  The proliferation of liquid argon time projection chamber detectors makes the
characterization of the dielectric properties of liquid argon a critical task.
To improve understanding of these properties, a systematic study of the
breakdown electric field in liquid argon was conducted using a dedicated
cryostat connected to the MicroBooNE cryogenic system at Fermilab. An electrode
sphere-plate geometry was implemented using spheres with diameters of 1.3 mm,
5.0 mm, and 76 mm. The MicroBooNE cryogenic system allowed measurements to be
taken at a variety of electronegative contamination levels ranging from a few
parts-per-million to tens of parts-per- trillion. The cathode-anode distance
was varied from 0.1 mm to 2.5 cm. The results demonstrate a geometric
dependence of the electric field strength at breakdown. This study is the first
time that the dependence of the breakdown field on stressed cathode area has
been shown for liquid argon.
