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 of ALL nuclear reactions – Lowest for sizeable reaction rate • What is BAD about this reaction? – => activation of confining vessel and resultant radioactivity – energy must be thermally converted (inefficiently) to electricity – must be separated from seawater – 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 • What is BAD about this reaction? – High required (why?) – Difficulty of confinement (technology immature relative to )

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---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

• ? • => 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 can be recovered via wall aborption and conversion • But really must Run at lower 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 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

• 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. " 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 ? • What about ?

April 20, 2006: 22.012 Fusion Seminar (MIT) FormationFormation ScenarioScenario

• Form cold target plasma in axial guide field • Blast target plasma with 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 • 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 !!?? • What gross stability issues would worry you? • Kink & Sausage modes (toroidal varieties) – gross MHD instability like Z- • Proposed to be “solved” via high energy, large orbit ions – system surely NOT MHD -- and possibly feedback control • Efficiency requirements for 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)