Polarized laser-induced fluorescence technique to measure localized electric field induced perpendicularly to magnetic field in the plasma-edge

Abstract

To develop a direct measurement of the weak electric field induced in a magnetically confined edge plasma, the crossed magnetic- and electric-field effect was investigated on the linear polarization of laser-induced fluorescence. The fluorescence was induced by laser excitation of metastable He atoms 2 1S to 4 1D state due to Stark-induced electric dipole and electric quadrupole moments. In a model-type experiment carried out in a cylindrical hollow cathode plasma column, the radial distribution of the polarization of fluorescence was analyzed to determine the electric field dependence. The measured degree of polarization agreed with estimates from a theoretical model based on the selection rules of the spontaneous radiative transition. The minimum detectable electric field is estimated to be a few tens of V/cm in our scheme. We briefly discuss techniques to increase our measurement sensitivity by the interference between both the forbidden transitions in the laser excitation.