Modelling the effects of misaligning the probe beam and magnetic field in Doppler backscattering measurements

Modelling the effects of misaligning the probe beam and magnetic field in Doppler backscattering measurements

Modelling the effects of misaligning the probe beam and magnetic field in Doppler backscattering measurements 150 150 amit lakhani
UKAEA-CCFE-CP(19)55

Modelling the effects of misaligning the probe beam and magnetic field in Doppler backscattering measurements

The use of Doppler Backscattering (DBS) in spherical tokamaks is challenging since the magnetic pitch angle can be large (up to 35, compared to 15 in standard tokamaks like JET). Moreover the pitch angle varies both spatially and temporally. Hence, the probe beam is generally not perpendicular to the magnetic field. This misalignment, which affects the backscattered signal, can be empirically optimised with 2D beam steering [1]. However, empirical optimisation is inefficient, requiring repeated pulses with different diagnostic settings, and may not always be possible. Hence, it is important to develop a model to quantitatively account for the effect of the misalignment on the backscattered signal, avoiding the need to optimise empirically. We use beam tracing (Torbeam [2] as well as a newly written code) and the reciprocity theorem [3] to derive a model for the backscattered power and its dependence on the mismatch angle. Our model works for both the O-mode and X-mode in tokamak geometry.

Collection:
Conference
Journal:
Publisher:
Conference:
14th International Reflectometry Workshop, Swiss Plasma Center of the Ecole Polytechnique Fédérale de Lausanne, Switzerland, 22-24 May 2019
Published date:
14/07/2020
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