Electron and Ion Induced Electron Emission from Volumetrically Complex Materials for Plasma Facing Surfaces

Materials with architectured surfaces have been shown to reduce electron and ion-induced electron emission yield (SEE and IIEE respectively) because their micro-cavities can trap emitted electrons. These materials are useful for various plasma applications including electric propulsion devices, propulsion testing facilities, and confining surfaces for nuclear fusion. Electron emission can contribute to the degradation of plasma facing surfaces due to an excess of outgoing negative flux which may reduce the sheath and cause rapid wearing of the walls. In addition, electron emission can contribute to increased instabilities, anomalous cross-field currents, and cooling of bulk plasmas. In fusion energy devices, secondary electrons quickly cool down the bulk plasma, reducing the fusion reaction capability of the device. In both of these applications, understanding the nature of SEE from textured surfaces can lead to designing materials with well-characterized electron emission properties. In this work, the SEE yield of carbon foams with a fixed ligament to pore diameter ratio (aspect ratio, ??_R = 0.2) is evaluated experimentally as a function of foam pore and ligament geometries using scanning electron microscopy. An analytical view factor-based model is also developed (Analytical Model for Particle Sputtering and Electron emission, AMPS-E) to validate electron emission yield trends from foams for both SEE and IIEE. Foam transparency to primary particles to a flat backplate is established and derived as a metric for incorporating foam geometric dimensions and thickness into one parameter. It is found that increased transparency increases the electron emission yield by up to 7%. Foam parameters for electron emission yield reduction of up to 38% are reported compared to flat surfaces for SEE and IIEE yield.