A METHOD TO MAINTAIN SUPERFLOW AT CONSTANT-PRESSURE DRIVE

In order to study the effects of thermal and quantum fluctuations on t he intrinsic critical velocity for vortex production, it is advantageo us to drive superfluid He-4 through a small aperture at constant press ure head. For 2 pi phase slip events, this gives a constant Josephson frequency and a corresponding constant vortex nucleation rate. In the usual relaxation transient experiments, the pressure head decays in ti me preventing observation at fixed nucleation rate. Therefore, we have developed a constant pressure-head drive system using feedback contro l. The technique, which senses the pressure difference across a flow a perture with a capacitive pressure gauge, can maintain pressure drives as low as 0.5 mPa (f(j) = 35 Hz).