The intrinsic critical velocity near T-lambda

We present preliminary measurements of the intrinsic critical velocity for vortex production due to flow through a submicron aperture at temperatures from 1.65K to T-lambda - T = 3 x 10(-3)K and fluid drive pressures (Josephson frequencies) from 0.5mPa (f(j) = 35Hz) to 250mPa (f(j) = 17kHz). The critical velocity is probed using a constant pressure drive technique which maintains a constant phase slip nucleation rate for several minutes. The measured critical velocity qualitatively agrees with a model of thermally activated vortex half rings.