Stability of Superflow inHe3-A

The stability condition is derived for uniform bulk superflow along a uniform texture in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$--$A$. The weak-coupling free-energy parameters in the dipole-locked Ginzburg-Landau regime give stable flow, but a 10% departure from these values can give instability. When present, the instability can reflect the absence of any activation-energy barrier against decay of the superflow through the appearance of vortex textures near the surface. We relate these conclusions to the hydrodynamic instability reported by Hook and Hall.