The chemistry of hydrogen plasmas plays a vital role in applications such as the deposition of ultra hard diamond like films, or the production of silicon layers in thin film solar cells. Understanding the reaction mechanisms of hydrogen plasmas is also essential for the development of negative hydrogen ion sources for nuclear fusion reactors. The plasma studied in this thesis is a magnetized hydrogen plasma expansion. This plasma is bright red for the first 20 cm, after which a sharp transition to a blue afterglow is observed. In the blue afterglow population inversion of the higher excited states of atomic hydrogen is observed. The cause of the sharp transition to the blue afterglow was, until now, not understood. The reaction mechanisms of the plasma have been studied by analyzing the excited state densities of atomic hydrogen. The densities of the excited states have been measured with Optical Emission Spectroscopy (OES) and Tunable Diode Laser Absorption Spectroscopy (TDLAS). TDLAS also measures the gas temperature of the plasma. The processes populating the excited states have been determined from Atomic State Distribution Functions (ASDFs). The ASDFs showed the presence of 3 different plasma regions. In the first 7 cm of the expansion dissociative recombination is important. After 7 cm the reactions shift to atomic ion recombination. In the blue afterglow the dominant population process is molecular ion recombination, which mainly produces the higher excited states. The production of highly excited states leads to the population inversion and the emission of blue light in the afterglow. It was found that the transition from atomic to molecular ion recombination was caused by a Double Layer (DL) at the red to blue transition. The strong local electric field of the DL can lead to a local increase in asymmetric charge exchange, thereby causing the transition from atomic to molecular ion recombination, which causes the blue afterglow.
|Date of Award||31 Aug 2010|
|Supervisor||W.E.N. van Harskamp (Supervisor 1) & Richard A.H. Engeln (Supervisor 2)|