Critical velocity of vortex nucleation in rotating superfluid 3He-A

We have measured the critical Velocity upsilon(c) at which He-3-A in a rotating cylinder becomes unstable against the formation of quantized vortex lines with continuous (similarity-free) core structure. We find that upsilon(c) is distributed between a maximum and a minimum limit, which we ascribe to a dependence on the texture or the orbital angular momentun axis (I) over cap(r) in the cylinder. Slow cooldown through T-c in rotation yields (I) over cap(r) textures for which the measured upsilon(c)'s are in good agreement with the calculated instability of the expected (I) over cap texture.