Polyatomic Ion-Induced Electron Emission (IIEE) in Electrospray Thrusters

• Main Authors: Jared M. Magnusson, Adam L. Collins, Richard E. Wirz
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Aerospace
• Subjects: electrospray; lifetime; overspray; secondary electron emission; facility effects; kinetic emission
• Online Access: https://www.mdpi.com/2226-4310/7/11/153

To better characterize the lifetime and performance of electrospray thrusters, electron emission due to electrode impingement by the propellant cation 1-ethyl-3-methylimidazolium (EMI+) has been evaluated with semi-empirical modeling techniques. Results demonstrate that electron emission due to grid impingement by EMI+ cations becomes significant once EMI+ attains a threshold velocity of ∼<inline-formula><math display="inline"><semantics><mrow><mn>9</mn><mi mathvariant="normal">e</mi><mn>5</mn><mrow><mi mathvariant="normal">c</mi><mi mathvariant="normal">m</mi><mo>/</mo><mi mathvariant="normal">s</mi></mrow></mrow></semantics></math></inline-formula>. The mean secondary electron yield, <inline-formula><math display="inline"><semantics><mover><mi>γ</mi><mo>¯</mo></mover></semantics></math></inline-formula>, exhibits strong linearity with respect to EMI+ velocity for typical electrospray operating regimes, and we present a simple linear fit equation corresponding to thruster potentials greater than 1<inline-formula><math display="inline"><semantics><mi mathvariant="normal">k</mi></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mi mathvariant="normal">V</mi></semantics></math></inline-formula>. The model chosen for our analysis was shown to be the most appropriate for molecular ion bombardments and is a useful tool in estimating IIEE yields in electrospray devices for molecular ion masses less than ∼<inline-formula><math display="inline"><semantics><mrow><mi>s</mi><mi>c</mi><mi>i</mi><mi>e</mi><mi>n</mi><mi>t</mi><mi>i</mi><mi>f</mi><mi>i</mi><mi>c</mi><mo>−</mo><mi>n</mi><mi>o</mi><mi>t</mi><mi>a</mi><mi>t</mi><mi>i</mi><mi>o</mi><mi>n</mi><mo>=</mo><mi>f</mi><mi>a</mi><mi>l</mi><mi>s</mi><mi>e</mi><mn>1000</mn><mi mathvariant="normal">u</mi></mrow></semantics></math></inline-formula> and velocities greater than ∼<inline-formula><math display="inline"><semantics><mrow><mi mathvariant="normal">e</mi><mn>6</mn><mrow><mi mathvariant="normal">c</mi><mi mathvariant="normal">m</mi><mo>/</mo><mi mathvariant="normal">s</mi></mrow></mrow></semantics></math></inline-formula>. Droplet-induced electron emission (DIEE) in electrospray thrusters was considered by treating a droplet as a macro-ion, with low charge-to-mass ratio, impacting a solid surface. This approach appears to oversimplify back-spray phenomena, meaning a more complex analysis is required. While semi-empirical models of IIEE, and the decades of solid state theory they are based upon, represent an invaluable advance in understanding secondary electron emission in electrospray devices, further progress would be gained by investigating the complex surfaces the electrodes acquire over their lifetimes and considering other possible emission processes.