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Digital Physics: Science, Technology and Applications Prof. Kim Molvig April 20, 2006: 22.012 Fusion Seminar (MIT) DDD-T--TT FusionFusion D +T → α + n +17.6 MeV 3.5MeV 14.1MeV • What is GOOD about this reaction? – Highest specific energy of ALL nuclear reactions – Lowest temperature for sizeable reaction rate • What is BAD about this reaction? – NEUTRONS => activation of confining vessel and resultant radioactivity – Neutron energy must be thermally converted (inefficiently) to electricity – Deuterium must be separated from seawater – Tritium must be bred April 20, 2006: 22.012 Fusion Seminar (MIT) ConsiderConsider AnotherAnother NuclearNuclear ReactionReaction p+11B → 3α + 8.7 MeV • What is GOOD about this reaction? – Aneutronic (No neutrons => no radioactivity!) – Direct electrical conversion of output energy (reactants all charged particles) – Fuels ubiquitous in nature • What is BAD about this reaction? – High Temperatures required (why?) – Difficulty of confinement (technology immature relative to Tokamaks) April 20, 2006: 22.012 Fusion Seminar (MIT) DTDT FusionFusion –– VisualVisualVisual PicturePicture Figure by MIT OCW. April 20, 2006: 22.012 Fusion Seminar (MIT) EnergeticsEnergetics ofofof FusionFusion e2 V ≅ ≅ 400 KeV Coul R + R V D T QM “tunneling” required . Ekin r Empirical fit to data 2 −VNuc ≅ −50 MeV −2 A1 = 45.95, A2 = 50200, A3 =1.368×10 , A4 =1.076, A5 = 409 Coefficients for DT (E in KeV, σ in barns) April 20, 2006: 22.012 Fusion Seminar (MIT) TunnelingTunneling FusionFusion CrossCross SectionSection andand ReactivityReactivity Gamow factor . Compare to DT . April 20, 2006: 22.012 Fusion Seminar (MIT) ReactivityReactivity forfor DTDT FuelFuel 8 ] 6 c e s / 3 m c 6 1 - 0 4 1 x [ ) ν σ ( 2 0 0 50 100 150 200 T1 (KeV) April 20, 2006: 22.012 Fusion Seminar (MIT) Figure by MIT OCW. ReactivityReactivity forfor protonprotonproton-Boron--BoronBoron FuelFuel 4 3 ] c e s / 3 m c 6 1 - 0 2 1 x [ ) ν σ ( 1 0 0 100 200 300 400 500 600 700 T1 (KeV) Figure by MIT OCW. April 20, 2006: 22.012 Fusion Seminar (MIT) ComparisonComparison ReactivitiesReactivities 8 ] c e s 6 / 3 m c 6 1 - 4 0 1 x [ ) ν σ ( 2 0 0 50 100 150 200 T1 (KeV) Figure by MIT OCW. 4 ] c e 3 s / 3 m c 6 1 - 0 2 1 x [ ) ν σ ( 1 0 0 100 200 300 400 500 600 700 T (KeV) 1 Figure by MIT OCW. April 20, 2006: 22.012 Fusion Seminar (MIT) AvailabilityAvailability ofof FuelFuel • Protons? • => Overwhelmingly available in seawater (deuterium extraction not even required!) • Boron? • Widely found in nature as alkali or alkaline borates or as boric acid (Boron11 constitutes 80.2% of the natural abundance) April 20, 2006: 22.012 Fusion Seminar (MIT) PowerPower DensityDensity ComparisonComparison p - 11B has almost 3 times the alpha energy of DT, so even with ½ the reactivity it produces LARGER alpha heating than DT => in that sense self-sustaining fusion is easier to maintain if high temperatures can be stably confined. Example Numbers: Compare to losses (Bremstrahlung): Whoops! April 20, 2006: 22.012 Fusion Seminar (MIT) HowHow toto beatbeat BremstrahlungBremstrahlung losses?losses? • Some Radiation can be recovered via wall aborption and conversion • But really must Run at lower electron temperature: Te < Ti •Example: Te = 85 KeV Ti = 235 KeV 3 PB =16 Watts / cm 3 PFus = 20 Watts / cm • Still marginal Power balance April 20, 2006: 22.012 Fusion Seminar (MIT) ITERITER ComparisonComparison forfor ReferenceReference What losses dominate in this Tokamak scheme? April 20, 2006: 22.012 Fusion Seminar (MIT) RequirementsRequirements forfor AneutronicAneutronic FusionFusion • Magnetic Pressure must B2 > (n T + n T ) balance particle pressure for 8π e e i i B2 confinement: ≈ 248 KeV @ n =1×1015 & B =10Tesla 8πn • High beta plasma • Te < Ti • Highly efficient direct energy recovery system (High recirculating power levels) April 20, 2006: 22.012 Fusion Seminar (MIT) FieldField ReversedReversed ConfigurationConfiguration (FRC)(FRC) Diagram removed for copyright reasons. See Figure 1 in Rostoker, N., A. Qerushi, and M. Binderbauer. "Colliding Beam Fusion Reactors.“ Journal of Fusion Energy 22, no. 2 (June 2003): 83-92. April 20, 2006: 22.012 Fusion Seminar (MIT) QuestionsQuestions • What about “negative” moving ions? • What about electrons? April 20, 2006: 22.012 Fusion Seminar (MIT) FormationFormation ScenarioScenario • Form cold target plasma in axial guide field • Blast target plasma with proton and Boron beams at high energy • This provides fuel, initial heating and confining current (electrons confined by positive electrostatic potential) • Assume power balance all works out and one has a “win” that is self-sustaining thermonuclear fusion • Collect energy (direct electric conversion) from escaping alphas April 20, 2006: 22.012 Fusion Seminar (MIT) FRCFRC FormationFormation Figure removed for copyright reasons. A different view of the FRC configuration. April 20, 2006: 22.012 Fusion Seminar (MIT) Problems?Problems? • Do you recognize this geometry from schemes presented during semester? • It’s an elongated Tokamak tiped sidewise with no toroidal magnetic field!!?? • What gross stability issues would worry you? • Kink & Sausage modes (toroidal varieties) – gross MHD instability like Z-pinch • Proposed to be “solved” via high energy, large orbit ions – system surely NOT MHD -- and possibly feedback control • Efficiency requirements for ion beam systems, alpha and radiation energy recovery are daunting • Some FRC properties demonstrated experimentally but BIG scale ups in all physical parameters will be required April 20, 2006: 22.012 Fusion Seminar (MIT) TheThe ChallengeChallenge April 20, 2006: 22.012 Fusion Seminar (MIT).
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