has been used to determine the polarization evolution on the Poincare sphere including collisional attenuation. Computer codes have been utilized to calculate the ellipticity, e, and the rotation angle of the vibrational ellipse, <£, of the polarization. Analyses have been carried out to evaluate the measurements on both ISX-B and TFTR tokamaks. As expected, it is found that the effect of ellipticity of the polarization is negligible for the measurements on ISX-B. For TFTR however, because of the large size, high plasma current, and the double-path of the probing beam in the plasma, large rotation angle and ellipticity are expected. The pertinent parameters for TFTR are major radius, R = 265 cm; minor radius, a = 110 cm; central electron density, n„ = lO'Vcm3; plasma current, Ip = 2.5 MA; toroidal field, Br = 5.2 T; wavelength, A = 119 /j.m. In this case, the maximum </> is approximately 15° with a maximum ellipticity of 0.045 for double path of the beam. The output signals of the interferometer detector, Vi; and polarimeter detector, Vp, can be expressed by the following relations. where Vio and Vp„ are the calibration constants for the interferometer and polarimeter, respectively, i/> is the phase shift, and and are given by It can ben seen in Eqs. (1) and (2) that both the amplitude and the phase of the signals depend strongly on the ellipticity of the polarization which cannot be measured easily due to limitations on the experimental techniques. An appropriate evaluation of the measured data is, therefore, necessary. For present TFTR operations (Dec. 15, 1984), R = 265 cm, a = 110 cm, n0 = 5 x 1013/cm3, Ip = 1 MA, BT = 3 T. The maximum is approximately 3°, and the maximum e is 1.6 x IO3. The error of density measurement due to neglecting of the ellipticity is very small (0.02 percent), and the error of Faraday rotation measurement is less than 0.048 percent. EXPERIMENTS AND DISCUSSIONS The interferometer/polarimeter system on ISX-B consists of a pair of cw 671 GHz iodomethane lasers, optically pumped by separate CO2 lasers. The smm cavities are tuned such that the two oscillate at frequencies differing by Af of the order of 1 MHz. The linearly polarized beam of the source laser is passed through a ferrite polarization modulator, a mechanical polarization rotator into the dielectric waveguide, and is then divided into five beams which are projected through the plasma. Emerging from the plasma chamber, each beam enters again into a waveguide and is directed onto a signal detector. Part of the beam from the reference laser is mixed first in a reference detector with a portion of the source laser, which is split off before passage through the modulator, and the remainder is guided to the signal detector to mix with the probing beam. Schottky diodes are utilized for all detectors. The output of the reference detector is a sinusoid at frequency Af and is used as reference signal for phase detection. The output of each signal detector is filtered, amplified, and fed into a digital phase detection circuit to extract the phase shift due to plasma density. An envelope detection circuit is utilized to demodulate
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