The so-called ASDEX pressure gauge is a hot cathode ionization gauge specially designed for operation in strong magnetic fields and under high noise conditions. It has been employed for investigations of total particle fluxes in or nearby high recycling zones in several tokamaks in the pressure range 10(-5) to 10(-1) mbar, by analyzing the ion current. However, partial pressures are also of great importance, e.g. the density of helium in pumping ports, because the helium ash removal is a crucial issue for a burning plasma. The partial pressure of hydrogen atoms in molecular hydrogen is a parameter which can be easily compared with the results of neutral gas simulations, and it is therefore an important parameter for the validation of such calculations. A spectroscopical approach using the photons produced in the gauge has been implemented to investigate such issues and to distinguish different atomic species. The degree of dissociation of hydrogen can be determined analysing the Ly(alpha) intensity using an oxygen cell as high transparent narrow band filter for Ly(alpha) light. Due to the lack of such a convenient filter for the resonant He lines in the VUV the He content in the recycling gas has to be measured with visible HeI lines.
