Award Title: Fusion-Enabled Pluto Orbiter and Lander Report Type: NIAC Phase II Final Report Principal Investigator: Ms. Stephanie Thomas, PSS ([email protected]) Co-Investigators: Mr. Michael Paluszek, PSS ([email protected]); Dr. Samuel Cohen, PPPL ([email protected]) Institutions: Princeton Satellite Systems, Inc., 6 Market St. Suite 926, Plainsboro, NJ 08536, (609) 275-9606, www.psatellite.com; Princeton Plasma Physics Lab, P.O. Box 451, Princeton, NJ 08543-0451, (609) 243-2000 Period of Performance: May 9, 2017 through May 9, 2019 Report Date: May 15, 2019 NASA Award Number: 80NSSC18K0040 NASA Innovative Advanced Concepts (NIAC) Direct Fusion Drive (DFD) is a unique fusion engine concept based on the Princeton Field-Reversed Configuration (PFRC) fusion reactor conceived by Dr. Sam Cohen of the Princeton Plasma Physics Laboratory. DFD would enable the Pluto orbiter and lander context mission and more broadly enable true "rapid transit" to outer-planet and near interstellar space. The truly game-changing levels of thrust and power in a modestly sized package could integrate with our current launch infrastructure while radically expanding the science capability of these missions. Our Phase I was our first funded work on the DFD, with previous work at PSS occurring only under internal R&D. We established the feasibility of our Pluto mission trajectories using straight-line and planar models, including a departure spiral from Earth and insertion at Pluto. We developed our first thrust and specific impulse model using the results of the UEDGE multi-fluid code. Our specific power model was improved. During this Phase II effort, we continued our efforts to increase the fidelity of the designs for the RF, magnet, and shielding subsystems. Dedicated thrust augmentation experiments were run on the PFRC experiment, using a supersonic gas puffing valve. For the first time, we analyzed the design of a closed-loop operation mode and estimated the hardware that would be required for a dual-mode engine. In an exciting new development, we have invented a new thermophotovoltaic thermal conversion method that has the potential to have efficiencies of a Brayton or Stirling system. Our report presents details of these analyses. Our roadmap to bringing DFD to flight predicts that with sufficient support, a first flight unit could be built by 2040. We anticipate that three machine generations are required before this point: a ~1 T PFRC-3 machine hitting new plasma temperature and density levels, a ~5 T PFRC-4 machine with first demonstration of D-3He fusion, and a flight. (The current experiment, PFRC-2, is limited to about 0.1 T). In order to achieve a flight in 2035-2040, the TRL of the supporting systems must be increased in parallel, including low mass radiators, cryogenic propellant storage, and large (>100 kW) thermal conversion systems. Fortunately, many of these systems are dual-use and are required for other technologies including fission systems, so DFD would contribute to and benefit from those programs. This NIAC support and the results of our work have led to multiple follow-on contracts. We won two NASA STTRs focused on DFD subsystems, one on the RF system and one on the superconducting magnets, and our magnet STTR is now midway through a Phase II. We will be receiving a superconducting test magnet for experiments at PPPL this summer. In addition, we won an ARPA-E OPEN grant; this is the first time that OPEN has included fusion technologies, and we are part of a cohort of three alternative fusion companies now supported directly by DOE. We are very optimistic that if we are able to meet our experimental milestones in the next 12-18 months, we will be competitive for additional DOE grants to build PFRC-3. Table of contents: 1 Introduction - 9 2 Methods and Assumptions - 11 2.1 Work Plan - 11 2.2 Princeton Field-Reversed Configuration (PFRC) Reactor - 12 2.3 Modeling Tools - 14 3 Results - 16 3.1 Superconducting Magnets - 16 3.2 Shielding Design - 32 3.3 Synchrotron Modeling - 41 3.4 Thrust Augmentation Experiments - 51 3.5 RF Modeling - 56 3.6 Closed-Loop Operation - 59 3.7 Updated DFD and Mission Design - 76 3.8 Conclusions - 83 4 Bibliography - 87 5 Acronyms - 92 Appendix A. PFRC Technical Description - 93 Our NIAC mission context is delivery of a Pluto orbiter with a lander, with