A hydrogen pellet is injected into a plasma in the Large Helical Device (LHD). Strong radiation from a dense plasma formed around the pellet, the so-called 'pellet cloud', is observed and its spectrum is analysed. Emission lines of neutral hydrogen exhibit Stark broadening profiles and the electron density is evaluated through comparison with theoretical data (e.g. 2.1 × 1023 m−3). The continuum radiation is dominated by two components which correspond to radiative recombination and radiative electron attachment, respectively, the latter of which yields negative ions. From the absolute intensity and its dependence on the wavelength of the continuum radiation, the electron temperature (1.02 eV), the atom density (1.7 × 1025 m−3) and the observed plasma volume (1.6 × 10−5 m3) are determined. These results indicate that the plasma state is close to complete LTE (local thermodynamic equilibrium). The Balmer-α line profile is deformed owing to the reabsorption effect. With the help of one-dimensional radiation transport calculation, the plasma thickness in the direction of the observation (2.4 × 10−3 m) is estimated. The result suggests that the pellet cloud is extended anisotropically in the direction perpendicular to the observation.
Study of the possibility for increasing the emission of soft x-rays from the plasma of a low-energy vacuum discharge triggered by a laser