Enhanced production of negative ions in low-pressure hydrogen and deuterium pulsed discharges: Theoretical calculations

Abstract

We show that the experimental enhancing of negative-ion production in pulsed discharges recently obtained in different laboratories can be rationalized on the basis of a theoretical model we developed.PACS number(s): 52.20.Fs, 52.75.d

Vibrational kinetics, electron dynamics and elementary processes in H2and D2plasmas for negative ion production: modelling aspects

“…In Fig. 5d, the dashed line has been obtained by decreasing by a factor 8 the rate coefficient for the deactivation of vibrationally excited molecules on the metallic walls, the vertical dashed line indicating the onset of the post-discharge regime [36]. It should be noted that the theoretical EEDF, obtained by the solution of the Boltzmann equation, has been experimentally reproduced by using Langmuir probe technique [38], [39].…”

“…The H + 3 ion concentration decays and a condition is reached in which there is charge neutrality between the H + 3 and the H − , since the plasma becomes almost electron-less [60]. This kind of effect is completely explained in terms of self-consistent plasma dynamics which cannot be observed by uniform models; nevertheless, we expect that a partial contribution comes from vibrational kinetics mechanisms as shown in [61]. The behaviour of the collected ions on the electrodes with time (figure 11(b)) shows the post discharge phase in which the ambipolar potential ceases to hold the negative ions inside the source and there is a growth of the negative-ion current towards the walls.…”

“…Mosback et al [34], who also developed a phenomenological model to explain the experimental results, and by Fukumasa and Shiroda [35] (enhancement by a factor 4 and duration of approximately 100 µs). On the other hand the code developed in Bari was used [36] to qualitatively reproduce the complex phenomenology of pulsed discharges. The results are summarized in Figs.…”

Physics aspects of negative ion sources

“…The vibrational distributions were calculated well before they could be measured [18]. The afterglow peak was also analysed by modelling soon after its observation [22]. The effect of caesium seeding was also modelized [21] Numerous phenomena such as electron-molecule collisions, vibration-translation exchange and molecule-surface interactions are important for negative ion sources.…”