Study On Output Characteristics Of Far-infrared DCN Laser

The electron density is an essential parameter in thermonuclear fusion research, for which can be measured mainly by the technique based on transmission characteristic of electromagnetic waves in plasmas. As a diagnostic means, the choice of wavelength is limited, and the sub-millimeter wave ranging from 0.1 mm to 1 mm fits for the requirement of plasma density measurement in tokamaks. Far-infrared interferometer is widely used for diagnosing plasma electron density in tokamaks. The study of Sub-millimeter lasers has become more and more important as the light source of the interferometer. The DCN laser has the highest specific output power among known sub-millimeter electric-discharge lasers. In this M.Sc thesis, principle of propagation of electromagnetic waves in plasmas and the density measurement by means of FIR laser interferometer were presented briefly. A CW sub-millimeter wave-guide discharge-pumped DCN laser operating on 190 and 195 μm lines was developed and all parts of the DCN laser were described in detail. The laser cavity is a 54 mm diameter, 3.4 m long Pyrex wave-guide with a gilded glass plane mirror at one end and a metal mesh at the other end. Optimization of various parameters, such as the pressure, the discharge current and the wall temperature were studied in experiments. When the laser work with mixture gas of N2, CD4 and D2, no brown polymer deposit was found after long time discharge and the output of the DCN laser is stable. The lasing modes and the propagation property of the laser were studied,by a series of experiments, the modes corresponding to 195μm and 190μm lines are observed and separated well. The diameter and position of the beam waist were obtained and the output beam profile is found to be qualitatively Gaussian in the far field of the DCN laser. According to the Gaussian beam theory of propagation, the optical design of the interferometer was presented. A single-channel horizontal FIR DCN laser interferometer was set up and the structure of the interferometer system was described, and some experiments were performed on the interferometer. The study in this thesis shows the feasibility of the multi-channel interferometer on the EAST tokamak.