Interchange instabilities in a partially ionized plasma

In a partially ionized plasma, the charged particle population is coupled to the neutrals through charge exchange, ionization and recombination. An interchange instability is identified in which the driving factor is the neutral pressure gradient and the dominant collisional process is charge exchange. Regimes are considered in which the mean free path for neutral collisions with ions λni is small in comparison to the macroscopic length scales. The instability is analogous to a gravitational interchange mode with an effective gravity geff=νinVn where νin is the collision frequency of ions with neutrals. The neutral flow Vn=−(Mniνin)−1dpn/dx results from a balance between the neutral pressure gradient and collisional friction with the ions. An arbitrary kyλni dispersion relation is derived using fluid equations to describe the ions and the Boltzmann equation to describe the neutrals. This dispersion relation contains viscous and inertial effects and is substantially altered in the presence of realistic parallel wavelength due to a coupling between the unstable interchange mode and a stabilizing shear Alfvén mode. Reasonable conditions under which the modes may exist are examined and the possible relevance to divertor plasmas is considered.