the lander being powered from orbit. Direct Fusion Drive (DFD) provides high thrust to allow for reasonable transit times to Pluto while delivering substantial mass to orbit: 1000 kg delivered in under four years using 5-10 N thrust. As much as 500 kW might be available to the payload, for communications and power beaming. In contrast, the recent New Horizons mission required a 10 year transit time for a flyby mission with the spacecraft having only about 200 W of power. Our Phase I NIAC was our first funded work on the DFD, with previous work at PSS occurring only under internal R&D. PFRC was previously supported at PPPL at a low level under DOE funding. This supported the construction and operation of the PFRC-1, followed by the PFRC-2. These machines used repurposed water-cooled magnets with a specially designed vacuum vessel and RF subsystem. PFRC-2 has held the world record for pulse length in an FRC, and for a long time was the only fusion experiment in the US using high-temperature superconductors, which are utilized in the passive flux-conservers. PFRC-1 and PFRC-2 established that the "odd-parity" RF heating method heats electrons efficiently. Dr. Cohen, the inventor of the PFRC, saw the potential for PFRC to be used as a space propulsion device. PSS collaborated with Dr. Cohen on a number of conference papers about Direct Fusion Drive leading up to our NIAC Phase I award. 2.1 Work Plan Our Phase I findings indicated that our baseline mission trajectories to Pluto were feasible. Lower cost launch vehicles could be used if the DFD vehicle were launched first into LEO. It would then depart from LEO using a spiral trajectory. Our mass studies indicated that we need to more carefully account for the mass of the magnets, as both superconductor and its support structure are important mass drivers. The RF subsystem is not a mass driver but does drive the total engine efficiency. The Phase II research objectives are focused on those subsystems that have the most unknowns after Phase I in terms of their design specifications and contributions to the engine mass. The Phase II study objectives: - continue research on the superconducting magnets - refine the shielding analyses - investigate a highly efficient RF subsystem design concept using switching (class E) amplifiers - create a synchrotron model that is specific to our magnetic configuration - analyze the nozzle magnet geometry and its effect on thrust - evaluate experiments relating to the thrust augmentation system. 2.2 Princeton Field-Reversed Configuration (PFRC) Reactor The Direct Fusion Drive concept is an extension of ongoing fusion research at Princeton Plasma Physics Laboratory dating to 2002. PFRC machines employ a unique radio frequency (RF) heating method invented by Dr. Samuel Cohen. Approximately 35% of the fusion power goes to thrust, 30% to electric power, 25% lost to heat, and 10% is recirculated for the RF heating. 2.3 Modeling Tools 1. LSP - 3-D electromagnetic particle-in-cell code for Large Scale Plasma simulations. 2. UEDGE - 2D multi-species fluid code predicting electron and ion temperatures. 3. RMF - Hamiltonian code modeling single-particle heating in 3-D with specified magnetic fields. 4. ATTILA - radiation transport environment modeling radiation effects, neutron transport, activation, and defect generation. 5. FieldCoils - determines magnetic field magnitudes and lines. 6. GS Solver - determines plasma current assuming ideal MHD equilibrium. 7. ANSYS - used for structural calculations of the magnets. 8. Spacecraft Control Toolbox: Fusion Propulsion Module (MATLAB utilities). Superconducting Materials Properties: Low-temperature superconductors and high-temperature superconductors operated at low temperatures will have similar total neutron fluence limits on the order of 1e23 n/m2. High-temperature superconductors see a performance increase at moderate fluence of about 2e22 n/m2 but degrade at higher fluences. Consultant Contract – Dr. Minervini: Dr. Minervini developed a pancake magnet sizing model suitable for HTS tape. For a nozzle coil field target of 16-20 T for a reactor-scale PFRC-4, operating current near 400 A requires temperatures below 20 K. Grad-Shafranov Solver: Iteratively computes plasma currents and induced magnet currents. Spacing between axial magnets must generally be less