New Challenges in DEC Research for Space Applications: High-density Regime and Beam Neutralization

New Challenges in DEC Research for Space Applications: High-density Regime and Beam Neutralization A. G. Tarditi Electric Power Research Institute, Knoxville, TN (USA) 14 th US-Japan IECF Workshop - College Park, MD 14 Oct. 2012

New Challenges in DEC Research for Space Applications: High-density Regime and Beam Neutralization A. G. Tarditi Electric Power Research Institute, Knoxville, TN (USA)

Introduction: Rationale for Direct Conversion of Nuclear Energy in Space

Space Exploration Needs Incremental modifications of existing space transportation designs can only go so far and so slow? Aerospace sector needs new propulsion technologies

Space Exploration Needs (II) Design a spacecraft architecture that, for a given payload, enables the most capable missions Focus on minimal overall system specific mass a (kg/ kw) Choose highest energy density source (fusion is just second to matter-antimatter annihilation but antimatter production/handling energy cost is prohibitive) and a propulsion scheme with a minimal-mass and highest-efficiency in propellant acceleration

Seeking Ideal Space Propulsion The most efficient propulsion system will utilize the highest energy density source and the simplest propulsion configuration Utilize fusion products directly for production of thrust Low-Mass Fusion Core Fusion Products Exhaust

The Need for Fusion Space Technology Even if a fusion reactor were to be available today, its successful application to space propulsion would be constrained by the requirements of integration with an electric thruster End-to-end system mass and efficiency is ultimately all that matters if a significant step-change in the potentials of space travel is to be achieved Key figure of merit: specific mass a [kg/kw]

The Need for Fusion Space Technology (II) Design of a viable fusion reactor for civil power generation (e.g., ITER- program) driven by engineering constraints and economic and, above all, political issues These issues are very different than those that must be considered in a space power system. Furthermore, a large plasma confinement D-T machine with neutron-to-electric, heat conversion technology is unlikely to be able to suit space travel.

The Need for Fusion Space Technology (III) Finally, as the goal of a terrestrial power reactor is nearing reality, even larger fusion research resources will likely be taken away from the advanced fusion concepts Development of lighter fusion technologies and neutron-free electric power generation may be further delayed Thus it would be unwise for the space community to wait for a power-utility-driven fusion reactor to become available before engaging in developments tailored for space applications.

Fusion Propulsion Ideal case: Low-kg/kW fusion core => Fusion Products => Exhaust Fusion Reactor? Plasma Jet Thrust

Fusion Propulsion: Direct Approach Electric Power Auxiliary Systems Fusion Reactor Fusion Products Beam Beam Conditioning System Exhaust Plasma exhaust production in a direct fusion propulsion scheme

Fusion Propulsion: Indirect Approach Electric Power Auxiliary Systems Exhaust Fusion Reactor Plasma Accelerator Magnetic Nozzle Electric power production and plasma generation/acceleration in an indirect fusion propulsion scheme

Basic Terminology A jet of particles (beam) has velocity v and a mass flow equal to dm/dt (kg/s) Thrust: F th =v (dm/dt) Specific impulse (conventionally expressed in seconds): I sp =v/g 0, where g 0 is the Earth gravity acceleration The exhaust jet power (ultimately, all that matters): P jet =vf th

Mission-Driven Fusion Propulsion Design For a given mission (e.g. Mars) and given power and initial mass, there is an optimal specific impulse profile that allows the fastest transfer In the gravity-free approximation, it can be shown that the optimal specific impulse (I sp ) is proportional to the trip time (shorter trips will require more thrust, less I sp ) [Moeckel, 1972] For reasonable travel in the Solar System the optimal I sp is in the 10 4 s range W. E. Moeckel, J. Spacecraft, 6 (12), 863 (1972) and NASA-TN D-6968 (1972)

Mission-Driven Fusion Propulsion Design Fusion products from main aneutronic reactions: p + 11 B => 3 4 He +8.7 MeV 2.9 MeV a-particle speed 10 7 m/s (simplification: each a-particle is considered having an energy of 2.9 MeV) D + 3 He => p (14.7 MeV) + a (3.7 MeV) 3.7 MeV a-particle speed 1.3 10 7 m/s 14.7 MeV proton speed 5.3 10 7 m/s These reactions give a specific impulse in the 10 6 s range; too high for most practical purposes

Direct Fusion Propulsion Example The most straightforward approach to aneutronic fusion propulsion is to collect and collimate the reaction product particle flow (in general isotropic) and re-direct it in the direction for thrust. For the p- 11 B aneutronic fusion reaction (the best truly aneutronic candidate) the energy of the a-particle products is in the order of 3 MeV. This gives an a-particle ejection speed of about 1.2 10 7 m/s like. Assuming a 100 MW net fusion power ouput, there will be a flow of about 2 10 20 particles/s carrying this power. The thrust T that this flow can produce is equal to the mass flow (kg/s) times the speed (assuming the ideal case of laminar flow). The mass flow for a-particles is about 7 10-27 kg/particle 10 20 particles/ s=7 10-7 kg/s. The resulting thrust is then T= 7 10-7 kg/s 1.2 10 7 m/s 8 N and the specific impulse is I sp 1.2 10 6 s.

