| This thesis describes various interactions between a superfluid He-4 vortex and its bounding surface.;We report a discrepancy between existing theory and our own experimental observations of vortex energy loss in a vibrating wire cell. In a computational investigation we find that as the vortex moves and reconnects with pinned mesh vortices it dissipates energy at a rate exceeding that expected from mutual friction alone.;We describe measurements in a vibrating wire cell which examine the effects of geometry on vortex stability as measured by depin temperature. The predominant effect is a base end instability, which we control for by comparing stability of only the base end of various graduated diameter cells. The second largest effect is the precise heating history. Slow annealing may decrease the microscopic mesh present on the cell wall after rotation and stabilize the vortex. Choosing the lowest vortex depin temperature for each cell accounts for this training effect Though the conventional model of vortex stability focusing on line energy is generally followed, topography plays a small but distinguishable role in vortex stability.;We find two different stable circulation values near N=1 in cells containing a bump terminus, indicating two stable pin locations. The higher temperature cluster corresponds to a slightly larger circulation value at the bump apex, and the lower temperature, less stable, to a smaller circulation, on the bump edge. We also report on the ability of a vortex to pin on a bump located along the cell wall. There the vortex pins more strongly at the edge of the bump than at its apex. This is counter to predictions from line tension alone and indicates that topography may directly affect vortex stability.;Surprisingly, we find that N=1/2 pins, observed at the diameter transition in the graduated diameter cells, are more stable than N=1 pins in the same cell. We are able to verify some known trends for energy dissipation, period, and cell radius. Interestingly, these effects seem to occur far before the transition region is reached; perhaps the velocity field is altered over a larger length scale than that of the transition region. |
