Full wave modeling of the cross-polarization scattering diagnostics for the NSTX-U tokamak*

Cross-polarization scattering (CPS) diagnostics measures the local, internal magnetic fluctuations in a tokamak plasma by scattering the incident microwave beam into the orthogonal polarization [1]. A new four-channel CPS diagnostic has been designed and installed on the NSTX-U tokamak. This work presents the efforts in modeling the CPS using COMSOL, a commercial multiphysics software suite. Using the realistic NSTX-U equilibrium and plasma parameters, a 2D full wave model of CPS has been developed which captures the essential physics of the electromagnetic scattering by magnetic fluctuations. The results from full wave modeling are then compared to the GENRAY ray tracing model. Several important effects such as the polarization mismatch at the edge and wavevector matching at the cut-off surface are explored using this model to understand their effect on the measurement of magnetic fluctuations. These modeling results are validated using the experimental measurement of magnetic fluctuations from devices such as DIII-D and MAST-U. The modeling efforts will guide the successful operation of the CPS diagnostic for the NSTX-U plasmas by identifying favorable conditions for the measurement of magnetic fluctuations.

1. T. Lehner et al., Europhys. Lett. 8, 759 (1989)