Real-time detection and correction of frequency sweeping non-linearities of FMCW reflectometry microwave sources

Abstract

FMCW reflectometry density profile measurements will be used to complement or replace magnetic measurements for plasma position control in future plasma fusion reactors, such as ITER and DEMO. Although FMCW reflectometry is a well established diagnostic technique, on present experimental devices with short discharges (typically of a few seconds), on-line measurement quality validation and correction is not a common practice. One of the technique's main requirements is to ensure that the microwave sources always produce a linear frequency sweep within the probing frequency range. Failing to do so leads to inaccurate density profile reconstruction and, hence, to erroneous plasma position feedback to the tokamak control loops. Traditionally, static calibrations are used to build the voltage ramps driving the reflectometer's VCO (Voltage Controlled Oscillator) sources. By using extra signals like frequency markers or calibrated delay lines, the experimental reflectometry measurements can be further calibrated, off-line, to minimize frequency sweep linearity related distortions. We herein propose a technique to monitor and re-calibrate in real-time the linearity of the frequency sweeps of FMCW reflectometers. It can be applied in the long duration discharges of future devices either to correct thermal or electronic drifts of the probing frequency sources or to generate alarm signals when measurement anomalies or fundamental diagnostic malfunctions are detected. Experimental data is presented showing the benefits of the proposed technique.