The Electrodeless Lorentz Force (ELF) Thruster

1. The Electrodeless Lorentz Force (ELF) Thruster Thomas Weber

2. Contents • Introduction • Motivation • Power Systems • Thruster Goals • ELF Thruster • Background • Operation • Features • Research Plan • Tasks completed to date. • Tasks yet to be completed.

3. Introduction

4. Introduction • Spaceflight goals: • Short trip time: • Large ΔV. • Short timeframes. • Large payload. • High payload/propellant ratio. • Lightweight power and propulsion system. • Small power and propulsion system.

5. Introduction • Requirements: • Propulsion system: • High power operation. • High specific impulse. • Lightweight. • Small size. • Power System: • High power output. • High energy density. • Lightweight. • Small size.

6. Introduction • Rocket Equation: • Thrust: • Power: • Rule of thumb: Exhaust velocity should be roughly the mission ΔV.

7. Introduction • Available Options: • Propulsion Systems: • Chemical: • High power. • Low specific impulse. • Low energy density. • Nuclear Thermal: • High power. • High energy density. • Low specific impulse. • Electric: • High specific impulse. • Low power.  Seek to improve this.

8. Introduction • Available Options: • Power Systems: • Solar: • Linear scaling. • Works well out to Mars (44% solar intensity), but not so well at Jupiter (3.7% solar intensity). • Technology rapidly improving. • Nuclear: • Non-linear scaling. • Does not scale well to low mass systems. • Controversial to launch. • Technology relatively mature. • Only option for deep space missions. • Works in dark and high radiation environments.

9. Introduction • Launch Capacity: • Space Shuttle: • 24400 kg to LEO • Ares V: • 130000 kg to LEO

10. Introduction

11. Introduction • Power system: • Near-term missions will have power levels in the tens to hundreds of kilowatts. • It is conceivable that a spacecraft will be able to produce tens of megawatts of power in the near future. • Thruster Requirements: • Goals for the near-term: • Power levels of tens to hundreds of kilowatts. • Exhaust velocity: 15 – 40 km/s. • Efficiency greater than 50%. • Ability to use any type of fuel. • Lightweight (< 100 kg). • Small size. • Long-term goals: • Power levels up to tens of megawatts. • Efficient operation at higher exhaust velocities (~ 80 km/s).

12. The ELF Thruster

13. ELF Thruster • Background: • The ELF thruster stemmed from research being done on the High Power Helicon (HPH) Thruster. • HPH used Rotating Magnetic Fields (RMF) to excite a magnetohydrodynamic wave along an axial magnetic field to accelerate a propellant. • It was found that the thruster worked better at higher powers, at which the rotating field ceased to be a perturbation and became comparable to the axial field. • ELF is intended to further increase the rotating field, resembling the RMF current drive scheme investigated in fusion research. • At this point enough current is driven to achieve a Field Reversed Configuration (FRC) plasmoid (rather than a diamagnetic stream of plasma).

14. ELF Thruster • Field Reversed Configuration: • Plasmoid: A coherent structure of plasma and magnetic field. • A azimuthal current produces a confining magnetic field. • Forms closed field lines. • Gas pressure is balanced with magnetic pressure, held by a flux conserving boundary.

15. ELF Thruster • Rotating Magnetic Field (RMF): • RMF drives electrons azimuthally, giving rise to a net current.

16. ELF Thruster • ELF Operation: • Neutral gas is injected through an Magnetoplasmadynamic Arc (MPD), filling the cone. • The MPD is fired, introducing a small amount of plasma into the cloud of neutrals. • The RMF is fired and the oscillating current gets larger with time. • As the current increases, neutral particles are ionized and current is driven. • When enough current is driven, the axial field reverses, and an FRC is formed. • As the FRC grows, the axial field is compressed against the flux conserving boundary and pushes against the FRC, eventually ejecting it, possibly through additional magnetic nozzle.

