Innovative Doppler backscattering spectra analysis methods applied to comb reflectometer measurements in ASDEX Upgrade

The W-band tunable 7-channel Comb reflectometer (P. A. Molina Cabrera et al., Rev. Sci. Instrum. 94, 083504 (2023)) is used for simultaneous measurements of Doppler backscattering (DBS) spectra at several radial locations in ASDEX Upgrade, enabling the study of dynamical events such as the L-H transition. To facilitate the analysis of the resulting large volume of spectra from short time windows with a limited signal/noise ratio, the common DBS spectra fitting approaches are examined in terms of how reliably they can be automated and improvements are proposed.

Specifically, the common fit of the asymmetric decomposition of DBS spectra which avoids the m=0 reflection component is improved by additionally enforcing the positivity of the latter. This avoids common issues of convergence to unrealistic solutions when the width of the Doppler peak becomes comparable to its shift. Additionally, the formulation of the problem as a forward model enables the use of a non-gaussian likelihood and an associated chi-squared-like statistic more appropriate for measured spectra (S. Baker and R.D. Cousins, Nuc. Inst. And Methods in Phys. Research 221 (2), 437-442 (1984)), which in turn is shown to improve the fit accuracy. This method may be of interest to other spectroscopists in general, since it tackles the common issue of separating a passive an active line, as in CXRS.

Finally, the dynamics and temporal ordering of DBS spectra parameter changes across the L-H transitions are presented.