The Electrodeless Lorentz Force (ELF) Thruster Thomas Weber

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

Introduction

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

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

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

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

Introduction Available Options: Power Systems: Solar: Nuclear: 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.

Introduction Launch Capacity: Space Shuttle: Ares V: 24400 kg to LEO

Introduction

Introduction Power system: Thruster Requirements: 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).

The ELF Thruster

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).

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.

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

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.

ELF Thruster ELF Advantages: Overall ELF Goals: 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.

Research Plan

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.

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.

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.

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.

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.

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

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.

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.

Questions