The Effects of Space-Charge Limited Electron Emission on the Plasma Sheath

Theory and experiments are presented that explore the space-charge limited regime of plasma sheaths surrounding electron emitting surfaces. The separation point technique and the inflection point probe methods for measuring the plasma potential were compared to the floating potential of a highly emissive probe in a Hall thruster plasma. The inflection point techniques' measurement of the plasma potential was ~2Te/e above the floating potential measurement, supporting fluid theory predictions. The separation point technique was inconsistent with every other technique and did not accurately measure the plasma potential. A virtual cathode was observed in a multidipole chamber plasma sheath near a grounded, metal boundary coated with a dielectric. A combination of secondary electron emission and a relatively high density of primary electrons formed the space-charge limited sheath indicated by the virtual cathode. Low neutral pressure, primary electron energy, and discharge current allowed the virtual cathode to form. The discharge current greatly affected the sheath potential, reducing it as the current increased. A kinetic theory of space-charge limited electron emitting sheaths is presented which accurately treats the loss of plasma electrons to the boundary and the velocity distribution of emitted electrons. By considering those electrons lost to the wall, the predicted sheath potential was reduced by 10%. Using a kinetic description of the emitted electrons, assuming a half-Maxwellian distribution, greatly affects the sheath potential. It is shown that the kinetic theory predicts that the sheath potential goes to zero as the plasma potential. To test this kinetic theory of emissive sheaths, time-resolved measurements of the emissive sheath potential were made in the afterglow of a capacitively coupled plasma. The results showed that as the plasma electron temperature cooled and approached the emitted electron temperature, the normalized sheath potential was drastically reduced and went to zero when the two temperatures were equal, qualitative supporting the emissive sheath kinetic theory. The emissive sheath potential was unexpectedly large when the plasma electron temperature was larger than the emitted electron temperature by a factor of 4.