Influence of wall material on VUV emission from hydrogen plasma in H-source

The study of VUV emission from a hydrogen plasma produced in a filament discharge in a magnetic multicusp device showed that the use of tantalum and tungsten filaments leads to significant differences in the spectra. The effect of the filament material is interpreted in terms of the fresh film of this material, deposited on the wall. The synthetic spectrum convoluted with our apparatus function for the conditions of this experiment (gas temperature 500 K, electron energy 100 eV) agrees roughly well with the spectrum obtained with tungsten covered walls, but not with the spectrum obtained with tantalum covered walls. We show that in the case of tungsten covered walls the E-V singlet excitation is indeed a two-step Franck-Condon transition, going through either B or C state from an initial H2 molecule with v''=0, added to a Franck-Condon transition to highly excited states cascading to the B or C states. The excitation process to high v'' states in the case of tantalum covered walls is a three step process, in which the first step is the formation by recombinative desorption on the wall of a vibrationally excited molecule with v''=1 or 2, which serves as the initial molecule in the subsequent E-V excitation through the B state. The results indicate a larger recombination coefficient of atoms on the tantalum covered wall