Calibration of the Joint European Torus energetic ion and alpha particle collective Thomson scattering diagnostic receiver

Abstract

The receiver of the Joint European Torus (JET) energetic ion and alpha particle collective Thomson scattering diagnostic is calibrated assuming blackbody emission from the torus vacuum vessel (VV) and using electron cyclotron emission (ECE). The 32 receiver channels are absolutely calibrated with a mechanical chopper in the quasioptical arm of the receiver, alternating the receiver view between the torus vacuum at 320 °C and room temperature. This calibration is noisy due to the small difference between torus and room temperatures. A more accurate relative calibration is achieved with the ECE during plasma shots. The intensity of the ECE is found to be a smooth function of frequency, which enables the combination of the ECE calibration with the VV calibration. The accuracy of the absolute VV calibration is hereby improved to nearly the same standard as the relative ECE calibration. ECE signals measured by the calibrated receiver agree well with standard JET ECE diagnostics. Based on mathematical considerations presented here, noise is injected into the receiver final intermediate frequency stage during VV calibrations to provide more bit transitions for accurate analog-to-digital (AD) conversion of the low level calibration signals. This yields a resolution which is better than 1% of an (AD) step.