Theoretical analyses for influences of initial operating parameters on charged-particle transport characteristics in a low-pressure double-wall bounded decaying plasma

The influencing mechanisms of the initial operating parameters including the initial plasma density, electron temperature, plasma width, and externally applied voltage on ion extraction characteristics are studied based on a simplified one-dimensional analytical model for a double-wall bounded decaying plasma system. Firstly, a criterion of the dimensionless plasma width (L_\rm min) which is normalized with the Debye length for evaluating the state of the sheath expansion is proposed for the first time. Secondly, if the plasma width is larger than the value of L_\rm min, a complete sheath expansion process including both the supersonic and subsonic sheath expansion phases occurs during plasma decaying. Consequently, on the one hand, the total ion extraction time and the fraction of the extracted ions at the end of stage II, i.e., the sheath expansion and ion rarefaction wave propagation stage during the whole ion extraction process, can be determined by the initial operating parameters; and on the other hand, it is estimated that about 70% of ions can be extracted at the end of the second stage from the bulk plasma with a consumption of about 62.5% of the total ion extraction time within the parameter ranges studied in this paper. This research is helpful for a deep understanding of the physical mechanisms of the charged-particle transport, as well as for guiding parameter optimizations with better ion extraction performances in practice.