Transport of a cathodic arc plasma in a straight, magnetized duct

Measurements are presented of the transport of a supersonic, cathodic-arc plasma through a straight, magnetized duct. These measurements are compared to previous work on curved ducts, in order to illuminate the effect of duct curvature on the transport. The axial ion flux through the straight duct decays as ions are lost to the walls. This decay is exponential, with a scale length of seven duct radii; this is two to three times longer than in most experiments on curved ducts. The scale length is independent of the magnetic field strength for fields from 5-40 mT. (For this range of magnetic fields, the electron Larmor radius varies from 0.03-0.003 duct radii; while the ion Larmor radius varies from 4-0.5 duct radii.) This differs from previous experiments with curved ducts, where the attenuation length generally increases with magnetic field. Also in contrast to experiments on curved ducts, biasing the duct wall to positive voltages similar to the ion energy produces only a slight decrease in the ion losses to the wall. The observed scale length for ion loss and its independence from the magnetic field strength are in quantitative agreement with a plasma fluid simulation. Differences in plasma transport through straight and curved ducts are discussed.