Ultra-high Specific Impulse Lithium-fueled Ion Thruster for Interstellar Precursor Mission Concepts

Design, fabricate, and test a lithium-fueled, gridded ion thruster with a specific impulse of 60,000 seconds. This specific impulse represents an 10x improvement over the state of the art and has the potential to enable deep space missions with velocities that are ten times greater than currently possible. Such missions could include a mission to 500 astronomical units (AU) from Earth with a trip time of roughly 12 years, 3.6-year rendezous missions to Pluto, or 6-month flight times to Jupiter rendezvous. The thruster is being designed to operate at up to 100 kW at 60,000 s and will be tested in JPL's unique condensible-metals vacuum test facility.

The development of a lithium-fueled, gridded ion thruster that provides a specific impulse of 58,000 s takes advantage of the unique properties of lithium. Lithium’s light atomic mass enables a very high specific impulse at a reasonable accelerating voltage, ~12 kV. In contrast, to obtain a specific impulse of 58,000 s with xenon would require a net accelerating voltage of 230 kV which would require an enormous mass of power conversion hardware and thermal radiator mass. The lithium-based system eliminates these masses along with their associated inefficiencies and waste heat. In addition, lithium can be stored as a solid at room temperature. This is critical to minimize the tank mass required to contain the propellant. Importantly, while lithium can be stored as a solid, it is easily melted, vaporized, and ionized. Lithium’s ionization properties are critical to our overall system architecture. Lithium has a first ionization potential of 5.4 eV, less than half that of xenon, making it easy to ionize. However, it has a double ionization potential of 76 eV making it virtually impossible to doubly ionize in our application.  This is expected to enable the thruster to be operated with nearly 100% ionization of the propellant effectively eliminating the loss of un-ionized lithium atoms from the thruster and the corresponding production of charge-exchange ions outside of the thruster. This has two important implications in our application. First, charge-exchange ions are responsible for the primary wear-out failure mode for gridded ion thrusters. The virtual elimination of these ions should enable the development of lithium-fueled ion thrusters that have extremely long lifetimes, much longer than state-of-the art ion thrusters. Second, the charge-exchange plasma adversely interacts with high-voltage solar arrays in solar electric propulsion systems. The effective elimination of the charge-exchange plasma in our lithium-ion thrusters will greatly mitigate this interaction. The 100-kW Li-fueled thruster will have a diameter of just 250 mm, produce a beam current of 8 A at 12,000 V. The discharge current is expected to be approximately 80 A. Successful development and operation of this thruster is expected to pave the way for the development of a much higher power version with power levels in the multi-megawatt range needed for rapid transportation throughout the solar system.