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Measurement of linear polarization in the HeI 21P–31D emission line in an electron cyclotron resonance discharge plasma

The magnitude and direction of linear polarization in the HeI 21P–31D emission line are simultaneously measured using a beam-splitting-type polarization spectroscopy system. The polarization measured in the steady-state phase of an electron cyclotron resonance (ECR) discharge produced in a simple cusp field is directed closely perpendicular to the magnetic field at the ECR surface, and the degree of polarization increases up to 8% when the neutral pressure is reduced to less than 23 mPa. These results suggest that the cause of the polarization is attributable to impact excitation by high-energy electrons produced at the ECR surface.