Development of laser-induced polarization fluorescence spectroscopy for measuring electric fields in magnetic confinement fusion plasma edges

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We have developed a high-sensitive method for measuring the electric field in plasmas, which relies on polarization of laser-induced fluorescence (LIF) by the excitation of He atoms through forbidden transitions (e.g. 2^1S→4^1D). Purposes of this work are to elucidate the effect of collisional disalignment due to electrons in plasmas on the LIF polarization method and to examine whether the method utilizing the interference between Stark and electric quadrupole (QDP) amplitudes can be available or not under such high magnetic field as in magnetic confined fusion plasmas.Time decay of LIF linear polarization was measured as a function of electron d ensity ne in a Penning discharge plasma. The disalignment rates obtained were proportional to ne. From the linear dependence the rate coefficient for disalignment due to electronic collisions w as deduced to be (4.9±0.5)x10^<-11> m^3/s. The upper limit of elect ron densities for application of the polarization method was discussed.Excitation spectra of circularly polarized LIF were observed by scanning the laser wavelength covering the Zeeman splitting under high magnetic fields (500 G< B< 1500 G) perpendicular to the electric field in the sheath of a magnetron plasmas. It was found that the anisotropic population (orientation) was formed among magnetic sublevels with opposite sign in 4^1D. This shows that the Stark-QDP interference takes place even under the cond ition of the selective excitation to each sublevel by a laser with a narrow spectral width as compared to the Zeeman splitting.