Development of Diagnostic System for Electric Field Distribution in Plasmas

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We have developed a new diagnostic system to measure spectroscopically the spatial distribution of electric fields in plasmas such as various kinds of discharge ones and also the fusion plasmas.1. First of all, we have improved theoretical calculations concerning the measurements. For the hydrogen atom in the plasmas, intensity ratio of- the forbidden transition 3P_<1/2>-2P_<3/2> and the allowed one 3P_<1/2>-2S_<1/2> was numerically calculated as a function of the electric field strength according to the Stark effect. The results shows that the ratio increases to about 0.5 for the field strength of as low as 50V/cm. For the helium atom, the forbidden transition probability 2^1S-3, 4^1D were calculated as functions of the field strength for various values of the exciting dye-laser power as well as the corresponding quadrupole transition probability which is independent of the electric field. Then the intensity ratio of both induced-fluorescences associated with the transitions gives us the electric field strength without any intensity calibration.2. The present diagnostic system consists of mainly a YAG laser-pumped dye-laser for exciting the atoms, a spectroscopic system including optics for spatial measurement, and a data-processing system. Computer programs for control of the devices and for the data processing were developed.3. Measurements of electric field distribution in a cathode dark-space in a hollow cathode discharge plasma were demonstrated. The negative glow potential obtained from the measured field distributions were in good agreement with probe measurements.4. A similar but complicated diagnostic system has been also developed for fusion plasmas, by using a helium neutral beam probe.5. As a future plan, measurement of polarization of the fluorescences will be performed by using the present diagnostics system. This measurement will give us more detailed informations about weak electric fields.