Showing papers on "Polarimetry published in 1985"

Magnetic field structures in active compact radio sources

Abstract: The analysis of simultaneous multifrequency linear polarimetry data between 1.4 GHz and 90 GHz for about 20 active, compact radio sources at six epochs from 1977 December 10 1980 July is presented. In addition, monthly 8 Ghz polarization data on the same sources were examined. The general polarization characteristics of these sources can be well described in terms of magnetic fields which are largely turbulent and slightly anisotropic. The magnetic field symmetry axes are generally aligned with the source structural axes on the milli-arcsecond scale (OJ 287 is a notable exception.) Monte Carlo calculations indicate that observed polarization variations and in particular rotator polarization events can be produced in this model as a consequence of random walks generated through evolution of the turbulent magnetic field. 43 references.

Chapter 5 – The Accurate Determination of Optical Properties by Ellipsometry

Abstract: Publisher Summary Electromagnetic theory aids in understanding the different techniques used to determine optical properties of materials. It is noted that if the electromagnetic wave is reflected by a smooth surface, the outgoing wave can be represented in the absence of anisotropic effects in another local coordinate system. Wherein, both the local coordinate systems the x axes are in the plane of incidence, the y axis is perpendicular to the plane of incidence, and the z axes define the plane of incidence. Four parameters—namely, two amplitudes and two phases are necessary to completely describe the incoming wave to describe the sample. The properties of the sample therefore are obtainable, if the properties of both incident and reflected waves are known. Reflectometry and ellipsometry are essentially two techniques for obtaining this information. For isotropic samples ellipsometry is strictly a nonnormal-incidence technique, while reflectometry and polarimetry can be performed at either normal or nonnormal incidence. Reflectometry deals with intensities and is therefore a power measurement, while ellipsometry deals with intensity-independent complex quantities and is therefore more nearly analogous to an impedance measurement.

Stokes-spectro-polarimetry with a two-dimensional diode array

Abstract: A new apparatus is described which measures the Stokes profiles by means of a two-dimensional 100 × 100 detector array. On-line data processing allows the immediate visualization of the profiles corresponding to several spectral lines observed simultaneously along one spatial direction on the solar disk. An improved method for the accurate compensation of the telescopic birefringence with a Bowen compensator is realized. Three different methods for the investigation of fluxtube properties are discussed, based on the analysis of V profiles and illustrated by first measurements with the apparatus. Two characteristics of V profiles; excess polarization and zero crossing wavelength of the V profile show strong spatial variations in plages and network indicating different dynamical conditions for individual fluxtubes.

On the viability of exploiting L-shell fluorescence for X-ray polarimetry

Abstract: It has been suggested that one may build an X-ray polarimeter by exploiting the polarization dependence of the angular distribution of L-shell fluorescence photons. In this paper we examine, theoretically, the sensitivity of this approach to polarimetry. We apply our calculations to several detection schemes using imaging proportional counters that would have direct application in X-ray astronomy. We find, however, that the sensitivity of this method for measuring X-ray polarization is too low to be of use for other than laboratory applications.

Far-infrared polarimetry/interferometry for internal magnetic field measurement on ZT-40M

Abstract: The measurement of internal magnetic field profiles may be a very important step in the understanding of magnetic confinement physics issues. The measurement of plasma-induced Faraday rotation is one of the more promising internal magnetic field diagnostics. This thesis describes the development of a heterodyne polarimeter/interferometer for internal poloidal magnetic field measurement on ZT-40M. Heterodyne techniques were employed because of the insensitivity to spurious signal amplitude changes that cause errors in other methods. 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. Grazing incidence motions of the constrained diagnostic access on ZT-40M. Grazing incidence reflections on metallic surfaces of the diagnostic ports caused polarization changes that affected the measurement accuracy. Installation of internally threaded sleeves to baffle the reflections eliminated the sensitivity problem, and allowed useful Faraday rotation measurements to be made. Simultaneous polarimetric and interferometric measurements have also been demonstrated. The ability to assemble a working heterodyne polarimeter/interferometer is no longer in question. The extension of the present system to multichord operation requires increased laser power and efficiency.

Far‐infrared polarimetry/interferometry for internal magnetic field measurement on ZT‐40M (abstract)

Abstract: An 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. This measurement performed on several chords yields information on the density and poloidal field profiles. 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. On ZT‐40M, amplitude changes exceeding 50% have been observed, thus precluding any other method for our application. We use a 185‐μm laser, a rotating grating for heterodyning, and GaAs photoconductors and Schottky diodes for detection. An instrumental sensitivity of 3 mrad at a time resolution of 25 μs has been demonstrated, however vibration‐induced structure limits the sensitivity to ∼50 mrad. Efforts to reduce each of these numbers are in progress. The simultaneous measurement has been made on ZT‐40M, and effort is turning to system refinement and scaling up to multich...

Far‐infrared imaging of tokamak plasma

Abstract: A 20-channel interferometer has been developed which utilizes a linear, one-dimensional microbolometer array to obtain single-shot density profiles from the UCLA Microtor tokamak plasma. The interferometer has been used to study time-dependent phenomena in the plasma density profile. Observations of the sawtooth instability clearly show the growth of the m=0 mode from a localized oscillation (r=1 cm) on axis to an oscillation of the entire plasma. Also, measurements during the initial startup phase of the discharge show evidence of hollow density profiles. In addition, a simultaneous measurement of the poloidal magnetic field has been developed which provides 20 channels of polarimetry. Interferometry and polarimetry both use the same imaging system and the spatial resolution of both measurements has been tested using plastic and crystal-quartz test objects. The signal-to-noise ratio for the polarimeter has also proved adequate for the expected Faraday rotation angle (alphamax=7°, Ip=70 kA, n=5×10^13 cm^−3).

Polarimetry of cosmic sources of hard gamma-ray emission

Abstract: The possibility of the polarimetry of cosmic sources of hard gamma-ray emission using telescopes containing spark chambers, based on the dependence of the anisotropy of the azimuthal distribution of the planes of e(+ or -) pairs on the degree of linear polarization, is discussed. An estimate is made of the photon statistics required for the measurement of polarization and it is concluded that such measurement is already feasible at the level of sensitivity of the Gamma-1 and GRO telescopes now being developed. 6 references.

Instrumentation for submillimeter polarimetry.

Abstract: During the last two years three instruments were built and operated for detection of polarization in the submillimeter to millimeter wavelength bands. In principle, simply rotating a polarizing grid in front of the detector would be sufficient to determine the state of linear polarization. In practice severe systematic problems are found with this approach. Everything in the light path has potential for inducing polarization. The telescope, apertures in the lightpath, and the Winston light collectors all introduce systematic errors. (The polarization/depolarization induced by these devices is due to diffraction and the finite conductivity of the metals used). Two of the polarimeters are for use on the KAO; the third is for the IRTF on Mauna Kea. The airplane polarimeters, M1 and M2, were specifically designed to minimize the systematic errors. The ground based polarimeter uses our f/35 photometer with an external polarizing grid as the analyzer. With all three instruments the key to success is the data collector and analysis scheme.