HIGH-SPATIAL-RESOLUTION INTERFEROMETRY AND POLARIMETRY IN HOT PLASMAS

Interferometry is a standard technique for measuring the line-integrated electron density of magnetically confined plasmas. It is based on the fact that an electromagnetic wave, on its passage through a plasma, experiences a phase change with respect to the vacuum situation. Moreover, the plane of polarization of the wave will in general rotate because the plasma is birefringent. The total (Faraday) rotation angle of the wave, which can be measured by polarimetric techniques, is proportional to the line-integrated value of the magnetic field component parallel to the beam. The optimum wavelength for interferometry/polarimetry, for diagnosing magnetically confined plasmas is in the far-infrared and infrared spectral region. To invert the line-integrated data of the electron density and magnetic field component to local values, without using too stringent assumptions on the plasma symmetry, it is necessary to probe the plasma along multiple chords. In many confinement devices this is done by using a limited number of discrete probing chords. In recent years much attention has been devoted to the development of multichannel interferometer/polarimeter systems with a much better coverage of the plasma, which could be achieved by expanding the probing beam either to a slab- or a fanlike beam, covering almost the complete minor cross section of the plasma. A much higher number of detectors can be implemented in this way, thus substantially improving the spatial resolution of the measurements. In this paper the present status of interferometer/polarimeter systems featuring slablike or fanlike probing beams will be reviewed. Results of detailed measurements that have been performed to study the effect of magnetohydrodynamic activity (in particular sawteeth), disruptions, additional heating, and pellet injection on the electron density distribution, will be used to illustrate the merits of interferometry/polarimetry with high spatial resolution. © 1995 American Institute of Physics.