Direct Fusion Propulsion Example (II) Mars rendez-vous trip: for a constant acceleration, gravity-free environment the choice of the desired trip time determines the maximum peak velocity that needs to be achieved ( delta-v ). Assuming an initial spacecraft mass (high Earth orbit) of 350 metric tons (mt) and a final (payload) mass of 35 mt, A constant acceleration, variable specific impulse 50 days trip to Mars requires a maximum specific impulse of 5300 s, and an initial thrust of about 13000 N [Moeckel, 1972] From this example it is clear that the direct utilization of the fusion product in the propulsive jet exhaust does not fit the required scenario. To generate a thrust in the order of 10,000 N with the specific impulse provided by the 3 MeV a-particles a jet power more more than 100 GW, clearly beyond any feasible projection at this time.

The Need of Direct Energy Conversion Both direct and indirect fusion propulsion approaches are relying heavily on the energy conversion from fusion products Aneutronic, non-ignited fusion will likely require a large amount of re-circulated electric power Lack of direct energy conversion would cause an unacceptable penalty in the obtainable specific mass

Fusion Products Direct Conversion into Electricity

DEC into Electricity: Traveling Wave + + + + + Input Power: Ion Beam Output Power: AC/RF Travelling Wave Direct Energy Converter (TWDEC) concept [Momota, 1990, 1992] converting kinetic energy of MeV ion beam into electric energy (AC or RF, in the MHz range)

Inside the TWDEC Grids produce beam density modulation ( bunching ) The bunched beam loses energy as it passes through a series of electrodes inducing an alternating potential Momota s TWDEC Conceptual scheme

TWDEC Operation No high-voltage electrodes, collects the energy of the beam through a series of electrode pairs, each at a smaller alternating potential. Electrodes capacitively coupled to a densitymodulated (bunched) beam of charged particles Beam bunches travel through of properly spaced electrodes inducing an alternating potential Alternating current has several advantages over DC in terms power conditioning and distribution

Exploring New TWDEC Features Improve TWDEC efficiency and reduce weight for high-power devices: design for higher density beams and hollow, grid-less electrodes. Low-density, space charge limited, ion beams, only able to support a low power densities For large fusion reactors a low-power density TWDEC requires a very large structure (additional engineering constraints and a large mass penalty

Exploring New TWDEC Features (II) Achievable efficiency: the latest composite-cycle steam powered turbine generator can achieve efficiency up to 60%. Thus a large-scale development program for DEC technologies would be justified if the perspective of reaching conversion efficiencies near 90% would appear, at least in principle, realistic. A gridless TWDEC will reduce losses, especially at high power

TWDEC Experiments Review

TWDEC for ARTEMIS Mirror Reactor ARTEMIS Reactor Design [Momota, 1993] D- 3 He protons: 14.7 MeV => v=5.3 10 7 m/s Design for 183 MW fusion energy provided by a proton flow of f=7.8 10 19 particle/s Assuming a beam radius of r=5 m, S=pr 2 =78 m 2, from f=n v S it is found n = f/v S = 1.87 10 10 m -3 : large electrodes, low density

High-Density Fusion Products Beam

TWDEC Processes at High Density The capacitance of a sphere of radius R b near a large flat conductor at distance d is considered, neglecting for now the effect of the hole. For d>> R b the capacitance is approximated by [Kaiser, 2005] Ion bunch approaching electrode The potential induced by the ion bunch will be then That is, the electrostatic coupling between the bunch and the electrode decreases with the distance and increases with the charge Q b, thus higher density would improve the coupling

TWDEC Modulator Application of TWDEC Simulator to End-loss Flux of GAMMA 10 Tandem Mirror [Takeno, 2011a], [Takeno, 2010a] Bias-type TWDEC: electrodes biased with variable negative voltage: repelling electrons and accelerating incident ions Relative energy spread can be controlled by the adjustment of the negative bias resulting in improved efficiency

TWDEC Small Experiment Analysis Performance analysis of small-scale experimental facility of TWDEC [Kawana, 2008] Current density j=0.13 A/m 2 gives n =j /Q He v = 9 10 11 m -3

