|M.Sc Student||Igal Kronhaus|
|Subject||Field Emission Cathode for Use in Electric Propulsion|
|Department||Department of Aerospace Engineering||Supervisors||Professor Emeritus Guelman Mauricio|
|Dr. Alexander Kapulkin|
|Full Thesis text|
Electric propulsion systems are being adopted for a growing number of spacecraft designs, enabling substantial reduction in required propellant mass, allowing for increased payload mass and useful life span. Contemporary electric thrusters utilize hollow cathodes as electron emitters. These thermionic cathodes have long activation time and limited number of ignitions. An alternative mechanism for electron emission is field emission, where a strong electric field near a conductive surface, enables electrons to tunnel through the bulk material into the vacuum. Such cathodes offer instantaneous ignition and power and propellant savings. In this thesis a comprehensive survey of available field emission cathode technology is conducted, indicating the advantage gained in using Carbon nanotubes as the field emitters. The excellent field emission properties of nanotube are attributed to their sharp geometry. Numeric investigation is carried out to obtain the relation between nanotube array geometry and the current emission. The results from the finite-element electrostatic model allow rough estimate of array layout and size.
When integrating the field emission cathode with Hall type electric thrusters the high operating voltage of field emission cathodes can seriously degrade thruster performance. To overcome this limitation in the present work a new approach to the design of field emission cathode for use in Hall thrusters is developed. An acceleration-deceleration electrode system is considered, where a high potential electrode provides the electric field necessary for carbon nanotubes to emit; followed by a second electrode which decelerates the electrons. To mitigate power loss due to returning currents a multi aperture cathode is considered together with ionization of propellant. The small Debye length of the plasma allows the independent investigation of a single sub-cathode. Current density - voltage characteristics of the sub-cathode are obtained using analytic electron fluid model. The one dimensional model is then augmented by more accurate numerical simulations in 2D axial symmetric geometry. These are performed using a Particle-in-Cell computer program (OOPIC), modeling both field emission and the ionization of the Xenon gas.
The results obtained from the numeric simulations show that a 1 Ampere cathode, composed of 1000 apertures, cover an area of ~20 cm2 (total cathode mass is less than 10 gram). The cathode requires Xenon mass flow rate of 0.11 mgram/sec and 15 W of power to operate, comparable to an advanced hollow cathode, but enables virtually instantaneous ignition and easy scaling for low power thrusters.