 GS/s in the CTS system at ASDEX Upgrade. CTS spectra with frequency resolution in the range of 1 MHz are then obtained through direct digitization and Fourier analysis of the CTS signal. We here describe the design, calibration and operation of the fast receiver system and give examples of measured bulk ion CTS spectra showing the effects of changing ion temperature, rotation velocity and plasma composition.

Recent development of collective Thomson scattering for magnetically confined fusion plasmas

“…In the following we will focus on O to O-mode scattering. The CTS receivers at ASDEX Upgrade are heterodyne radiometers and are discussed in reference [21,22].…”

Consistency between real and synthetic fast-ion measurements at ASDEX Upgrade

“…We note from Figure 1(e) that the background-subtracted CTS signal is indeed negligible at large frequency shifts during phases with no NBI heating (and hence with no supra-thermal ions in the plasma). Further proof-of-principle measurements and first applications of the new CTS dual-receiver technique can be found in [16,30], while the acquisition, calibration, and background subtraction of time-resolved CTS spectra are described in detail in [16,31].…”

“…The CTS signal is absolutely calibrated using two blackbody sources at different temperatures [31]. The measurements discussed here employed the new dual-receiver background technique described in [16], with a passiveview receiver system whose field of view does not intersect the probing beam.…”

“…The CTS system at AUG employs a gyrotron which generates mm-wave probing radiation at f gyr 105 GHz [12,[14][15][16]31]. The CTS signal is absolutely calibrated using two blackbody sources at different temperatures [31].…”

First operations with the new Collective Thomson Scattering diagnostic on the Frascati Tokamak Upgrade device

“…The CTS diagnostic of FTU has been recently improved with a new fast data acquisition, implemented for the first time in ASDEX [19], to allow studying the rapidly modulated emissions correlated with rotating islands. This system allows spectral reconstruction by direct FFT of the intermediate frequency signal and is added in parallel to the preexisting multi-channel spectrum analyzer, which was originally installed in FTU for thermal ion distribution measurements.…”