Diffusion-driven currents in a Z-pinch

A model of a Z-pinch and related systems such as Extrap is considered where there is a hot fully ionized plasma core which is impermeable to neutral gas and being surrounded by a partially ionized cool boundary layer. The pinch is driven by an imposed toroidal (axial) electric field. The model should apply both to linear geometry at sufficiently long axial lengths and to toroidal geometry at large aspect ratios. The recirculation of matter in the boundary layer generates diffusion-driven 'bootstrap-like' currents, both in the toroidal and the poloidal (transverse) directions. The largest toroidal bootstrap currents arise at relatively low plasma densities, high temperatures of the core edge and weak toroidal magnetic fields. There is a low-density limit at which the core becomes 'permeable' to neutral gas and where the bootstrap currents and plasma-neutral gas interaction are distributed all over the plasma body. The toroidal bootstrap current reinforces the externally driven pinch current, and the poloidal bootstrap current produces a diamagnetic effect. The diffusion-driven current system is expected to become disturbed when the pinch radius approaches its upper limit, as defined by a magnetic separatrix or a metal limiter