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Electrodynamic Tether System Analysis Comparing Various Mission Scenarios

Electrodynamic Tether System Analysis Comparing Various Mission Scenarios. Keith R Fuhrhop and Brian E. Gilchrist University of Michigan. Introduction. Electron Emission Theory & Space Charge Limits Thermionic Cathodes Field Emitters Hollow Cathodes ED Tethers System Integration

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Electrodynamic Tether System Analysis Comparing Various Mission Scenarios

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  1. Electrodynamic Tether System Analysis Comparing Various Mission Scenarios Keith R Fuhrhop and Brian E. Gilchrist University of Michigan

  2. Introduction • Electron Emission Theory & Space Charge Limits • Thermionic Cathodes • Field Emitters • Hollow Cathodes • ED Tethers System Integration • System Simulations

  3. Thermionic Emission • 2 Step Process – Thermionic Emitter then Electron Gun • TC production • Overcome Fermi Energy • Boils off Low E Electron • EG emission • To Overcome SCL • EG needs more E • Thermionic Emitter • Electron Gun • Richardson Eq.

  4. Field Emission • Quantum Mechanical Tunneling Effect • 108 - 109 V/m E-field • 107 tips / cm2 • No Heaters or Gas Required • Many Emitter Types • Spindt type, carbon nanotubes, BN nanostructure (UM) • Fowler-Nordheim Eq.

  5. Hollow Cathodes • Setup • TC Emission • Xe Ionized Gas • Stats • Fuel Flow Rate (4–14 sccm) • Potentials (10-40 V) • Diameter of Keeper (1-12 cm) • Double Sheath Possible • 2 Potential Drops • Across the Xe gas then Sheath

  6. Space Charge Limit 1-d C-L Law (vacuum gap) • Only so many Electrons can emitted at a time • Plasma Parameters Determine • Sheath • Emission Area • Emission e- Energy 3-d C-L Law (vacuum gap)

  7. De-boost Tether Example • Tether electromotive force (EMF) drives current through tether • Vemf = (vxBNorth)• l • Geomagnetic field, BNorth (0.18 - 0.32 Gauss) • Orbital velocity, v (~7500 m/s @ 300 km alt) • Vemf (35 – 250 V/km) along tether of length l • Electrons collected from ionosphere along positively biased upper bare tether and returned to ionosphere at lower end • Current I produces magnetic force (drag thrust) dF on each tether section of lengthdl:dF = dlIxBNorth. • Current magnitude varies along tether • Current magnitude determined by • Available EMF and tether resistance • Bare-tether electron collection efficiency • Electron ejection efficiency at lower end

  8. Grounded Cathode (HC’s) S/C Surface is negative by HC bias Grounded Gate (TC & FEA) S/C at floating potential Vemf powers emitter If can’t emit current then emmiter cathode or spacecraft pulled very negative Series - Bias Grounded Gate (TC & FEA) S/C at floating potential Emitter not Powered by Vemf Easily Control Emitter Potential Requires non-tether power source If can’t emit current then emitter spacecraft pulled very neg. Configurations Series – Bias Grounded Gate Grounded Tip Grounded Gate

  9. Differences in Mission Objective Tether length: 5005 m Geometry: Single Line Bare vs. Insulated: 50% Bare Boost vs. De-boost: Both cases Orbital Parameters: 0o Latitude, 35o Inclination HVPS: 2000 V S/C Surface area: Next Slide Emission device: TC, FEA, or HC Models: IRI-2001, MSIS-86, IGRF-91 Test Dates: 1-1-06 (Min) & 7-15-01 (Max) EDT Simulation System Setup

  10. EDT Simulation System Setup 2 Total Mass = 1055 kg Total SA = 15.622 m2 Ballistic Coeff. = 30.697

  11. Simulation Analysis 1 • Max boost [N]: • High Density: ~0.56 HC, ~0.51 FEA, ~0.076 TC • Low Density: ~0.048 HC, ~0.046 FEA, ~0.033 TC • Fewer TC Emitters: Potential near Max • System correspondingly reacts

  12. Simulation Analysis 2 • Total Power ( = PHVPS) [W]: • High Density: ~7800 HC, ~7300 FEA, ~1700 TC • Low Density: ~418 HC, ~ 421 FEA, ~462 TC • TC has weakest boost & uses most power (Min) • Same issue with Fewer Emitters in Max Case

  13. Simulation Analysis 3 • Efficiency = Orbit Power / Supplied Power • Identical (Min) • Dip being investigated (Max)

  14. Simulation Analysis 4 • Boost / Power [N/W] • HC most Efficient • Within 25% of max value through 2000 km • Investigating max density case

  15. Simulation Analysis 5 • Max boost [N]: • High Density: -0.57 HC, -0.52 FEA, -0.11 TC • Low Density: -0.038 HC, -0.036 FEA, -0.024 TC • Fewer TC Emitters: Potential near Max • System correspondingly reacts

  16. Simulation Analysis 6 • Total Power ( = PEMF) [W]: • High Density: ~7390 HC, ~6760 FEA, ~1580 TC • Low Density: ~330 HC, ~320 FEA, ~280 TC • Near equivalent power (Min) • Same issue with Fewer Emitters in Max Case

  17. Simulation Analysis 7 • Boost / Power [N/W] • TC most efficient until 400 km in high density case • Efficiency reaches min at 500 km then goes up • FEA highest after ~1300 km in low density case

  18. Conclusion • TC’s • Not Very Effective • FEA’s • Nearly identical to HC performance • HC’s • Proven, Most Powerful & Efficient • Requires use of a consumable gas! • Future Work: • Analysis on Other EDT Mission Objectives • Further analysis on current work

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