Ray-tracing Analysis for Cross-polarization Scattering Diagnostic on MAST-Upgrade Spherical Tokamak

TL;DR

This paper uses 3D ray-tracing simulations to analyze the scattering signals in a combined Doppler backscattering and cross-polarization system on MAST-U, aiding the measurement of plasma turbulence and magnetic fluctuations.

Contribution

It introduces a modified GENRAY 3D ray-tracing approach to evaluate the scattering contributions and detectable magnetic fluctuation wavenumbers in the CPS system on MAST-U.

Findings

Identifies the wavenumber ranges detectable by CPS in simulated plasmas.

Simulates beam propagation and scattering contributions along the entire trajectory.

Provides insights into the system's capability to measure magnetic turbulence.

Abstract

A combined Doppler backscattering/cross-polarization scattering (DBS/CPS) system is being deployed on MAST-U, for simultaneous measurements of local density turbulence, turbulence flows, and magnetic turbulence. In this design, CPS shares the probing beam with the DBS and uses a separate parallel-viewing receiver system. In this study, we utilize a modified GENRAY 3D ray-tracing code, to simulate the propagation of the probing and scattered beams. The contributions of different scattering locations along the entire beam trajectories are considered, and the corresponding local $\tilde{\mathbf{B}}$ wavenumbers are estimated using the wave-vector matching criterion. The wavenumber ranges of the local $\tilde{\mathbf{B}}$ that is detectable to the CPS system are explored for simulated L- and H-mode plasmas.