Abstract
The straight EXTRAP experiment [Phys. Scr. 16, 147 (1977); Nucl. Instrum. Methods 207, 223 (1983)] is modeled as a magnetohydrostatic equilibrium with cylindrical symmetry and a plane magnetic field. The plasma is surrounded by a vacuum that extends to infinity, and in which four axial rod currents are placed symmetrically. The plasma current density is either parallel or antiparallel to the rod currents, thus having one sign throughout. In the stability analysis, the configuration is assumed to have a finite axial length (the boundary conditions appropriate to a rigid perfect conductor are imposed at the two ends), and the rods are assumed to be permeable to the magnetic field. According to ideal magnetohydrodynamics, such an equilibrium is globally unstable to flute modes and/or slip modes unless the rod currents vanish (circular Z pinch). This is due to the increase away from the center of the octupole field of the rods.
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