LAUSR

A deep and narrow potential structure has been measured along the last magnetic field line to exit the source tube of an expanding plasma device. The potential structure is identified as an electric triple layer and features a potential dip with Full Width Half Maximum ~70 Debye lengths (16 mm), depth of ~25 V () and its width and magnitude are seen to decrease with increases in argon pressure. The triple layer acts as a potential barrier for field-aligned electron transport, decelerating hot electrons exiting the plasma source before reaccelerating into the expansion chamber those that are able to overcome the triple layer potential, explaining the topography of high density plasma conics published previously. Measurements of the electron energy probability function at locations upstream and downstream of the potential structure show two-temperature electron populations with break energies of 41.5 V and 22 V, respectively, values that are consistent with the plasma potential measurements along the field line. Under these experimental conditions, the dielectric wall of the source tube charges to significantly negative potentials and the proximity of the field line to this negatively charged wall appears to be the cause for the triple layer.