Electron thermodynamics and ion transport in the magnetic nozzle of electrodeless electric thrusters*

Electric Propulsion (EP), an efficient way of moving a spacecraft, relies upon the ejection of ions to generate a net momentum. The large velocities of ions makes EP much more efficient than chemical propulsion from a propellant consumption viewpoint. Contrary to the well-established gridded ion engine and Hall-effect thruster technologies, electrodeless thrusters have no electrode in direct contact with the plasma discharge. In addition, such thrusters expel a current free quasi-neutral plasma to generate thrust, which prevents the use of an external neutralizer. Electrodeless thrusters, which encompass RF, helicon, electron cyclotron resonance and VASIMR thrusters, therefore offer attractive features in terms of reliability, lifetime and propellant options. One of the main element of an electrodeless thruster is the magnetic nozzle. A magnetic nozzle (MN) acts similarly to the solid nozzle of a chemical rocket: It guides particles and efficiently converts thermal energy into kinetic energy. In most devices the thermal energy reservoir is the electron temperature. In that case the energy transfer between hot electrons and cold ions is quite complicated and mostly originates in the creation of an ambipolar electric field. After a brief introduction to EP fundamentals, electron properties and transport of ions in the magnetic nozzle of various electrodeless devices are discussed in light of recent experimental results. All devices have been operated with xenon and krypton as propellant. This study especially focuses on the impact of the magnetic field strength, the MN geometry (shape, throat location), the propellant flow rate and the power level on the electron thermodynamics, described by means of the isentropic exponent, the ion acceleration, flow velocity and detachment.