Tilting instability of a cylindrical spheromak

The stability of a low‐beta spheromak with a perfectly conducting cylindrical boundary of length L and radius R is analyzed in terms of force‐free fields with J = λB(λ = const). The axisymmetric equilibrium is found unstable to tilting when the elongation L/R is larger than about 1.67. Numerical solutions of the time‐dependent ideal magnetohydrodynamic equations confirm this result.