A Hybrid X-Mode Reflectometry First Fringe Detection Algorithm For Jet

At the Joint European Torus (JET), the presently implemented density profile by X-mode Frequency Modulated Continuous Wave reflectometry system, known as KG10, is not living up to its full potential. This is due to an unreliable first fringe detection, and excessive filtering of the full signal which can result in profiles with little detail. This paper aims to address the first of these problems, the first fringe detection. Since no documentation is available regarding the implementation currently in use, there are no assurances about how the present algorithm works, but a glance at successive profiles highlights its unreliable nature. Typically, first fringe detection algorithms used in X-mode reflectometry rely on detecting at which point the beat signal’s power increases significantly. The method introduced in this work uses the signal’s power as the main discriminant, but also takes into consideration the spectral behavior of the signal in those instances where the power alone doesn’t relay a consistent detection. Large deviations between consecutive sweeps of the reflectometer are prevented, thus yielding a stabler detection of the first fringe. As an extra problem, JET’s FM-CW system displays low power at the end of the Q-band and at the beginning of the V-band, where both bands overlap, between 51 and 53GHz. This causes a lack of reliability on the power based methods, leading to an increased prevalence of the frequency based ones. This hybrid power-frequency method yields promising results, providing a solid platform for subsequent relevant physics results.