Overview of FRC Propulsion and Materials Research Authors: - David Kirtley and George Votroubek (MSNW LLC, Redmond, WA 98052, USA) - John Slough, Samuel Andreason, and Chris Pihl (Plasma Dynamics Laboratory, University of Washington) - Aydin Tankut and Fumio Ohichi (Materials Science Department, University of Washington) - Richard Milroy, Brian Nelson, and Eric Meier (Plasma Science and Innovation Center, University of Washington) - Alan Hoffman, Kenneth Miller, and Daniel Lotz (Redmond Plasma Physics Laboratory, University of Washington) Presented at: AFOSR Propulsion Materials Workshop, November 4, 2010 - Basics of FRC Physics - Summary of Propulsion Activities - Summary of Related DOE Activities - Related Materials Interests - Current Materials Research Program - Needs and Discussion Equilibrium relations and parameters: - R: null radius - rs: separatrix radius - rc: coil radius - xs: rs/rc - External Field, FRC Closed Poloidal Field, Radial Pressure Balance, Axial Pressure Balance, and Flux conservation. Rotating Magnetic Field Formation: - m=1, 'saddle' coils positioned radially external to axial field coils. - Two oscillators phased at 90° produce constant amplitude Bω. - Synchronous electron motion: jθ(r) = e * ne * ω * r - Decreases circuit requirements (100% solid state) - Decreases radiation losses (operation on heavy gases) - Increases plasma currents and acceleration force - Minimizes wall interaction - RMF generated plasma current from synchronous electrons in a steady magnetic field with conical geometry. Operating Steps: 1. Rotating Magnetic Fields (RMF) form high-density, FRC plasmoid. 2. FRC grows and accelerates driven by RMF-generated currents and steady field. 3. FRC expands as ejected, converting thermal energy into directed kinetic energy. Advantages: - Vast Operating Range: 10-100 kW, 1000-6000 s Isp in a single thruster. - Technology Scalable: 0.1 to 1000s of kW, 10^18 to 10^20 m^-3 operating densities. - Low Mass Thruster and PPU: 1-2 kW/kg including PPU. - Efficient Ionization: Rapid, high-temperature, and magnetically isolated. - COTS Electronics: Low voltage, solid-state switching. - Long Life and Any Propellant: Electrodeless and magnetically isolated. Status: - Basic operation and performance demonstrated with earlier ELF program. - Current program aims to develop from 6.1 to 6.2 and prove a steady state thruster. - Initial design and advanced antenna geometry demonstrated. - Neutral entrainment chamber constructed and passive operation underway. - Modeling effort contracting is underway. Issue: Plasma formation is the primary loss mechanism for ALL electrostatic or electromagnetic propulsion systems. It prevents operation at lower specific impulse, lowers efficiency at all exit velocities, and requires high mass propellants. Solution: Entrain neutrals in an acceleration field after formation. - Benefit: Dramatic increases in thrust-to-power (T/P) across all specific impulses and propellants, including Air. - Dynamic formation and acceleration of an FRC shown with 4 field coils. A 1 kW-scale FRC thruster for deep space missions: - 200-2000 Watt FRC thruster (3” diameter, 4” long, 0.2-2 Joule). - Very long lifetime, throttle-able power, deep space propulsion. - Dramatically lower mass than existing electric propulsion (EP). - In-situ operation on ambient propellants. - First demonstration of FRC formation at <1 Joule. - Plasmoid achieved 1000-6000 s Isp in Xenon, Hydrazine (simulant). - First demonstration of steady state operation (50 plasmoids at 2 kHz). - Demonstrated technology scaling and circuit efficiencies (10 nH stray inductance). - Plasma Liner Compression (PLC): High energy magnetic compression, Xenon plasma ions at >2 keV at end regions. - Pulsed High Density (PHD): FRC formation and collision, >10 MW plasma, >10 GW/m^2 transient energy loading. - Foil Liner Compression (FLC): Metallic liner implosion, intense transient neutron and UV radiation pulse. - Translation Compression and Sustainment (TCSU): Steady State RMF-Formed FRCs at 10 MW energies, low density requiring low recycle/impurity rates. - Translation, Compression, and Sustainment Experiment (TCS): Study formation and sustainment of RMF-driven FRCs directly via RMF alone or by translating and expanding theta-pinch formed FRCs from LSX/mod. Fundamentally Non-Equilibrium Transient Loading: - Pulsed devices have transient wall loading of optical radiation, electric fields, and ions. Determining equilibrium temperature, non-linear sputtering effects, gas deposition, and recycling is critical. Wall Chemistry for Reactive Gases: - FRC thrusters and fusion devices operate on chemically reactive gases (O2, N2, H2). High-temperature ion-wall chemical sputtering can exceed kinetic sputtering rates. Effects of Magnetized Plasma: - Magnetically confined ions with pulsed fields (300-3000 Gauss), large gyroradii, and large electric fields at the wall (up to kV levels) alter wall