Artemis TWDEC 2D Simulation Two-Dimensional Analysis of Energy Conversion Efficiency for a Traveling Wave Direct Energy Convertor [Shoyama, 1996] Simulated 20% efficiency loss for ion collision to the grids: investigating disc-shaped electrodes for loss reduction

TWDEC Energy Spread Control Improvement of Cusp Type and Traveling Wave Type Plasma Direct Energy Converters Applicable to Advanced Fusion Reactor [Takeno, 2010b], [Takeno, 2011b] Conceptual illustration of fan-type TWDEC Experimental dual-beam TWDEC: as in fan-type TWDEC, two electrode arrays are in the optimal positions for each beam energy Electrically connected corresponding electrodes simulate curved surface electrodes Provides option for processing different ion energy in p- 11 B

TWDEC Energy Spread Control Improvement of Cusp Type and Traveling Wave Type Plasma Direct Energy Converters Applicable to Advanced Fusion Reactor [Takeno, 2010b] CUSPDEC with low-energy ion recovery Two-stage conversion using CUSPDEC with improved thermal ion conversion scheme Similar setup proposed for ion beam generation under Univ. of Illinois / NASA JSC collaboration

Charge Separation vs. Beam Density Studies of Charge Separation Characteristics for Higher Density Plasma in a Direct Energy Converter Using Slanted Cusp Magnetic Field [Taniguchi, 2010] CUSPDEC experimental device The dependence of electron transmission ratio on gradient of the field line CUSPDEC used to study charge separation vs. plasma density

Present Efficiency Limiting Factors Beam energy spread limits conversion efficiency (as some particles get out of phase with the traveling wave) Fan-shaped beam technique to compensate for thermal spread effect [Yasaka, 2009] works with ions following single-particle trajectories Efficiency improves with rejection of thermal electrons (reducing ion thermal spread) Particle-grid collisions energy loss estimated in the 10% range (increasing with power)

References [Momota, 1992] Fusion Technology, 21,2307 (1992) [Kawana, 2008] Energy Conversion and Management 49 (2008) 2522 2529 [Shoyama, 1996] J. Plasma Fusion Res. 72(5), 439 (1996) [Takeno, 2011a] Proc. 15th Int. Conf. on Emerging Nuclear Energy Systems, May 15-19, 2011, San Francisco, CA, page TuP-16 [Takeno, 2011b] Proc. 15th Int. Conf. on Emerging Nuclear Energy Systems, May 15-19, 2011, San Francisco, CA, page TuP-17 [Takeno 2010a] J. Plasma Fusion Res. Series, 9, 202 (2010) [Takeno 2010b] 23rd IAEA Fusion Energy Conference, Paper ICC/P7-02 [Taniguchi, 2010] J. Plasma Fusion Res. Series, 9, 237 (2010) [Yasaka, 2009] Nucl. Fusion 49, 075009 (2009)

Modeling TWDEC Bunch to Rim Electrode Coupling

Targeting TWDEC High-Efficiency Exploring co-existing ion and electron bunches to reduce particle diffusion (longer TWDEC=more efficiency) Electrons and ion beam modulated separately, bunched co-axially injected (ion ring bunch) Exploring the limit of higher density for improved particle-field coupling (more efficient coupling per stage requires shorter length) Investigating use of hollow electrodes with bunch strong focusing to eliminate particle-grid collisions

Bunch-Electrode Capacitive Coupling Let C bg be the capacitance between a grid electrode and the closest ion bunch. At any given time a charge Q b will induce a potential V= Q b /C bg. The capacitance is obviously time-dependent, as the bunch travels, first approaching, then moving away from the electrode.

Bunch-Electrode Capacitive Coupling (II) TWDEC electrostatic induction: consider the capacitances between a traveling bunch and the two adjacent electrodes Bunch Electrode

Bunch-Electrode Coupling Effect Highest potential difference between two adjacent electrodes as the bunch passes through one of them Zero potential difference when the bunch is half-way in between the two electrodes.

Bunch Approaching Electrode Estimate of the capacitance C bg : approximate the ion bunch be by a spherical charge distribution of radius R b The grid electrode will be first approximated by a conducting plane with a circular hole, corresponding to the cross section of the beam.

Bunch Approaching Electrode (II) The capacitance of a sphere of radius R b near a large flat conductor at distance d is considered, neglecting for now the effect of the hole.

