Theoretical and numerical comparison of different poloidal correlation reflectometry schemes

Rahoitus

To tackle the limitation of the linear model, full-wave modelling with CUWA was performed for raw ELMFIRE data and the resulting CCF is presented on Fig. 8. Once again, we observe a strong decay of correlation. While the nonlinear scattering theory predicts that the four antenna configuration should be preferable [4], in our case both are at the noise level, although the four antenna configuration results in a marginally better CCF. To confirm that this effect was caused specifically by nonlinear effects, the amplitude of density perturbations was decreased tenfold and another set of full-wave computations was performed. The resulting correlation function is shown in Fig. 9 and has the same behavior as the linear CCF in Fig. 4. Thus the nonlinear effects are capable of distorting correlation between the scattering signals and obstructing measurements. Conclusions Theoretical analysis of the PCR diagnostics in the linear approximation and slab geometry was performed. Analytical expression for PCR CCF was obtained. The equivalence of the three and four antenna configurations was confirmed. Theoretical results were verified with both linear and full-wave modelling. The computations have also demonstrated decorrelation of signals in the nonlinear scattering regime. Geometrical effects were also shown to play a significant role in PCR measurements for the geometry of a small tokamak like the FT-2. Acknowledgements This work was prepared under support of the Russian Science Foundation grant 17-12-01110. The full-wave computations were supported by the Ioffe Institute. References