Problemi di Fisica per il Diploma d'Ingegneria Elettronica e Biomedica

Edizioni Libreria Progetto, Padova (2000

Data analysis for a rotating quarter-wave, far-infrared Stokes polarimeter

Data analysis techniques are reviewed and extended for the measurement of the Stokes vector of partially or completely polarized radiation by the rotating quarter-wave method. It is shown that the conventional technique, based on the Fourier analysis of the recorded signal, can be efficiently replaced by a weighted least-squares best fit, so that the different accuracy of the measured data can be taken into account to calculate the measurement errors of the Stokes vector elements. Measurement errors for the polarization index P and for the azimuth and ellipticity angles \u3c8 and \u3c7 of the radiation are also calculated by propagation error theory. For those cases in which the above technique gives a nonphysical Stokes vector (i.e., with a polarization degree of P > 1) a constrained least-squares best fit is introduced, and it is shown that in this way a Stokes vector with P = 1 (rather than P 64 1) is always obtained. In addition an analysis technique useful to remove from the measured data systematic errors due to initial misalignment of the rotating quarter-wave axis is described. Examples of experimental Stokes vectors obtained by the above tech- niques during the characterization of components for a far-infrared polarimeter at \u3bb = 118.8 \u3bcm for applications in plasma diagnostics are presented and discussed. Finally the problem of the experimental determination of physically consistent Mueller matrices (i.e., of Mueller matrices for which the transformed Stokes vector has always P 64 1) is discussed, and it is shown that for simple Mueller matrices of the ABCD type, whose elements can be determined by the measurement of a single Stokes vector, the imposed P 64 1 constraint gives a sufficient condition for physical consistency. On the other hand, the same constraint, when imposed to the set of four basic Stokes vectors conventionally measured for the determi- nation of a full 16-element Mueller matrix, gives only a necessary but not a sufficient condition

On the calibration of polarimetric Thomson scattering by Raman polarimetry

Polarimetric Thomson scattering (TS) is an alternative method for the analysis of Thomson scattering spectra in which the plasma temperature Te is determined from the depolarization of the TS radiation. This is a relativistic effect and therefore the technique is suitable only for very hot plasmas (Te > 10 keV) such as those of ITER. The practical implementation of polarimetric TS requires a method to calibrate the polarimetric response of the collection optics carrying the TS light to the detection system, and in particular to measure the additional depolarization of the TS radiation introduced by the plasma-exposed first mirror. Rotational Raman scattering of laser light from diatomic gases such as H2, D2, N2 and O2 can provide a radiation source of predictable intensity and polarization state from a well-defined volume inside the vacuum vessel and is therefore suitable for these calibrations. In this paper we discuss Raman polarimetry as a technique for the calibration of a hypothetical polarimetric TS system operating in the same conditions of the ITER core TS system and suggest two calibration methods for the measurement of the additional depolarization introduced by the plasma-exposed first mirror, and in general for calibrating the polarimetric response of the detection system

Characterization of fast microchannel plate photomultipliers for the ITER core LIDAR Thomson scattering system

In the new ITER core LIDAR Thomson scattering system under development, detectors with pulse response as low as 330 ps and 10 mm diameter active area, covering with good sensitivity the spectral range 380\u20131100 nm will be necessary. For the visible region 400\u2013750 nm, fast microchannel plate photomultipliers with a GaAsP photocathode that fulfill the characteristics of speed, sensitivity and active area are already commercially available and have been recently characterized for operation in the JET edge LIDAR Thomson scattering system. In this paper we present a further characterization of these devices to evaluate their usefulness in the more demanding operating conditions of the ITER core LIDAR Thomson scattering system. The characteristics of these detectors with regard to linearity, gain recovery time, pulse repetition rate, quantum efficiency and response time for large F-number illumination, have been measured. Linearity and recovery time data have been interpreted according to a new, time dependent, microchannel plate operational model. The results show that these detectors are suitable also for use in ITER