Improved accuracy and robustness of electron density profiles from JET’s X-mode FMCW reflectometers
JET’s frequency-modulated continuous wave (FMCW) reflectometers have been operating well with the current design since 2005 and density profiles are being automatically calculated intershot since then. However, the calculated profiles had long suffered from several shortcomings – poor agreement with other diagnostics; sometimes inappropriately moving radially by several centimeters; elevated level of radial jitter; and persistent wriggles (strong unphysical oscillations). This contribution goes through several aspects improved in the data analysis that resulted in significant suppression of all these issues. Starting with improving the equilibrium reconstruction that estimates the background magnetic field, also adding a ripple correction in the reconstructed magnetic field profile, and new inner-wall reflection positions estimated through ray-tracing, these changes not only improve the agreement of reconstructed profiles to other diagnostics but also solve density profile wriggles that were present during band transitions. Other smaller but also persistent wriggles were also suppressed by applying a localized correction to the measured beat frequency where persistent oscillations are present. Lastly, the burst analysis method, as introduced in [P. Varela et al 2006 Nucl. Fusion 46 S693], has been implemented to extract the beat frequency from stacked spectrograms. Due to the strong suppression of spurious reflections, the radial jitter that sometimes would span several centimeters has been strongly reduced. The stacking of spectrograms has also been shown to be very useful for stacking recurring events, like small gas puff modulations, and extracting transport coefficients that would otherwise be below the noise level.