The ZT-40M instrument, which is under construction, employs a second wavelength to correct for room vibrations in the 0.01 fringe density range. The CTX instrument has been operating with eight chords since March 1984. High reliability is achieved with a temperature-controlled wave guide CO/sub 2/ laser operating on a single mode at intermittent duty. The real-time electron density readouts provide a considerable savings in ADC channels and computation time when compared with quadrature phase detection more » schemes. Heterodyne signals detected with room-temperature HgCdTe photoresistors are demodulated with digital phase detectors that can track several fringes per microsecond.
The systems are double pass, Mach-Zehnder, heterodyne, interferometers (that employ a single germanium Bragg cell and 2/sup n-italic//sup +1/-1 (n-italic is an integer) 50% beamsplitters to produce 2/sup n-italic/ interferometer chords), and a single reference beam with matched optical path lengths. « lessĪ versatile, multichord CO/sub 2/ interferometer design with a wide dynamic range is adapted to several different experiments. Error propagation in the inversion process is being evaluated. The simultaneous measurement has been made on ZT-40M, and effort in turning to system refinement and scaling up to multichord operation. Efforts to reduce each of these numbers are in progress. mu.s has been demonstrated, however vibration-induced structure limits the sensitivity to approx. An instrumental sensitivity of 3 mrad at a time resolution of 25. The authors use a 185-.mu.m laser, a rotating grating for heterodyning, and GaAs photoconductors and Schottky diodes more » for detection. On ZT-40M, amplitude changes exceeding 50% have been observed, thus precluding any other method for their application. Heterodyne techniques are employed for the polarimetry as well as the interferometry because they are insensitive to spurious amplitude changes that can hamper other methods. This measurement performed on several chords yields information on the density and poloidal field profiles. The extension of the present system to multichord operation requires increased laser power and = ,Īn important aspect of this method is to measure the density simultaneously with the Faraday rotation in order to infer the magnetic field from the rotation. The ability to assemble a working heterodyne polarimeter/interferometer is no longer in question.
Simultaneous polarimetric and interferometric measurements have also been demonstrated.
Installation of internally threaded sleeves to baffle the reflections eliminated the sensitivity problem, and allowed useful Faraday rotation measurements to be made. Grazing incidence reflections on metallic surfaces of the diagnostic ports caused polarization changes that affected the measurement accuracy. Grazing incidence motions of the constrained diagnostic access on ZT-40M. Initial problems in polarimetric sensitivity were observed that were ultimately found to be related to discharge-induced motions of the constrained diagnostic access on ZT-40M. Heterodyne techniques were employed because of the insensitivity to spurious signal amplitude changes that cause errors in other methods. This thesis describes the development of a heterodyne polarimeter/interferometer for internal poloidal magnetic field measurement on ZT-40M. The measurement of plasma-induced Faraday rotation is one of the more promising internal magnetic field diagnostics. The measurement of internal magnetic field profiles may be a very important step in the understanding of magnetic confinement physics issues.