The Lithium Lorentz Force Accelerator (LiLFA) as one of the most promising candidates for planetary exploration and heavy payload orbit raising missions. Although initial experimental data on the LiLFA obtained at the Moscow Aviation Institute is promising , little is understood concerning the basic physics at play in such devices. Therefore, no systematic optimization of design or operating conditions has been achieved. While the extensive database on gas-fed magnetoplasmadynamic thrusters (MPDTs), the precursor to the LiLFA, offers a starting point, fundamental differences in cathode design, propellant type and injection, and current attachment in the LiLFA require new theoretical models to be developed and tested. As an initial step in this direction, The Jet Propulsion Laboratory (JPL) has proposed and begun work on the necessary testing facilities, the lithium feeding system, and the design of a 0.5 MWe thruster model
They currently have two LiLFA experimental models in our laboratory at Princeton. The Open Heat Pipe LiLFA (OHP-LiLFA) designed and built by Thermacore Inc. and EPPDyL and, the workhorse of the present phase of our research, the 30kW MAI-LiLFA, designed and built at the Moscow Aviation Institute.
The 30kW version of the LiLFA is the only one operated these days.
An investigation of the scaling of thrust efficiency with the applied magnetic field in applied- field magnetoplasmadynamic thrusters (AF-MPDTs) is carried out in order to provide guidelines for scaling and controlling AF-MDPT performance. Thruster voltage measurements were made at diff erent current, applied magnetic fi eld and mass flow rate levels in a 30 kW lithium-fed AF-MPDT. The efficiency was then calculated using the voltage data along with a semi-empirical thrust formula derived and verified previously for the same thruster. The non-useful voltage component (the voltage associated with the thruster’s power losses) was found to scale linearly with current and applied magnetic field and inversely with mass flow rate. This behavior was attributed to electrode sheath effects and decreased conductivity with increasing applied magnetic field. The efficiency was found to increase with applied magnetic fi eld for all current and mass flow rate values and the enhancement of the efficiency by the applied magnetic field was found to be greater when the mass flow rate is reduced. The observed minimum in the efficiency vs current curve was related to interplay between the components of the thrust and was shown experimentally and analytically to increase with increasing applied field and decreasing mass flow rate.
Magnetoplasmadynamic thrusters (MPDTs) are a subclass of plasma thrusters with an overwhelmingly electromagnetic acceleration mechanism involving the interaction of a current between an anode and a cathode and a magnetic fi eld which could be applied or induced by the current itself. This interaction gives rise to a Lorentz force density (f = j B) that accelerates propellant downstream and out of the thruster. The thrust generation mechanism of the self-fi eld MPDT is well understood and was characterized by Maecker and Jahn and analyzed by Choueiri. High thrust and thrust density are also the big advantages that MPDTs have over other types of electric propulsion devices, such as the Hall thruster or the ion thruster. MPDTs promise a wide range of thrust levels (100 mN – 100 N) that depends on the power level, along with high speci c impulse (1000-5000 s) a high thrust efficiency, (1025% with argon and up to 60% with lithium propellant), and the ability to process 100’s of kW in a single compact device.
It has been well established that the addition of an applied magnetic eld to the thruster increases its performance signi cantly. This is often necessary at low power levels (below 100 kW) where the current is too low for the self-induced magnetic eld to be sufficient. Thrust, efficiency and speci c impulse tend to increase with the applied magnetic fi eld intensity. It has been observed4 that the thrust increases linearly with the product JB, where J is the total current applied to the thruster and B is the value of the applied magnetic fi eld measured at the solenoid’s center. The detailed physics behind the acceleration mechanism in applied magnetic fi eld MPDT (AF-MPDT) is not yet fully understood and further experimental research is needed.
The focus of ongoing studies on AF-MPDTs is on the most promising variant called the Lithium Lorentz Force Accelerator (LiLFA). The LiLFA is a steady state AF-MPDT that uses lithium as a propellant and a multi-channel hollow cathode. Lithium has great potential for two main reasons: 1) Lithium’s first ionization potential (5:4 eV) is signi cantly lower than that of other, commonly-used propellants such as argon (15:7 eV), xenon (12:1 eV) or hydrogen (13:6 eV), while lithium’s second ionization potential is signifi cantly higher than that of these propellants. Therefore the frozen flow losses are lower in lithium-fed MPDTs.