The Effect of the Propagation Mode of a Laser Wave in an Interferometer Diagnostics in Determining of Electron Density of Damavand Tokamak Plasma and Calculation of the Measurement Error

Interferometer is the one of tools to determine the plasma characteristics. In this paper, Firstly the regime of propagation mode for an infrared laser interferometer in plasma medium of Damavand tokamak is determined by considering the tokamak magnetic field range and the propagation angle. At the same time, the role of laser wavelength of diagnostic system in changing the propagation mode is examined. Results indicate, in a certain magnetic field, by increasing the laser wavelength the limitation of the quasi- longitudinal propagation mode becomes smaller, conversely that of the quasi-transverse propagation mode becomes larger. The introduced error in determining the refractive index with comparison between the refractive index of the ordinary mode and averaged refractive index from the Altar- Appleton- Hartree dispersion relation depends on the wave propagation angle and also the strength of the magnetic field. In a certain laser wavelength, increment of magnetic field increases the error value in phase measurement. On the other hand, in a specified magnetic field, by increasing of the laser wavelength, the error of phase measurement increases. Also, in a given propagation angle, by increasing the laser wavelength the error value in phase measurement significantly increases.