Secondary Electron Emission from Complex Surfaces

Materials with complex surfaces have been shown to reduce secondary electron emission (SEE) yield because their micro-cavities can trap emitted electrons. These materials are useful for various plasma applications including fusion energy, electric propulsion, and manufacturing. Secondary electrons significantly affect the life of plasma facing surfaces and plasma performance due to their ability to modify the local sheath and cool the bulk plasma. In addition, SEE can contribute to plasma instabilities and anomalous cross-field currents. 

The objective of this work is to investigate and characterize SEE from complex material surfaces using an open cell and volumetrically reticulated structures, i.e., foams. Such materials have recently demonstrated a sustained reduced sputter yield of up to 80 % over a 15-hour exposure to 300 eV plasma ions and are of interest for plasma-facing component design. In this work, similar foams made of carbon are evaluated for their SEE properties using a technique for SEM evaluation as described by Ottaviano, Banerjee, and Raitses. These foams are characterized as a function of varying pore and ligament dimensions and angle of incidence of the primary electrons. In addition, it is shown how the presence of a partially exposed backplate placed behind the foam may significantly affect SEE trends. 

Results thus far show that SEE yield is reduced by up to 35% of its planar value for foams with 40 micron ligament diameters and 200 micron pore diameters, and a volume fill density of 3%. A modified angular dependence is found as a function of backplate exposure to the primary electron beam. The results are compared with other complex carbon surfaces such as carbon velvets, as well as with recent computational and analytic models. 

Ottaviano, A., Banerjee, S., and Raitses, Y., Journal of Applied Physics, Vol. 126, No. 22, 2019, p. 223301.