Speaker
Valerian Hongjie Chen
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1011.pdf
Effects of misaligning the probe beam and magnetic field in Doppler
backscattering measurements
V.H. Chen1,2 , F.I. Parra1,2 , J.C. Hillesheim2
1 Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, UK
2 CCFE, Culham Science Centre, Abingdon OX14 3DB, UK.
The Doppler Backscattering (DBS) microwave diagnostic enables the non-perturbative char-
acterisation of density fluctuations (1 . k⊥ ρi . 10) and flows, both at the edge and the core of
the plasma. The large magnetic pitch angle (up to 35◦ , compared to 15◦ in standard tokamaks
like JET) and the time-varying magnetic equilibrium make the use of DBS in spherical toka-
maks challenging. Due to spatial variation, it is not possible to simultaneously achieve align-
ment between the probe beam and electric field for all launch frequencies. 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 diag-
nostic 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 used beam tracing [2] and the reciprocity theorem to derive an analytic model for the
backscattered power and its dependence on the mismatch angle. Unlike previous work on reci-
procity [3], our model works for both the O-mode and X-mode in tokamak geometry. Our more
general model can be implemented numerically, allowing the misalignment of DBS measure-
ments to be accounted for. The results are compared to scans of the toroidal launch angle from
MAST data. With insight from our model, we also assessed the measurement capabilities for
the planned MAST-U DBS system.
Acknowledgements
This work has been funded by the RCUK Energy Programme [grant number EP/P012450/1].
In order to obtain further information on the data and models underlying this work, please
contact PublicationsManager@ukaea.uk. V.H. Chen’s DPhil is funded by a National Science
Scholarship from A*STAR, Singapore.
References
[1] J.C. Hillesheim, N.A. Crocker, W.A. Peebles, H. Meyer, A. Meakins, A.R. Field, D. Dunai, M. Carr, N.
Hawkes, and MAST Team, Nuclear Fusion 55 (7), 073024 (2015)
[2] E. Poli, A.G. Peeters, and G.V. Pereverzev, Computer Physics Communications 136 (1-2), 90-104 (2001).
[3] E.Z. Gusakov, and A.V. Surkov, Plasma Physics and Controlled Fusion, 46 (7), 1143 (2004).
