Lithium Lorentz Force Accelerator (LiLFA)

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Motivation

The promise of achieving high thrust density coupled with high specific impulse is the motivation behind research into the lithium Lorentz force accelerator (LiLFA). The combination of these thruster characteristics enables the cost saving benefits of electric propulsion to be applied to missions requiring short transit times, such as manned missions to Mars and beyond. The LiLFA provides a higher thrust density than any currently flying electric propulsion device and is realistically only limited in thrust by the amount of available power.

In one sentence: We want to send people to Mars, but we don't want to spend a lot of money.

Figure 1 - The LiLFA firing on May 7, 2015.

Thrust Generating Mechanisms

The LiLFA is a type of Applied-Field Magnetoplasmadynamic Thruster (AF-MPDT) using lithium as a propellant, as lithium has several advantages over more conventional propellants, such as xenon, krypton, or argon. Lithium has a lower first ionization potential (5.4 eV) than commonly used propellants (12.1 eV for xenon), which means more power can go to thrust generation instead of ionization. Additionally, it has a high second ionization potential compared to other propellants (75.6 eV compared to 21.0 for Xenon), reducing frozen flow losses. Lithium also has the benefit of reducing the work function of tungsten (the material from which the cathode is made) from 4.5 to 2.1 eV when barium is used as an additive, decreasing erosion and increasing the lifetime of our thruster.

There are three primary mechanisms through which thrust is generated, each of which is outlined below.

  1. Self-Field Component
  2. In the simplest configuration, MPDT's operate in what is known as the self-field mode, which is illustrated in Fig. 2. In this configuration, the thrust is generated by the cross product of the current travelling radially from the anode to the central cathode, and the magnetic field generated by that current travelling through the cathode, resulting in an axial force.

    While Self-Field MPDT's are simplest in design, they require MW levels of power, which are beyond the abilities of any present day space-based power supplies. Application of an external magnetic field can reduce the power requirements, allowing operation in the 10s to 100s of kW. In the applied-field configuration, an additional thrust generating mechanism exists and is outlined in section 2.

    The self-field component of thrust has been shown to be \(T_{SF} = bJ^2\), where \(J\) is the current through the electrodes and \(b\) is a geometric scaling factor.

    Figure 2 - Diagram of a SF-MPDT with an annular anode and central cathode. The current is shown in red, the magnetic field is blue, and the cross product of the two is purple.
  3. Applied-Field Component
  4. The cross product of the current between the electrodes with an axial diverging magnetic field results in a force that acts to swirl the plasma. The mechanism by which this swirling motion generates thrust is an area of active research here at the EPPDyL. The general idea is that the swirling motion of the charged particles through the expanding magnetic field lines results in the conversion of kinetic energy perpendicular to the field lines into parallel kinetic energy because of adiabatic invariants.

    The applied-field component of thrust has been experimentally shown to be \(T_{AF} \propto kJB_A\), where \(B_A\) is the applied magnetic field strength and \(k\) is a constant.

    Figure 3 - Diagram of an AF-MPDT with an annular anode and central cathode. The applied magnetic field is generated by the solenoid wrapped around the thruster. The current is shown in red, the applied magnetic field is blue, and the cross product of the two is purple.
  5. Cold Gas Component
  6. Because we are expanding a gas through a nozzle, there is also thrust generated that depends on our mass flow rate (\(\dot{m}\)), the gasdynamic pressure (\(P\)), and the area on which that pressure is pushing (\(A\)). The cold gas component of the thrust is then given by \(T_{CG} = c_s \dot{m} + PA\), where \(c_s\) is the speed of sound of the propellant.


Current Research

Research on the LiLFA is presently being conducted on the following topics:

  • Development a new feed system using a \(J\times B\) pump.
  • Development of a new method for monitoring lithium flow rate.
  • A new probe, the Dynamic Resistance Probe (DRP), has been created and successfully tested for the first time (May 2015) to measure the deposition rate of lithium. Further testing will show if this probe is sensitive enough to measure deposition rates upstream of the thruster where attachment to spacecraft is a concern.

  • Thrust measurements.
  • A new method of measuring thrust, isolating the component of force on the solenoid, has been implemented into the existing thrust stand. This will help reveal the mechanism through which the applied-field generates thrust by providing a targeted measurement of the constant \(k\) described in the above section.

  • Onset.
  • When operating at high currents for a given propellant mass flow rate, MPDT's encounter a major performance-limiting phenomenon known as "onset." This critical operating regime is characterized by large voltage oscillations, spotty anode attachment, and increased erosion of the anode material. While this phenomena has been researched extensively in the pulsed, self-field configuration, the physical mechanism responsible for this transition has yet to be confirmed. Furthermore, little research has been been carried out for applied-field thrusters, particularly in steady-state operation. We aim to identify and investigate this physical mechanism, and how the applied field affects this physical model.


Facilities

The EPPDyL is presently (2016) the only facility with an operating LiLFA. We currently have two LiLFA experimental models in our laboratory. 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.

We fire the thruster in a lithium-resistant stainless steel vacuum chamber pictured in Figure 4. The chamber is actively cooled, which causes the propellant to condense on the surfaces of the tank. This helps maintain a low background pressure. We typically operate in the \(10^{-5}\) Torr regime. We achieve this pressure using the combination of a roughing pump, a roots blower, and a diffusion pump.

Figure 4 - Vacuum apparatus housing the LiLFA at the EPPDyL.

Lithium handling is performed using our glovebox, pictured in Figure 5, which is filled with argon to prevent any contamination of the lithium by reaction with the air.

Figure 5 - Glovebox for handling lithium.

We also possess the appropriate fire- and chemical-resistant clothing, as well as a supplied-air breathing system, for cleaning the contaminated chamber after firing, some of which is shown in use in Figure 6.

Figure 6 - Cleaning the vacuum chamber after firing the LiLFA.


LiLFA Publications


Contact

Currently at Princeton: Former students:
  • Mike Hepler
  • Dan Lev
  • Andrea Kodys
  • Leonard Cassady
  • Kamesh Sankaran
Former Visiting Researchers:
  • Gregory Emsellem (Laboratoire de Physique des Milieux Ionisés)