17. ELF Thruster • ELF Advantages: • Pulsed: • Operation is the same at any power level. • Very high and very low power levels are easily attainable. • No inefficient start-up. • Plasmoid (vs. plasma stream): • No magnetic detachment issues. • Electrodeless: Any type of propellant may be used. • The plasma is isolated from the device. • Low Voltage: • Lower mass components and insulation. • Less transformer mass. • Overall ELF Goals: • Develop a new type of thruster with all the above advantages. • Study FRC formation with RMF. • Study FRC translation and expansion as it moves out of the RMF region.

18. Research Plan

19. Research Plan – Completed Work • Construct ELF vacuum system: • Assemble carriage, chamber, G-10 walls, pumps, gauges, support structure. • Design and construct drift magnets, flux conservers, diagnostics. • Data acquisition and triggering system: • Set up CAMAC/Labview system for taking data. • Design and construct appropriate filters and attenuators. • Set up system for appropriate RMF triggering. • Write MATLAB calibration files.

20. Research Plan – Completed Work • Construct ELF thruster body: • Design and construct magnets, flux conservers, and diagnostics. • Wrap RMF antennae and eliminate coupling. • Construct MPD/vacuum boundary for small end. • Construct power system: • Design and construct capacitor bank/switching assemblies for slow magnets. • Re-use MPD power source from past PI testing. • Refurbish RMF wheels from TCS.

21. Research Plan – Completed Work • Construct pendulum to measure thrust: • Design and construct pendulum body. • Calibrate with known mass. • Write calibration routine. • Integrate into DAQ. • Complete background work on HPH project: • Test HPH thrust on NASA Glenn thrust stand. • Confirm that the pendulum is accurate. • Re-create HPH operating conditions in ELF cone with identical pendulum and measure thrust.

22. Research Plan – Completed Work • Improve on the HPH thrust and find a good operating condition: • Determine which style of MPD provides best performance. • Use FIG to determine neutral gas flow characteristics. • Use thrust data to determine whether it is better to use a short or long burst of RMF power. • Find optimal operating conditions in Nitrogen, “Seattle” air, and Oxygen.

23. Research Plan – Completed Work • Iterate ELF Design: • Change to a different gas injection/PI system. • Design and construct straight section with magnets, flux conservers, and diagnostics. • Construct support structure. • Add additional RMF antennae and ringing capacitors. • Redesign RMF drivers to reach higher powers. • Develop additional diagnostics to measure energy consumption and exhaust characteristics.

24. Research Plan – Completed Work • ELF Results: • Demonstrated FRC survival outside RMF region. • Demonstrated FRC acceleration throughout magnetic nozzle region. • Demonstrated FRC creation, acceleration and ejection in Nitrogen, Air, and Oxygen. • Measured FRC velocity of 15 – 40 km/s. • FRC density range of 1018 – 1020 [m-3]. • Directly measured thrust of >1mN-s per FRC

25. Research Plan – Not Completed • Study “single shot” mode operating conditions: • Determine new neutral flow characteristics. • Study partial pre-fill of tube for more massive FRC ejection. • Find optimal operating conditions in Nitrogen, Argon, Deuterium, Xenon, etc. • Study “burst mode” operating characteristics: • Rebuild triggering system to operate in burst mode. • Using single shot data as a guide, study burst mode. • Find peak impulse/energy conditions. • Determine performance over a range of exhaust velocities.

26. Research Plan – Not Completed • Develop dynamic acceleration system: • Develop driver boards. • Construct support structure. • Construct cone and diagnostics. • Study dynamic acceleration: • Install cone and carriage equipped with dynamic acceleration hardware. • Install periphreal systems: charging, triggering, etc. • Install large diameter bias coils. • Design and construct power supply for bias coils. • Study single shot mode. • Study burst mode. • Operate in the same gases as used in passive ejection scheme. • Find peak impulse/energy. • Determine performance over a rang of exhaust velocities.

27. Questions