Ion-induced electron emission from surfaces bombarded by an emim-bf₄ electrospray plume

Experiments have been conducted at the University of Illinois Urbana Champaign Electric Propulsion Laboratory to study the ion-induced electron emission behavior of surfaces being bombarded by cations and anions in an [EMIM][BF4] electrospray plume. The surfaces tested include 6061 aluminum, carbon graphite, 316 stainless steel, and grade 5 titanium. Operation in cation mode resulted in yields from 0.5-2.5 emitted electrons per incident ion for emitter voltages of 2.2-3.0 kV. Additionally, the energy of electrons emitted from surfaces bombarded by cations increases linearly with emitter voltage and ranges from 80-165 eV depending on the surface material. A hysteresis effect is identified in the suppression of electron emission during cation bombardment and strongly depends on the target material and emitter voltage. Further, results indicate that 85-95% of the current from electron emission in cation mode travels upstream and is collected by the emitter. Operation in anion mode resulted in much lower yields of < 0.2 emitted electrons per incident ion over the range of 2.2-3.0 kV emitter voltages. However, evidence is presented showing that anion yields can increase to > 1 when surfaces are negatively biased.