Universal diagram for regimes ofZ-pinch stability

The regimes for the applicability of various theoretical models for the stability of a Z pinch under pressure balance are shown to be clearly delineated in a diagram of ln(I⁴a) vs lnN, where I, a, and N are the current, pinch radius, and line density, respectively. In particular, the most unstable regime where ideal magnetohydrodynamics applies is shown to be restricted to a small-wedge-shaped region bounded by resistive, viscous, anisotropic, and finite Larmor radius effects. Recent experimental results of anomalous stability can be interpreted in terms of resistive or large-ion Larmor-radius effects.