Determination of Damavand Tokamak Laser Interferometer System Characteristics by Means of Identifying and Evaluating the Sources of Error in Plasma Electron Density Measurement

Laser interferometer system is a diagnostic tool used to measure the electron density in the tokamak plasma. The system consists of various components such as the laser, the detector, the optics, the control unit and data acquisition unit, etc. The first step in setting up the system for experimental purposes is to characterize each of its components according to the tokamak parameters and the research goals. The laser is one of the most important components of this system and once chosen, affects the selection of other components. For this reason, this component has been the first to be studied in Damavand tokamak system. The laser wavelength can be selected from microwave to far-infrared and infrared regions depending on the plasma volume, the magnetic field and electron density. In this paper, laser characterization is accomplished by means of identifying sources of error in the measurement of electron density in the Damavand tokamak plasma. Resolution of two-color laser interferometer, refraction phenomenon, Faraday rotation effect, vibration limit, transparency of the vacuum window for laser wavelength, the noise due to the compensation wavelength and the extraordinary mode propagation are identified as the sources of error in density measurements. Then, the wavelength dependence on these phenomena is found. Results indicate despite the fact that the dual CO2 laser system is poor at effective phase resolution, density resolution and the signal to noise ratio, in comparison with other combinations of laser wavelength, its small refraction, tiny Faraday rotation angle and more stable interference signal make it a suitable choice for electron density measurement in Damavand tokamak.