Production of low-electron-temperature electron cyclotron resonance plasma using nitrogen gas in the mirror magnetic field

An electron cyclotron resonance (ECR) plasma with a low electron temperature (< 2eV) and high electron density (∼ 10₁₂ cm^-3) was realized in a mirror magnetic field for Ar/N₂ or N₂ gas. It was found based on the particle and power balance that the decrease in the electron temperature was due to the magnetic-mirror confinement depending on the collision cross section between electrons and neutral particles. The experimental and numerical results suggest that diluting with N₂ gas, whose excitation cross section peaks at a low electron energy of about 2 eV, in the mirror magnetic field enables us to reduce the electron temperature efficiently in an ECR plasma.