interaction and thruster lifetime. Optical and Nuclear Radiation: - Pulsed optical and UV/neutron radiation causes contamination, sputtering, and thermal loading at the neutral gas boundary. - Magnetic Isolation dramatically limits plasma-wall interaction. - Lifetimes must be demonstrated for ELF and EMPT development programs. - Erosion of insulator (investigating if an insulator is strictly required). - RF coupling to high-temperature insulators (dissipation). - Material selections: Quartz (low thermal conductivity), Boron Nitride (BN), Aluminum Oxide (AlO), Silicon Nitride (SiN). - Key thruster elements: Quartz/SiN/insulator walls, Aluminum flux conservers/reflectors/heatsinks, and Copper RMF antennas. First Wall Material Coatings & Preparation: - Ta, SiO2 coatings for surface gas loading. - Steel and quartz chemistry during reversals; siliconization, atomic oxygen loading, and Ti-gettering. Diagnostics: - Surface Analysis System (SAS): Cylindrical Mirror Analyzer (CMA), X-Ray Photoelectron Spectroscopy (XPS), Plasma Chemical Vapor Deposition (PCVD, RGA), Auger Electron Spectroscopy (AES), and vacuum materials handling. - TCS and UW Facilities: He Glow Discharge Cleaning (He-GDC), Scanning Electron Microscopy (SEM), Multi-point Thomson Scattering (MTS), in-situ optical first wall diagnostics, and scrape-off layer Langmuir probes. Primary Materials Concerns: Impurities from wall materials and gaseous wall loading, chemical and kinetic sputtering, gas implantation (recycling, embrittlement), and neutron activation. UW Experimental Efforts: - Wall materials selection, first wall processing, high energy divertor studies, neutron studies, and DPA limits. PSI-Center Modeling Efforts: - NIMROD FRC modeling of scrape-off layer, wall layer, divertor flow, and first wall interactions. Key Publications: - A. Tankut et al., 'Wall conditioning in TCSU and its effect on plasma performance', Journal of Nuclear Materials, 2010. - A. Tankut et al., 'An XPS study on the evolution of type 304 stainless steel surface during routine TCSU operation', JNM, 2010. - A. Tankut Doctoral Thesis (2009) and APS/ICCM presentations (2008). Core Science Question: What are the average temperature and sputtering effects of non-equilibrium, chemically-reactive, transient plasma-wall interaction in a highly magnetized plasma? Proposed Plan: - Leverage University of Washington DOE program diagnostics, hardware, and personnel for space propulsion. - Add interface to the Neutral Entrainment experiment to utilize existing SAS hardware. - Execute MSE post-doc investigations on erosion, deposition, surface chemistry, and material selection. - Utilize PSI-Center boundary condition modeling via NIMROD and implement UEDGE Tokamak code for ELF thruster configurations. Impurity Analysis in TCS: - Radiated power vs. total input power showed a significant fraction of input power was radiated due to impurities, impeding FRC physics studies. - Surface evolution of Stainless Steel (SS) was tracked through standard cleaning, He-GDC, atmospheric exposure, and plasma discharge. Ti-Gettering Performance: - Ti-films deposited on SS and quartz with deposition rate controlled by filament current. - Ti-gettering reduced Fe-O compounds on the surface and trapped volatiles from stainless steel. - Inhibited release of H2O and drastically reduced impurity radiation and hydrogen recycling during plasma operation. Program Plan: - Year 1: Initial thruster prototype testing in pseudo-steady state mode, multi-discharge single gas puff tests, chamber effect investigations, and AFRL validation testing. - Year 2: Complete demonstration thruster and nose-to-tail electronics package, in-situ performance validation, and comprehensive AFRL/MSNW testing. Key Demonstrated Results: - First demonstration of non-inductive formation, acceleration, and ejection of FRC plasmoids. - Specific impulse of 1000-6000 s achieved in Air, Xenon, and Monopropellant. - Produced >1 mN-s impulse per pulse on Air propellant. - Direct thrust measured using a ballistic impulse pendulum (calibrated at NASA GRC). - Average power 25-50 kW steady state (5-50 J discharges). - Peak measured efficiency ~50% in Xenon (theoretical 87%); no noticeable erosion observed in multiple discharge tests. - Helicon Thruster: Relies on electrothermal heating and nozzle expansion; exhibits detachment and beam spread challenges. - High Power Helicon Thruster: Lower frequency RF yields high beta plasma with axial Jθ x Br expansion. - Electrodeless Lorentz Force (ELF) Thruster: Lower frequency rotating field creates synchronous theta electron motion, forming a magnetized FRC plasmoid. Strong axial Jθ x Br forces eject the plasmoid cleanly with no detachment issues, converting thermal to directed kinetic energy. - Magnetically Accelerated Plasmoid Propulsion: Peristaltic sequencing of axial flux coils accelerates the FRC further while entraining neutral gas, optimizing thrust and efficiency.