Modeling the influence of anode-cathode spacing in a pulsed discharge nozzle

The pulsed discharge nozzle (PDN) is a spectrochemical source that is designed to produce and cool molecular ions in an astrophysically relevant environment in the laboratory with limited fragmentation. In order to gain a better understanding of the PDN and to optimize the yield of molecular ions and radicals in the PDN source, a parameter study of the influence of the interelectrode distance on the plasma properties is carried out by means of a discharge model, providing a qualitative as well as a quantitative picture of the plasma. We model the electron density and energy, as well as the argon ion and metastable atom number density for various interelectrode distances. The results reveal that increasing the interelectrode distance does not significantly influence the plasma at the cathode and at the anode. However, a positive column forms between the electrodes, which increases in length as the interelectrode distance increases. This is an additional evidence that the PDN is a glow discharge. This positive column does not contribute significantly to the formation of metastable argon atoms. Because metastable argon is thought to be the primary agent in the formation of molecular ions through Penning ionization of the neutral molecular precursor there is no benefit to be expected from an increase of the interelectrode distance. In fact, electron impact dissociation of the molecules in the column might even make the source less efficient for longer column lengths. The simulations presented here provide physical insight into the characteristics of interstellar species analogs in laboratory experiments