Line Spectra Observation of the Rydberg Helium Atoms due to Volumetric Recombination in the RF Plasma Source DT-ALPHA

Keeping compatibility between steady state gas puffing and stable radio frequency (RF) discharge, helium recombining plasma production was achieved in an RF plasma device. In this experiment, axial position of orifice, which suppresses backflow of secondary gas, was modified to increase electron density at a test region. Changing neutral pressure at the test region from 11 Pa to 21 Pa, the electron temperature, the electron density and the wavelength spectrum were measured. The electron temperature decreased with increasing neutral pressure and finally becomes about 3 eV. The electron density shows similar pressure dependence as the electron temperature. When the neutral pressure increases to 15 Pa, the line spectra from highly excited helium atoms were clearly observed. The electron temperature estimated from these line spectral intensities is about 0.05 eV, which indicates that the electron density reduction is caused by volumetric recombination occurring at the periphery of the plasma column.