Bunch Approaching Electrode (III)

Bunch through Extended Electrode Highest potential difference between two adjacent electrodes as the bunch passes through one of them: model with concentric cylinder (coax cable) with length >> radius

Bunch through Extended Electrode (II) Approximate the capacitance between the bunch and the passing-through electrode is by the capacitance of two concentric cylinders (coax cable) C max = where l th is the axial length of the cylindrical electrode, r ele is its radius, l th << r ele and r b is the bunch radius

Bunch through Extended Electrode (III) Example, with l th =10 cm, r ele =1 cm and r b =0.9 cm it is found C max =53 pf The induced potential on the electrode for a given charged bunch of charge Q b will be V ele =Q b /C max a-particle bunch density 10 15 m -3, bunch length l th =10 cm, 2.54 10 10 particles per bunch and Q b = 8.1 10 9 C The induced potential on the electrode for a given charged bunch of charge Q b will be V ele =Q b / C max =154 V

Neutralization Overall beam (partial) neutralization may allow higher charge density and larger coupling with the electrodes (more energy transfer per electrode) Co-existing electron and ring-ion bunch formed in the modulator section: overall neutral beam but local charge separation due to modulator potential well + + + + + + v i - - - - - - v E + + + + + - - - - - -

Neutralization (III) Maintain an anisotropic distribution for the electron bunch population: explore the possibility of triggering a Weibel instability to generate an EM wave Couple EM mode with resonant circuit to improve beam energy extraction in addition to the ES capacitive coupling

Neutralization (II) External axial magnetic field to control radial charge separation and produce concentric (fully or partially) bunches Ion bunch with higher charge density in the outer rim may improve coupling with hollow electrodes, eliminating grids for higher efficiency Electrode

Modeling and Simulation Plan Tool: PIC code (initially 2D, r-z, for azimuthally symmetric configurations Start with electrostatic model (much faster) Externally imposed modulation field and magnetic field Test first single particle trajectory, then increase density until collective effects become relevant Study electrostatic instabilities and axial magnetic field stabilizing effect

Fusion Products Direct Conversion into Exhaust Thrust (NIAC 2011 Study NASA Grant)

Old School : Slush Plasma Propellant The a s are injected into a denser, cold plasma (or gas After exchanging momentum and energy the propellant will be faster and warmer a-beam A magnetic nozzle will redirect (most of) the thermal energy into the direction of thrust

Fusion Energy to Thrust Direct Conversion Fusion Reactor Electric Power Slow/Dense Ion Source Dense Slow Ion Bunch Exhaust Ion Beam Collector Fast Ions TWDEC Fast Ion Bunch Fast-to-Slow Bunch Energy Transfer (TWDEC=Travelling Wave Direct Energy Converter) Neutralizing Electrons System concept

Converting Beam Energy into Thrust Two basic processes operate concurrently: 1. Fast-to-Slow Bunch electrostatic energy exchange Fast Ion Bunch Slows Down Slow, Dense Bunch Speeds Up Electric Field 2. Magnetic Piston effect created by fast beam bunches confined into a spiral trajectory j Slow, Dense Bunch Speeds Up Fast Ion Bunch Slows Down v bunch

Fast-to-Slow Bunch Energy Exchange Bunch Formation (Hollow Electrodes) Slow Dense Bunch Speeds Up Fast Ion Bunch Fast Ion Bunch Slows Down Electric Field Guide Magnetic Field Coil

Magnetic Piston : an Old Concept Concept illustration (from W.B. Kunkel, Plasma Physics in Theory and Applications, 1966)

Magnetic Piston: 1) Beam Injection STEP 1. Injecting fusion products with a large angle w.r.t. the axis of a solenoidal magnetic field: the longitudinal speed will be reduced and particles follow a spiral orbit B Collimated a-beam The gyro-radius for a 2.9 MeV a-particle in a 1 T field is about 0.25 m. Bunching can allow for non-adiabatic injection required to capture the ions.

2) Formation of Current Layer STEP 2. With a collimated, pencil-beam injection, the accumulation of ion bunches forms a current ring j B Traveling Storage Ring

3) Magnetic Field Increase STEP 3. As more particles are collected the current in the layer increases that, in turn, increases the magnetic field j

Magnetic Piston Pushing Target Ion Bunch j Slow Dense Bunch Speeds Up v FastBunch Fast Ion Bunch Slows Down

Summary Analysis of the conditions for improving the TWDEC efficiency Exploring the limit of higher density to improve TWDEC particle-field coupling Next experiments to focusing on the efficiency vs. density trend, rather than on the maximum efficiency obtainable

Summary (II) Exploration of new space propulsion concepts for suitable for aneutronic fusion Fusion energy-to-thrust direct conversion: turn fusion products kinetic energy into thrust Fusion products beam conditioning: specific impulse and thrust compatible with needs practical mission