Continuum emission as a diagnostic for the electron density at Pilot-PSI

Abstract

Optical emission spectroscopy was used to observe the continuum emission at 700 – 900 nm in a hydrogen plasma at Pilot-PSI in order to extract the electron density out of the absolute intensity of the emission. The validity of the continuum emission diagnostic was judged by comparison with Thomson scattering, which acts as a reliable tool for measuring the electron density. We observed the absolute value of the continuum emission about an order of magnitude higher than the Thomson scattering profile. Moreover, the emission profile over the radius of the plasma beam appears to be hollow, which is not to be expected from the Thomson data. The overcalculation of the continuum intensity is most likely caused by an emission band of molecular hydrogen(715 -765 nm), whereas the hollow emission profile is explained by considering the density of molecular hydrogen, which dominates the emission profile above an electron density of ne = 5 ∙ 1019 m−3 and hence causes a hollow profile. In conclusion, the 700 – 900 nm regime in a hydrogen plasma is not suitable for extracting the electron density accurately from the continuum emission.