than one-half the magnet radius. For a 6-magnet FRC design with LTS axial coils (4.2 K) and HTS nozzle (16 T, 10 K), total magnet mass is 391 kg (cryocooler mass 712.8 kg). If all coils are HTS at 10 K, magnet mass is 230 kg (cryocooler mass 518.75 kg). Cryocooler sizing functions based on ter Brake's survey were implemented: mass = alpha(T) * Qc^beta(T). Shielding is a primary driver of specific power. Neutrons from D-3He and suppressed D-D reactions produce substantially less neutron wall loading (1–6 kW/m2) compared to D-T fusion. LiH is considered for neutron moderation due to its low density (0.78 g/cm3, one-third of B4C) and high elastic scattering cross-section for fast neutrons (2.5 MeV). 10B4C or 6LiH is used for thermal neutron capture. Mean free path for fast neutrons in LiH and B4C is ~3.6 cm; 10 cm of shielding provides ~93% neutron attenuation. A 10 cm LiH shield for a 1 MW reactor has a mass of ~181 kg (plus 19.4 kg W x-ray absorber), ensuring a 10-year magnet lifetime at <5e22 n/m2 fluence limit. 0D, 1D, and 2D models of synchrotron emission from the DFD were developed. In an FRC, the magnetic field varies strongly with radius, being lowest at the core where density and temperature are highest. Reflectivity of the first wall to microwave (~100 GHz) radiation is critical. A copper coating (R ~ 0.95 to 0.99) reflects synchrotron radiation back into the plasma, reducing net synchrotron loss to 25–31% of total fusion power while raising electron temperature self-consistently to ~55–57 keV. Experiments on PFRC-2 investigated gas injection into the Source End Cell (SEC) using a fast supersonic puff valve to study scrape-off-layer (SOL) flow and plasma-mediated gas transport. Results confirmed that plasma significantly accelerates neutral gas flow through the chamber (5 ms response versus hundreds of ms for neutral gas alone), demonstrating proof-of-principle for scrape-off-layer thrust augmentation. Exploration of high-efficiency switching RF heating systems using Class E and inverse Class E amplifiers operating at 1 MHz with GaN transistors and k-Core graphite thermal spreaders. These enable theoretical efficiencies up to 100%, handling 10 kW per board to drive the rotating magnetic field (RMF) antennae. For orbit or power-only mode, the DFD operates closed-loop where propellant is captured and recycled via a divertor chamber (~1 m length/diameter). Superpermeable metallic membranes (e.g. Pd, Nb, Fe) allow hot atomic hydrogen (>1 eV) to pass through with >90% transmission while rejecting impurities, replacing 275 kg of turbopumps with just 20 cm2 of membrane (~1 kg). A novel thermophotovoltaic (TPV) conversion concept was developed pairing a molecular working fluid (such as CO2 at 2000 K) emitting sharp spectral lines with a low bandgap photovoltaic cell (PbSe, 0.27 eV) behind a sapphire window, achieving predicted conversion efficiencies of 25–43% in a solid-state system. Updated reference designs were established: - 1 MW DFD Engine: 1345 kg total dry mass, 0.75 kW/kg specific power, 4–5 N thrust, 8000–8500 s Isp, 5.0 T magnetic field, 0.25 m plasma radius, 0.9 m length. - 10 MW DFD Engine: 9400 kg total dry mass, 1.07 kW/kg specific power, 35–55 N thrust, 9900–12000 s Isp, 5.2 T magnetic field, 0.3 m plasma radius, 5.0 m length. Mission applications: - Pluto Orbiter & Lander: 1000 kg payload delivered in 5 years with 0.6 MW power (or under 4 years with two 1 MW engines). - Mars Fast Transit: 40 Mg payload in 110 days (or 45 days with higher specific power/aerobraking). - Jupiter: 1 Mg payload in 1.2 years. - 125 AU Interstellar Precursor: 1 Mg payload in 10 years. - Mercury Transfer: 1000 kg payload in <140 days (0.9 MW engine, 21 km/s delta-V). Conclusions: Follow-on funding was secured via NASA STTR Phase II for superconducting magnets, ARPA-E OPEN grant ($1.25M), and provisional patents on Magnetic Dipole Cancellation and Direct Thermal Energy to DC Power Conversion. A four-phase roadmap projects an operational 1 MW flight engine by 2035–2040 through progressive testbeds (PFRC-2, PFRC-3A/B, PFRC-4). Appendix A Summary: PFRC operates using clean D-3He fuel with odd-parity RMF heating, yielding 99% charged particle power, high plasma beta (~0.88), minimal neutron wall loading, and magnetic nozzle thrust augmentation providing variable thrust and specific impulse.