  High voltage breakdown in liquid argon is an important concern in the design
of liquid argon time projection chambers, which are often used as neutrino and
dark matter detectors. We have made systematic measurements of breakdown
voltages in liquid argon along insulators surrounding negative rod electrodes
where the breakdown is initiated at the anode. The measurements were performed
in an open cryostat filled with commercial grade liquid argon exposed to air,
and not the ultra-pure argon required for electron drift. While not addressing
all high voltage concerns in liquid argon, these measurements have direct
relevance to the design of high voltage feedthroughs especially for averting
the common problem of flash-over breakdown. The purpose of these tests is to
understand the effects of materials, of breakdown path length, and of surface
topology for this geometry and setup. We have found that the only
material-specific effects are those due to their permittivity. We have found
that the breakdown voltage has no dependence on the length of the exposed
insulator. A model for the breakdown mechanism is presented that can help
inform future designs.
