Beam model of Doppler backscattering
Abstract
We use beam tracing  implemented with a newlywritten code, Scotty  and the reciprocity theorem to derive a model for the linear backscattered power of the Doppler Backscattering (DBS) diagnostic. Our model works for both the Omode and Xmode in tokamak geometry (and certain regimes of stellarators). We present the analytical derivation of our model and its implications on the DBS signal localisation and the wavenumber resolution. To determine these two quantities, we find that it is the curvature of the field lines that is important, rather than the curvature of the cutoff surface. We proceed to shed light on the hitherto poorlyunderstood quantitative effect of the mismatch angle. Consequently, one can use this model to correct for the attenuation due to mismatch, avoiding the need for empirical optimisation. This is especially important in spherical tokamaks, since the magnetic pitch angle is large and varies both spatially and temporally.
 Publication:

arXiv eprints
 Pub Date:
 September 2021
 arXiv:
 arXiv:2109.10973
 Bibcode:
 2021arXiv210910973H
 Keywords:

 Physics  Plasma Physics