Plasma chemistry and kinetics in a magnetized hydrogen plasma expansion : a study of negative ions

Abstract

Studying hydrogen plasmas is of great importance from a fundamental point of interest since hydrogen is the most abundant molecule in the known universe and from a theoretical point of view the simplest to work on. In industry, hydrogen containing plasmas are used for example in surface modification, thin film deposition and creating negative hydrogen ions in neutral beam injectors for future fusion devices. In this work the hydrogen plasma is created by a cascaded arc plasma source which expands in a low pressure surrounding. When an external magnetic field is applied to this expansion a confined plasma column is created with two distinct color regions. At a specific distance from the source of expansion a sharp transition from a red light emitting plasma (dominated by H_ emission) to a blue light emitting plasma (dominated by H_, H and H_ emission) occurs. The main research question was to understand the kinetics in the plasma expansion. Since atomic processes alone cannot explain the distinct emissions observed in the two different regions, molecular processes such as dissociative recombination and processes involving negative hydrogen ions were suspected to be key in the understanding of the underlying mechanisms. Therefore, the relevance of this work was to underpin the importance of these molecular processes in atomic regimes of hydrogen containing plasmas both by simulation and experiment. A Collisional Radiative model (CR-model), which includes molecular processes, was used to simulate the spatially resolved excited state densities. This CR-model assumes quasi steady state and requires as input the temperature of the species (the heavy particle temperature Tgas and the electron temperature Te) and the density of the species (the electron density, the atomic hydrogen density, the molecular hydrogen density, the positive ion densities of H+, H+2 and H+3 and the negative ion density). The output of the CR-model gives excited state densities of atomic hydrogen which are compared with the measured excited state densities as determined using two diagnostics: tunable diode laser absorption spectroscopy for the first excited state n=2 and absolute optical emission spectroscopy for n=3-9. Only when the two molecular mutual neutralization processes of H+2 and H+3 with H- are included in the CR-model, good agreement for all the investigated excited state densities of n=2 up to n=9 was obtained. Since we suspected that the excited states of atomic hydrogen are mainly populated by processes involving negative hydrogen ions a novel photo-detachment technique was developed. This technique uses a laser to photo-detach all present negative hydrogen ions in the detection volume in combination with an optical detection setup to monitor time dependently the change of Balmer line emission. We have shown that the atomic mutual neutralization process of H+ with H-mainly populates the excited state n=3 in the red light emitting plasma and that a branching ratio of the molecular mutual neutralization process of H+2 with H- mainly populates the excited state n=3 up to 9 in the blue light emitting plasma. We have also shown that there are processes involving negative ions that do not lead to the population of excited states, namely the molecular mutual neutralization process of H+3 and H- and a branching ratio of the molecular mutual neutralization process of H+2 and H-.The main conclusion of all the presented work is that we now have a much better understanding of the kinetics of weakly magnetized expanding hydrogen plasmas. The two distinct color regions that are resulting from specific Balmer line emission can be explained by populating molecular processes in the plasma, i.e. dissociative recombination and processes involving negative hydrogen ions.