The effect of neutral-gas depletion on the plasma density and momentum

Space and laboratory plasmas can be significantly affected when ionization is so intense that the neutral-gas density is modified. Our study of the coupled dynamics of plasma and neutral-gas has predicted previously-unexpected phenomena. Such are the decrease of plasma density due to neutral-gas depletion; despite the increase in plasma production as deposited power is increased [1], and the transition from neutral-gas depletion to neutral-gas repletion as neutral-gas collisionality with ions is reduced [2]. Two new theoretical predictions related to neutral-gas depletion will be described in the talk. First, it has been recently shown that the 2D plasma transport along and across magnetic field in the presence of neutral-gas depletion may result in a hollow cylindrical profile of the plasma density, in which the plasma density has a minimum at the cylinder axis [3]. We examine further the possibility of two different steady-states of the plasma and neutral-gas in the presence of a magnetic field and neutral-gas depletion, for the same input parameters. A second issue is related to the amount of momentum carried by plasma flowing out of a plasma source, which determines the efficiency of such a source as a thruster. We have recently calculated that momentum when the plasma is collisionless [4]. Taking into account the neutral-gas dynamics, we show that the carried momentum when the plasma is collisional is larger. \\[4pt] [1] A. Fruchtman, G. Makrinich, P. Chabert, and J. M. Rax, Phys. Rev. Lett. \textbf{95}, 115002 (2005). \\[0pt] [2] A. Fruchtman, G. Makrinich, J.-L. Raimbault, L. Liard, J.-M. Rax, and P. Chabert, Phys. Plasmas \textbf{15}, 057102 (2008). \\[0pt] [3] A. Fruchtman, Phys. Plasmas \textbf{17}, 023502 (2010). \\[0pt] [4] A. Fruchtman, IEEE Trans. Plasma Sci. \textbf{36}, 403 (2008).