A mock-up experiment for the ITER polarimeter

In this paper we describe an experimental test of a polarimetric set-up very similar to that foreseen for ITER, that we have assembled using part of the FIR polarimeter of the RFX experiment. We have used a CO2 pumped, CH3OH FIR laser (λ = 118.8 μm), a 31m-long, nitrogen filled, 10 mirror, optical transmission line and a metallic corner-cube retroreflector to simulate the operating conditions of the future ITER FIR polarimeter. The polarization state of the beam has been modulated by a rotating quarter-wave plate and the effect of the plasma has been simulated by a combination of a half-wave and a quarter-wave plates. A non-polarizing beam-splitter, made from a square metal mesh has been used to separate the forward and return beams and the FIR radiation has been detected by pyroelectric detectors. The detector outputs have been recorded by a digital oscilloscope and numerically processed by a digital lock-in technique. In a series of experiments of increasing complexity, ranging from the simple check of the polarization modulation to a final arrangement with the polarization modulated beam propagating along the entire optical beam-line, we have experimentally demonstrated that the set-up as proposed is suitable for operation in ITER. In addition, we have demonstrated that the polarization changes anticipated in the ITER plasma can be measured with the accuracy and the time resolution close to those required.