This article describes the realization and testing of the electronic system which forms part of the multichannel far-infrared (FIR) polarimeter for the RFX machine,a plasma confinement experiment with Reversed Field Pinch (RFP) configuration. The electronic system,which comprises the detectors,the signal-processing electronics,and the motion electronics for the half-wave plate movement,is described. Emphasis is placed in the analysis of the polarimeter signals,which permits an in-depth understanding of the performance of the data processing electronics and the role of the various sources of noise in the system. After a brief outline of the basic principle of the measurement,the choice of detectors and their characteristics are described in order to achieve the best performances at the FIR wavelength (λ =118.8 μm) of interest. Various tests,which are described,confirmed the need for a specifically designed pyroelectric detector capable of operating in the hostile magnetic environment near the machine. The processing of the raw polarimeter signals to produce the required sum and difference signals and to convert them into dc signals with 3 ms time constant is presented. These signals are synchronous with a chopper signal on the FIR beam and are subsequently fed to a lock-in amplifier. An accurate analysis of the data processing procedure is described,which helps to clarify the understanding of the output signals that are eventually recorded in the data acquisition system. In particular,various sources of noise,such as thermal noise of the detectors,laser fluctuations,spurious signals at harmonics of the chopper frequency,and phase jitter of the chopper,are evaluated,discussed,and compared with the observed signals. Finally,the control circuitry for the movement of the half-wave plates,both for manual control and for the programmed sequences of zero-search and calibration performed by a PLC control system,is described. Calibration curves obtained during experiments are also reported.
All Science Journal Classification (ASJC) codes
Zilli, E., Milani, F., O’Gorman, M., Giudicotti, L., & Prunty, S. L. (2001). Electronics and signal processing for the multichord far-infrared polarimeter of the RFX experiment. Review of Scientific Instruments, 72(11), 4125 - 4138. https://doi.org/10.1063/1.1412262